Alnus incana


Table of Contents


INTRODUCTORY


Gray alder thicket. Photo by Robert Vidéki, Doronicum Kft., Bugwood.org.


AUTHORSHIP AND CITATION:
Fryer, Janet L. 2011. Alnus incana. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
ALNINC
ALNINCR
ALNINCT

NRCS PLANT CODE [335]:
ALIN2
ALINR
ALINT

COMMON NAMES:
gray alder

speckled alder
tag alder

thinleaf alder
mountain alder

TAXONOMY:
The scientific name of gray alder is Alnus incana (L.) Moench (Betulaceae) [120,127,137,167,169,192,237]. There are 2 subspecies in North America:

Alnus incana (L.) Moench subsp. rugosa (Du Roi) R.T. Clausen, specked alder [120,137,192,205,237,255]
Alnus incana (L.) Moench subsp. tenuifolia (Nutt.) Breitung, thinleaf alder [120,167,179,192,355]

European gray alder, Alnus incana (L.) Moench subsp. incana, is native to western Europe [120,127]. It has been introduced in the northeastern United States [231]. Since variation in the subspecies is continuous rather than discrete, gray alder subspecies are sometimes difficult to tell apart ([127], review by [143]). Speckled alder and thinleaf alder intergrade where their ranges overlap, mostly in Saskatchewan and Manitoba [106,120,127,231].

In this review, gray alder refers to information that is general to the species. Speckled alder refers to A. i. subsp. rugosa, and thinleaf alder refers to A. i. subsp. tenuifolia.

Hybrids: Gray alder hybridizes with European alder (A. glutinosa), an introduced species [133]. Speckled alder hybridizes and intergrades with hazel alder (A. serrulata) [120,231,295]. Speckled alder × hazel alder hybrid swarms may occur in Massachusetts and elsewhere [127]. Thinleaf alder may hybridize with red alder (A. rubra) in Idaho [202].

SYNONYMS:
For speckled alder (Alnus incana subsp. rugosa):
Alnus incana (L.) Moench var. americana Regel [131]
Alnus rugosa (Du Roi) Spreng. [106,171,231,295,304,350,359]
Alnus rugosa (Du Roi) Spreng. var. americana (Regel) Fernald [37,304]
Alnus rugosa (Du Roi) Spreng. var. rugosa
Alnus rugosa (Du Roi) Spreng. forma hypomalaca Fern [37,304]

For thinleaf alder (Alnus incana subsp. tenuifolia):
Alnus incana (L.) Moench var. occidentalis (Dippel) C.L. Hitchc. [208,304]
Alnus incana (L.) Moench var. virescens S. Watson
Alnuspurpusii Callier
Alnus tenuifolia Nutt. [155,208,231,242]

LIFE FORM:
gray alder:
Tree-shrub

speckled alder:
Shrub

thinleaf alder:
Tree-shrub

DISTRIBUTION AND OCCURRENCE

SPECIES: Alnus incana
GENERAL DISTRIBUTION:
Distributions of gray alder and its subspecies. Maps courtesy of USDA, NRCS. 2011. The PLANTS Database. National Plant Data Team, Greensboro, NC. (14 June 2011).

gray alder

Gray alder occurs in North America and Europe [120,192,296]. It occurs in the northeastern and central regions of Europe, extending locally into the southern mountains [296].

States and provinces (as of 2011 [335]):
United States: AK, AZ, CA, CO, CT, IA, ID, IL, IN, MA, MD, ME, MI, MN, MT, ND, NH, NJ, NM, NV, NY, OH, OR, PA, RI, UT, VA, VT, WA, WI, WV, WY

Canada: AB, BC, LB, MB, NB, NF, NS, NT, ON, PE, QC, SK, YT

speckled alder
Speckled alder, the eastern subspecies, occurs at least as far west as Saskatchewan [120,192]. Little [230] places its distribution as far west and north as northwestern Yukon, central Northwest Territories, and central Nunavut. Its distribution is mostly continuous, with scattered populations in Michigan, Ohio, and West Virginia [230].

States and provinces (as of 2011 [335]):
United States: CT, IA, IL, IN, MA, MD, ME, MI, MN, ND, NH, NJ, NY, OH, PA, RI, VA, VT, WI, WV

Canada: AB, BC, NT, SK, YT

thinleaf alder
Thinleaf alder, the western subspecies, has a mostly continuous distribution. Populations are scattered and sometimes isolated in the southern Sierra Nevada [179,192,229,229]. It has been reported in Baja California, although it may no longer occur there [358].

States and provinces (as of 2011 [335]):
United States: AK, AZ, CA, CO, ID, MT, NM, NV, OR, UT, WA, WY

Canada: LB, MB, NB, NF, NS, ON, PE, QC, SK



SITE CHARACTERISITICS AND PLANT COMMUNITIES:
Gray alder prefers moist to mesic sites throughout its distribution but occurs in a wide variety of plant communities within that moisture gradient. Its symbiotic relationship with nitrogen-fixing actinomycetes helps enrich soils; this affects plant community composition and succession. After fallen gray alder leaves and roots die and decay, their nitrogen is released back into the soil and becomes available to other plants [171,180,215]. An English poet wrote:

The alder, whose fat shadow nourisheth.
Each plant set there to him long flourisheth.
     óWilliam Brown, Brittania's Pastorials. 1613.

On the Tanana River floodplain of central Alaska, nitrogen input to thinleaf alder ecosystems due to this symbiosis was 4,734.5 kg/ha/year, averaged over 10 years [339]. In the Adirondack Mountains of New York, ecosystem nitrogen input from speckled alder-actinomycete symbiosis averaged 37 kg/ha/year. A Wisconsin study found a range of 1 to 4 kg/ha/year [180]. Gray alder is especially valuable in high-elevation riparian areas, where nitrogen is often limiting [215]. Site characteristics and plant community information follows by subspecies. Speckled alder
Site characteristics: This subspecies dominates on wetlands [131,255,359], although some speckled alder-dominated sites may dry by the end of the growing season [160]. Speckled alder grows on streambanks, lakeshores, and wet roadsides and in riparian forests, swamps and swamp edges, bogs, fens, bog or fen borders, margins of wet fields, and swales [120,127]. It often forms dense thickets [120,255]. Speckled alder communities often flood during spring run-off [331]. In boreal ecosystems, the black spruce (Picea mariana)/speckled alder forest cover type occurs on waterlogged sites and sites with gently flowing groundwater [86]. In Michigan, speckled alder grows along lakes and streams and in "extensive mucky" swamps [350]. A forestry note reported that in Minnesota, speckled alder occurs on moist to mesic sites but was most common on moist sites [16]. In Ohio, it is mostly confined to glaciated areas in the northern part of the state [37]. Speckled alder occurs in wetlands and by streambanks in the Northern Great Plains [219]. It may not tolerate flooding for long periods, however [205], and needs drainage when growing in swamps [171].

Speckled alder commonly occurs or codominates with willows (Salix spp.), a mixture that may be controlled by length of flooding season. Willows may tolerate longer flooding seasons than speckled alder. A laboratory study showed speckled alder was less tolerant to continuous flooding than willow species. Speckled alders suffered 33% average mortality when water levels were kept above the root crown for 2 years, while speckled alders experienced no mortality when the water level was maintained below the root crown for the same length of time [196].

Soils: Speckled alder tolerates a wide variety of soil textures and pH but is most common on nutrient-poor soils. It grows in sandy loam [49,160], chalky, rocky till, and mucky soils [160]. In Ontario, speckled alder is an indicator of fine loamy-clayey soils with thick black humus; it is also an indicator of poor to rich fens with moderately decomposed organic soils [188]. In swamps of northern Michigan, speckled alder communities tended to occur on relatively well-drained, sandy loams, while black ash/speckled alder communities occurred mostly on poorly drained, silty clay loams [275]. In the Catskill and Adirondack mountains, speckled alder grows in limestone-, gneiss- and anorthosite-derived soils [205]. In black ash-paper birch communities of the Great Lakes states, speckled alder was noted on minerotrophic peat substrates [31], and it grew in coarse woody peats in northern white-cedar (Thuja occidentalis) swamps of northern Wisconsin [67]. It occurred on glacial till in central New York; soils were neutral to "somewhat alkaline" [176]. It is reported on soils with pH from 3.4 to 7.7 in Ohio (review by [160]). In Wisconsin, speckled alder thickets occurred on peats of pH 5.5 [85]. In jack pine (Pinus banksiana) stands in Minnesota, speckled alder was most common on medium-quality silvicultural sites (18% frequency). It also occurred on good-quality sites (7% frequency) but not on poor-quality sites (Hansen 1946 cited in [39]). In an assessment of Michigan shrub habitats, speckled alder occurred on the moistest, coolest, and most nutrient-poor sites when compared to other shrubs [36]. However, speckled alder also occurs on nutrient-rich sites [204]. On the Saskatchewan River Delta, speckled alder was positively associated with relatively nutrient-rich, shallow peat fens and levees. Soil water was alkaline and high in calcium and magnesium [97].

Elevation and topography: Speckled alder ranges from near sea level on the North Atlantic coast to about 2,710 feet (825 m) in the Appalachians [127]. It grows on low-elevation alluvial soils in Nova Scotia [295] but at "moderately high elevations" elsewhere in Canada [304]. It is restricted to high elevations in the southernmost portion of its range [160]. In a tamarack bog in Itasca State Park, Minnesota, speckled alder was more abundant on hummocks (35% cover and 10% frequency) than in depressions (2% cover and <1% frequency) [142].

Plant communities: Speckled alder occurs in forest, shrub, and herbaceous communities. It is an important component of northern spruce-fir (Picea-Abies spp.) [22,247], red-white-jack pine (Pinus strobus-P. resinosa-P. banksiana) [69], Atlantic white-cedar (Chamaecyparis thyoides) [106], and mixed-hardwood [341] forests, especially at forest and bog ecotones [247]. It also occurs in elm-ash-cottonwood (Ulmus-Fraxinus-Populus spp.) galleries and forests [108]. In Newfoundland, it occurs in rich fens dominated by pale sedge (Carex livida) [289]. Species diversity is generally high in wetland communities where speckled alder is an important to dominant component of the vegetation (for example, [33,114,356]).

Regional descriptions of plant communities with speckled alder and site characteristics of speckled alder habitats follow.

Great Lakes: Speckled alder is common in coniferous, mixed conifer-hardwood, shrubland, and graminoid wetland communities in the Great Lakes region. It is common, for example, in the understories of red pine forests and plantations. American hazelnut (Corylus americana) and beaked hazelnut (C. cornuta var. cornuta) usually dominate or codominate the understories [46]. In Voyageurs National Park, Minnesota, black spruce/speckled alder communities occurred on nutrient-poor, wet sites [207]. On the Lake Agassiz Peatlands Natural Area of Minnesota, speckled alder was abundant (covered 0.25% to 0.5% of an area) in rich swamp forests dominated by northern white-cedar, black ash (Fraxinus nigra), tamarack (Larix laricina), or white spruce (Picea glauca). It was not abundant in poor swamp forests, bogs, fens, or heaths [163]. In northern Wisconsin, speckled alder was important to dominant in northern white-cedar swamps [67]. It was also important to dominant in eastern redcedar (Juniperus virginiana) bogs of Ohio [125]. On the shores of Lake Superior, a speckled alder/red raspberry /fowl mannagrass-reed canarygrass (Rubus idaeus/Phalaris arundinacea-Glyceria striata) thicket community occurred on secondary dunes, lees of high dunes, and sand flats. This community type had the highest species richness of 4 types identified [33].

Speckled alder is occasional to invasive on open fens and bogs of the Great Lakes states. In northeastern Ohio, it is characteristic in sedge (Carex) fens. Bottlebrush sedge (C. hystericina), hairy sedge (C. lacustris), and tussock sedge (C. stricta) are typical dominants [301]. Speckled alder often dominates marginal fen or marginal bog zones, which separate the mires from the upland communities surrounding them [73].

Northeast: Speckled alder grows in coniferous forests, mixed and hardwood forests, shrublands, and open wetlands in the Northeast.

Maine: On Peak's Island, Portland, speckled alder was characteristic in the tree strata of an upland red maple (Acer rubrum) forest, parts of which had burned 3 times in 24 years, and of an unburned balsam fir (Abies balsamea) forest. At the time of the survey, speckled alder was the 2nd most common taxon on 7-year-old burned sites in the red maple forest (97% relative density and 13% frequency on 10 m² plots) and the 4th most common in the balsam fir forest (56% relative density and 16% frequency). Speckled alder clumps were also frequent in a nearby red maple swamp, where speckled alder dominated poorly drained areas of the swamp [79].

New England: In pitch pine (Pinus rigida)-hardwood swamp mosaics of New Jersey, speckled alder was a minor to codominant component of red maple-black tupelo-sweetbay/coastal sweetpepperbush (Nyssa sylvatica-Magnolia virginiana/Clethra alnifolia) swamps. It was most important on continuously flooded sites, declining on sporadically flooded or dry sites [112].

Southeast: Speckled alder occurs in mixed-hardwood, pine (Pinus spp.), and shrubland communities of the Southeast. In a 1930 study, it was a component of a climax American chestnut-yellow-poplar-eastern hemlock (Castanea dentata-Liriodendron tulipifera-Tsuga canadensis) cove forest in the Black Mountains of North Carolina [91]. Yellow-poplar became the primary dominant in many cove forests after chestnut blight decimated American chestnut populations [341]. In Georgia, speckled alder occurs in pond pine (Pinus serotina)-shrubland and bay (Magnolia and Persea spp.)-shrubland bogs. Shrub diversity is generally high [356].

The following vegetation classifications describe plant communities, and some of their distinguishing site characteristics, where speckled alder is dominant. In state and province lists, plant communities are arranged geographically from north to south and west to east.

Great Lakes Northeast Southern Appalachians Southeast
Canada      

Great Lakes
Great Lakes, general Michigan Minnesota Wisconsin Northeast
Eastern North America, general Maine New York New England Southern Appalachians
West Virginia Southeast
Georgia Canada
Boreal North America, general Saskatchewan Ontario Quebec Maritime provinces Newfoundland

Thinleaf alder
Site characteristics: Thinleaf alder is most common on wet to moist sites (review by [143]). It is a frequent component of streamside vegetation throughout mountainous regions of western North America [120]. It is considered an indicator of riparian or subirrigated sites on the Shoshone National Forest, Wyoming [172]; of moist, well-drained sites—especially streambanks and springs at low elevations—in central Oregon [203]; and of moist to wet soils in subboreal spruce (Abies spp.) and pine (Pinus spp.)-spruce ecosystems of British Columbia [23]. Riparian sites with thinleaf alder may experience frequent flooding and/or scouring (for example, [104,114]).

