Celtis occidentalis


Table of Contents


INTRODUCTORY


Photo © Andrew Hart

AUTHORSHIP AND CITATION:
Gucker, Corey L. 2011. Celtis occidentalis. 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:
CELOCC

NRCS PLANT CODE [193]:
CEOC

COMMON NAMES:
common hackberry
bastard elm
nettle-tree
northern hackberry

TAXONOMY:
The scientific name of common hackberry is Celtis occidentalis L. (Ulmaceae) [57,69,100].

Although common hackberry and sugarberry (C. laevigata) are self compatible and could potentially hybridize, reports of natural hybrids were lacking as of 1990 (review [107]), and artificial crosses produced no seed [205]. Suspected common hackberry × netleaf hackberry (C. laevigata var. reticulata) hybrids were reported in Hemphill County, Texas [168].

SYNONYMS:
Celtis occidentalis var. canina (Raf.) Sarg [72,128,174]
Celtis occidentalis var. crassifolia (Lam.) A. Gray-Gates [72,118,174]
Celtis occidentalis var. georgiana (Small) Ahles
Celtis occidentalis var. occidentalis [153]
Celtis occidentalis var. pumila (Pursh) A. Gray-Fernald [72,128,174]
Celtis pumila Pursh [84,123]

LIFE FORM:
Tree-shrub

DISTRIBUTION AND OCCURRENCE

SPECIES: Celtis occidentalis
GENERAL DISTRIBUTION:
1971 USDA, Forest Service map provided by [189]

Common hackberry is most common in the midwestern United States [150]. It is sparingly distributed in Manitoba, Ontario, and Quebec [54,123], rare along the Atlantic coast from Massachusetts to Virginia [51], occasional in Tennessee, North Carolina, and Georgia [208], and restricted to Hemphill County in the Texas panhandle [168]. The map above illustrates the North American distribution of common hackberry as of 1971.

Below are the states and provinces reporting common hackberry populations (as of 2011 [193])*:
United States: AL, AR, CO, CT, DC, DE, FL, GA, IA, IL, IN, KS, MA, MD, MI, MN, MS, MT, NC, ND, NE, NH, NJ, NM, NY, OH, OK, PA, RI, SC, SD, TN, TX, UT, VA, VT, WI, WV, WY
Canada: MB, ON, QC

*Distribution information available in the reviewed literature together with the large distances between common hackberry populations identified on the 1971 map [189] and in the 2011 list [193] suggest very limited common hackberry distributions in the following states and provinces: CO, FL, MT, NM, TX, UT, WY, MB, ON, and QC.

SITE CHARACTERISTICS AND PLANT COMMUNITIES:
Common hackberry tolerates a range of climatic and soil conditions. Although most common along rivers and streams [150], it also occurs in open woodlands, rocky hillsides, limestone outcrops, and sand barrens throughout its North American range [72,105,123,128,137,178]. Habitat restrictions or preferences have been reported in some areas. In Michigan, common hackberry is common in rich, moist woodlands along rivers and in stream valleys and ravines but is considered rare in drier habitats [200]. In the western part of its range, it is restricted to well developed river valleys, northern slopes, or protected ravines and does not occur on windswept sites [106].

Climate: Temperature and precipitation vary widely throughout common hackberry's range. In the Great Plains, an annual temperature range of 140 °F (78 °C) is common. Between common hackberry's northern and southern habitats, annual precipitation abundance, distribution, and delivery differences are great. In the Great Plains, annual precipitation can average 14 inches (360 mm) and in the Southeast can average 59.8 inches (1,520 mm) (review by [107]). The number of frost-free days ranges from 120 to 250 where common hackberry grows. However, when experimentally planted in shelterbelts in the Northern Great Plains, common hackberry was susceptible to early-fall freezing, killed back by late-spring frosts, and suffered considerable winterkill. Common hackberry establishment was generally poor in the Northern Great Plains where the annual frost-free period was 127 to 139 days and annual precipitation averaged 13.6 to 16.2 inches (345-412 mm) [64].

Common hackberry is somewhat drought tolerant [52]. After a study of trees in Nebraska and Kansas before and after the severe drought of the 1930s, researchers reported that common hackberry "endured drought especially well". In ravines in Kansas, common hackberry growth averaged 0.6 inch (17 mm) in a wet year and 0.5 inch (12 mm) in a dry year. Growth rate differences were greater in dry than mesic ravines [9]. In southeastern South Dakota, 96% of common hackberry trees remained alive through the 1934 to 1939 survey period, which included 2 years of above-average drought intensity [129]. However, after studies at North Dakota's Mandan Experimental Station, common hackberry was not recommended for Northern Great Plains sites without favorable moisture [65].

Soils: Although common hackberry tolerates a range of soil conditions, growth is typically best in fertile, moist but well-drained soils [106]. Mollisols and Entisols are common where common hackberry grows; Inceptisols are less common (review [107]). Common hackberry is common on limestone soils [150], and in Pennsylvania, it is considered an indicator of high pH (7.2) soils (review [107]). A coal mine revegetation guide reports that common hackberry's lower pH limit is 5.0 [199]. Although common hackberry may grow best in nutrient-rich soils, it also grows, albeit very slowly, on "almost sterile soils". On sites with very unfavorable soil conditions, it may take 15 years for common hackberry to increase 1 inch (2.5 cm) in diameter [150].

Common hackberry growth and soil relationships have been evaluated in detail in Indiana. Based on General Land Office survey records of presettlement Indiana, common hackberry was most important on alluvial soils with a median pH of 7.1, high levels of nitrogen (0.105%) and available water, low clay content (23%), but without a B horizon [39]. In postsettlement woodlands of eastern Indiana, common hackberry was significantly (P<0.05) associated with swampy to semiswampy, fertile, silty clay loams and very fertile silt loams [16].

