Salix discolor



INTRODUCTORY


  Dennis Kalma, 2006
AUTHORSHIP AND CITATION:
Gucker, Corey L. 2007. Salix discolor. 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:
SALDIS

NRCS PLANT CODE [88]:
SADI

COMMON NAMES:
pussy willow
American pussy willow
glaucous willow
large pussy willow

TAXONOMY:
The scientific name of pussy willow is Salix discolor Muhl. (Salicaceae) [36,38,49,92]. Pussy willow belongs to the Caprisalix subgenus and the Vetrix section of the Salicaceae family [42].

Infrataxa: The following varieties are recognized by some systematists:
   Salix discolor var. latifolia [11,27,60,84,92,94]
   Salix discolor var. discolor [11,92,94]

Hybrids: Voss [92] considers pussy willow hybridization common, but others have identified many potential barriers to natural hybridization [63,64,65].

Hybrids with Bebb willow (S. bebbiana) are reported [35,50], and some suggest that hybrids with prairie willow (S. humilis) [36] and Scouler willow (S. scouleriana) are possible [71].

SYNONYMS:
Salix discolor var. overi Ball [92]=
   Salix discolor [49]

Salix discolor var. prinoides (Pursh) Anderss. [84,94]=
   Salix discolor [49]

LIFE FORM:
Shrub-tree

FEDERAL LEGAL STATUS:
No special status

OTHER STATUS:
Information on state-level protected status of plants in the United States is available at Plants Database.


DISTRIBUTION AND OCCURRENCE

SPECIES: Salix discolor
GENERAL DISTRIBUTION:
Pussy willow is common throughout the southern half of Canada and the north-central and northeastern portions of the United States. The westernmost and southernmost distribution of pussy willow in the United States is not entirely clear [54]. There is general agreement that pussy willow occurs from Montana and Wyoming east through Illinois, Kentucky, and West Virginia [10,25,53,60,84]. However, Froiland [34] indicates that pussy willow occurs as far west as the Cascade Mountains of Oregon and Washington. Some researchers report pussy willow in Idaho [2,54], but these claims are not substantiated by floras from these areas. Some authors indicate that the southern US distribution includes Nebraska, eastern Tennessee, and western North Carolina [2,54]. The distribution of pussy willow varieties is not well described. The US Geological Survey provides a distributional map of pussy willow.

HABITAT TYPES AND PLANT COMMUNITIES:
The most typical pussy willow habitats include seral communities that dominate after the colonization of open water by emergent vegetation and forests following canopy-opening disturbance events. Pussy willow is shade intolerant, prefers moist site conditons, and rarely persists beyond the water's edge in climax forest vegetation. Several seral communities where pussy willow is a dominant are described below.

United States:

Canada:

Alberta:

Manitoba: Quebec: Saskatchewan:

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Salix discolor

 

Sherburne National Wildlife Refuge

GENERAL BOTANICAL CHARACTERISTICS:
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g., [10,23,24,38,92]).

Pussy willow is highly variable in its identifying characteristics. Typically it grows as a shrub or small tree to heights of up to 30 feet (8 m) [42]. Willows, in general, grow rapidly and produce extensive root systems [19]. Pussy willow trunks may be single or clustered with stout ascending branches and branchlets [16,42,80]. A large trunk circumference of 61.8 inches (157 cm) was reported for a North American pussy willow (Hightshoe cited in [81]). Bark is slightly fissured and ridged [80]. First-year pussy willow twigs are hairy but become glabrous with age [23,92]. Twigs are flexible and average 1.5 mm in diameter [80]. Large buds (up to 1 cm long) are present and conspicuous along the stems [36,84].

Pussy willow leaves are deciduous, simple, and alternate [16]. Leaf shape varies from oblong to elliptic, and blades are typically 2 to 5 times as long as they are wide [10,24,77]. Blade length varies from 1.2 to 5.1 inches (3-13 cm), and petioles measure 0.2 to 0.8 inch (5-20 mm) long [53,84]. Leaf margins are typically toothed, but the pattern is inconsistent [10,23,38]. Flowers occur in dense catkins. Catkin buds are covered in dense silky hairs and are the best known pussy willow feature [33]. Mature fruiting female catkins are typically 1 to 4.7 inches (2.5-12 cm) long, while mature fruiting male catkins measure 0.6 to 2 inches (1.5-5 cm) long [77]. Historic reports from an early beekeeper indicated that a single staminate pussy willow catkin contained 270 individual flowers and a pistillate catkin had 142 flowers [19]. Neither the age nor location of the catkin-providing shrub was reported. Pussy willow produces long-necked capsule fruits that measure 0.2 to 0.5 inch (5-13 mm) long [53,80]. Seeds are small (1.2-1.6 mm long and 0.5 mm in diameter) and have rings of short, stiff and long, silky hairs at the base [80].