Soils: Thinleaf alder grows on a variety of soil textures and nutrient levels. It grows in coarse-textured soils but is well-adapted to cold, "heavy" soils (review by [143]). It grows well in sand [199,339], and it is common on sandbars [339]. In the Trout Creek Mountains, thinleaf alder grows in sand- and siltbanks about 2.6 feet (0.8 m) above stream channels [114]. Soils supporting thinleaf alder are often rocky in mountainous areas [318]. In Montana, thinleaf alder communities typically develop on cobble and gravel, accumulating organic debris over time [149]. Nutrient levels of soils supporting thinleaf alder vary from poor to rich, although they are typically poor (review by [184]). Thinleaf alder typically establishes on poor, skeletal soils in primary succession [149], but it usually enriches in soils in which it grows [149]. In the understories of coniferous forests in Blue Mountains, thinleaf alder is an indicator of productive sites [144].

Elevation and topography: In the conterminous United States, thinleaf alder is primarily restricted to mid- to high-elevation mountains, mountain valleys, and mesic canyons [155,208,231,241], although it grows on low-elevation sites in Alaska and Canada [231]. In the United States, thinleaf alder ranges in elevation from near sea level in Alaska to over 10,000 feet (3,000 m) in Colorado and Arizona [127]. Thinleaf alder populations apparently do not have exacting elevational requirements. Thinleaf alder cuttings have been successfully transplanted onto sites that vary greatly in elevational range [256].

Elevational range of thinleaf alder by state
State Range (feet)
Arizona 5,000-9,000 [195,228]
California 3,900-7,900 [167]
Colorado 5,000-10,000 [155]
New Mexico 6,000-9,500 [96,228,241];
1,745-2,438 in Jemez Mountains [6]
Nevada 5,000-8,000 [193]
Texas 5,000-8,000 feet [348]
Utah 4,100-9,005 [355]

Thinleaf alder presence is likely more associated with moist to mesic conditions than with aspect alone. In the Wasatch Mountains of Utah, thinleaf alder was common in the bottom of Cold Canyon [272], but it was not noted on higher, north- or south-facing slopes, which may have drier soils [272]. In Lassen Volcanic National Park, California, the location of willow-thinleaf alder communities was not significantly associated with aspect, but it was positively associated with relatively moist conditions (P=0.01). Willow-thinleaf alder communities were most common on steep slopes, depressions, and valley bottoms (P≤0.002). The authors attributed their presence on steep slopes to the prevalence of intermittent streams in avalanche chutes and their presence in low areas to the prevalence of concave drainage channels [288].

Plant communities:
Thinleaf alder in a mixed-conifer forest on the El Dorado National Forest, CA. Photo © 2009 Dr. Mark S. Brunell.

Thinleaf alder prefers mesic to moist plant communities. These include the moist slopes of coniferous forests, riparian areas, wet meadows and grasslands, and fen and bog margins [127]. Site moisture and elevation are critical in determining relative dominance of thinleaf alder; the surrounding vegetation usually tolerates drier conditions. On the shore of Slave Lake in Alberta, thinleaf alder codominates the understory of white spruce-paper birch communities on gravel or sand beaches that are 300 feet (90 m) or less from the lake [227]. In the Trout Creek Mountains of Nevada, thinleaf alder communities are concentrated in low-elevation (4,920-5,300 feet (1,500-1,615 m)), narrow riparian corridors. Mountain big sagebrush (Artemisia tridentata subsp. vaseyana) typically dominates vegetation beyond the corridors, and plant species diversity is lower outside than within the riparian corridors [114]. Willows (Salix), red-osier dogwood, common snowberry (Symphoricarpos albus), and horsetails (Equisetum) codominate with thinleaf alder in many plant communities throughout much of thinleaf alder's distribution.

Regional descriptions of plant communities with thinleaf alder and site characteristics of thinleaf alder habitats follow.

Alaska: Thinleaf alder is reported on glacial shrublands (see Successional Status), riparian and wet-upland shrublands, and spruce-birch (Picea-Betula spp.) swamps. Thinleaf alder-willow and resin birch-thinleaf alder-willow cover types are common near timberline in interior Alaska. These communities form thickets that may be "extremely dense" or open and interspersed with reindeer lichens (Cladonia) and heath (Ericaceae) [357]. On the Tanana River of central Alaska, 5-year-old thinleaf alder communities on sandbars were codominated by feltleaf willow (Salix alaxensis), sandbar willow (S. interior), and barrenground willow (S. brachycarpa subsp. niphoclada). Balsam poplar, paper birch (B. papyrifera), and white spruce grew in the understory; meadow horsetail (E. pratense) and variegated scouringrush (E. variegatum) were dominant herbs [339]. See Successional Status for more information on this study.

Pacific Northwest: Thinleaf alder is a component of and sometimes forms glades within coniferous forests of the Pacific Northwest; it is also important to dominant in riparian corridors. Thinleaf alder is prevalent in ponderosa pine (Pinus ponderosa), Douglas-fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta), and fir-spruce (Abies-Picea spp.) communities [184]. Thinleaf alder glade openings generally occur on wet to mesic sites where thinleaf alder outcompetes conifers for soil moisture [92].

California: Thinleaf alder grows in coniferous, western hardwood, riparian, and other wetland communities in California. The photo above shows a typical Californian thinleaf alder habitat. Thinleaf alder is frequently found on the edges of ponds, bogs, and fens [242]. In the Klamath Mountains and Sierra Nevada, it grows on the margins of California pitcher plant (Darlingtonia californica) bogs [221]. Prior to extensive water diversions, thinleaf alder was likely important in quaking aspen-black cottonwood (Populus balsamifera subsp. trichocarpa) riparian corridors leading to Mono Lake, which is situated in the arid Mono Basin [320].

Northern and Central Rockies: Thinleaf alder is reported in coniferous forests, riparian galleries and shrublands, and wetland edges. In western Montana, thinleaf alder is frequent in Douglas-fir, western hemlock-western redcedar (Tsuga heterophylla-Thuja plicata), and grand fir (Abies grandis) forests. Surveys in the Blackfoot and Flathead valleys indicate that most thinleaf alders in these forest types grow as shrubs [121]. In southwestern Colorado, thinleaf alder grows in mixed-conifer riparian woodlands dominated by white fir (A. concolor), southwestern white pine (Pinus strobiformis), interior ponderosa pine (P. ponderosa var. scopulorum), and blue spruce (Picea pungens) [14].

In the Rocky Mountains, thinleaf alder-dominated riparian communities tend to occur on an elevational gradient just above narrowleaf cottonwood (Populus angustifolia) communities [281]. In western Colorado, Baker [15] noted that thinleaf alder was nearly always present at varying frequencies, and sometimes dominant, along narrowleaf cottonwood and blue spruce riparian forests. Thinleaf alder, red-osier dogwood, and water birch (Betula occidentalis) formed an often impenetrable shrub layer, with thinleaf alder dominant along stream margins and becoming less frequent with distance from the streams [15]. Thinleaf alder dominates in the upper reaches of the narrowleaf cottonwood-Scouler willow (S. scouleriana) formation in Boulder County; it may also dominate in canyon bottoms [364]. Its distribution extends into the mountain sagebrush-mountain grassland zone, mostly above 5,000 feet (2,000 m), in the Northern Rocky Mountains (reviews by [184,256]). In the Sawtooth National Recreation Area, Idaho, thinleaf alder dominates high-gradient streams (those on steep slopes or with rapid water flows) [173]. In northwestern Montana, thinleaf alder community types occur from 3,760 to 6,700 feet (1,150-2,040 m) elevation on moist stream edges, overflow channels, and slope seeps [32]. Thinleaf alder is common in many riparian shrublands dominated by other species, especially willows, red-osier dogwood, and Wood's rose (Rosa woodsii) (for example, [149,284]).

Southwest: In the Southwest, thinleaf alder occurs in wetlands within desert ecosystems and at high elevations. It may be a component of or locally dominant in riparian scrublands of Arizona; these scrublands are usually dominated by Bebb willow (Salix bebbiana), Scouler willow (S. scouleriana), and/or other willow species [40]. Thinleaf alder is an obligate riparian taxon in New Mexico, where it may dominate riparian shrublands solely or codominate with willows [96,276]. Its relative dominance generally increases with increasing elevation [95]. Thinleaf alder is a common shrub component of narrowleaf cottonwood communities of Arizona, New Mexico [20], and southern Colorado. Within wet areas, it sometimes grows as low as the pinyon-juniper (Pinus-Juniperus spp.) zone in northern Arizona [184]. In Great Sand Dunes National Monument, Colorado, it is an important component of narrowleaf cottonwood/Kentucky bluegrass (Poa pratensis) riparian communities [243]. Its distribution extends into the Engelmann spruce-corkbark fir zone (Picea engelmannii-Abies lasiocarpa var. arizonica), from 7,000 to 9,000 feet (2,000-3,000 m)), in the Rincon Mountains of Arizona (review by [184]). Thinleaf alder may be locally important to dominant in alpine riparian communities of the Southwest [40].

The following vegetation classifications describe plant communities, and some of their distinguishing site characteristics, where thinleaf alder is dominant or is a community type indicator. In state and province lists, plant communities are arranged geographically from north to south and west to east.

Boreal North America, general West, general Alaska Pacific Northwest
California Southwest Great Basin Northern and Central Rockies
Northern Great Plains Canada    

Boreal North America, general West, general Alaska
Pacific Northwest
Pacific Northwest, general Oregon Washington California
Southwest
Southwest, general Arizona New Mexico Great Basin
Nevada Utah Northern and Central Rockies
Rocky Mountains, general Idaho Montana Wyoming Colorado

Utah (see entries in Great Basin)

Northern Great Plains
Great Plains, general Canada
Yukon Saskatchewan

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Alnus incana
GENERAL BOTANICAL CHARACTERISTICS:
Male and female catkins of speckled alder.
Photo © 2005 Louis-M. Landry.
Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (for example, [120,137,167,169,192,237]).

Gray alder is a tree or shrub, growing from 15 feet to 82 feet (4.6-25 m) tall. [120]. Bark is smooth and thin [155,348], often with conspicuous lenticels [106] (see photo in Fire Effects and Management). The wood is soft [171]. Leaves are oblong and serrated at the margins [208,355]. The inflorescences are small, naked [131] catkins. Male catkins grow in clusters of 2 to 4. They are 0.8 to 3 inches (2-8 cm) long and pendulous at maturity. Female catkins are woody and resemble cones, growing in clusters of 2 to 6 [106,120,237]. The "cones" are 0.4 to 0.8 inch (1-2 cm) long at maturity [155]. The fruits are described as either irregular samaras [120] or nutlets with small, narrow wings [106,154,171,179,242,345]. They hold 1 to 4 seeds/cone scale [137,155,318]. The seeds lack endosperms, so the cotyledons are relatively small [154]. The root system is shallow and spreading [171]. Roots are typically infected with nitrogen-fixing, actinomycete bacteria [171,205,215,261]. A review reported that thinleaf alder fixes more nitrogen than Sitka alder (Alnus crispa subsp. sinuata) and quantities similar to those of red alder [143].

Gray alder is adapted to periodic flooding in spring or other run-off periods [23,204,267], although it cannot tolerate long periods of inundation. In the laboratory, speckled alder growth and root development were "severely reduced" when water levels were at or above the root crown for 30 days or more (P<0.05) [196,267]. A review ranked gray alder more flood tolerant than cottonwoods, birches, and elms but less tolerant than willows [130].

Morphological characteristics of the gray alder subspecies overlap (review by [143]). Thinleaf alder is typically more tree-like than speckled alder. The subspecies also differ in bark, leaf shape, and leaf margin characteristics [120,127,318].

Speckled alder is a spreading shrub [120] or small tree [131], growing up to 30 feet (9 m) tall [120] and 4.7 inches (12 cm) in diameter. Typically multistemmed with crooked branches, it is "very crooked" in form as a small tree [106], and only assumes tree form on high-quality sites [171]. Its common name refers to the lenticels that give a characteristic "speckled" look to the bark [81]. In a speckled alder community by a small stream in upstate New York, speckled alder stems averaged 14 years old, ranging from 7 to 31 years old. Stem density averaged 7,850 stems/ha [331]. In central New York, age of mature stems ranged from 10 to 25 years. Based on stem sprouting vs. stem mortality rates, the author estimated maximum age of speckled alder clones at about 100 years [176].

Thinleaf alder is an open, spreading shrubby tree or shrub, growing from 15 to 39 feet (4.6-12 m) tall [120,345] and usually less than 4 inches (10 cm) in stem diameter [215]. It often forms thickets along streams [179,231,242,345], although on upland sites it usually grows in discrete, shrubby clumps [208]. Thinleaf alder stems on sandbars of the Tanana River, central Alaska, averaged 14 years old [339]. Morris and others [259] provide a key for identifying thinleaf alder and other western shrubs in winter.

Thinleaf alder is frost-tolerant [23,204].

Raunkiaer [290] life form:
Phanerophyte
Geophyte

SEASONAL DEVELOPMENT:
Gray alder's catkins are formed the previous growing season and are exposed in buds during winter. In spring, gray alder flowers before leaf-out and stem elongation [120,171,176]. Male catkins drop in late spring, shortly after pollination [176,261]. There is "considerable" leaf drop in summer and fall [106]. Seeds disperse in fall or winter, although female catkins remain on the plant [171,176,261]. Both male and female catkins form the year prior to opening, so in fall, gray alders usually display immature male and female flower clusters, immature fruits, and old female catkins [171,176,261]. Phenology of speckled and thinleaf alder is as follows:

Speckled alder

Area Event Season
Illinois flowers May-June [255]
Atlantic coast flowers March-April [105]
central New York flowers April
seed disperses winter; often over snow [176]
Adirondack Mountains sheds pollen late March-April [205]
flowers late April-May [64]
Blue Ridge Mountains flowers May-June [359]
Northeast flowers March-May [237]
Canada flowers March-May [154]
Quebec, Gaspe Peninsula flowers March-May
fruit ripe August-October
seed disperses fall-winter [257]
 
Thinleaf alder
Area Event Season
Alaska flowers May-June [345]
California seeds ripe August-September [254]
Colorado, South Platte River Basin seeds disperse 2 pulses: early June & late August [253]
Idaho flowers March-April
fruit ripens August-September [154]
Montana flowers March-April
fruit ripens August-September [154]
New Mexico flowers April-June [241]
Nevada flowers April-June [193]
Oregon flowers March-April
fruit ripens August-September [154]
Great Basin flowers February or later [261]
Northern Great Plains flowers May
fruits August-September [219]
North-central Great Plains flowers late April
fruits September [318]
Great Plains flowers April-June [137]
REGENERATION PROCESSES:
Gray alder establishes from seed and spreads vegetatively. Both strategies are important to its regeneration (review by [143]). Seedling establishment is important in primary succession, while vegetative regeneration is more important after top-killing events [160]. For established populations, vegetative regeneration appears more common than seedling establishment. For example, in the Trout Creek Mountains of Nevada, "very mature" thinleaf alder in the largest size class (>7.1 inches (18 cm) basal diameter) comprised 90% of the population, while seedlings (<0.1 inch (0.3 cm) tall) represented about 2% of the population [114].