Soil moisture and flooding: Because common hackberry is a common floodplain species, many studies have investigated its soil moisture preferences and flood tolerance. After reviewing surveyor notes from the early 1800s in the prairie-forest border region of eastern Illinois, researchers assigned common hackberry a moisture adaptation value of 3, where 1 represented a tolerance of excessive soil moisture and 10 represented the dry end of the soil moisture scale [99].

Tolerance of excessive soil moisture or flooding likely increases with common hackberry age. Seedlings are much more sensitive to saturation than trees (review [107]). In the Trelease Woods of east-central Illinois, common hackberry trees occurred in all soil types but dominated in Humic Gley soils with standing water at or above the soil surface throughout the spring. Sapling densities were lowest in Humic Gley soils and highest in Brunizem soils that were saturated until late spring. Seedling densities were lowest in Humic Gley soils and highest in Transitional soils with maximum drainage and a water table below the solum all spring and summer [63]. Along the Wabash and/or Tippecanoe rivers in Indiana, common hackberry seedlings and saplings were killed by submergence during high flood levels in June [117]. In the greenhouse, first-year common hackberry seedlings survived 60 days in saturated soils. Saturated seedlings were shorter than control seedlings, although not significantly. A few seedlings died when removed from saturated conditions [87]. In central Illinois, common hackberry saplings and seedlings were more than twice as abundant in wet-mesic than dry-mesic, mesic, or wet sites [5].

In general, common hackberry trees are more common in upper, less frequently flooded sites than lower, frequently flooded sites. Along the lower Chippewa River in northwestern Wisconsin, common hackberry was most common on river terraces and floodplain forests that were 13 feet (4 m) or more above river level [13]. In streamside forests in the Sangamon River Basin of Illinois, common hackberry was a dominant species in areas between sites receiving substantial flooding and sites receiving almost no flooding (flooding frequency 0%-3%) [19]. In streamside forests in Piatt County, Illinois, common hackberry dominated sites that were flooded 1.5% to 3% of a 55-year time period and was present, but much less common, in areas flooded for 18% of the same time period [18]. In secondary forests along the Raritan River of New Jersey, common hackberry occurred in high (>11 feet (3.4 m)) but not low floodplain sites. High sites had sandy loam soils and were flooded less than 1 day each year. Low sites had predominantly clay loam soils and were flooded 0.7 to 18 days per year [61]. After surveying ranchers and farmers in North Dakota and Montana, researchers found that those who planted common hackberry on sites with a shallow water table ranked its windbreak performance lower than those who planted it on sites with deeper water tables [192].

Common hackberry trees are unlikely to survive 4 years of continual flooding. One year following a flood that resulted in a little more than 230 days of inundation on the northern Mississippi River, 62.5% of common hackberry trees and 81.8% of common hackberry saplings were dead [212]. The Pere Marquette Wildlife Experimental Area in western Illinois was completely inundated in 1939. About 33% of common hackberry trees in the mud were dead within 3 years of inundation, and 66% were dead within 5 years. About 33% of common hackberry trees in the water were dead within 2 years, and all were dead within 3 years [211]. On the Upper Mississippi River in Illinois and Iowa, common hackberry trees survived 3 years of constant flooding, and growth was described as "fair" in the 2nd year of flooding [73].

Plant communities: Throughout its range, common hackberry is rarely dominant and rarely occurs in pure stands [150] but is often reported at low frequency and importance levels in a variety of bottomland and floodplain forest types [108,127]. Common hackberry often occurs in bottomland and oak-hickory (Quercus-Carya spp.) forest types in the Upper Gulf Coastal Plain, Central Till Plains, Coastal Plains and Flatwoods, western Gulf Coastal Plain, Mississippi Alluvial Basin, and Blackland Prairie ecoregions [134]. Although not typically a community dominant, common hackberry is considered a prominent species in the following forest types recognized by the Society of American Foresters (review [107]): sugarberry-American elm-green ash (Ulmus americana-Fraxinus pennsylvanica) type from eastern Texas to Illinois and east (common hackberry replaces sugarberry in the northern range of this type) [97,139], sugar maple-basswood (Acer saccharum-Tilia americana) type in the Central Hardwoods region [70], American beech (Fagus grandifolia)-sugar maple type in the Midwest [169], and sycamore-sweetgum (Platanus occidentalis-Liquidambar styraciflua)-American elm type in the northern Mississippi Valley [110].

In the Great Plains and Great Lakes regions, where common hackberry is most abundant and widely distributed, it is often associated with floodplain, bottomland, or other riparian deciduous forests. At the fringes of common hackberry's range, habitat relationships become less predictable. Although common hackberry is mentioned as a minor component in many forest types throughout its range, the information below focuses on those studies or surveys where common hackberry was considered important or predominant.

Great Plains: Common hackberry is most commonly described in hardwood gallery, floodplain, or bottomland forests along streams and rivers throughout the Great Plains [36,164,188]. Common dominants or associates in these forests include: green ash, elm (Ulmus spp.) [8,115,164], hickory [50], basswood [202], pecan (C. illinoinensis) [115], eastern cottonwood (Populus deltoides) [102,109], black willow (Salix nigra) [109], sugar maple [118], sycamore, and sweetgum [187].

In some woodlands and gallery forests in South Dakota, Kansas, and Missouri, common hackberry occurs with oak species, primarily bur oak (Q. macrocarpa), chinkapin oak (Q. muehlenbergii), and swamp white oak (Q. bicolor) [60,82,101,164].