The following floras provide specific, local information useful for distinguishing pussy willow from similar looking sympatric species [16,77,92]. For systematic information on pussy willow varieties, see Braun [11] and Seymour [77].

Hybrids: Pussy willow has been described as freely hybridizing [92]; however, many barriers to natural hybridization such as flowering date, pollination and seed incompatibility, inviable seedling production, and reduced fertility are reported for many potential pussy willow hybrids [3,63,64,65]. It is possible that the highly variable nature of pussy willow shrubs has been erroneously attributed to hybridization.

RAUNKIAER [73] LIFE FORM:
Phanerophyte

REGENERATION PROCESSES:
Pussy willow regenerates asexually by sprouting [71,74,93] and sexually through seed production and germination [62,79].

Pollination: Pussy willow flowers are insect and wind pollinated. Bumblebees, butterflies, flies, and ants visit willow flowers for nectar and pollen. In many areas, willows provide the first and most important bee forage [19]. During pollination and hybridization experiments with pussy willow in the Central Research Forest near Ottawa, Ontario, Kevan (personal communication in [3]) observed many insects visiting pussy willow. However, shrubs isolated from insect visitors produced abundant seed, indicating that fertilization occurred without insect visitors [3].

High-sugar nectar is produced by pussy willow. Five pussy willow catkins collected from a Maine bog had about 200 nectar-bearing flowers each and averaged 2.2 μL of nectar and 1.4 mg of sugar. The amount of sugar in pussy willow catkins was the highest of the 17 associated species evaluated [41].

Breeding system: Pussy willow shrubs are dioecious [80] and protandrous [3].

Seed production: Studies on pussy willow seed production are lacking. A review reports that "optimum" seed production occurs when shrubs are 8 to 25 years old [74]. In seral communities on wet sites that eventually support eastern white pine (Pinus strobus) communities in northern Minnesota, pussy willow produced a "great abundance" of seed. Seeds were so numerous that they blanketed the soil surface [79].

Seed dispersal: Small willow seeds are easily dispersed by wind [19]. A review by Rawson [74] reports that pussy willow seeds are dispersed by both wind and water. On the reclaimed Fresh Kills Landfill on Staten Island, New York, a single pussy willow seedling emerged, and the nearest seed source was 942 feet (287 m) away [75].

Seed banking: Willow seeds are typically viable for just a few days [19]. In the only experiment on pussy willow seed found for this review (2007), a rapid decrease in germination rate occurred after freshly harvested seed was kept for 2 weeks at room temperature [62].

Germination: Studies of pussy willow seed germination are lacking. Pussy willow seed collected in Edmonton, Alberta, germinated at a 50% level when fresh [62]. Based on observations made in pussy willow habitats, Stallard [79] suggested that seeds germinated best on wet or moist soils but would not germinate in submerged soils.

Seedling establishment/growth: No research describes the factors affecting pussy willow seedling establishment. It is reported, though, that 2- to 3-year-old pussy willow seedlings survive submergence [79]. In a shrub-carr adjacent to White Clay Lake in Wisconsin, pussy willow cover was significantly greater on alluvial than histic surface soils (P<0.01). Alluvial soils received recent sediment deposition and were litter-free. The researcher suggested that the leaf litter and grasses that covered histic soils may have restricted seedling establishment [48].

Vegetative regeneration: Pussy willow regenerates vegetatively from basal sprouts [74], and broken branches that lodge in moist soil may root and grow. There are reports of willows used as fence posts that rooted and grew into shrubs or trees [19].

Aboveground stem death promotes basal sprouting. Ewing [32] describes pussy willow postfire sprouting as prolific. Stem death from borer beetles also encourages sprouting [93]. Two years after pussy willow shrubs were cut down in north-central British Columbia, sprouts were over 8.2 feet (2.5 m) tall [71].

SITE CHARACTERISTICS:
Pussy willow occurs along waterways, shorelines, and swamp margins and in low-lying portions of poorly drained thickets, meadows, sloughs, and forest openings throughout its range [9,25,28,33,53,80].