Pollination and breeding system: Gray alder is monoecious [171,318]. The flowers are wind-pollinated [81,261], so most plants are cross-pollinated. However, thinleaf alder self-pollinates rarely (review by [154]).

Genetic differences among populations are generally small in species with wind-dispersed pollen and seeds (for example, [168]), such as gray alder. Allozyme studies of speckled alder in central Quebec showed high rates of gene flow and weak genetic differentiation among 4 populations [34].

Spatially, the genetic make-up of individuals in speckled alder thickets may be random. Studies of 4 speckled alder thickets in New York showed clones were randomly distributed, and clumps of single genotypes were not aggregated. Thus, the author concluded that although speckled alder regenerates vegetatively, sexual regeneration was driving the genetic structure of these 4 populations [175].

Seed production: Gray alder first produces seed at 25 years or younger. There are usually 1 to 4 years between large seed crops (review by [154]). Thinleaf alder produces "abundant" seed (reviews by [143,256]). Mean annual seed rain of thinleaf alder in white spruce stands of interior Alaska was 745 seeds/m² [366].

Seed dispersal: Wind and water disperse thinleaf alder seeds ([81,93], reviews by [35,312]). In waterways of Alaska, thinleaf alder seeds stayed afloat for "long periods of time" (Densmore 1976 personal observation [93]). On a floodplain of Little Otter Creek in Vermont, speckled alder seed was found in floodwaters but not in seed rain deposited on soil. The surrounding plant community was a red maple-sugar maple (Acer saccharum) forest [178].

Thinleaf alder may establish from seed in crown-stored "cones" after disturbances such as fire ([256], review by [35]), road construction, logging, and mining [256].

Seed banking: Gray alder has a transient seed bank; seed longevity in the field or in water is short [344]. Even in dry storage, seeds do not remain viable for more than 2 years [154]. In a Freeman maple-white ash (Acer × freemanii-Fraxinus americana) swamp in New York, speckled alder seed density was 0.01 seed/120 cm². Speckled alder had 0.9% cover in aboveground vegetation [28]. In dry Douglas-fir forests of south-central British Columbia, thinleaf alder seed was present in the soil seed bank on unlogged and unburned sites. Thinleaf alder seed was not found in the soil seed bank on adjacent sites that were either clearcut 5 or 10 years prior or burned at low or high severity 5 years prior. Thinleaf alder was not present in aboveground vegetation on any of the study sites [317].

Germination: Gray alder seeds are usually nondormant ([27,300], review by [19]) and, under favorable conditions, may germinate immediately after dispersal (review by [143]). However, some seed lots may require a stratification period, from a few days to over winter (review by [154]). Moist soil ([71], review by [143]) and temperatures from approximately 50 to 77 °F (10-25 °C) [154] are required for germination.

Gray alder seed viability is generally low (review by [143]), but preliminary studies suggest that light enhances germination ([93], review by [154]). Some laboratory studies found only 5% viability in thinleaf alder seeds (review by [143]). In greenhouse trials, speckled alder showed low seed viability (4-42%). Germination of filled seed varied from 0 to 50%, with presoaking and light increasing germination rates [27]. For thinleaf alder seeds collected near Fairbanks, germination averaged 90% for seeds cold-stratified in light and 5% for seeds cold-stratified in dark. Unstratified seed showed 100% germination in light and ≤13% germination in dark [93]. Gray alder seed may germinate without light, however. In a laboratory study, speckled alder showed no significant difference in time to germination and germination rate in dark vs. light, averaging 10 days to germination, and 36% germination, in both light and dark [246].

Seedling establishment and plant growth: Gray alder seedling establishment may be rare except in primary succession or on disturbed, open sites. In central New York, 4 populations of speckled alder showed no seedling establishment over 3 years [176]. In a Freeman maple-white ash swamp in New York, speckled alder establishment averaged 10 seedlings/100 m². One-year-old seedlings averaged 5.2 inches (13.1 cm) tall; 2-year-old seedlings averaged 13.5 inches (34.3 cm) tall [29].

Soil disturbance and/or exposed mineral soil favor gray alder establishment ([71], review by [143]). In Michigan, the margins of American beaver ponds were favorable sites for speckled alder germination and establishment [222]. In a Swedish field experiment, European gray alder seedling survival was higher in mineral soil (34%) than in humus (9%) [300].

Gray alder seedling establishment on new sandbars and on banks with receding floodwaters is common, although other substrates also provide favorable establishment sites. After a flood on the Connecticut River, for example, speckled alder seedlings were noted in a sugar maple swamp in an oxbow [170]. On Lassen National Forest, thinleaf alder established most often near wide stream channels on sand- or gravelbars. Thinleaf alder seedling establishment was negatively correlated with canopy cover (P=0.02) and litter depth (P=0.002) and positively correlated with solar radiation (P=0.002) [297]. In a greenhouse experiment in France, flooded European gray alder seedlings grew fastest on moist, sandy loams that were flooded and drained daily [177]. In boreal quaking aspen-paper birch ecosystems near Slave Lake, Alberta, thinleaf alder established on decaying logs and stumps [226]. Gray alder germinants apparently tolerate slightly drier conditions than willow germinants [114].

Gray alder seedlings grow rapidly under favorable conditions ([298], review by [154]). Monsen and others [256] report that thinleaf alder seedlings are "very competitive and vigorous. Once established, few plants can grow as rapidly". In central New York, annual growth increments of 4 speckled alder populations ranged from 0 to 2.8 inches (1.1 cm). Growth rate was generally greatest in midsized stems (0.8-1.5 inches (2.0-3.9 cm) DBH, P<0.01). In one population, gypsy moths removed nearly 100% of foliage in July, but stems leafed out again in late summer. Slowed growth and/or stem mortality was anticipated as a result of the defoliation but was not recorded [176]. Speckled alder seedlings proved "remarkably tough when placed in the field" after growing in a greenhouse. Ninety-seven percent survived their first year [27].

Speckled alder may grow rapidly after canopy removal (review by [143]). MacLean [236] reported that after clearcutting in boreal quaking aspen-paper birch-spruce communities in Ontario, understory speckled alder gained height growth quickly and spread vegetatively. From small clumps with few stems, speckled alder typically formed a tall, closed-shrub canopy within 10 to 12 postharvest years. Some regeneration from seed was also apparent. Best growth occurred on moist to very moist clay loams [236]. Speckled alder also grew rapidly following clearcutting on black spruce peatlands in Ontario. Ten years after harvest, speckled alder averaged 5 to 6 feet (1.5-1.8 m) tall and 16,000 to 30,000 stems/acre. Speckled alder crown closure ranged from 40% to 80% [347].

Colonizing gray alders may facilitate subsequent gray alder establishment and colony expansion. Below receding glaciers above Glacier Bay, Alaska, thinleaf alder established as widely scattered individuals. These individuals were centers of aggregation from which thinleaf alder thickets spread [75].

Ungulate browsing can substantially reduce growth of thinleaf alder on heavily browsed sites. On big game winter rangelands near the Clearwater River in northern Idaho, thinleaf alder's annual biomass accumulation averaged 0.23 g/m² in ungulate exclosures. Study plots outside exclosures, which were subject to moose, elk, and mule deer foraging, did not contain thinleaf alder, Rocky Mountain maple (Acer glabrum), or several other important browse species [4].

One study found midsized speckled alder sprouts suffered the least mortality among size classes. Over 3 growing seasons in New York, short speckled alder stems (<4.6 feet (1.4 m) tall, minimal DBH) died most often, while midsized stems (6.6-13 feet (2.0-3.9 cm) DBH) showed less mortality than larger sprouts. Sampling was conducted in 4 early-successional speckled alder thickets. Variation in stem mortality and growth was greater within populations than across sites (P<0.05 for all variables) [176].

Vegetative regeneration: Cloning is apparently the primary means of spread in established stands of gray alder (review by [143]). Gray alder sprouts from the root crown [35,38,175,208]. It can also sprout from roots, including root offsets [175,176,298], and layers. Root sprouting and layering are apparently less important than root-crown sprouting [176], although root sprouts distant from the parent plant have been noted for both speckled alder and European gray alder (review by [143]). Haeussler and others [143] report that sprouting "can be expected" after mechanical site preparation. In a Magellan's sphagnum bog in Ohio, speckled alder sprouted after cut-stump herbicide applications, growing up to 3 feet (1 m) in one growing season [9].  Gray alder does not spread rapidly, but its clones can be long-lived. In a central New York study, 4 speckled alder populations were monitored for 4 years. There was no seedling establishment on study plots during that time. Clump sprout production and stem mortality were variable within and among populations, although no clumps died out. No lateral extension of clones via root sprouting occurred. Annual sprout production averaged about 3 live stems/clump [176].

In a study comparing root anchorage of riparian species in Italy, European gray alder was less resistant to uprooting by flood than Lombardy poplar (Populus nigra) or Elaeagnus willow (Salix elaeagnos) [191]. This relative ease of uprooting may allow for vegetative spread of gray alder when root fragments are torn off, distributed downstream, and sprout.

SUCCESSIONAL STATUS:
Gray alder tolerates full sun to light shade. It is an important colonizer in primary succession. It is also successionally important after stand-replacing events such as fire and logging [108] and in canopy gap succession [209]. Its ability to fix nitrogen (see Site Characterisitics and Plant Communities) can enhance site quality for later-successional species. Plant response to fire provides information on gray alder and postfire succession.

Speckled alder
Speckled alder prefers open sites [106,171,205] but tolerates moderate shade (Shirley 1932 cited in [39]). Studies in Michigan found that it occurred on open to closed sites but was most common on lightly shaded, cool sites [16,36]. In southern-boreal, quaking aspen forests of Quebec, speckled alder was associated with relatively high light transmission at 7 to 13 feet (2-4 m) above ground level (P<0.001). Thirty percent of full sun was the highest light level achieved on most sites [18].

Speckled alder is an important shoreline and meadow colonizer, and it is successional in mires and other wet to damp areas throughout its range [120]. In Michigan, it colonizes sand dunes by Lake Huron [13]. In New York, it is a characteristic taxon on unstable beachgrass (Ammophila breviligulata) dunelands [293]. On an estuary on the St Lawrence River, Quebec, speckled alder seedlings and saplings showed 28% cover, the greatest of any colonizing shrub taxa. Drifting ice, wave action, and frequent flooding inhibited establishment of less tolerant shrubs along the shoreline [24]. By upland oak-hickory communities, floodplains with speckled alder generally succeed to sycamore, elms, and red maple (review by [160]). Speckled alder may be invasive in sedge (Carex spp.) wetland meadows of Wisconsin, especially with fire exclusion [294]. Speckled alder is noted in the older zones of bogs in Michigan [350]. In bog succession, speckled alder generally establishes after litter from mosses and low ericaceous shrubs has formed a peat layer. Without disturbance, trees such as tamarack and black spruce establish after speckled alder, willow, and other shrubs [303]. Speckled alder may be the pioneer woody taxon in swamps that succeed to red maple [176].

The successional pathway of acidic mires and other wetland communities may alter after speckled alder establishment [9,38]. Since speckled alder is a nitrogen fixer, acid-loving bog and fen species may be replaced successionally by species with higher nutrient requirements. On a kettle bog in Ohio, speckled alder established on a Magellan's sphagnum mat and was displacing acid-loving herbs such as purple pitcher-plant (Sarracenia purpurea) and round-leaved sundew (Drosera rotundifolia) [9]. Speckled alder is more characteristic of fen and bog margins than of the mires themselves, although it sometimes establishes in mires with peaty soils [89]. On the Lac St-Francois National Wildlife Area in Quebec, speckled alder invaded a water sedge-hairy sedge (C. lacustris) fen. Over several decades, speckled alder became dominant over about 10% of the wetland that had formerly been sedge fen [38]. Woody invasion of herbaceous bogs and fens in eastern North America has been attributed to fire exclusion [9,38], although altered hydrologic regimes may also play a role [38]. A 1938 study of the speckled alder-willow high bog association in Cheboygan County, Michigan, found speckled alder and willows were casting dense shade and becoming "scraggly". They were being replaced successionally by red-osier dogwood, black spruce, tamarack, and northern white-cedar. Charred logs and stumps in the bog indicated that fire had cleared the area of trees in the past [110]. In contrast to speckled alder replacing herbs successionally in wetlands, beaked sedge has established and spread in speckled alder-withe-rod (Viburnum nudum var. cassinoides) communities of east-central West Virginia [90].

In forests, speckled alder may be important after logging, insect attacks, and/or disease. On logged black spruce forests of northeastern Ontario, speckled alder was most common in forests <40 years old, forming dense shrub layers of up to 570 to 600 stems/0.01 ha. In older stands, speckled alder was less important, forming a "diffuse canopy" on most sites. It remained dense in seeps and by open water, however. The authors surmised that speckled alder was probably present in low numbers in the prelogged forest and increased rapidly after logging [43,44]. Speckled alder was prevalent after an eastern spruce budworm attack and subsequent clearcutting in balsam fir-paper birch communities in Ontario. Speckled alder density ranged from 109 stems/ha to 6,667 stems/ha on 8- to 12-year-old clearcuts [158]. In early secondary succession, deforestation due to Dutch elm disease tended to favor speckled alder and other shrubs [108]. In hardwood swamps of central New York, speckled alder was "frequently encountered" in gap succession following death of overstory American elms to Dutch elm disease. It occurred in both small gaps created by the death of single trees and in larger gaps created by deaths of multiple trees [174].

Speckled alder is generally unimportant in late-successional forests. It was a mostly minor component of late-successional eastern hemlock-black spruce-red maple forests of northeastern Pennsylvania, attaining high density and cover only in canopy gaps [100]. However, a 1934 publication reported speckled alder as characteristic of climax balsam fir forests in Itasca County, Minnesota. It was also present in earlier succession [136].

Speckled alder may be of minor importance in old field succession. It formed thickets on old fields in New York [49,175,176]. In southeastern Ontario, speckled alder seedlings were found 19 years after abandonment of ploughed hay fields, although in low numbers (<1% frequency). Speckled alder was also present in adjacent silver maple/roundleaf serviceberry (Acer saccharinum/Amelanchier sanguinea) forest. It presumably established in the old field from wind-blown seed originating from the forest [84]. In an old-field study, speckled alder was an early-seral shrub on the Piedmont Plateau of North Carolina. It preferred wet bottomlands that had been used as hayfields, sometimes forming dense, shoulder-high thickets. Speckled alder was also noted in old fields 34 and 45 years after abandonment on a site succeeding to loblolly pine (Pinus taeda) forest. Its density in the loblolly pine habitat ranged from 1.9 stems/16 m² on a 34-year-old field to 4.1 stems/16 m² on a 45-year-old field. In an adjacent old field on logged streambanks succeeding to paper birch, speckled alder was present 6 and 14 years after abandonment at densities of <2 stems/16 m². In old fields succeeding to mixed-hardwood bottomlands, it was found in 8- and 32-year-old fields (densities not recorded). None of the old-field plant communities surveyed was older than 55 years [270].