Though pure common hackberry stands are rare, a nearly pure grove was found in northwestern Nebraska. In this area, common hackberry is generally of "secondary importance" and averages only 4 trees per acre in deciduous woodlands. However, a large common hackberry grove occurred on a sand terrace 25 to 35 feet (7.6-11 m) above a stream in White Clay Canyon. In the canyon there were 49 common hackberry trees in 0.1 acre (0.04 ha). The origin or potential persistence of this monotypic stand was not discussed [190].

At its extreme northwestern distribution, common hackberry is described as an early-seral forest species. It grows to only 30 to 70 feet (10-20 m) tall in open areas within the sand ridge forests that separate the bays of Lake Manitoba [123].

Great Lakes: Common hackberry is most often described in lowland hardwood and other deciduous riparian woodlands in the Great Lakes [49,117,136,182,186,191,203] but is also reported in nonriparian deciduous woodlands, dry oak-hickory forests, and in sand dune communities [38,63,86,103]. The number of canopy species reported in these riparian woodlands can be as high as 26 [182] and typically include those listed for the Great Plains region. The same overstory species are common in the nonriparian Great Lake woodlands, which occupy wet-mesic soils ([63,80,86], review by [159]).

In the northern Great Lakes area, common hackberry is described in dry habitats. In Michigan, it is characteristic but grows as a shrub in the basswood-maple (Acer spp.) community on the sand dunes of Lake Michigan [38]. On Pelee Island, Ontario, Canada's southernmost alvar, common hackberry occurs in dry forests dominated by white oak (Q. alba) and shagbark hickory (C. ovata) [103].

Northeast: In Pennsylvania and New York, common hackberry is described as an early colonizer of grasslands and cleared forests. In Pennsylvania, it is a common woody species associated with xeric, sideoats grama (Bouteloua curtipendula)-dominated, limestone prairies [114]. On Robins Island, Long Island, New York, common hackberry and black cherry (Prunus serotina) are the only trees in American beachgrass (Ammophila breviligulata) dune communities, which occur in low-lying areas [32]. In Tottenville, Staten Island, New York, common hackberry dominated the overstory of a young forest on a site that was cleared for development 60 years earlier. Common hackberry was abundant in the overstory and understory. Researchers described this forest community as unusual [74].

Southern Appalachians: In the southern Appalachians, common hackberry is typical in bottomland or wetland forest types with most of the same species reported in the Great Plains region [37,46,56,58]. However, there are several nonriparian habitats and unique species associations in the southern Appalachians. On the Kentucky Karst Plain in Kentucky and Tennessee, common hackberry occurred in 5 community types on dry to wet soils but was most common in a juniper (Juniperus spp.)-hickory-common hackberry community type on shallow limestone soils [14]. At the Stones River National Battlefield in Tennessee, common hackberry was dominant in the black walnut-Ohio buckeye (Juglans nigra-Aesculus glabra)-common hackberry and shingle oak-Shumard oak (Q. imbricaria-Q. shumardii)-chinkapin oak/common hackberry/heartleaf nettle (Urtica chamaedryoides) forest types [144]. In other southern US forests, common hackberry has been reported in Ashe juniper (Juniperus ashei) types [122].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Celtis occidentalis
Photo © Chris Evans, River to River CWMA, Bugwood.org
GENERAL BOTANICAL CHARACTERISTICS:
Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [69,72,178,200]).

Aboveground description: Common hackberry typcially grows as a broad tree measuring around 50 feet (15 m) tall and 20 inches (50 cm) in diameter [54,150,174]; however, size and growth form can vary with site conditions [26,57,69,72]. Common hackberry trees may reach 110 feet (35 m) tall and 6 feet (1.8 m) in diameter [52]. However, reports of trees this large were rare, and even on well-suited sites, trees 80 feet (24 m) tall and 24 inches (61 cm) in diameter were considered large [150]. In bottomland hardwood forests along major rivers in Missouri, Iowa, and Illinois, the largest common hackberry tree was 85.3 feet (26 m) tall with a 19.5-inch (50 cm) DBH [35]. Common hackberry bark is thick, deeply furrowed, and develops warty cork projections with age [52,174].

Common hackberry is symmetrical and open branched, with larger branches 26 to 33 feet (8-10 m) above ground [69,174]. The crown is ascending with spreading branches, which droop at the tips [33,54]. Common hackberry is sensitive to mites (Eriophyes spp.), which cause witches' brooms or thick clusters of branches and twigs [54,93]. It produces simple leaves, which are arranged alternately, measure 1.2 to 6.7 inches (3-17 cm) long, and are about twice as long as they are wide [33,51,153,178]. Leaves have serrate margins with 10 to 40 teeth per side, at least on the upper half [51,57,72]. Leaves are triangular with uneven sides, long tapered tips, and 3 prominent veins from the same basal point [54,69,128,174]. Common hackberry trees produce both male and female flowers. Pistillate and staminate flowers are usually solitary but also occur in clusters of up to 3. Pistillate flowers are borne in the axils of new leaves, and staminate flowers are borne at the ends of new branches [54,170,174,178]. Common hackberry produces solitary, single-seeded drupes that are 8 to 11 mm in diameter [72,174,178]. The fruity flesh of the drupes is sweet and edible but very thin [26,54,178].

The size and growth of common hackberry trees vary from site to site, and leaf and stem characteristics can vary within a single tree [26,69]. Dwarf forms are reported on shallow limestone soils [26]. In other xerophytic habitats, including dunes and rocky areas, common hackberry has been described as a "struggling" shrub growing in clumps and reaching only 13 feet (4 m) tall [84]. Leaf size, shape, and texture are also variable [52,200]. Common hackberry leaves have been described as thin and smooth to firm, veiny, and scabrous [69]. On dry sunny sites, leaves are often scabrous [26].