Climate: Continental climates prevail in most of pussy willow's range, but climates in coastal habitats of the northeastern United States and southern Canada are likely moderated by maritime influences. It is difficult to summarize the entire range of climatic conditions experienced throughout pussy willow's range. The prevailing climates spanning a portion of pussy willow's distribution are summarized below.

In pussy willow habitats of northern Minnesota, rainfall averages 23 to 32 inches (580-810 mm)/year. More rain typically falls in the west than in the east. About 75% of the annual precipitation falls during the growing season from April to September. Winter often brings heavy snow. High and low temperatures of 104 F (40 C) and -76 F (-60 C), respectively, are reported in northern Minnesota [32]. In the Black Hills of South Dakota, where pussy willow occurs, winters are cold, and summers are warm. Annual precipitation averages 14 to 17 inches (360-430 mm) in the plains and up to 28 inches (710 mm) at high-elevation sites. Most precipitation (65-75%) occurs during the April to September growing season. More than 100 inches (2,500 mm) of snow may fall at high-elevation sites. A low temperature of -52 F (-47 C) is reported for the southern Black Hills, and a high of 112 F (44 C) is reported from Belle Fourche in the north. In the Black Hills, pussy willow was most common on north- or east-facing slopes [34]. In pussy willow habitats of the Inga Lake area of British Columbia, winters are long and cold with moderate snowpack, and summers are warm with frequent thunderstorms [39].

Elevation: Very few floras or other references report elevational distributions for pussy willow. It is described from 2,000 to 8,000 feet (600-2,000 m) in the Rocky Mountain region [26]. In the Black Hills of South Dakota, pussy willow is considered fairly common at elevations of 5,000 to 6,600 feet (1,500-2,000 m) [34]. In the Adirondacks, pussy willow occurs at 100 feet (30 m) elevation along Lake Champlain and at 2,878 feet (877 m) on Haystack Mountain; a pussy willow Bebb willow hybrid occupies a site at 3,380 feet (1,030 m) on Noonmark Mountain [50].

Soils: Pussy willow is commonly found on poorly-drained soils with nearly neutral pH. Along Lake Champlain in the Adirondacks, pussy willow was common on poorly drained gneiss, anthracite, and limestone soils [50]. In a shrub-carr adjacent to White Clay Lake in Wisconsin, pussy willow cover was significantly greater on alluvial than histic surface soils (P<0.01). Available phosphorus and nitrogen were greatest in alluvial soils, and histic soil surfaces were covered with grasses and leaf litter [48]. Pussy willow was most common in very eutrophic (pH 7-7.9), wet (depth to water: 7.9-15 inches (20-39 cm)) peatlands near Candle Lake in central Saskatchewan, but also occurred in very moist (depth to water: up to 31 inches (79 cm)) peatlands with pH levels of 6 to 6.9 [47]. On the Saskatchewan River Delta, pussy willow occurs in low- and tall-shrub community types with shallow peat (1-3 feet (0.3-1 m)) over silty clay. These soils have a pH of 6.6 to 7 [20]. In the willow-alder tall shrub vegetation on rarely flooded levees of the Peace-Athabasca Delta, soils average 6.5 pH, 18% organic matter, and 82% mineral material of which 24% is sand, 49% is silt, and 27% is clay [21]. Pussy willow habitats in north-central British Columbia are on subxeric to subhydric, coarse to moderately fine soils [71].

Flooding tolerance: Along Midvale Creek in East Glacier, Montana, beaver dams created ponds and inundated riparian vegetation. Pussy willow was "thriving" after 5 years of inundation [1].

SUCCESSIONAL STATUS:
Pussy willow is shade intolerant [50,74] and restricted to seral communities that dominate after the colonization of open water by emergent vegetation [9,18] or sites where disturbances have opened the forest canopy [14]. High levels of disturbance are tolerated [6,72]. In climax forests, pussy willow rarely exists beyond the water's edge of streams, lakes, and swamps in the absence of canopy-opening disturbances that may include fire, flooding, or logging.

Shade: Pussy willow is characterized as a "very" shade-intolerant, pioneer species in a review by Rawson [74]. A study of regenerating clearcut stands in New Brunswick showed that pussy willow abundance was greatest on disturbed sites with open canopies [72]. After tree death opened the canopy of a tamarack (Larix laricina) bog in Itasca State Park, Minnesota, large increases in pussy willow occurred [46]. In the understory of 30-year-old eastern white pine-red maple (Acer rubrum) woodland in northwestern Vermont, pussy willow shrubs were dying [43].