Thinleaf alder
Thinleaf alder is moderately shade tolerant. Thinleaf alder is adapted to nearly all types of disturbance [151], including severe disturbance [32], and is most common in early succession. It can grow in forest understories, but it is found more often on open sites. Sprouts may tolerate shade better than seedlings (review by [143]). To date (2011), most successional studies on plant communities with gray alder had been conducted on Alaskan sites in primary succession.

Thinleaf alder is an early-successional shrub in riparian zones in primary succession [93]. The willow/alder stage typically forms on bare floodplains [362]. On the Tanana River in interior Alaska, it typically establishes on bare to nearly bare, recently deposited alluvium [1]. Feltleaf and/or sandbar willow may establish first [1,55]. Thinleaf alder and balsam poplar dominate in midsuccession, after which balsam poplar and finally, white spruce, dominate the overstory [1]. A 5-year-old thinleaf alder sandbar community on the Tanana River was a "nearly impenetrable" thicket of thinleaf alder, feltleaf willow, sandbar willow, and barrenground willow. The shrubs were uniformly about 10 feet (3 m) tall, 0.8 inch (2.0 cm) in stem diameter, and averaged 49,699 stems/ha. Balsam poplar, paper birch, and white spruce grew in the understory; meadow horsetail and variegated scouringrush were dominant herbs. In 15-year-old stands, feltleaf willow was overtopping thinleaf alder, and shrub density declined to 2,827 stems/ha. Groundlayer vegetation was nearly all meadow horsetail [162]. By Glacier Bay, Alaska, thinleaf alder occurs about 15 to 20 years after glacier recession, in the "late pioneer" stage. About 25 to 30 years after recession, thinleaf alder forms closed stands, presenting an "almost impenetrable barrier". At 30 to 35 years, black cottonwoods begin to establish [337].

According to a 1923 study by Cooper [76], thinleaf alder may be abundant even in late succession after glacier recession. Herbs, especially dwarf fireweed (Chamerion latifolium), dominated the northernmost, pioneer community, but thinleaf alder and willows were also establishing. Feltleaf willow-Sitka willow-thinleaf alder communities occurred in isolated patches and on midlatitude landscapes; midlatitude sites had a longer period of recession than the northern sites. Thinleaf alder was "nearly everywhere dominant" and eventually overtopped the willows. Sitka spruce forests occurred in the southernmost portion of the landscape, which had the longest time since glaciation. Thinleaf alder persisted in these late-successional forests, typically at greater abundance than in the pioneer stage [76].

The pattern of willows, and sometimes cottonwoods, establishing before thinleaf alder is typical in riparian succession. On bare gravel bars of Meadow Creek on the Starkey Experimental Forest, Oregon, thinleaf alder established at lower densities (0.96 stem/50 m²) than sandbar willow and black cottonwood [60]. Along the Animas River in southwestern Colorado, narrowleaf cottonwood/thinleaf alder communities occur upland from narrowleaf cottonwood/tickle grass communities and are considered a latter successional community type than the narrowleaf cottonwood/tickle grass community [351].

Thinleaf alder shrub communities are initiated and maintained by disturbances that are often severe. Historically in northwestern Montana, disturbances have included placer mining, ice jams, log transport [32,151], and fire. On Emigrant Creek near Burns, Oregon, thinleaf alder colonized a new alluvial fan 6.5 years after deposition following an intense thunderstorm. Cattle had grazed the area for at least 30 years [145]. Thinleaf alder snowslide communities in the Blue Mountains are maintained by avalanches and soil slippage [144]. Along the San Miguel River in southwestern Colorado, thinleaf alder shrub communities occupied less area than that of later-successional, narrowleaf cottonwood/thinleaf alder communities. Thinleaf alder shrubland also had shorter flood-return intervals (averaging <10 years) than those of narrowleaf cottonwood/thinleaf alder communities (averaging about every 50 years) [126].

Thinleaf alder may facilitate establishment of later-successional riparian species, likely due in part to its ability to fix nitrogen. On floodplains of interior Alaska, for example, balsam poplar and feltleaf willow establish in nitrogen-enriched soil beneath thinleaf alder, although root crowding and shading by thinleaf alder may interfere with growth of later-establishing species on many floodplain sites (reviews by [53,54]). On floodplains near Fairbanks, there was a "marked" increase in exchangeable soil potassium, calcium, magnesium, manganese, and phosphorus within 5 years of thinleaf alder establishment, but there was no consistent increase in soil mineral content from 5 to 20 years after thinleaf alder establishment. Soil pH decreased beneath thinleaf alder stands over 20 years, while soil cation exchange and organic matter increased. At study year 20, most of the total aboveground plant biomass (~43 kg/ha of ~48 kg/ha total) and basal area (7,142 stems/ha of 7,241 stems/ha total) was thinleaf alder [338].

Experiments on the Tanana River floodplain showed changes in soil nutrient dynamics and soil microbe community composition as aboveground succession proceeded from thinleaf alder to balsam poplar. As balsam poplar gained dominance, soil carbon became increasingly more available and soil nitrogen became increasingly less available to soil microbes. Thus, the soil biota changed from carbon-limited microbes under thinleaf alder dominance to nitrogen-limited microbes under balsam poplar dominance [70].

Thinleaf alder occurs in all stages of forest succession, although it is most prevalent in early forest succession. It pioneers in forest communities of British Columbia, sometimes persisting in mature forests [23]. Thinleaf alder is seral in Douglas-fir, spruce, and other coniferous forests in the West (for example, [150]). In forested habitats, the thinleaf alder shrub community is usually an early- to midsuccessional seral stage that arises after severe disturbance. Conifers or taller hardwoods typically replace thinleaf alder successionally [151]. Near Slave Lake in Alberta, thinleaf alder sprouts were more common on sites logged 28 years prior than adjacent sites burned by wildfire 28 years prior (P<0.05). Study sites were dominated by quaking aspen, balsam poplar, and paper birch [225]. Thinleaf alder often dominates the understory of midseral spruce/bluejoint grass forest habitat types in Montana [151]. In northwestern Montana, thinleaf alder was abundant in Engelmann spruce/skunk cabbage riparian communities from early-seral (logged and heavily grazed) to late-seral and climax stages. It dominated early stages of succession in Engelmann spruce/field horsetail and Engelmann spruce/bluejoint reedgrass riparian communities [32]. In central Alaska, thinleaf alder thickets are often replaced successionally by balsam poplar, which in turn is replaced by white spruce, then black spruce [339]. Thinleaf alder occurred late in the succession of white spruce-Engelmann spruce forests on the east slope of the Rocky Mountains in Alberta [78]. By the Peace River in northern Alberta, thinleaf alder was dominant in old-growth white spruce forests. It was most common in canopy gaps, although it persisted in the understory. It also occurred in resin birch clearcuts [332]. In central Alberta, thinleaf alder was more common in mature quaking aspen-balsam poplar-white spruce forest than on edges of 16-year-old clearcuts (P=0.05) [153].

Browsing pressure may alter succession in forest ecosystems with thinleaf alder. Browsing may favor thinleaf alder at the expense of more palatable browse species. In spruce-birch taiga forests of interior Alaska, browsing pressure by moose favored thinleaf alder and quaking aspen over more palatable willow species [52]. In northern Idaho, ungulate browsing helps maintain shrubfields, which contain thinleaf alder and other seral shrubs. Heavy browsing, however, may accelerate succession to conifer species, which are less palatable [4].

FIRE EFFECTS AND MANAGEMENT

SPECIES: Alnus incana
Thin bark of a mature speckled alder. Photo by Robert Vidéki, Doronicum Kft., Bugwood.org.
FIRE EFFECTS: Immediate fire effect on plant: Gray alder's thin bark [155,348] does not insulate it well from fire damage. Fire generally top-kills gray alder ([38,164,346], reviews by [35,92,312]).

Postfire regeneration strategy [319]:
Tree with adventitious buds, a sprouting root crown, and root sprouts
Tall shrub, adventitious buds and a sprouting root crown
Geophyte, growing points deep in soil
Crown residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on- or off-site seed sources)
Ground residual colonizer (on site, initial community)

Fire adaptations and plant response to fire:
Fire adaptations: Gray alder has many adaptations that can aid its postfire recovery. It sprouts from the root crown and/or roots after top-kill. Root crown sprouting is most common (see Vegetative regeneration), while root sprouting may occur when fire is severe enough to kill the root crown. Gray alder may also establish from seed; this may include seed dispersed from on-site, crown-stored "cones" ([256], review by [35]), wind- or water-dispersed seed originating from off-site parents, or seed in the transient soil seed bank . Some of gray alder's adaptations to riparian environments, including tolerance of high light and the ability to sprout after repeated top-kill, may also benefit gray alder in early postfire environments [209].

Plant response to fire: Speckled alder
Speckled alder sprouts from the root crown after top-kill by fire. Its postfire sprouting response is likely strongest on sites that are moist during the growing season [51]. In a review of fire responses of plant taxa in northern coniferous forests, Heinselman [164] classified speckled alder as fire-tolerant.

A prescribed April fire top-killed most speckled alders in a study site on the Allegheny River floodplain of Pennsylvania. Open areas with individual trees burned well, but dense stands of speckled alder carried fire mostly on the edges. The fire top-killed 55% of speckled alder stems. By postfire month 9, most burned speckled alders were sprouting at or within 2 inches (5 cm) of the soil surface, from near the bottoms of the stems or from the root crowns. However, sprouting in speckled alder was not related to fire; it occurred on burned and unburned stems at approximately the same rate, 90%. Sampled stems produced an average of 8 to 10 sprouts, with basal diameters ranging from 0.1 to 0.5 inch (3-12 mm) and heights ranging from 9 to 47 inches (22-120 cm). Twelve percent of speckled alder stems were browsed, probably by white-tailed deer and cottontails [49]. See the Research Project Summary of this paper for information on the burning conditions and effects of the fire on hawthorns (Crataegus sp.) in the same area.

A few studies show speckled alder may be more common in early than late postfire succession. In a fire chronosequence study in black spruce-jack pine ecosystems of northern Ontario, speckled alder was most common on wet peatlands burned 1 to 3 years prior (P=0.05). Burn ages ranged from 0 to 57 years [308]. Speckled alder reached greatest postfire abundance in postfire year 10 in red pine-black spruce forests of Saskatchewan [306]. In a chronosequence of 9 burned black spruce/mountain-laurel (Kalmia latifolia) forests in Terra Nova National Park, Newfoundland, speckled alder was present only on a 20-year-old burn, with 2% mean cover. Burn ages ranged from 1 to 38 years [30]. In quaking aspen, jack pine, and black spruce swamp communities of Itasca County, Michigan, speckled alder occurred in 7 of 9 burns surveyed. The burns were from 1 to about 58 years old [135]. A review of fire succession in balsam fir forests of the Northeast reported that speckled alder thickets may replace balsam fir after fire and that those thickets may persist for decades before trees establish [128].

Frequent fire likely favors speckled alder over conifers. Speckled alder thickets may form after stand-replacing fire in northern white-cedar communities [186]. In a northern white-cedar swamp in Michigan, speckled alder was most abundant in an area that burned 3 times in 30 years. The authors reported that dense growth of speckled alder and willows now occur on sites where the northern white-cedars experienced the greatest fire kill [110].

Speckled alder increased over 10 postfire years on a hybrid spruce clearcut that was slashburned at low to moderate severity. The site was on the Mackenzie Forest District in subboreal British Columbia. Speckled alder was the dominant shrub before and after the fire, with abundance at postfire year 10 exceeding its prefire cover and frequency. Speckled alder's prefire cover and frequency were 4.6% and 50%, respectively. At postfire year 10, it had 8.9% cover and 67% frequency. Overall, shrub height was more in postfire year 10 than before fire, primarily due to the rapid postfire growth of speckled alder and Rocky Mountain maple. See the Research Paper of Hamilton's [146] study for details on the fire prescription, fire behavior, and postfire responses of other species in the plant community.

Two studies suggest that speckled alder responds similarly to fire and logging. Pooled across plant communities and stand ages, there was no significant difference in speckled alder frequency between wildfire-burned (37.0%) and logged (31.4%) stands in Minnesota. Surveys were conducted in naturally regenerated quaking aspen, jack pine, and black spruce communities that were 25 to 100 years old [292]. In a black spruce forest in west-central Quebec, there was no significant difference in speckled alder cover on the edges of burns and clearcuts. Burns were 3 to 4 years old, and clearcuts were 2 to 5 years old [152].

Both wildfire and clearcutting apparently favored speckled alder; its average frequency was higher than that of any other deciduous shrub after both disturbances in surveys in northeastern Ontario [57]. Speckled alder was more frequent after clearcutting than after wildfire, but a longer period of postdisturbance succession on burned vs. clearcut stands makes direct comparisons difficult. Time since fire averaged 89.6 years, while time since logging averaged 22.9 years: Given speckled alder's role in early succession in this area, greater abundance in the younger, logged communities seemed likely. On clearcut stands (n=131), speckled alder averaged 6.9% frequency in the canopy and 18.47% cover in the subcanopy. Both wildfire and clearcutting apparently favored speckled alder; its average frequency was higher than that of any other deciduous shrub after both disturbances [57].

Thinleaf alder
Thinleaf alder sprouts from the root crown after top-kill by fire ([38,164,346], reviews by [35,92,312], observations reported in [119]). Stem density may increase after fire due to multiple sprouts arising from single root crowns (review by [312]). A review of fire responses of British Columbian shrubs states that thinleaf alder is "set back by moderate or severe fires" [159]. An anecdotal account from central Oregon reports that in thinleaf alder-dominated riparian communities, thinleaf alder withstands low-severity surface fires but is killed by more severe fires [200]. Root sprouting is possible, however, and further investigations are needed to ascertain thinleaf alder's response to moderate and severe fires.

Thinleaf alder establishes on burns from on- or off-site, wind- or water-dispersed seed (reviews by [35,312]). Wind may disperse seed onto burns from "considerable distances" [312]. Seedlings may also establish from seed in crown-stored "cones" after late summer fires (reviews by [35,312]). Viereck and Schandelmeier [346] reported that fire usually kills crown-stored seed in Alaskan alders such as thinleaf alder, but alder seeds are typically blown onto burns from off-site parents. Alders in lightly burned areas may produce "large numbers" of seeds in early postfire years [346]. Thinleaf alder may establish in early postfire years after stand-replacement fires in lodgepole pine. In the Bitterroot Mountains of northern Idaho, thinleaf alder often establishes from wind-blown seed around postfire year 3 [217]. Establishment from soil-stored seed may also occur (see Seedling establishment).