Belowground description: Common hackberry often develops deep, widespreading root systems. The root systems of several common hackberry trees were evaluated from excavated trees in Nebraska and North Dakota. At the North Dakota Agricultural College and Experiment Station in Fargo, a maximum root length of 41.5 feet (13 m) and a maximum depth of 5 feet (1.5 m) were reported for a 35-year-old, 35-foot (11 m) tall common hackberry tree growing in clay soil [210]. For 32 common hackberry trees excavated from several sites in Nebraska, the ratio of root depth to tree height averaged 0.5, meaning that roots extended 0.5 feet (0.15 m) deep for every 1 foot (0.3 m) of tree height. The ratio of lateral root spread to tree height averaged 2.0. Ratios of root depth to root spread for 2- to 5-year-old common hackberry trees averaged 0.39, for 6- to 10-year-old trees averaged 0.55, for 11- to 25-year-old trees averaged 0.14, and for 26- to 49-year-old trees averaged 0.23. A 7-year-old, 6-foot (1.8 m) tall common hackberry tree excavated from clay soil with a 15-foot (4.6 m) water table in Lancaster County produced roots that extended 6 feet (1.8 m) deep and 8 feet (2.4 m) laterally. A 25-year-old, 19-foot (5.8 m) tall tree excavated from silt loam soil with a 45-foot (14 m) water table in Nance County produced roots that extended 9 feet deep and a little over 34 feet (10 m) laterally [171].

Lifespan: Common hackberry is relatively short-lived. Trees typically live 150 to 200 years [54,107,120]. In a presettlement blue ash (Fraxinus quadrangulata)-oak remnant in Kentucky's Inner Bluegrass region, the oldest common hackberry was a little over 200 years old [30].

Raunkiaer [154] life form:
Phanerophyte

SEASONAL DEVELOPMENT:
Common hackberry produces flowers at the same time as leaves, which is typically in April or May [33,51,57,72,138,174]. Flowers are typical in early April in the southern part of common hackberry's range and in late May in the northern part of its range [106]. Fruits are often ripe in September or October but remain on the tree until winter or spring [33,93,107,174]. In Kansas, common hackberry fruits remained on the tree an average of 132.2 days and were available for foraging from 21 August to 10 March [173].

REGENERATION PROCESSES:
Common hackberry reproduces by seed and can regenerate from sprouts following top-kill. These topics are discussed in detail below in Seedling establishment and Vegetative regeneration. Breeding system and pollination: Common hackberry is polygamomonoecious [72,206], and flowers are wind pollinated [54].

Seed production: Although several sources report that common hackberry produces large seed crops almost annually ([25,54,150], US Forest Service as cited in [53]), detailed studies on the abundance and variation in common hackberry seed production were lacking as of 2011. In a study of plant food production on a wildlife refuge near Salem, Missouri, there were18 pounds (8 kg) of common hackberry seed/acre, and 95.7% of seeds were sound [45]. In sand ridge forests by Lake Manitoba, which represents common hackberry's northwesternmost distribution, common hackberry trees are small (30-70 feet (10-20 m) tall) and normally fail to produce fruit. However, researchers observed some common hackberry seedlings and suspected that seed may be produced in hot summers [123].

Seed dispersal: Common hackberry seeds are dispersed by gravity [62] as well as fruit-eating birds, mammals, and reptiles [54,177].

In 5- to 40-year old fields in Tennessee, researchers used 2 types of traps to distinguish airborne and gravity-borne seed dispersal. Gravity traps were mounted above ground and designed to prevent bird perching. In 5- to 15-year-old stands, there were 7,360 common hackberry seeds/ha collected from gravity traps and no common hackberry seeds collected from airborne traps. In 20- to 40-year-old stands, there were 25,020 common hackberry seeds collected from airborne traps, and no common hackberry seeds collected from gravity traps [62].

On Sandy Hook, an isolated peninsula off the New Jersey shore, common hackberry was thought to have originated from bird-dispersed seeds [172]. Common hackberry establishment at Point Pelee and other locations near Lake Erie may also be from seeds dispersed by birds migrating north [54]. Birds take common hackberry fruits in the fall and winter. In Kansas, common hackberry fruits averaged 132 days on the tree, and most were removed in the winter [173]. In central New Jersey, fall and winter use of fruits by birds was evaluated periodically from 1977 to 1986. Use of common hackberry fruits was documented from seed traps, bird feces, and stomach contents; abundance of seeds dispersed or frequency of seed use was not reported [204].

Seed banking: In the few studies that address seed banking by common hackberry, persistence in the soil was short-lived. In a field study in Orange County, Indiana, a known quantity of common hackberry seeds was scattered in the forest litter in a plot lacking nearby seed-bearing common hackberry trees. After 1 year, common hackberry germination was 34%, and after 2 years, germination was 20% [34]. When dry common hackberry seeds were stored in a sealed container and kept at 41 °F (5 °C), some seed was still viable after 5.5 years [206]. However, when soil samples were collected from mixed-deciduous forests in southwestern Ohio and on the eastern shore of Virginia, no common hackberry seedlings emerged. In Ohio, common hackberry was described as a dominant aboveground species, but it failed to emerge from forest soil samples collected on 2 March [116]. In Virginia, the relative density of common hackberry was 0.6% in the overstory, and no common hackberry seedlings emerged from 20 soil samples [140].