Disturbance: Disturbed sites are common pussy willow habitat, and shrubs withstand high levels of disturbance. In north-central British Columbia, pussy willow is common on disturbed sites [71]. After an ice storm in Missouri and Illinois, pussy willow shrubs showed little injury [17]. On the northern shore of the upper St Lawrence estuary of southern Quebec, pussy willow occurs on floodplains subject to ice flow scouring and high levels of deposition. In this highly disturbed area, the age of the oldest pussy willow was 20 years [6]. In shrub-carrs of southeastern Wisconsin, pussy willow was more common on lightly and very lightly disturbed than on moderately and severely disturbed sites. However, soil properties differed with degree of disturbance and may have affected abundance differences more than disturbance level. Severely disturbed sites were relatively dry and lightly disturbed sites were relatively wet. The water retaining capacity of lightly disturbed sites was about 20% higher than that of severely and moderately disturbed sites [93].

Primary succession: Pussy willow typically colonizes open waters after soils are deposited and/or water levels are lowered from the death and accumulation of emergent vegetation. Several studies summarize this successional process. In western Canada, sloughs are first colonized by emergent vegetation. Vegetative material accumulates as pondweed (Potamogeton spp.) grows and dies, and water levels become more shallow. Pussy willow and other willows colonize the slough following the establishment of emergent vegetation. Pussy willow is "quickly" replaced by quaking aspen unless there are periods of excessive moisture. Rates of colonization and replacement were not provided [9]. Willow-bluejoint reedgrass vegetation represents the last low-moor series in the hydrarch succession of lakes and sloughs in central Alberta. Cottonwoods (Populus spp.) dominate the next successional stage [61].

Pussy willow is common in southern shrub-carr communities in Wisconsin that represent an intermediate stage in primary hydrosere succession of lakes and ponds. These communities follow the sedge meadow stage and, in the absence of disturbances such as fire, are eventually replaced by wetland hardwood forests. Shrub-carrs can persist for 50 years or more, and tree invasion is often slow [18]. White [93] indicates that pussy willow is typically one of the last shrubs to appear in the shrub-dominated community in hydrarch primary succession in southeastern Wisconsin. In northern Minnesota, similar colonization and succession processes in open water are described by Ewing [32] and Stallard [79].

Secondary succession: Disturbance tolerance and shade intolerance make pussy willow a common early- to mid-seral species along floodplains, moist prairie sites, and logged and burned forests.

Floodplain: After dredging operations, mud was deposited on the sand flats along eastern New York's Hudson River. In 1935 sand flats were very sparsely vegetated, with an uneven scatter of eastern cottonwood (P. deltoides) and low herbaceous cover. Ten years later pussy willow occurred on the sand flats [57]. Pussy willow also occurred on alluvial fan and delta sites exposed after the draining of 2 northern Minnesota lakes. Newly created habitats were visited 9 to 15 years after their exposure. Pussy willow was among the sparse vegetation on drained portions of Bass Lake. Fourteen to 15-year-old pussy willow shrubs occurred on the drained south end of Sunken Lake in west-central Itasca County [66].

Prairie: Pussy willow populations at the edges of swamps may increase in size and spread into adjacent prairies, but without disturbances such as fire, pussy willow will be replaced by taller tree species such as quaking aspen. Initially quaking aspen establishment is slow, and if fire occurs when quaking aspen trees are very young, pussy willow will remain among the dominant woody vegetation. Visits to brush prairie vegetation in northwestern Minnesota revealed numerous clumps of pussy willow that were up to 47 years old in postfire aspen-dominated stands. Pussy willow was sparse in drier, older (6-45 years old) aspen stands but abundant in the same stands in the wettest areas: it is likely that pussy willow survived and sprouted best on the wettest burned sites [32].

Forest: Recently disturbed forests provide important pussy willow habitat. Pussy willow appears soon after the herbaceous stage of secondary succession on wet soils in climax eastern white pine forests in northern Minnesota. Pussy willow importance decreases as conifer species such as balsam fir (Abies balsamifera), northern white-cedar (Thuja occidentalis), and black spruce (Picea mariana) become established. Pussy willow is not typically present once the forest canopy closes [79]. In Itasca County, pussy willow was characteristic of shrub-dominated communities that follow the postfire herbaceous-dominated stage in pine, hardwood, and swamp forests [37]. In British Columbia's Inga Lake area, pussy willows from 10 to 20 feet (4-6 m) tall occurred in quaking aspen/cream pea (Lathyrus ochroleucus) and quaking aspen/twinberry honeysuckle (Lonicera involucrata) communities that dominated sites burned 36 years earlier [39].