In a greenhouse study using soil from a wildfire-burned area near Slave Lake in Alberta, thinleaf alder sprouted from the roots but did not emerge from the seed bank. Soil samples were collected a week after the fire (cut in blocks 27.4² inches (177 cm²) across and least 4 inches (10 cm) deep). Thinleaf alder sprouted from soil blocks where fire had been "intense", averaging 23% cover after 2 years in the greenhouse. Thinleaf alder did not sprout from soil blocks collected from lightly burned or unburned sites. The plant community was mixed hardwood-conifer dominated by quaking aspen [224].

After summer wildfires on the Plumas National Forest, California, fewer thinleaf alders sprouted on upland sites—where fire crowned—than on gravelbars, where fire was less severe. Postfire response of vegetation on 2 streams, Third Water and Fourth Water creeks, was evaluated in postfire year 1. On the 1st and 2nd terraces above the creeks' gravelbars, the plant community was an incense-cedar-white fir/Eastwood manzanita-huckleberry oak (Arctostaphylos glandulosa-Quercus vaccinifolia) forest. Thinleaf alder dominated the gravelbars [197].

For each topographic position, mean relative density of thinleaf alder and percent of burned thinleaf alders that sprouted along Third and Fourth Water creeks in postfire year 1 [197]
Topographic position Gravelbar 1st terrace 2nd terrace Riparian-zone slope
Site % of total % sprouting % of total % sprouting % of total % sprouting % of total % sprouting
Third Water Creek
n=47 plants
100 100 48 30 38 54 34 67
Fourth Water Creek
n=25 plants
23 100 40 90 not present 4 0

On hybrid spruce-Engelmann spruce plantations in subboreal British Columbia, thinleaf alder abundance was similar on clearcuts prepared for planting by either prescribed burning or mechanical treatment (blading). Across treatments, mean thinleaf alder cover ranged from 17% to 25% on plantations <7 years old [147].

In a study of disturbed forests near the Peace River in northern Alberta, thinleaf alder was strongly positively associated (P<0.001) with frequently flooded sites (~1-10 years) but not with recent burns (5- to 20-year-old) or older burns (>20 years). Most (48%) of the white spruce and hardwood forests originated after flooding; fewer regenerated after logging (32%) or stand-replacing fire (19%). Thinleaf alder was recorded only on flooded sites. It was typically present in the earliest seral stages after flooding, establishing on gravelbars, levees, and the lowest stream terraces [332].

Thinleaf alder may occur in late postfire succession. On 12 white fir sites in Sierra County, California, that had experienced stand-replacing wildfires, thinleaf alder occurred only on study plots that had burned 80 years or more prior to the study. Time since fire ranged from 5 to 277 years [72].

In sagebrush ecosystems of the West, mesic thinleaf and other alder shrublands may establish after fire, or they may be self-sustaining [322].

FUELS AND FIRE REGIMES:

Fuels: Because gray alder adds nitrogen to soils and typically grows in moist soils, gray alder communities are usually highly productive [132]; hence, their fuel loads can be large. Gray alder may produce a "dense" litter layer [353]. However, because gray alder prefers moist sites, in most years gray alder communities may act as firebreaks. Riparian zones with gray alder are often buffer zones where upland fires decrease in severity or stop [285]. Speckled alder and thinleaf alder communities are placed in fuel model 0 [282] and fire group 0 [312], respectively; these plant communities do not burn readily [282,312]. Communities in fuel model 0 are described as vegetation types in which fire will not carry due to saturated ground or standing water, discontinuous fuels, and/or lack of ladder fuels [282]. However, with dry weather and the accumulation of dry fuel, riparian areas can become corridors for fire spread [285].

Speckled alder: A prescribed burning guideline for Ontario reports that speckled alder communities do not burn well under conditions needed for prescribed fires. McRae [248] found that in boreal black spruce-white spruce-quaking aspen forests of Ontario, prescribed fires did not typically spread into drainageways dominated by speckled alder. Because there was no combustible slash, these drainages acted as fire barriers [248]. An extension publication recommends speckled alder as an ornamental due to its low flammability [7].

Several publications provide help for estimating fuel loads in speckled alder habitats. See these publications: [314,329] for information on models to estimate speckled alder biomass in the northern United States and Canada. Buech and Rugg [47,48] provide equations to predict aboveground biomass of speckled alder. Their model was developed in northern Minnesota [47,48]. A photoseries for assessing fuels in coniferous forests of northern Ontario provides size class breakdowns and photos for live and woody surface fuels in black spruce, black spruce-tamarack, black spruce-jack pine, black spruce-poplar (Populus spp.), balsam fir-black spruce, paper birch-white spruce, and poplar-white birch stands [321], all of which may include speckled alder as a component of the vegetation (see Site Characteristics and Plant Communities).

In the laboratory, speckled alder's heat of combustion and total heat release were about average for shrubs of the Northeast [94].

Several studies provide what may be representative examples of fuel loads in speckled alder habitats. In a speckled alder/goldenrod (Solidago spp.) community in upstate New York, speckled alder production averaged 730 g/m²/year. Leaves and twigs comprised 42% of total production; 25% was in the boles; 21% in branches; and 12% in fruits. Herbaceous production averaged 241 g/m², yielding a total aboveground production of 971 g/m² for the speckled alder community [331]. A New York study found total speckled alder production averaged 2,225 g/m², 21% of which was roots [349].

In commercial forests in Michigan, speckled alder biomass was least in oak-hickory and greatest in tamarack stands [273]. In a similar study across commercial forests in the Northern Great Lakes (northern Minnesota, northern Wisconsin, and the Upper Peninsula of Michigan), speckled alder biomass was least in jack pine and greatest in balsam fir stands [313]. No information on stand ages was available.

Speckled alder biomass by forest type
Forest type Michigan [273] Northern Great Lakes [313]
Biomass (lbs/acre)
oak-hickory 13 no data
maple-birch 35 239
elm-ash-maple 607 2,584
aspen 134 1,989
paper birch 283 552
red pine 24 383
jack pine 71 153
eastern white pine 118 3,368
white spruce 260 1,758
northern white-cedar 433 4,221
balsam fir 644 5,482
black spruce 1,703 4,090
tamarack 2,607 3,037

In Nova Scotia, speckled alder biomass averaged 83 kg/ha in dense mixed-conifer, 140 kg/ha in dense mixed-hardwood, and 1,150 kg/ha in open mixed-conifer forests [328].

Thinleaf alder: Thinleaf alder thickets are somewhat fire-resistant because the duff is usually cool and moist, and the undergrowth is sparse [92]. The generally moist conditions in thinleaf alder stands usually inhibit fire spread (review by [312]). Even when fires are severe in upland areas, riparian zones may remain unburned or burn with lower severity. However, fires may burn severely in riparian areas in drought years [315]. A fire management plan for Lassen National Park, California, reported that natural fires seldom ignite in black cottonwood-thinleaf alder woodlands; instead, those woodlands are generally firebreaks. However, old stands with diseased and downed trees may be "quite flammable", especially if white fir or red fir has formed ladder fuels in the understory [181].

In many forested riparian areas, live and dead fuel structure and loads have altered due to fire exclusion. Arno and Harrington [12] report that in riparian ponderosa pine communities of the West, where thinleaf alder is known to occur, most sites are dominated by dense, shade-tolerant trees such as grand fir, and the shrub and ground layers are sparse.

On 3 thinleaf alder wetland communities in west-central Montana, total aboveground plant productivity ranged from 3,320 to 4,820 lbs/acre. Of that total, shrubs averaged 316 lbs/acre, forbs averaged 1,200 lbs/acre, and graminoids averaged 2,713 lbs/acre [286].

Several studies described below provide what may be representative examples of fuel loads in thinleaf alder habitats by the Tanana River in Alaska. Yarie and Mead [363] present models to estimate twig and leaf biomass of thinleaf alder and other species of the Tanana River Basin.

On sandbars of the Tanana River, total production in 5-year-old thinleaf alder thickets averaged 20,000 kg biomass/ha (including root biomass). Production peaked in 15-year-old thickets at an average of 65,000 kg/ha, while 20-year-old thickets averaged 95 kg/ha [339]. On thinleaf alder-dominated floodplains of the Tanana River, thinleaf alder biomass was almost 5 times greater in 20- vs. 5-year-old stands. Codominant willows and horsetails comprised much smaller portions of the total plant community biomass (3.2-8.7%) [339].

Distribution of biomass (kg/ha) in thinleaf alder-dominated stands of different ages [339]
Sample 5 years 15 years 20 years
Thinleaf alder biomass* 8,751 27,810 42,741
% of total plant community biomass 42.9 43.5 45.1
Total down woody fuel biomass not available 10,464 16,389
Total litter biomass 4,340 4,134 4,390
Total biomass 20,388 63,876 94,689
*Above- & belowground biomass mass was measured for live plants.

After late February logging, a prescribed broadcast burn in July reduced fuels in a white spruce/thinleaf alder/feathermoss (Hylocomium sp.) forest on the Tanana River floodplain. The fire "substantially" decreased organic material on the soil surface and exposed mineral soil. Down and dead woody fuels were reduced an average of 67%, mean depth of the forest floor was reduced 43%, and the soil organic layer was reduced 2.9%. See Zasada and Norum [367] for information on the fire prescription and fire weather conditions.

Fire regimes: Gray alder experiences a wide variety of fire regimes across its broad distribution in North America. For example, ponderosa pine woodlands in the Pacific Northwest and California historically experienced mostly low-severity surface fires with an average return interval of 13 years [210], while mixed-hardwood-spruce forests of the Northeast historically experienced mostly stand-replacement fires at intervals of 400 years or more [212]. As an early-successional species that sprouts after fire, gray alder is likely to flourish in wetland plant communities where fire is frequent or severe enough to maintain or create open conditions. This is likely true across the species' worldwide distribution. Scots pine-Norway spruce (Pinus sylvestris-Picea excelsa) forests of Scandinavia, where European gray alder is an important component of the vegetation, historically experienced frequent, low-severity surface fires and mixed-severity fires under a fire regime similar to that of ponderosa pine and mixed-conifer forests of the western United States. Frequent understory fires helped maintain European gray alder in the understory (review by [10]). Speckled alder: Little information was available on fire frequency and fire behavior in riparian and wetland plant communities where speckled alder is an important to dominant member of the plant community. One study, near Thunder Bay in Ontario, found wildfire did not carry well once it reached a speckled alder community. A stand-replacement wildfire burned a mixed hardwood-coniferous forest, but the fire slowed or stopped in riparian zones dominated by speckled alder. Speckled alder showed scorching and damage to top branches but was otherwise unharmed, and "little damage was evident in the riparian zone" [209]. Research is needed on fire regimes of riparian and wetland plant communities in the eastern United States.

Summaries of several fire studies in plant communities where speckled alder was important or dominant follow.

Red-white-jack pine ecosystems of the Great Lakes states historically experienced frequent, low-severity surface fires. Frequent fires prevented development of late-successional tree species such as white spruce and red maple, and also inhibited spread of understory shrubs including speckled alder, mountain alder (A. viridis subsp. crispa), beaked hazelnut, and American hazelnut. Heinselman [165] reported that in these ecosystems, both the late-successional tree and the shrub layers are maturing and expanding under fire exclusion. A fire history study on Pictured Rocks National Lakeshore, Michigan, found red-white-jack pine shoreline forests with speckled alder had a mean fire-return interval of 21.8 years before European settlement, with fire-return intervals lengthening after the early 18th century. At the time of study (1985), the area had not experienced fire for the past 84 years [232].

A study of fire scarring in red and jack pines near Lake of the Clouds in northern Minnesota found fire-return intervals ranging from 16 to 62 years between 1691 and 1818. Speckled alder was a dominant shrub in the area. Over the previous 1,000 years, lake sediment analyses showed a mean fire-return interval of about 70 years; speckled alder pollen was present in most sediment cores sampled across the 1,000 years [323]. A similar lake sediment study in Boundary Waters Canoe Area also found a mean fire-return interval of about 70 years over the past 1,000 years; speckled alder pollen was present in all cores sampled across that time. The extant plant community was a quaking aspen-paper birch/speckled alder forest [324].

Red spruce-balsam fir-white spruce forests of the Northeast historically experienced infrequent, stand-replacement fires at 150- to 300+-year intervals. Surface fires were "extremely uncommon". Speckled alder is an early-successional species in these forests [211].

The black spruce/speckled alder forest cover type of boreal North America is reportedly a stable type that regenerates into forests of similar composition after fire [87]. Information on fire regimes for this type was not found in the literature. In general, boreal black spruce forests experience stand-replacement fires at intervals of 50 to 150 years [165].

A study by Jacobson and others [182] suggests that historically, infrequent fires may have reduced encroachment of speckled alder and other woody species into open mires in the Northeast. Frequent fires, however, may favor sprouting woody taxa such as speckled alder. A study at Crystal Fen in northern Maine suggested that frequent fires combined with high drainage favored encroachment of speckled alder and other woody species. Charcoal analysis of the fen showed fire was uncommon before a railroad was constructed in the area. Fire frequency increased after 1937, when a drainage ditch was excavated next to a railbed, drying out vegetation, and sparks from diesel-powered trains ignited fires frequently. A survey of tree ages showed that "many" speckled alder, northern white-cedar, and other woody species established between 1937 and 1950. Fire frequency dropped after 1950, when diesel engines replaced steam engines, ignition sources became uncommon, and for undescribed reasons, the fen began flooding again. Increased moisture and no fire, however, did not open the fen: Woody species continued establishing on areas of the fens that, based on pollen and charcoal records, historically had few to no woody plants. The authors recommended restoring the historical drainage pattern, cutting woody species, and/or introducing prescribed fire to restore the open fen [182].

As of 2011, almost no literature was available on fire regimes of pine or hardwood ecosystems of the Southeast where speckled alder formed a substantial component of the vegetation. A review reported that pond pine and bay (Magnolia and Persea spp.) shrubland bogs, of which speckled alder is a component, burn at irregular, 5- to 50-year intervals. Nearby bog communities with low shrub diversity tend to have less frequent fire-return intervals than the bog communities with high shrub diversity [356].

See the Fire Regime Table for speckled alder for further information on fire regimes of vegetation communities in which speckled alder may occur.

Thinleaf alder: Historical fire regimes for riparian zones of the western United States are not well studied [307,311], although more is known of fire regimes in riparian and wetland areas of the West than those of the East. Historically, thinleaf alder glades and thickets in the Northern Rocky Mountains probably burned less frequently than surrounding coniferous forests, but at "higher intensity" than surrounding vegetation. Davis and others [92] speculated that thinleaf alder thickets may be maintained by infrequent fire. In most years, thinleaf alder thickets in riparian zones do not burn [92]. Similarly, thinleaf alder stands growing on seeps and springs may burn rarely [200].