Germination: Emergence and establishment are best if common hackberry seeds are buried shallowly by soils 0.5 inch (1.3 cm) deep or less. Stratification and pulp removal increased the germination of common hackberry seed in the laboratory [150]. At the Oklahoma Agricultural Experiment Station, germination increased by about 65% with stratification [7]. A review reports that dormancy can be overcome by temperatures of 41 °F (5 °C). Germination of stratified common hackberry seeds averaged 47% after 60 days at 86 °F (30 °C) daytime and 68 °F (20 °C) nighttime temperatures [25]. When seeds were kept at 41 °F (5 °C) for 60 to 90 days before germination tests, common hackberry germination averaged 39% after 37 days in the laboratory [206]. In experiments conducted by Adams [6], however, common hackberry germination was high regardless of warm or cold pregermination storage conditions for dry seeds. Seed germination was 16% when ripe seed was sown outside on 10 October. Germination was 72% when ripe seed was sown in the greenhouse on 10 October. Germination was 90% when dry common hackberry seeds were stored outdoors or indoors until planting on 6 March [6]. In later germination experiments, Taylor [185] found that common hackberry seeds germinated best when fruits were depulped and fermentation occurred before or in the early stages of afterripening. He suggested that in the field, fruit pulp would be removed naturally by soil organisms. Seeds germinated better when entire fruits were planted and the fermentation process produced temperatures of 95 to 110 °F (35-43 °C) than when temperatures from the fermentation process were suppressed. During these experiments, the researcher also found that common hackberry seed coats are brittle. Seeds were damaged and unviable after going through a macerator [185].

Removal of the pulp from common hackberry seeds may increase germination (review [25]), but in the only study that evaluated the differences in germination with and without passage through the digestive system, germination was significantly (P<0.001) lower for passed seeds. Germination of common hackberry seeds collected from northern raccoon scat was 38%, while that for uningested seeds was 81.2% [44]. The occurrence of common hackberry seeds from fecal samples passed by wild-captured eastern box turtles in Missouri was 2.8%, but germination of passed seeds was not tested [177].

Seedling establishment and plant growth: In the reviewed literature (as of 2011), the only characteristics evaluated in any detail in relation to common hackberry seedling establishment and growth were canopy conditions and litter depths. Field studies indicate that common hackberry seedlings establish well beneath closed canopies [88,89], although establishment may be better in canopy openings [124,130]. Burial in moderate to heavy litter may improve seedling establishment [88].

Studies in bottomland forests in southern Illinois suggested that common hackberry germinated and established under a wide range of soil and site conditions. The density of 1- to 2-year-old common hackberry seedlings was 68, 180, and 94 seedlings/acre where litter depths were <0.5 inch (1.3 cm), 0.5 to 2 inches (1.3-5 cm), and >2 inches (5 cm), respectively [88]. Seedlings established and survived beneath closed canopies. Frequency of 1- to 5-year-old common hackberry seedlings increased with successional advancement. There were no common hackberry seedlings on newly developed sandbars or sites recovering from recent, major disturbances. Frequency of seedlings was 0.6% on old fields without trees larger than 4.5-inch (11.4 cm) DBH. Frequency of seedlings was 3.2% in early-seral cottonwood-willow (Populus-Salix spp.) and 13.4% and 9.2% in midseral mixed hardwood and softwood stands, respectively [89].

Studies in deciduous forests in Iowa and dense shrublands in Kentucky reported better common hackberry seedling establishment within canopy openings or clipped areas. In southeastern Iowa, abundance of common hackberry seedlings and saplings was greater beneath dead American elm canopies than live forest canopies. Common hackberry seedlings and saplings were 30% to 33% greater beneath dead American elms than beneath live trees. The researcher suggested that use of dead American elms by resting seed-eating birds may have initiated the regeneration beneath dead trees. There were fewer young, avian-dispersed tree species under living than dead trees [130]. On the Northern Kentucky University campus, common hackberry seedlings did not occur in dense Amur honeysuckle (Lonicera maackii) shrublands, but in plots that were cut repeatedly over 4 years, seedling frequency was 22%, and density was 0.2 seedling/m² [124].

Plant growth: In a review of the life history traits of trees, common hackberry was rated a 4, where 1 represented very slow growth and 5 represented very fast growth [120]. Several sources indicate that common hackberry growth is most rapid in early development [75]. Growth is typically most rapid at 20 to 40 years old [106,150]. Studies suggest that growth rate typically increases with increasing DBH until diameters reach about 11 to 11.8 inches (28-30 cm), when growth rates slow [90,96]. On harsh sites and heavily shaded sites, common hackberry growth rates may slow but persistence is likely [141,145,150,195].

Hackberry seedlings may reach 6 to 12 inches (15-30.5 cm) in their first year [150] and may be impacted by herbivory. On the Manassas National Battlefield in Virginia, during a time when white-tailed deer exceeded the estimated carrying capacity by about 42 deer/km², survival of hackberry seedlings was lower on unprotected than protected plots [160].

Growth changes with tree size: Common hackberry growth rates were reported from streamside forests in Illinois and from its northeastern limit in Quebec. Along Hickory Creek in Illinois, common hackberry averaged 1.9 mm of radial growth/year [17]. Along the Sangamon River also in Illinois, common hackberry growth and mortality were monitored for 3 years. During this time period, no common hackberry trees died, and the radial growth rate for common hackberry trees averaged 3.4 mm/year. Growth rate steadily increased as DBH increased from about 2.8 to 11 inches (7-28 cm) [96]. Common hackberry growth and regeneration were evaluated in 2 populations near Lake St Paul in southwestern Quebec. At the northernmost site, the oldest common hackberry tree was 54 years old, and its DBH was 13 inches (33 cm). The largest common hackberry tree had a 13-inch (33 cm) DBH and was 48 years old. Annual radial growth at the St Paul site averaged 1.90 mm. At the Godefroy River site, the oldest common hackberry tree was 114 years old, and its DBH was 12.8 inches (32.5 cm). The largest common hackberry tree had a DBH of 14 inches (36 cm) and was 70 years old. Annual radial growth at the Godefroy site averaged 1.13 mm [90].