Pussy willow was present only in logged black spruce stands in northeastern Ontario lowlands when logged and unlogged stands were compared. Stands were cut up to 56 years earlier. Pussy willow cover was greatest (3%) in speckled alder/deciduous leaf litter communities that dominated sites logged an average of 9.3 years before. Pussy willow cover averaged 1.3% on speckled alder/fir (Abies spp.)-black spruce/herb rich vegetation on sites logged an average of 35.5 years earlier. In stands logged an average of 1.6 years earlier, pussy willow cover was 0.4%. In black spruce/bog Labrador tea (Ledum groenlandicum)/sphagnum vegetation that dominated 28.8-year-old stands, pussy willow cover was 0.2%. Ordination of the sites revealed that speckled alder/deciduous leaf litter and speckled alder/fir-black spruce/herb rich communities were most environmentally similar, suggesting that pussy willow cover differences were better attributed to site differences than time since logging [14].

SEASONAL DEVELOPMENT:
Pussy willow flowers in early spring (March-May), and flowers appear before the leaves. Fruits ripen soon after flowering [42,74].

Reproductive phenology of pussy willow by state, province, or region
State/province/region Reproductive phenology (flowering/fruiting/seed dispersal)
Adirondacks fruits ripen in late spring [16]
Atlantic coast from New York to Newfoundland catkins emerge in April [28]
Great Plains (north) flowers mid-April-early May; fruits mid-May-early June [53]
Illinois catkins March-May [60]
Illinois (Morton Arboretum's East Woods) early flower date was 30 March and late flower date was 8 May after 15 years of observations [95]
Plains (north-central) flowers in May; fruits in June [80]
New York (Fire Island, Suffolk County) flowers in April [27]
North Dakota (Fargo) average flowering date from 3 or more years was 23 April [82]
Northeast flowers in March-April [74]
West Virginia flowers in March-May [84]
British Columbia (north-central) seed dispersal begins in May [71]

Flowering was observed in pussy willow grown from natural cuttings in a common garden with other willows near Maple, Ontario. The pussy willow flowering period was short, and averages ranged from 6.4 to 12.7 days over the 3 years of study. Early-flowering willows were reproductively isolated from late-flowering species in all but 1 of 3 years, suggesting that annual weather patterns may affect hybridization potential. Over the 3-year study, researchers recorded the appearance of male and female flowers at the same time, female flowers first, and male flowers first. The earliest flower receptivity date was 6 April, and the latest receptivity date was 2 May. Flower buds were produced the season before flowering, and researchers presumed that flower development was controlled by "temperature thresholds" or "accumulated heat units" [65].


FIRE ECOLOGY

SPECIES: Salix discolor
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: Pussy willow survives top-kill by fire through basal sprouting [93]. Repeated prairie fires encourage prolific sprouting [32]. Seedling establishment on burned sites was not discussed in the available literature, but given a seed source and appropriate germination conditions, is possible.

Fire regimes: Sun and moist site conditions are necessary components of pussy willow habitats. Pussy willow requires canopy-opening disturbances such as flooding, ice scouring, fire, or logging to persist in most habitats. In the absence of canopy-opening disturbances, pussy willow is replaced by quaking aspen, poplar, spruce (Picea spp.), fir, or pine (Pinus spp.) woodlands or forests. In a Wisconsin brush savannah, pussy willow was more frequent on sites burned 11 times in 15 years than on adjacent unburned sites [5]. Likely pussy willow would persist indefinitely in habitats with a fire-return interval that maintained open forest or woodland canopies. Habitats with long fire-return intervals that allow forest and woodland succession to proceed would likely provide only temporary pussy willow habitat.

The following table provides fire regime information that may be relevant to pussy willow:

Fire regime information on vegetation communities in which pussy willow may occur. For each community, fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Models [52]. These vegetation models were developed by local experts using available literature, local data, and/or expert opinion as documented in the PDF file linked from the name of each Potential Natural Vegetation Group listed below. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
Northern Rockies Northern Great Plains Great Lakes Northeast Southern Appalachians
Northern 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 Rockies Shrubland
Riparian (Wyoming)
Mixed 100% 100 25 500
Northern Rockies Forested
Ponderosa pine-Douglas-fir Replacement 10% 250   >1,000
Mixed 51% 50 50 130
Surface or low 39% 65 15  
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
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
Central tallgrass prairie Replacement 75% 5 3 5
Mixed 11% 34 1 100
Surface or low 13% 28 1 50
Northern tallgrass prairie Replacement 90% 6.5 1 25
Mixed 9% 63    
Surface or low 2% 303    
Southern tallgrass prairie (East) Replacement 96% 4 1 10
Mixed 1% 277    
Surface or low 3% 135    
Oak savanna Replacement 7% 44    
Mixed 17% 18    
Surface or low 76% 4    
Northern Plains Woodland
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    
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
Jack pine-open lands (frequent fire-return interval) Replacement 83% 26 10 100
Mixed 17% 125 10  
Great Lakes Forested
Conifer lowland (embedded in fire-prone system) 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 pine forest, jack pine Replacement 67% 50    
Mixed 23% 143    
Surface or low 10% 333
Maple-basswood-oak-aspen Replacement 4% 769    
Mixed 7% 476    
Surface or low 89% 35    
Red pine-white pine (frequent fire) Replacement 38% 56    
Mixed 36% 60    
Surface or low 26% 84    
Great Lakes pine forest, eastern white pine-eastern hemlock (frequent fire) Replacement 52% 260    
Mixed 12% >1,000    
Surface or low 35% 385    
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
Northeast Forested
Eastern white pine-northern hardwoods Replacement 72% 475    
Surface or low 28% >1,000    
Northern hardwoods-spruce Replacement 100% >1,000 400 >1,000
Northeast spruce-fir forest Replacement 100% 265 150 300
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 Grassland
Bluestem-oak barrens Replacement 46% 15    
Mixed 10% 69    
Surface or low 44% 16    
Eastern prairie-woodland mosaic Replacement 50% 10    
Mixed 1% 900    
Surface or low 50% 10    
*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[40,51].

POSTFIRE REGENERATION STRATEGY [83]:
Tall shrub, adventitious buds and/or a sprouting root crown

FIRE EFFECTS

SPECIES: Salix discolor
IMMEDIATE FIRE EFFECT ON PLANT:
Pussy willow is normally top-killed by fire but survives by sprouting [32,87,93].

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
No additional information is available on this topic.

PLANT RESPONSE TO FIRE:
Basal sprouting is typical following fires that kill aboveground pussy willow stems [93]. Ewing reports that willows are top-killed by repeated prairie fires but sprout prolifically after fire [32]. Research indicates that pussy willow survives repeated fire. In a Wisconsin brush savanna, pussy willow frequency was greater on sites burned 11 times in 15 years than on adjacent sites unburned for 35 years or more [5]. Observations made in postfire seral communities in northwestern Minnesota suggest that fires on dry sites may be more damaging to pussy willow than fires in wet habitats. Pussy willow was typically most abundant and persistent in wet postfire communities. However, observations came from various sites with different levels of canopy cover, which also affected pussy willow abundance [32].

In the Waubun Prairie Research Area dominated by bluestem (Andropogon spp.), Indiangrass (Sorghastrum nutans), and porcupine grass (Hesperostipa spartea), pussy willow was top-killed by fire, and sprouts were present in the first postfire year. Portions of the Prairie were burned in spring and portions in the fall. It was unclear whether pussy willow occurred on spring, fall, or spring and fall burned sites. Pussy willow was restricted to wet soils and to pothole borders [87].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
There are few fire effects studies on pussy willow. From the little information available, the following generalizations can be made: pussy willow survives most fires, postfire sprouts are often present by the first postfire year, and shrubs regenerate and persist on repeatedly burned sites. Additional information regarding effects of different fire seasons and fire severities on pussy willow are needed. Comparisons of postfire sprouting and long-term survival on dry versus wet burned sites would improve the understanding of pussy willow's response to fire.

Several studies indicate that pussy willow is common in postfire communities in the boreal forest; however, comparisons with unburned or prefire communities are lacking. Pussy willow occurred in the shrub-dominated successional stage after fires in black spruce swamps in Itasca County, Minnesota. There were 12 pussy willow clumps/100 m on sites visited 3 years after the last fire. Sites burned 2 to 3 times in 4 years. Pussy willow was also noted in shrub-dominated postfire communities in red pine (Pinus resinosa), jack pine (P. banksiana), highland hardwood, and fir-birch (Betula spp.) forest types [37]. On 2-year-old burned sites northwest of Ely, Minnesota, pussy willow was present. The boreal forest site burned in the Little Sioux Fire in mid-May. Postfire abundance was not quantified, but willows (pussy willow, Bebb willow, and sandbar willow (Salix interior)) were abundant enough to provide 25% of local moose diets from April to December [45].