In general, fires are less frequent in riparian and wetland areas of the West than in drier areas [12]. At low- to midelevations in the southern Cascade Range and Klamath Mountain regions of California, fire-return intervals in riparian areas overall are about twice those of surrounding areas, but fire intensity is generally greater when the riparian areas burn [307]. Fire-return intervals near intermittent and ephemeral streams are likely similar to those of surrounding areas [311]. In most years, perennial stream communities may serve as firebreaks (review by [311]). A fire management plan for Craters of the Moon National Monument, Idaho, states "there is no evidence that fire is necessary to maintain the (riparian) vegetation type in this area". However, the authors speculated that infrequent, periodic fire would probably improve the health and productivity of shrubs and that most shrubs, including thinleaf alder, would survive fires that were "not too frequent" [17]. Arno and Harrington [12] suggest that prior to 1900, riparian ponderosa pine communities in the western United States burned 2 to 5 times per century. Fire-return intervals in riparian and wetland communities may vary across time and space, however.

In riparian zones where thinleaf alder is known or likely to occur, fires tend to have longer return intervals, and burn at higher severity, than in upland sites ([115,268,310]), Arno 2001 personal communication [11]). This trend is not strong or consistent in all areas, however. Fire frequency and severity have been lower in some riparian areas compared to adjacent upland areas; other sites have similar fire histories on riparian and upland zones. Factors affecting fire-return intervals in riparian zones include soil and plant moisture, fire exclusion, livestock grazing, logging, damming and other water-flow regulation, and presence of invasive species that alter fuel characteristics [111]. Coniferous riparian forests may have historically been too moist to burn in some years; consequently, low- and moderate-severity fires in upland areas would often extinguish at the riparian zone. In an Alberta study of upland and riparian lodgepole pine, subalpine fir-Engelmann spruce, white spruce, and balsam poplar communities on the Jumping Creek Watershed, landform was the primary driver of fire frequencies. Using time-since-fire distributions, the authors found that riparian zones and the larger, entire-watershed area had similar mean fire frequencies. At a fine scale, however, fires were less frequent near stream channels with gravel- or sandbars, whereas terraces above straight streams without bars tended to burn at same frequency as the overall watershed. From 1851 to 1890, fire-return intervals averaged 71 years in riparian and 48 years in entire-watershed zones, respectively. After 1891, fire-return intervals lengthened to 208 and 178 years, respectively. The authors concluded that on the Jumping Creek Watershed, except for bars, areas adjacent to streams were just as likely to burn as upland areas [66]. Understory vegetation was not surveyed in the study, but thinleaf alder is known to occur in the area.

Two studies suggest a trend of longer fire-return interval in riparian Douglas-fir than upland Douglas-fir forests. In Douglas-fir forests on the Steamboat Creek Watershed in southwestern Oregon, mean fire-return intervals were longer in riparian than upland sites (37 vs. 31.5 years), but the difference was not significant. Fire-return intervals ranged from 4 to 167 years on riparian and 2 to 110 years on upland sites. The authors speculated that fires were historically common, of mixed severity, and patchy on their study sites, with a high likelihood of fire in both riparian and upland zones. They suggested that historically, many riparian forests burned when upslope fires backed into riparian zones [268]. On eastside riparian Douglas-fir forests in the Cascade Range of Washington, fire-scarred trees were consistently fewer in riparian than on upland sites. The authors concede that fewer fire scars does not necessarily mean fewer fires; fires in the riparian zone could have been mostly low-severity surface fires that caused less scarring than fires in upland areas, or fires could have been more severe, stand-replacement fires that left fewer standing live, fire-scarred trees than in upland zones. Plant association groups in riparian forests had 25% to 42% fewer wildfires than similar plant associations on sideslopes. Fire-return intervals ranged from 11 to 46 years in riparian forests and from 7 to 39 years on sideslopes. On sideslopes, stands on south-, east-, and west-facing slopes in riparian zones were older than those in nonriparian zones (P≤0.1); differences in stand ages between riparian and nonriparian sites on north-facing slopes were not significant [115].

On the Shasta-Trinity National Forest in the Klamath Mountains of California, fire-return intervals in riparian and upland mixed-conifer forests were compared. Intervals between fires were consistently longer in riparian vs. upland sites [310]:

Fire-return intervals of riparian and upland mixed-conifer sites in the Klamath Mountains [310]
Site Mean fire-return interval (range) Time period Number of stumps sampled
Root Creek,
riparian
33 years (7-56) 1673-1880 4
Root Creek,
upland
7 years (3-44) 1749-1924 14
North Fork Shotgun Creek,
riparian
16 years (5-56) 1710-1916 9
North Fork Shotgun Creek,
upland
8 years (4-64) 1622-1887 16

In a fire history study of Jeffrey pine, white fir, and mixed-conifer forests of the Sierra Nevada in which thinleaf alder was "highly abundant", fire-return intervals were usually similar on riparian and upland sites. On 36 paired riparian and upland sites on the Lassen and Tahoe National Forests, only 9 paired sites had significantly different fire-return intervals (P≤0.1). Fire-return intervals ranged from 15.3 to 86.5 years on riparian sites and from 10.0 to 56.3 years on upland sites. They averaged 30.0 and 27.8 years on riparian and upland sites, respectively. Most fire scarring (88% on riparian and 79% on upland sites) happened in late summer and early fall, after tree-ring growth had stopped [340].

Fires in the spruce-birch and aspen ecosystems of interior Alaska are usually stand-replacing and often extensive [234,344], burning "for days with high intensities, covering vast acreages" [357]. Fire frequency is poorly known but probably ranges from 75 to 150 years. In quaking aspen/thinleaf alder stands on Ester Dome, fire kill was the primary cause of tree mortality [206].

Near Rainbow Lake in Wood Buffalo National Park, Alberta, a fire history study found charcoal evidence of at least 12 local fires in 840 years, with a mean return interval of 69 years. In some cases, thinleaf alder pollen counts (a surrogate for thinleaf alder density) peaked about 35 years after a fire; however, thinleaf alder pollen counts declined after 6 of the local fires. The extant plant community is mixed hardwood-conifer. Thinleaf alder is most common at the lake margins, along with scattered black spruce and tamarack [216].

See the Fire Regime Table for thinleaf alder for further information on fire regimes of vegetation communities in which thinleaf alder may occur.

FIRE MANAGEMENT CONSIDERATIONS:
Early postfire environments generally favor gray alder establishment and growth. As a nitrogen fixer, gray alder can have important ecological effects on postfire plant and soil fauna succession. In riparian areas, sprouting gray alders help stabilize burned banks and shores. Gray alder may provide little protection in riparian areas burned by fire severe enough to kill the roots. Dead and decaying thinleaf alder roots seldom provide substantial protection from erosion during heavy spring run-off [200]. To date (2011), research on using prescribed fire in wetland communities with gray alder was limited, and further research is needed before guidelines for burning in gray alder riparian and wetland zones can be made. Speckled alder: Prescribed fire may help control speckled alder on silvicultural sites and on open mires. Related prescribed fire studies and management recommendations available are summarized here.

In the Great Lakes states, frequent prescribed fires (no return interval given) can help control speckled alder, beaked hazelnut, and American hazelnut in the understories of red pine plantations. Summer fires best prepare a mineral soil seedbed necessary for natural red pine regeneration, and the generally drier fuels results in severer fires and better shrub control than spring prescribed fires [46].

On plantations, interference of postfire speckled alder regeneration with conifer regeneration and growth may be minimal on moist lowland sites where fire has prepared a mineral seedbed for conifers. Speckled alder and mountain alder "grew well" on such sites 10 years after clearcutting and 8 years after a late spring (2 June 1970) prescribed fire on a jack pine/black spruce plantation in southeastern Manitoba. Chrosciewicz [69] concluded that even though "plant competition was generally much more severe" on moist lowland sites than on dry or fresh upland sites, the alders and other postfire regeneration "had no detrimental effect on regeneration of commercial tree species, including pine" on moist lowland sites. Postfire conifer establishment was poor on the upland sites, which the author attributed to lower than normal precipitation and consequently, drier than normal soils, during the first 2 postfire growing seasons [69]. Another study by Chrosciewicz [68], near Winnipeg, Manitoba, had similar results. Five years after low- and moderate-severity spring (late May) prescribed fires, shrub cover, including speckled alder, averaged 15% to 20% in regenerating black spruce stands. This "rather luxuriant vegetation" "seemed to provide valuable shelter to the spruce seedlings in their early development" [68]. See the Research Project Summary on Chrosciewicz's [68] research for information on the fire prescriptions, fire behavior, and for details of black spruce's postfire response.

Early spring burning—after snowmelt but before soil thaw—may help control speckled alder and other woody invaders in wetlands. Researchers found that early spring prescribed burning was more cost-effective than early and late summer cutting or late manual torching for reducing woody encroachment in a tussock sedge-bluejoint grass wetland meadow in Wisconsin. Additionally, it caused the least disturbance to groundlayer vegetation and to the soil. The researchers warned that due to sprouting of woody species (which occurred after all fire and cutting treatments), prescribed fire must be viewed as an ongoing treatment strategy. Speckled alder was a component of the woody vegetation, although Sandberg's birch (Betula × sandbergii) and red-osier dogwood had the most cover. Fire effects on speckled alder alone were not measured [294].

Managers of the Lac St-Francois National Wildlife Area in Quebec found that neither a single wildfire nor mechanical treatment (double-cutting shoots) controlled speckled alder invasion on an acidic, water sedge-hairy sedge fen in the short term, while stump applications of herbicide reduced speckled alder but also reduced nontarget plants. The wildfire had burned part of the fen prior to the study, and plots were established on sections of the burn that were "highly invaded by small to medium-sized individual alder shrubs". Speckled alder root crowns were located at or below the water table. All of 40 speckled alder individuals on study plots survived the fire and were sprouting "vigorously" in postfire year 1. For the double-cut treatment, stems were cut to the root crown in mid-June, with a 2nd cutting of new sprouts in early August. The next year, 80% of cut plants were sprouting "vigorously". While conceding that another cut would increase speckled alder mortality, the authors found cutting too ineffective and costly to justify further cuttings. Foliar applications of glyphosate in July gave 100% control of speckled alder but also killed more nontarget plants than considered acceptable (no limits given). For cut-stump glyphosate treatments, either August or March application gave good control (79% and 93%, respectively). The March treatment was preferred, because it was more effective and accessibility was greater on the frozen winter ground than on the wet summer ground [38]. The authors noted that based on other research [46,161], very frequent prescribed fires (1 or 2 fires annually over 5 years) may control speckled alder but would "carry very high operation costs" and possibly be detrimental to the fen. However, they considered prescribed fire a viable option for long-term control of speckled alder [38].

Thinleaf alder: Recommendations for prescribed fire use in thinleaf alder communities are few and sometimes contradictory. Prescribed fire in these communities may increase plant community productivity or conversely, be used as a tool to control thinleaf alder. Olason and Agee [268] note that although fires were probably historically frequent in riparian Douglas-fir forests of the Pacific Northwest, it may be necessary to "totally protect" riparian forests in areas where favorable salmonid habitat is limited. They suggest treating upslope forests with prescribed fire or fire-surrogate treatments, speculating that such treatments would make wildfires less likely to burn into moist portions of riparian zones [268].

Others suggest that excluding fire from riparian zones may be harmful in the long term. On the Payette National Forest, Idaho, a low-severity May prescribed fire in an upland subalpine fir-Engelmann spruce community burned only slightly into an alder (Alnus spp.)-dominated riparian zone. Comparing these results to late summer wildfires in the same communities, the authors found that with wildfires, the area burned in riparian communities was proportionate to that burned in the upland communities. While fire severity was generally lower in the riparian than the upland communities, portions of the riparian zones burned at high severity. The authors concluded that in riparian zones, spring prescribed fire is not an ecological surrogate for fall wildfire and that a "prescribed fire regime" of repeatedly burning upland communities while excluding fire from riparian forests may eliminate important natural disturbances from riparian and stream habitats [8]. Besides wildfire itself, these disturbances may include postfire erosion, altered stream channels, altered stream temperatures, and altered species composition of the aquatic animal communities [8,107].

Effects of prescribed fires in riparian and wetland zones may be difficult to predict. Prescribed fire may help control thinleaf alder and other woody invasives in freshwater wetlands [42]. Jacobson and others' study [182] provides management recommendations for reducing woody encroachment on open fens. However, in forests, thinleaf alder may increase after overstory removal by prescribed fire and/or cutting, especially on wet sites. Researchers in British Columbia report that although moderate to severe fires are needed to "set back the growth" of thinleaf alder, such fires rarely carry in the wet to moist sites where thinleaf alder usually grows [71].

Thinleaf alder-dominated communities may recover more quickly than adjacent upland communities. Six years after the Diamond Peak Wildfire Complex on the Payette National Forest, woody plant cover in thinleaf alder-dominated riparian communities was greatest on sites where fire severity was greatest (P=0.05). Across 6 riparian sites burned at varying severities, total woody cover ranged from a low of 8% in postfire year 2 to 30% in postfire year 6. The authors found that in postfire year 6, "little recovery had taken place in the upland (ponderosa pine) community" [287].

A May wildfire in a quaking aspen-balsam poplar/thinleaf alder-red alder community reduced nesting and hiding cover for ruffed grouse in the short term. Alders (thinleaf and red alder combined) averaged 8.0 feet (2.4 m) tall 1 year before and 3.5 feet (1.1 m) tall 1 year after the fire. Along a 2,000-foot (600 m) transect, alders <3 feet (2 m) tall averaged 35.8 stems/transect before fire and 3.8 stems/transect 1 year after fire. On drumming sites used by male ruffed grouse, alders <3 feet tall averaged 28.7 stems/transect before fire and 2.9 stems/transect after fire. In postfire years 1 and 2, estimated ruffed grouse numbers in burned areas averaged 50% of ruffed grouse numbers 1 year before fire. By postfire year 3, density of males in unburned drumming areas was almost 3 times that of males in burned drumming areas [99]. See the FEIS review of ruffed grouse for details.

MANAGEMENT CONSIDERATIONS

SPECIES: Alnus incana
FEDERAL LEGAL STATUS:
None

OTHER STATUS:
Information on state- and province-level protection status of plants in the United States and Canada is available at NatureServe.

IMPORTANCE TO WILDLIFE AND LIVESTOCK:
Gray alder provides winter browse, although wildlife may not prefer it (review by [143]). This preference may be unexpected because the foliage is high in nitrogen (for example, [180,235]). However, gray alder may be a secondary source of wildlife browse due to its wide distribution and local abundance [334]. Gray alder utilization is generally highest from late fall to early spring [334]. Songbirds eat gray alder seeds [215], and squirrels consume the catkins (review by [143]).

White-tailed deer and lagomorphs browse speckled alder [176]. It provides winter forage (review by [141]), but browsing wildlife are likely to avoid speckled alder if more palatable species are available. In a winter deer yard in Quebec, white-tailed deer browsed red-osier dogwood and pussy willow more than expected, and speckled alder less than expected, based on availability [41].

In cafeteria feeding trials in Wisconsin, captive ruffed grouse avoided speckled alder catkins, preferring those of quaking aspen and willows. The authors speculated that secondary metabolites in speckled alder catkins rendered the speckled alder catkins unpalatable [140].