Growth changes with site characteristics: Although common hackberry persists in infertile soils and beneath dense canopies, its growth is typically more rapid on fertile sites and in canopy openings. Extremely slow common hackberry growth has been reported on sites with infertile soils [150]. Based on studies of windbreak species used in the Great Plains, researchers reported good survival and some reproduction for common hackberry trees even with 24 years of intense crowding, but growth was greater and more rapid when trees were released [195]. In the Piedmont of North Carolina, common hackberry reproduction was reported in "overstocked", dense bottomland pine (Pinus spp.) forests [145]. In a bottomland hardwood forest in the Mermet Lake State Conservation Area in Illinois, density of common hackberry stems generally increased as overstory density decreased. Density of common hackberry was compared on increasingly disturbed sites 3 years following tornado damage. Density was 40 trees/ha in undisturbed, closed-canopy forest plots. Common hackberry trees did not occur on a plot with a partial canopy at the edge of the tornado damage. Density was greatest (217 trees/ha) on a severely damaged plot where nearly all the overstory was removed. Density was also high (141 stems/ha) in severely wind-damaged sites that had also been salvage logged [141].

Vegetative regeneration: Common hackberry sprouts from the root crown following top-kill [54,90], and many sources indicate that sprouting is much more likely for seedlings and small trees than large trees [106,107,150]. However, there were no reports on the specific ages and/or sizes at which sprouting potential decreases.

The lack of particulars in most studies that report common hackberry sprouts makes it nearly impossible to evaluate the production, survival, and abundance of vegetative regeneration as it relates to site or disturbance characteristics. In southwestern Quebec, common hackberry saplings of root sprout origin were described in common hackberry-dominated stands. The trigger for sprout production and age or size descriptions of the trees producing sprouts were not reported [90]. In Kankakee, northern Ohio, common hackberry in forests bordering fields is reported to sprout "tenaciously" after cutting [194]. Researchers conducting windbreak studies in the Great Plains suggested cutting low-vigor, stunted common hackberry trees in a 24-year-old windbreak to encourage growth from sprouts [195]. In the Cross Timbers of Oklahoma, up to 7% of common hackberry trees sprouted after herbicide treatments. Although the age of common hackberry was not provided, all woody vegetation on the site was described as brush [179].

SUCCESSIONAL STATUS:
Common hackberry occurs in early-seral and climax communities and tolerates open as well as dense, closed-canopy conditions. It colonizes old fields as well as recently disturbed sites and persists and reproduces in old-growth forests.

Common hackberry occurs in forest types that range from early seral to climax (review [107]). In southern Manitoba, common hackberry is described as an early forest species on open sites in Lake Manitoba's sand ridge forest [123]. In the Mississippi Alluvial Valley, the common hackberry-elm-ash bottomland hardwood type is considered early seral and increased in proportion of the valley area during "anthropogenic intrusion", which included timber harvests, livestock grazing, agricultural development, and altered flood patterns [162]. However, after evaluating environmental factors, species composition, and species abundance in the 100-year flood region along a portion of the lower Wisconsin River, researchers found that common hackberry was most likely to occur in older forests (regions forested for last 40 to 70 years). Common hackberry was consistently found with other late-seral species such as American hornbeam (Carpinus caroliniana), bitternut hickory (Carya cordiformis), and basswood [191]. In the prairie-forest border region of Wisconsin, common hackberry is considered a near-climax species. After evaluating species composition, dominance, constancy, and importance in many upland forests stands, researchers established that bur oak was the earliest seral species, and sugar maple was the latest seral species. Bur oak was assigned a climax adaptation number of 1 and sugar maple 10. Common hackberry was assigned a climax adaptation number of 8, but because of the low frequency of common hackberry, researchers were tentative about common hackberry's classification [43].

Common hackberry importance typically increases in deciduous forests as succession proceeds. In the Brownfield Woods in Illinois, common hackberry density increased from 1925 to 1975 as the total density of trees in the 59-acre (23 ha) woodland increased from 6,500 to 11,609 trees. Density of common hackberry was 2.3, 3.7, 4.1, 9.5, and 11.9 trees/acre in 1925, 1939, 1951, 1960, and 1975, respectively [135]. Increased common hackberry importance was also noted over a 50-year period (1926-1976) in a deciduous forest at Davis-Purdue Research Station in east-central Indiana. Density of common hackberry increased by 8.4 stems/ha over the study period, and researchers considered common hackberry a late-seral or climax species [147,148]. When forest composition and species abundance were compared in 1960 and 2000, researchers found that common hackberry increased in importance in a relatively undisturbed mature oak-hickory stand in Tippecanoe County, Indiana. Importance of common hackberry was 0.59 in 1960 and 7.22 in 2000. A corresponding increase in basal area did not occur [149]. For more information on the increased abundance of common hackberry as succession proceeds in the absence of major disturbances, see the discussion on Succession in the absence of fire.