On a 2-year-old burned site on Moose Hill in southwestern Nova Scotia's Queens County, the density of pussy willow was 0.5 individuals/6.6 6.6 foot quadrat and frequency was 11%, respectively. The site was a regenerating clearcut burned in late May and was dominated by eastern white pine and spruce that were about 8 feet (2 m) tall. The burned site was flat and poorly drained with the water table at about 19 inches (48 cm) deep [56]. In shrub and grassland alvars of eastern Ontario's Burnt Lands, pussy willow frequency was 12%. Alvars were likely white spruce climax forests that were cut and burned 37 years earlier [15].

Studies in the northern clay boreal forest of northern Ontario revealed that pussy willow was associated with a group of species reaching their greatest abundance on 4- to 11-year-old burned sites. Burned sites ranged from 0 to 57 years old. Through species classification analyses, pussy willow was found to be correlated with the "dry peat" species group on burned sites (P≤0.05). Researchers noted that most fires were "relatively intense". Study findings did not address whether or not pussy willow or other dry peat species were present or even somewhat abundant in younger and older postfire communities [78].

Repeated fire: The following studies indicate that pussy willow tolerates repeated burning. Pussy willow abundances were rarely very different between repeatedly burned and unburned sites. There have been several fires and studies in prairies and savannas of Wisconsin. Differences in the frequency of pussy willow between burned and unburned sites were greatest on the most frequently burned sites. On sites burned 1 to 4 times in a 5- to 6-year period, pussy willow frequency was slightly lower on burned than unburned sites [90]. In a brush savanna, pussy willow frequency was at least 25% greater on sites burned 11 times in 15 years than on adjacent unburned sites. Fires consumed all foliage and killed aboveground shrub and tree parts. Unburned reference sites had not burned for 35 years or more [5].

In the Crex Meadow area of northwestern Wisconsin, pussy willow was 2.5% more frequent on sites burned 1 to 4 times than on unburned sites. Pussy willow averaged 16.4% frequency on unburned jack pine-northern pin oak (Quercus ellipsoidalis) brush prairie savannas that had a partially closed canopy of 40% to 90% cover and were undisturbed for at least 25 years. Frequency averaged slightly lower (13.9%) on 33 sites burned in prescribed fires. Sites were typically burned 1 to 4 times in March or April. Burned sites averaged 1.9 fires and were visited 1 to 2 years after the last fire [90,91].

The fire effect on pussy willow was considered "neutral" in western bracken fern (Pteridium aquilinum)-grasslands in northeastern Wisconsin. The average frequency of pussy willow was just 1.1% greater on burned than unburned sites. Fifteen burned stands and 12 adjacent unburned stands were compared. Fires burned in March or April, and all but 3 sites burned once. The other 3 sites burned 2 to 3 times. Burned sites were evaluated the first postfire summer. Unburned stands had not burned for 25 years or more [89].

FIRE MANAGEMENT CONSIDERATIONS:
The above studies indicate that pussy willow is not adversely affected by fire. Repeated burning in pussy willow habitats will not likely remove this species. However, a lack of prefire or unburned comparisons makes predicting the effects of fire on pussy willow abundance difficult in most vegetation types. The gap in knowledge regarding pussy willow's postfire response on dry versus wet sites and in low-severity versus high-severity fires also makes postfire abundance predictions difficult. Additional and more detailed fire effects studies in pussy willow habitats are needed.


MANAGEMENT CONSIDERATIONS

SPECIES: Salix discolor
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
In a review, Rawson [74] reports that deer, moose, rabbits, and rodents browse pussy willow stems and that ducks and other water fowl feed on pussy willow catkins. Other studies indicate that pussy willow is also forage for cattle [58], caribou [7], muskrats [85], ruffed grouse [12], and Galerucella decora beetles [9]. Of these wildlife species, pussy willow is most important to moose and Galerucella decora beetles.

Livestock: Cattle browsed pussy willow in sedge (Carex spp.) meadows in southern Wisconsin's Lodi Marsh [58].

Native ungulates: Caribou browsed pussy willow in the summer and winter in Newfoundland [7], moose utilized pussy willow year-round in Minnesota [69], and white-tailed deer browsed pussy willow on winter ranges in Minnesota and Quebec [13,44].

Caribou: Rumina of Newfoundland caribou had a low frequency of pussy willow in the summer (7%) and in the winter (3%) [7].