Mule deer browse thinleaf alder sparingly [261,348], although thinleaf alder may be more important in winter [261]. Mountain goats also use thinleaf alder [240], and livestock may browse thinleaf alder heavily in riparian areas [256]

Arthropods and mollusks consume gray alder. Millipedes browse fresh speckled alder leaf litter [235]. Landsnails (Cepaea nemoralis) also browse speckled alder leaf litter [360]. Speckled alder is a host of alder aphids (Prociphilus), which feed only on alders (Alnus) [368]. Alder aphids are a larval food of the harvester butterfly (Feniseca tarquinius) [305].

Palatability and/or nutritional value: Gray alder's palatability is rated poor to fair for big game animals. Sprouts and young plants are preferred over older browse ([334], review by [141]). Its palatability for livestock is rated from poor in the Southwest to fair in the Pacific Northwest and California. Gray alder is generally more palatable to cattle than to domestic sheep and goats [334].

Songbirds, grouse, and American woodcocks eat gray alder buds, seeds, and catkins (reviews by [142,160]). See Grodzinski and Sawicka-Kapusta [139] for information on the nutritional value of gray alder seeds.

Speckled alder is not a preferred browse taxon; it is used primarily as winter forage. It was ranked very low in palatability among woody taxa (22nd among 23) that white-tailed deer used as forage [265]. On the Apostle Islands of Wisconsin, speckled alder was 1 of 4 taxa that white-tailed deer chose last as browse; 28 taxa were tested [21]. A habitat suitability model, developed for coniferous and aspen-birch ecosystems of the Great Lakes region, lists speckled alder as low-preference browse for moose during the growing season [5]. A study on Isle Royal, Michigan, found moose used speckled alder less than expected based on speckled alder's availability and nutritional content (P≤0.05) [25]. American beavers, however, used speckled alder as much as expected (P<0.05) [26].

Speckled alder is high in protein. See these sources: [25,235] for nutritional information on speckled alder browse. See this source: [140] for nutritional information on speckled alder catkins.

Thinleaf alder is generally unpalatable to wild ungulates [151,223], cattle, and horses. It is fairly palatable to domestic sheep [32]. Wild ungulates browse new thinleaf alder growth but generally avoid mature thinleaf alder [151] except as emergency winter forage. On summer rangelands in Tehama County, California, mule deer browsed thinleaf alder less than expected based on availability [223].

In Alaska, feltleaf willow and balsam poplar were more palatable to browsing mammals when grown in the shade of thinleaf alder than when grown in the open (Rohleder 1985 cited in [45]).

Cover value: Gray alder provides shade and hiding cover for many vertebrate species.

Speckled alder: In a Quebec study, speckled alder was the dominant streamside shrub in an ecosystem supporting brook trout, American mink, and Appalachian brook crayfish (Cambarus bartoni), the American mink's primary prey species in that stream [50]. The federally endangered bog turtle [336] is positively associated with speckled alder wetlands (review by [50]). In Alberta, white spruce/gray alder/horsetail/moss (Equisetum/Hypnum and Polytichum spp.) riparian communities provide shade and reduce water temperatures in trout streams [77].

Speckled alder is critical for American beavers. They use speckled alder for den and dam construction more than they use it as browse [103,222]. In Algonquin Provincial Park, Ontario, the number of speckled alder stems found in American beaver dams (403 stems) was over twice that of speckled alder stems found in their food caches (176 stems). Speckled alder was selected for dam construction about 4 times as often as spruces, which were the next most preferred [103]. In another Algonquin Provincial Park study, American beavers used speckled alder more than expected. Although speckled alder composed 25.6% of the standing vegetation, 49.2% of stems that American beavers felled were speckled alder. Speckled alder cover and basal area increased significantly with distance from American beaver dens (P<0.009) [101,102].

Speckled alder provides important cover for American woodcocks, ruffed grouse [99,266], and other birds. Speckled alder thickets provide critical feeding, nesting, molting, and premigratory staging habitat for American woodcocks ([252,258,266,291], Sheldon 1967 cited in [49]). In Michigan, ruffed grouse used speckled alder for cover in drumming sites (review by [160]). A study in eastern hemlock-red maple/speckled alder-mountain-laurel riparian corridors of Pennsylvania found the number of obligate wetland bird species, such as kinglets and flycatchers, was significantly higher in undisturbed speckled alder riparian corridors than in speckled alder-dominated riparian corridors disturbed by residential or agricultural development [83]. Niemi and Pfannmuller [266] provide a list of bird species using speckled alder-dominated wetlands as breeding and nesting habitat in Minnesota and Michigan.

Thinleaf alder: Thinleaf alder provides cover for snowshoe hares [353] and big game animals [261]. Mule deer, white-tailed deer, and elk use thinleaf alder communities for thermal and hiding cover [151]. American beavers use thinleaf alder to make their dens (review by [160].

Thinleaf alder provides forage and nesting sites for songbirds, upland game birds, and wetland birds (reviews by [143,149]).

Thinleaf alder provides cover for aquatic organisms. Dense thinleaf alder thickets provide thermal and shade cover for fish. Thinleaf alder and associated shrubs stabilize stream channels, and overhanging banks reduce erosion [32,151]. In Oregon, thinleaf alder on streambanks provide cover, food, and shade for salmonids [200]. In the Trout Creek Mountains, thinleaf alder-Wood's rose overstories shaded over 70% of the stream channels of 4 streams, while the more open thinleaf alder/woolly sedge communities shaded about 50% of the stream channels. Dense woolly sedge that grew over the streambank enhanced trout habitat [114].

Cattle loaf in thinleaf alder stands, although dense thinleaf alder thickets may impair access. Heavy cattle trampling may degrade thinleaf alder habitats by creating dish-shaped stream channels and reducing cover of palatable herbs [32,151].

VALUE FOR REHABILITATION OF DISTURBED SITES:
Gray alder helps protect and stabilize streambanks and other riparian areas [215,256,261] and is used for erosion control [348]. Banks stabilized with gray alders can withstand "relatively severe" spring run-off [200]. Gray alder's shade provides mesic habitats for groundlayer herbs [261]. Its symbiotic relationship with nitrogen-fixing bacteria makes it a good selection for planting in nitrogen-depleted soils [117]. Seeds and plants are commercially available [263].

Speckled alder: See these sources: [154,160] for propagation information.

Thinleaf alder: Thinleaf alder is used in revegetation projects [62,256]. It is recommended for riparian revegetation in the Intermountain West [59] and the Northern and Southern Rocky Mountains [58]. It is particularly recommended for high-gradient, high-velocity, low- to midorder streams [59]. See these sources: [104,154,183,256,309] for propagation information.

OTHER USES:
Gray alder wood has little commercial value due to the species' small stature. It is used for firewood [171] and for smoking salmon [345].

American Indians used gray alder medicinally (Moerman 1986 cited in [120]). They used the bark to make red dye [157,190,203,215,348].

OTHER MANAGEMENT CONSIDERATIONS:
Gray alder may compete with timber species, sometimes occupying highly productive sites [142]. When speckled alder is in the understory, clearcutting may result in dense speckled alder stands [160]. Dense stands may prevent or impede conifer regeneration, although scattered individuals present little threat to planted conifers (review by [143]). In northwestern Ontario, speckled alder occurred on wet to moist, nutrient-poor to nutrient-medium sites. It was implicated in competing with conifers for light and moisture but credited with fixing atmospheric nitrogen [51]. On the Tanana River floodplain in central Alaska, thinleaf alder inhibited growth of understory white spruce seedlings by shading and root competition. Thinleaf alder's dense litter layer may also inhibit conifer establishment [353].

When using mechanical treatment to control gray alder, the root crown must be destroyed to prevent sprouting. Stems in contact with soil may layer; layering is common after logging [51]. These sources: [38,51] provide information on controlling speckled alder with herbicides.

Although it is not heavily browsed when other forage is available, livestock concentrated in riparian areas sometimes overbrowse and trample thinleaf alder. In northeastern Oregon, thinleaf alder density was significantly greater on ungrazed gravelbar communities than on riparian sites with livestock (P<0.01) [138]. In extreme cases, thinleaf alder becomes scattered and short, with mostly broken branches, and streambanks of thinleaf alder communities degrade to dish shapes. In central Oregon, it took at least 5 years for thinleaf alder to recover from such overbrowsing, and at least 2 to 5 years for streams to reform banked channels, which had become dish-shaped with trampling [200]. Ehrhart and Hansen [113] provide management guidelines for grazing cattle in riparian zones.

APPENDIX: FIRE REGIME TABLE

SPECIES: Alnus incana
The following table provides fire regime information for plant communities in which gray alder is known to occur. In many cases, fire regimes in riparian and wetland communities where gray alder comprises a large portion of the vegetation or is dominant may differ from these usually larger, surrounding plant communities (see Fire regimes).

Speckled alder
Fire regime information on vegetation communities in which speckled alder may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models [214], which were developed by local experts using available literature, local data, and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
Great Lakes Northeast Southern Appalachians Southeast
Great Lakes
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Great Lakes Woodland
Great Lakes pine barrens Replacement 8% 41 10 80
Mixed 9% 36 10 80
Surface or low 83% 4 1 20
Jack pine-open lands (frequent fire-return interval) Replacement 83% 26 10 100
Mixed 17% 125 10  
Northern oak savanna Replacement 4% 110 50 500
Mixed 9% 50 15 150
Surface or low 87% 5 1 20
Great Lakes Forested
Northern hardwood maple-beech-eastern hemlock Replacement 60% >1,000    
Mixed 40% >1,000    
Conifer lowland (embedded in fire-prone ecosystem) Replacement 45% 120 90 220
Mixed 55% 100    
Conifer lowland (embedded in fire-resistant ecosystem) Replacement 36% 540 220 >1,000
Mixed 64% 300    
Great Lakes floodplain forest Mixed 7% 833    
Surface or low 93% 61    
Great Lakes spruce-fir Replacement 100% 85 50 200
Maple-basswood Replacement 33% >1,000    
Surface or low 67% 500    
Maple-basswood mesic hardwood forest (Great Lakes) Replacement 100% >1,000 >1,000 >1,000
Maple-basswood-oak-aspen Replacement 4% 769    
Mixed 7% 476    
Surface or low 89% 35    
Northern hardwood-eastern hemlock forest (Great Lakes) Replacement 99% >1,000    
Oak-hickory Replacement 13% 66 1  
Mixed 11% 77 5  
Surface or low 76% 11 2 25
Pine-oak Replacement 19% 357    
Surface or low 81% 85    
Red pine-eastern white pine (frequent fire) Replacement 38% 56    
Mixed 36% 60    
Surface or low 26% 84    
Red pine-eastern white pine (less frequent fire) Replacement 30% 166    
Mixed 47% 105    
Surface or low 23% 220    
Great Lakes pine forest, eastern white pine-eastern hemlock (frequent fire) Replacement 52% 260    
Mixed 12% >1,000    
Surface or low 35% 385    
Eastern white pine-eastern hemlock Replacement 54% 370    
Mixed 12% >1,000    
Surface or low 34% 588    
Northeast
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Northeast Woodland
Eastern woodland mosaic Replacement 2% 200 100 300
Mixed 9% 40 20 60
Surface or low 89% 4 1 7
Rocky outcrop pine (Northeast) Replacement 16% 128    
Mixed 32% 65    
Surface or low 52% 40    
Pine barrens Replacement 10% 78    
Mixed 25% 32    
Surface or low 65% 12    
Oak-pine (eastern dry-xeric) Replacement 4% 185    
Mixed 7% 110    
Surface or low 90% 8    
Northeast Forested
Northern hardwoods (Northeast) Replacement 39% >1,000    
Mixed 61% 650    
Eastern white pine-northern hardwood Replacement 72% 475    
Surface or low 28% >1,000    
Northern hardwoods-eastern hemlock Replacement 50% >1,000    
Surface or low 50% >1,000    
Northern hardwoods-spruce Replacement 100% >1,000 400 >1,000
Appalachian oak forest (dry-mesic) Replacement 2% 625 500 >1,000
Mixed 6% 250 200 500
Surface or low 92% 15 7 26
Beech-maple Replacement 100% >1,000    
Northeast spruce-fir forest Replacement 100% 265 150 300
Southeastern red spruce-Fraser fir Replacement 100% 500 300 >1,000
Southern Appalachians
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Southern Appalachians Woodland
Appalachian shortleaf pine Replacement 4% 125    
Mixed 4% 155    
Surface or low 92% 6    
Table Mountain-pitch pine Replacement 5% 100    
Mixed 3% 160    
Surface or low 92% 5    
Oak-ash woodland Replacement 23% 119    
Mixed 28% 95    
Surface or low 49% 55    
Southern Appalachians Forested
Bottomland hardwood forest Replacement 25% 435 200 >1,000
Mixed 24% 455 150 500
Surface or low 51% 210 50 250
Mixed mesophytic hardwood Replacement 11% 665    
Mixed 10% 715    
Surface or low 79% 90    
Appalachian oak-hickory-pine Replacement 3% 180 30 500
Mixed 8% 65 15 150
Surface or low 89% 6 3 10
Eastern hemlock-eastern white pine-hardwood Replacement 17% >1,000 500 >1,000
Surface or low 83% 210 100 >1,000
Red pine-eastern white pine (frequent fire) Replacement 38% 56    
Mixed 36% 60    
Surface or low 26% 84    
Eastern white pine-northern hardwood Replacement 72% 475    
Surface or low 28% >1,000    
Appalachian Virginia pine Replacement 20% 110 25 125
Mixed 15% 145    
Surface or low 64% 35 10 40
Appalachian oak forest (dry-mesic) Replacement 6% 220    
Mixed 15% 90    
Surface or low 79% 17    
Southern Appalachian high-elevation forest Replacement 59% 525    
Mixed 41% 770    
Southeast
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Southeast Shrubland
Pocosin Replacement 1% >1,000 30 >1,000
Mixed 99% 12 3 20
Southeast Woodland
Longleaf pine/bluestem Replacement 3% 130    
Surface or low 97% 4 1 5
Longleaf pine (mesic uplands) Replacement 3% 110 40 200
Surface or low 97% 3 1 5
Longleaf pine-Sandhills prairie Replacement 3% 130 25 500
Surface or low 97% 4 1 10
Pine rocklands Mixed 1% 330    
Surface or low 99% 3 1 5
Pond pine Replacement 64% 7 5 500
Mixed 25% 18 8 150
Surface or low 10% 43 2 50
Atlantic wet pine savanna Replacement 4% 100    
Mixed 2% 175    
Surface or low 94% 4     
Southeast Forested
Sand pine scrub Replacement 90% 45 10 100
Mixed 10% 400 60  
Coastal Plain pine-oak-hickory Replacement 4% 200    
Mixed 7% 100      
Surface or low 89% 8    
Atlantic white-cedar forest Replacement 34% 200 25 350
Mixed 8% 900 20 900
Surface or low 59% 115 10 500
Maritime forest Replacement 18% 40   500
Mixed 2% 310 100 500
Surface or low 80% 9 3 50
Loess bluff and plain forest Replacement 7% 476    
Mixed 9% 385    
Surface or low 85% 39    
Southern floodplain Replacement 7% 900    
Surface or low 93% 63    
*Fire Severities—
Replacement: Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed: Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low: Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [148,213].