Shade tolerance: Common hackberry occurs and reproduces in areas with canopy conditions ranging from full sun to nearly complete shade. On Hog Island, Virginia, common hackberry occurred in canopy gaps but not beneath dense wax-myrtle (Myrica cerifera) thickets. Common hackberry did not emerge from any soil samples taken from Hog Island [40]. In mesic, upland, old-growth forests of central Indiana, common hackberry trees and saplings occurred at both edge and interior sites, but their abundance was greater at edge than interior sites. Differences in the densities of common hackberry at edge and interior sites were greater for trees than saplings and greater on warm southern aspects than cool northern aspects. Photosynthetically active radiation was reduced to less than 2% of full sun within 7 feet (2 m) of the forest edge [29]. Early survey records (1805-1824) of central and eastern Illinois show that importance of common hackberry was greatest in shaded habitats. In McLean and Mason counties in Illinois, common hackberry was more common in open and closed forests than in prairies and savannas [157]. In the prairie-forest border region of eastern Illinois, researchers assigned common hackberry a shade adaptation value of 8, where an adaptation value of 1 represented an intolerance of shade and a value of 10 represented the greatest level of shade tolerance [99].

Several studies indicate good common hackberry survival but slow and suppressed growth in heavy shade. A review reports that common hackberry trees beneath dense overstory shade are "often poorly formed" (review [107]). In 24-year-old windbreaks, common hackberry survival was good even with intense crowding, but common hackberry trees grew larger and more rapidly in experimental release areas [195]. At the Mandan Experimental Station in North Dakota, common hackberry trees in interior rows were suppressed by 50- to 60-year-old trees in outside rows [65].

Disturbance tolerance: Common hackberry typically persists on disturbed sites, even repeatedly disturbed sites, although stem abundance may be decreased. From information solicited from foresters and field observations on uprooting and breakage, Xi and Peet [209] rated common hackberry the most damage-resistant tree among the 34 species evaluated 1 and 4 years following Hurricane Fran. Density of common hackberry stems was unchanged before and 3 months after a tornado in the Cross Timbers of north-central Oklahoma [167]. In the Mermet Lake State Conservation Area in Illinois, the density of common hackberry was compared on increasingly disturbed sites 3 years following a tornado and salvage logging in a bottomland hardwood forest. Density of common hackberry was 40 stems/ha on undisturbed, closed-canopy plots, 0 stem/ha at the edge of the tornado damage with a partial canopy, 217 stems/ha on tornado-damaged plots with almost no canopy, and 141 stems/ha on tornado-damaged plots that were salvage logged [141]. A long-term study of canopy survival and regeneration following a tornado in Boone County, Kentucky, suggests that common hackberry may suffer delayed canopy mortality, and large increases in recruitment on tornado-damaged sites may be short-lived. The study occurred after the tornado damaged a mesic, old-growth hardwood forest dominated by sugar maple. In the study area, the total density of trees before the tornado was 334/ha; this changed to 320/ha in the growing season following the tornado, to 242/ha 11 years after the tornado, and to 261/ha 20 years after the tornado. The density of common hackberry stems in the canopy and understory in the years following the tornado is summarized below [83].

Density (stems/ha) of overstory and understory common hackberry stems as time since tornado increased [83]
Time since tornado Same year 11 years 20 years
Trees 35 7 12
Stems >1.4 m tall; 3.82-9.99 cm diameter 97 14 0
Stems >15 cm tall; <3.81 cm diameter 250 754 2

Common hackberry survived repeated cutting along a powerline corridor in north-central Kentucky; however, stems were more frequent in the adjacent forest edge and forest interior. The powerline corridor was cut or mowed to ground level every 5 to 10 years, and the last cutting occurred at least 5 years before this study. Frequency of common hackberry stems less than 4 inches (10 cm) tall was 40% in the maintained corridor, 70% at the forest edge, and 80% in the forest interior. There were no common hackberry stems greater than 4 inches (10 cm) tall in the corridor, but frequency of this stem size was 40% to 65% at the forest edge and interior [125].

Studies suggest that common hackberry tolerates deer browsing; however, seedlings may be less tolerant than saplings. On Virginia's Manassas National Battlefield, where white-tailed deer populations exceeded the estimated carrying capacity by about 42 deer/km², common hackberry seedling survival was greater on protected than unprotected plots [160]. Along the Missouri and Platte rivers in southeastern Nebraska, density of small common hackberry trees (1.2-6 inches (3-15 cm) DBH) was about 3 times greater in areas with high white-tailed deer densities than in areas with low white-tailed deer densities. An earlier report from this study area indicated that "nearly all woody plants within reach of deer were stunted and deformed by repeated browsing by deer", suggesting that common hackberry was not avoided by deer but was likely resistant to deer use [76].

Old-field succession: Typically, common hackberry is rare in the most recently abandoned fields, and abundance increases as field age increases. However, there are reports of common hackberry seedlings in 0- to 10-year-old fields in the Georgia Piedmont [143], and the density of common hackberry stems was much more on 12-year-old than 30-year-old fields in western Tennessee. Density of common hackberry was 60 stems/ha in 12-year-old fields dominated by dense herbaceous cover and 10 stems/ha on 30-year-old fields dominated by a closed-canopy forest [166].

In old fields in Ohio, occurrence of common hackberry increased with time since abandonment. In Shawnee Lookout Park, common hackberry did not occur in a corn field abandoned for 3 years and was uncommon in a 25-year-old field. Common hackberry was important in an 80-year-old mixed mesophytic forest on silty clay loam soils but was uncommon in an oak-ash-maple forest on loam soils that was undisturbed for 120 years [92]. Soil differences between the oldest stands suggest that common hackberry's importance cannot be attributed to stand age alone. At Wright State University in southwestern Ohio, common hackberry frequency was less in 40-year-old fields than in fields that were 60 years or older [47]. Common hackberry was restricted to the oldest fields when a chronosequence of 5 upland sites with similar macroenvironments was compared in southwestern Ohio. Common hackberry did not occur in 2- or 10-year-old, herbaceous-dominated stands or in 50-year-old, Canada goldenrod (Solidago canadensis)-dominated fields with about 30% tree cover. Common hackberry was present in 90-year-old and old-growth deciduous stands on old-field sites [197].