Moose: A review reports that willows are browsed by moose throughout their range, and that pussy willow provides important moose winter forage in western Montana [68]. Willows (pussy willow, Bebb willow, and sandbar willow) made up 25% of the total moose diet from April to December on 2-year-old burned boreal forest sites northwest of Ely, Minnesota [45]. In northeastern Minnesota, upland willows (pussy willow, Bebb willow, and prairie willow) were utilized year round by moose. Use averaged 26% for the year and was slightly greater from October to December than from January to April and from June to September. When availability of the vegetation was evaluated with moose use, researchers suggested that upland willows were the most palatable. Pussy willow was consumed more than the other willow species when species were identifiable [69].

Deer: Pussy willow is not often the most palatable forage available to white-tailed deer but is an important winter food. In browse surveys conducted on the Apostle Islands of Lake Superior in northern Wisconsin, researchers grouped pussy willow in the 3rd choice browse group out of 4 possible groups [4]. In the Mud Lake National Wildlife Refuge of Marshall County, Minnesota, willows provided the bulk of the winter food consumed by white-tailed deer (about 45%), although willows were not the most palatable food available. Pussy willow was 1 of 10 willows present in the study area [44]. In the Rigaud white-tailed deer wintering area in southwestern Quebec, pussy willow use increased as winter progressed and with each progressive sampling year. In the 1st sampling winter, 13.5% of available pussy willow stems were browsed, and by the 3rd winter 21.4% of available stems were browsed [13].

Small mammals: Pussy willow occurred in muskrat "feeding platforms" (areas where muskrats bring food to consume) in the Corinna marsh area of central Maine, although pussy willow was not encountered in any quadrat during a vegetation survey of the study area [85].

Birds: Pussy willow was the 2nd most important winter food for ruffed grouse collected throughout Maine. Pussy willow buds and leaves made up 8.8% of the contents of digestive tracts taken in the winter and 3.6% of tracts collected in the spring. The researcher indicated that these percentages may include other willow species but that pussy willow was most common [12].

Insects: In western Canada, Galerucella decora beetles will defoliate pussy willow, their "favorite food plant", before moving on to other willow species [9].

Palatability/nutritional value: The nutritional content of pussy willow is reported for the terminal 3 inches of current year's stems collected in December in northeastern Minnesota: N=1.1%, P=0.2% K=0.7%, Ca=1.3%, and crude protein=8.2% [69]. Available palatability information was integrated into the above Importance to Livestock and Wildlife section.

Cover value: In a review, Rawson [74] reports that pussy willow thickets provide bird and mammal cover.

VALUE FOR REHABILITATION OF DISTURBED SITES:
Pussy willow and other willow species grow rapidly and produce extensive root systems that bind soil and decrease erosion [19], making them an obvious choice for site-appropriate revegetation and rehabilitation programs.

Researchers have used pussy willow in vegetative filter strip plantings between agricultural areas and waterways or wetlands to improve water quality, through the removal of nutrients and sediments produced by agriculture operations. Pussy willow stems can be harvested as early as 2 years after establishment. Selling pussy willow stems to florists soon after the establishment of filter strips can augment the cost of their construction. Rapid sprouting provides for a short-term and continued economic return [59].

OTHER USES:
Today pussy willow twigs with immature catkins are used in floral arrangements [33]. In the past, willow bark, which contains salicin (a close relative of acetylsalicylic acid) was used to treat variety of ailments such as fevers, pains, colds, and rheumatism [19].

OTHER MANAGEMENT CONSIDERATIONS:
Herbicide effects: Articles written between 1948 and 1950 provide information on the effects of herbicide on pussy willow shrubs in Connecticut [29,30,31].

Pests: Borers and beetles attack pussy willow. Pussy willow deaths from willow borers (Sternochetus lapathi) are occasional in the Black Hills of South Dakota [34]. In southeastern Wisconsin, the many tunnels made by borer beetle larvae weaken and kill pussy willow stems. Sprouting is common after stem death [93]. In western Canada, pussy willow is considered the "favorite food plant" of Galerucella decora beetles. Beetles will completely defoliate pussy willow before moving on to other willow species [9].

Estimating shrub dimensions and biomass: In northeastern Minnesota, equations were developed to estimate leaf, stem, current year's stem, and total biomass for willows from plant height, crown area, or stem diameter. Equations were developed from data collected in regenerating clearcuts, but the willow species were not identified [67]. Additional regression equations were developed from data collected in northeastern Minnesota to determine canopy area and volume from a variety of pussy willow measurements. Data used in equation development came from shrubs growing on well-drained soils, with a sparse to nonexistent overstory [70].


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