Thinleaf alder
Fire regime information on vegetation communities in which thinleaf alder may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models [214], which were developed by local experts using available literature, local data, and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
Pacific Northwest California Southwest Great Basin
Northern and Central Rockies Northern Great Plains    
Pacific Northwest
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Northwest Grassland
Marsh Replacement 74% 7    
Mixed 26% 20    
Bluebunch wheatgrass Replacement 47% 18 5 20
Mixed 53% 16 5 20
Idaho fescue grasslands Replacement 76% 40    
Mixed 24% 125    
Alpine and subalpine meadows and grasslands Replacement 68% 350 200 500
Mixed 32% 750 500 >1,000
Northwest Shrubland
Wyoming big sagebrush semidesert Replacement 86% 200 30 200
Mixed 9% >1,000 20  
Surface or low 5% >1,000 20  
Wyoming sagebrush steppe Replacement 89% 92 30 120
Mixed 11% 714 120  
Mountain big sagebrush (cool sagebrush) Replacement 100% 20 10 40
Northwest Woodland
Western juniper (pumice) Replacement 33% >1,000    
Mixed 67% 500    
Oregon white oak-ponderosa pine Replacement 16% 125 100 300
Mixed 2% 900 50  
Surface or low 81% 25 5 30
Pine savannah (ultramafic) Replacement 7% 200 100 300
Surface or low 93% 15 10 20
Ponderosa pine Replacement 5% 200    
Mixed 17% 60    
Surface or low 78% 13    
Oregon white oak Replacement 3% 275    
Mixed 19% 50    
Surface or low 78% 12.5    
Subalpine woodland Replacement 21% 300 200 400
Mixed 79% 80 35 120
Northwest Forested
Sitka spruce-western hemlock Replacement 100% 700 300 >1,000
Douglas-fir (Willamette Valley foothills) Replacement 18% 150 100 400
Mixed 29% 90 40 150
Surface or low 53% 50 20 80
Oregon coastal tanoak Replacement 10% 250    
Mixed 90% 28 15 40
Dry ponderosa pine (mesic) Replacement 5% 125    
Mixed 13% 50    
Surface or low 82% 8    
Douglas-fir-western hemlock (dry mesic) Replacement 25% 300 250 500
Mixed 75% 100 50 150
Douglas-fir-western hemlock (wet mesic) Replacement 71% 400    
Mixed 29% >1,000    
Mixed conifer (southwestern Oregon) Replacement 4% 400    
Mixed 29% 50    
Surface or low 67% 22    
California mixed evergreen (northern California and southern Oregon) Replacement 6% 150 100 200
Mixed 29% 33 15 50
Surface or low 64% 15 5 30
Mountain hemlock Replacement 93% 750 500 >1,000
Mixed 7% >1,000    
Lodgepole pine (pumice soils) Replacement 78% 125 65 200
Mixed 22% 450 45 85
Pacific silver fir (low elevation) Replacement 46% 350 100 800
Mixed 54% 300 100 400
Pacific silver fir (high elevation) Replacement 69% 500    
Mixed 31% >1,000    
Subalpine fir Replacement 81% 185 150 300
Mixed 19% 800 500 >1,000
Mixed conifer (eastside dry) Replacement 14% 115 70 200
Mixed 21% 75 70 175
Surface or low 64% 25 20 25
Mixed conifer (eastside mesic) Replacement 35% 200    
Mixed 47% 150    
Surface or low 18% 400    
Red fir Replacement 20% 400 150 400
Mixed 80% 100 80 130
Spruce-fir Replacement 84% 135 80 270
Mixed 16% 700 285 >1,000
California
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
California Grassland
California grassland Replacement 100% 2 1 3
Herbaceous wetland Replacement 70% 15    
Mixed 30% 35    
Wet mountain meadow-Lodgepole pine (subalpine) Replacement 21% 100    
Mixed 10% 200    
Surface or low 69% 30    
Alpine meadows barrens Replacement 100% 200 200 400
California Shrubland
Coastal sage scrub Replacement 100% 50 20 150
Coastal sage scrub-coastal prairie Replacement 8% 40 8 900
Mixed 31% 10 1 900
Surface or low 62% 5 1 6
Chaparral Replacement 100% 50 30 125
Montane chaparral Replacement 34% 95    
Mixed 66% 50    
California Woodland
California oak woodlands Replacement 8% 120    
Mixed 2% 500    
Surface or low 91% 10    
Ponderosa pine Replacement 5% 200    
Mixed 17% 60    
Surface or low 78% 13    
California Forested
California mixed evergreen Replacement 10% 140 65 700
Mixed 58% 25 10 33
Surface or low 32% 45 7  
Coast redwood Replacement 2% ≥1,000    
Surface or low 98% 20    
Mixed conifer (north slopes) Replacement 5% 250    
Mixed 7% 200    
Surface or low 88% 15 10 40
Mixed conifer (south slopes) Replacement 4% 200    
Mixed 16% 50    
Surface or low 80% 10    
Aspen with conifer Replacement 24% 155 50 300
Mixed 15% 240    
Surface or low 61% 60    
Jeffrey pine Replacement 9% 250    
Mixed 17% 130    
Surface or low 74% 30    
Mixed evergreen-bigcone Douglas-fir (southern coastal) Replacement 29% 250    
Mixed 71% 100    
Interior white fir (northeastern California) Replacement 47% 145    
Mixed 32% 210    
Surface or low 21% 325    
Red fir-white fir Replacement 13% 200 125 500
Mixed 36% 70    
Surface or low 51% 50 15 50
Red fir-western white pine Replacement 16% 250    
Mixed 65% 60 25 80
Surface or low 19% 200    
Sierra Nevada lodgepole pine (cold wet upper montane) Replacement 23% 150 37 764
Mixed 70% 50    
Surface or low 7% 500    
Sierra Nevada lodgepole pine (dry subalpine) Replacement 11% 250 31 500
Mixed 45% 60 31 350
Surface or low 45% 60 9 350
Southwest
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Southwest Shrubland
Southwestern shrub steppe Replacement 72% 14 8 15
Mixed 13% 75 70 80
Surface or low 15% 69 60 100
Southwestern shrub steppe with trees Replacement 52% 17 10 25
Mixed 22% 40 25 50
Surface or low 25% 35 25 100
Mountain sagebrush (cool sage) Replacement 75% 100    
Mixed 25% 300    
Southwest Woodland
Pinyon-juniper (mixed fire regime) Replacement 29% 430    
Mixed 65% 192    
Surface or low 6% >1,000    
Pinyon-juniper (rare replacement fire regime) Replacement 76% 526    
Mixed 20% >1,000    
Surface or low 4% >1,000    
Ponderosa pine/grassland (Southwest) Replacement 3% 300    
Surface or low 97% 10    
Bristlecone-limber pine (Southwest) Replacement 67% 500    
Surface or low 33% >1,000    
Southwest Forested
Riparian forest with conifers Replacement 100% 435 300 550
Riparian deciduous woodland Replacement 50% 110 15 200
Mixed 20% 275 25  
Surface or low 30% 180 10  
Ponderosa pine-Gambel oak (southern Rockies and Southwest) Replacement 8% 300    
Surface or low 92% 25 10 30
Ponderosa pine-Douglas-fir (southern Rockies) Replacement 15% 460    
Mixed 43% 160    
Surface or low 43% 160    
Southwest mixed conifer (warm, dry with aspen) Replacement 7% 300    
Mixed 13% 150 80 200
Surface or low 80% 25 2 70
Southwest mixed conifer (cool, moist with aspen) Replacement 29% 200 80 200
Mixed 35% 165 35  
Surface or low 36% 160 10  
Aspen with spruce-fir Replacement 38% 75 40 90
Mixed 38% 75 40  
Surface or low 23% 125 30 250
Stable aspen without conifers Replacement 81% 150 50 300
Surface or low 19% 650 600 >1,000
Lodgepole pine (Central Rocky Mountains, infrequent fire) Replacement 82% 300 250 500
Surface or low 18% >1,000 >1,000 >1,000
Spruce-fir Replacement 96% 210 150  
Mixed 4% >1,000 35 >1,000
Great Basin
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Great Basin Grassland
Mountain meadow (mesic to dry) Replacement 66% 31 15 45
Mixed 34% 59 30 90
Great Basin Shrubland
Wyoming big sagebrush semidesert Replacement 86% 200 30 200
Mixed 9% >1,000 20 >1,000
Surface or low 5% >1,000 20 >1,000
Wyoming big sagebrush semidesert with trees Replacement 84% 137 30 200
Mixed 11% >1,000 20 >1,000
Surface or low 5% >1,000 20 >1,000
Wyoming sagebrush steppe Replacement 89% 92 30 120
Mixed 11% 714 120  
Mountain big sagebrush Replacement 100% 48 15 100
Mountain big sagebrush with conifers Replacement 100% 49 15 100
Mountain sagebrush (cool sage) Replacement 75% 100    
Mixed 25% 300    
Montane chaparral Replacement 37% 93    
Mixed 63% 54    
Gambel oak Replacement 75% 50    
Mixed 25% 150    
Mountain shrubland with trees Replacement 22% 105 100 200
Mixed 78% 29 25 100
Black and low sagebrushes Replacement 33% 243 100  
Mixed 67% 119 75 140
Black and low sagebrushes with trees Replacement 37% 227 150 290
Mixed 63% 136 50 190
Curlleaf mountain-mahogany Replacement 31% 250 100 500
Mixed 37% 212 50  
Surface or low 31% 250 50  
Great Basin Woodland
Juniper and pinyon-juniper steppe woodland Replacement 20% 333 100 >1,000
Mixed 31% 217 100 >1,000
Surface or low 49% 135 100  
Ponderosa pine Replacement 5% 200    
Mixed 17% 60    
Surface or low 78% 13    
Great Basin Forested
Interior ponderosa pine Replacement 5% 161   800
Mixed 10% 80 50 80
Surface or low 86% 9 8 10
Ponderosa pine-Douglas-fir Replacement 10% 250   >1,000
Mixed 51% 50 50 130
Surface or low 39% 65 15  
Great Basin Douglas-fir (dry) Replacement 12% 90   600
Mixed 14% 76 45  
Surface or low 75% 14 10 50
Aspen with conifer (low to midelevations) Replacement 53% 61 20  
Mixed 24% 137 10  
Surface or low 23% 143 10  
Douglas-fir (warm mesic interior) Replacement 28% 170 80 400
Mixed 72% 65 50 250
Aspen with conifer (high elevations) Replacement 47% 76 40  
Mixed 18% 196 10  
Surface or low 35% 100 10  
Stable aspen-cottonwood, no conifers Replacement 31% 96 50 300
Surface or low 69% 44 20 60
Spruce-fir-pine (subalpine) Replacement 98% 217 75 300
Mixed 2% >1,000    
Aspen with spruce-fir Replacement 38% 75 40 90
Mixed 38% 75 40  
Surface or low 23% 125 30 250
Stable aspen without conifers Replacement 81% 150 50 300
Surface or low 19% 650 600 >1,000
Northern and Central Rockies
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Northern and Central Rockies Grassland
Northern prairie grassland Replacement 55% 22 2 40
Mixed 45% 27 10 50
Mountain grassland Replacement 60% 20 10  
Mixed 40% 30    
Northern and Central Rockies Shrubland
Riparian (Wyoming) Mixed 100% 100 25 500
Wyoming big sagebrush Replacement 63% 145 80 240
Mixed 37% 250    
Basin big sagebrush Replacement 60% 100 10 150
Mixed 40% 150    
Low sagebrush shrubland Replacement 100% 125 60 150
Mountain shrub, nonsagebrush Replacement 80% 100 20 150
Mixed 20% 400    
Mountain big sagebrush steppe and shrubland Replacement 100% 70 30 200
Northern and Central Rockies Woodland
Ancient juniper Replacement 100% 750 200 >1,000
Northern and Central Rockies Forested
Ponderosa pine (Northern Great Plains) Replacement 5% 300    
Mixed 20% 75    
Surface or low 75% 20 10 40
Ponderosa pine (Northern and Central Rockies) Replacement 4% 300 100 >1,000
Mixed 19% 60 50 200
Surface or low 77% 15 3 30
Ponderosa pine (Black Hills, low elevation) Replacement 7% 300 200 400
Mixed 21% 100 50 400
Surface or low 71% 30 5 50
Ponderosa pine (Black Hills, high elevation) Replacement 12% 300    
Mixed 18% 200    
Surface or low 71% 50    
Ponderosa pine-Douglas-fir Replacement 10% 250   >1,000
Mixed 51% 50 50 130
Surface or low 39% 65 15  
Western redcedar Replacement 87% 385 75 >1,000
Mixed 13% >1,000 25  
Douglas-fir (xeric interior) Replacement 12% 165 100 300
Mixed 19% 100 30 100
Surface or low 69% 28 15 40
Douglas-fir (warm mesic interior) Replacement 28% 170 80 400
Mixed 72% 65 50 250
Douglas-fir (cold) Replacement 31% 145 75 250
Mixed 69% 65 35 150
Grand fir-Douglas-fir-western larch mix Replacement 29% 150 100 200
Mixed 71% 60 3 75
Mixed conifer-upland western redcedar-western hemlock Replacement 67% 225 150 300
Mixed 33% 450 35 500
Western larch-lodgepole pine-Douglas-fir Replacement 33% 200 50 250
Mixed 67% 100 20 140
Grand fir-lodgepole pine-larch-Douglas-fir Replacement 31% 220 50 250
Mixed 69% 100 35 150
Persistent lodgepole pine Replacement 89% 450 300 600
Mixed 11% >1,000    
Whitebark pine-lodgepole pine (upper subalpine, Northern and Central Rockies) Replacement 38% 360    
Mixed 62% 225    
Lower subalpine lodgepole pine Replacement 73% 170 50 200
Mixed 27% 450 40 500
Lower subalpine (Wyoming and Central Rockies) Replacement 100% 175 30 300
Upper subalpine spruce-fir (Central Rockies) Replacement 100% 300 100 600
Northern Great Plains
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Northern Plains Grassland
Northern mixed-grass prairie Replacement 67% 15 8 25
Mixed 33% 30 15 35
Oak savanna Replacement 7% 44    
Mixed 17% 18    
Surface or low 76% 4    
Northern Plains Woodland
Oak woodland Replacement 2% 450    
Surface or low 98% 7.5    
Northern Great Plains wooded draws and ravines Replacement 38% 45 30 100
Mixed 18% 94    
Surface or low 43% 40 10  
Great Plains floodplain Replacement 100% 500    
*Fire Severities—
Replacement: Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed: Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low: Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [148,213].

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