Floodplain succession: In general, common hackberry is most common in rarely flooded, late-seral floodplain communities. Elevation, soil type, rate of sedimentation, and disturbances influence floodplain community composition and successional composition changes. Hodges [85] described general floodplain succession along major US river bottoms. At low-elevation, poorly drained bottomlands, the usual pioneer tree species is black willow. When clay sediments are rapidly deposited in the bottomlands, black willow is typically replaced by the American elm-green ash-common hackberry (Celtis spp.) type (common hackberry in the north; sugarberry in the south). When loam or sand sediments are rapidly deposited in the bottomlands, black willow is replaced by a boxelder (Acer negundo)-sugar maple-common hackberry type before being dominated by the American elm-green ash-common hackberry type. At higher-elevation, better-drained bottomlands, the usual pioneer tree species is eastern cottonwood. The American elm-green ash-common hackberry type replaces the eastern cottonwood community, provided the site does not experience a canopy-opening disturbance. With a canopy-opening disturbance in the eastern cottonwood type, a community dominated by sycamore, pecan, and elm may develop before succession to the American elm-green ash-common hackberry type [85].

Field studies indicate that common hackberry is most often associated with mature floodplain communities receiving limited flooding. On the Republican River in Clay County, Kansas, common hackberry established in 10- to 30-year old stands but was not dominant until stands were 100 years or older. Researchers predicted that common hackberry would persist and dominate climax stands, especially given the mortality of American elm from Dutch elm disease [21]. Researchers found that common hackberry was more likely in older forests when environmental factors, species composition, and species abundance were evaluated within the 100-year flood region on the lower Wisconsin River. Logistical regression models suggested that common hackberry was most likely in the floodplain regions that were forested for at least 40 to 70 years [191]. Along the lower Wisconsin River, common hackberry increased in importance and abundance from the 1950s to 2001. Researchers suspected that decreased flooding allowed for common hackberry increases, although selective logging of other trees species and Dutch elm disease may have also played a role [77]. On the Missouri River in Atchison County, Missouri, common hackberry occurred in floodplain forests on the east but not the west side of the river. West-side forests were young to intermediate aged and periodically flooded; east-side forests were mature and not flooded [24].

Succession in the absence of fire: An increased abundance of common hackberry is often reported when fires are excluded in prairies and oak savannas. In eastern Nebraska, common hackberry's relative importance was 0.1 based on records from the 1850s. Relative common hackberry importance was 5.2% between 1979 and 1983. Fire exclusion was suggested as the major reason for increased abundance of woody vegetation in the study area [161]. The Fitch Natural History Reservation in northeastern Kansas was a nearly treeless tallgrass prairie in 1948. After 50 years of protection from fire and other disturbances, the Reservation was described as a woodland. Common hackberry made up 5% to 9% of invading tree species [55]. On the Konza Prairie in northeastern Kansas, the area dominated by gallery forests was 388 acres (157 ha) in 1939 and more than 596 acres (241 ha) in 1985. Increased predominance of gallery forests dominated by common hackberry, bur oak, chinkapin oak, and American elm was associated with the absence of prairie fires [104]. On mesic sites in the Konza Prairie, common hackberry is replacing bur oak. Abrams [2] attributed these changes in species dominance to fire exclusion, which began with European settlement in the mid-1800s. In 1983 in the mesic gallery forest stands, the density of common hackberry seedlings and saplings was almost 15,000 stems/ha, and the density of young bur oak was only 187 stems/ha [2].

Common hackberry abundance typically increases in oak, oak-hickory, and deciduous woodlands that are protected from fire. In eastern North American oak woodlands, common hackberry commonly establishes in the understory of undisturbed, unburned oak forests, which leads to increased canopy closure and mesic conditions [42]. The Barton Woods of Mason County, Illinois, were mostly open-canopy bur oak savannas in 1940. In 1990, common hackberry and bur oak were codominant species; common hackberry dominated the smaller size classes and bur oak the larger. Researchers recommended selective cutting and fire to restore the savanna condition [131]. In an old-growth oak-hickory remnant in central Illinois, sugar maple, common hackberry, and basswood are common in the intermediate and suppressed size classes. Researchers suspect that without fire, the gaps created as oak and hickory trees die will be filled by sugar maple, common hackberry, and basswood [159]. In the Ozark Hills of Illinois, common hackberry was not reported in repeatedly burned, presettlement, oak-hickory forests but did occur with low importance in the tree, sapling, and seedling stages in fire-excluded, present-day, maple-beech forests [59].

Information related to succession in common hackberry habitats with fire is presented later. For a discussion on the effects of fire on common hackberry and common hackberry's response to fire, see Fire adaptations and plant response to fire.

FIRE EFFECTS AND MANAGEMENT

SPECIES: Celtis occidentalis
FIRE EFFECTS: Immediate fire effect on plant: Common hackberry seedlings and saplings are generally only top-killed by fire; larger and more mature common hackberry trees may or may not survive fire. Sprouting potential decreases as common hackberry trees increase in size [106,107,150], but experiments suggest that thick common hackberry bark may protect trees from lethal temperatures [196]. Additional studies are needed to determine at what age or size common hackberry fails to sprout following top-kill or is able to withstand fire.

Postfire regeneration strategy [175]:
Tree with adventitious buds and/or a sprouting root crown
Ground residual colonizer (on site, initial community)
Secondary colonizer (on- or off-site seed sources)

Fire adaptations and plant response to fire: Fire adaptations:
Photo © David J. Moorhead, University of Georgia, Bugwood.org

Controlled experiments suggest that common hackberry bark may protect internal tissues from lethal fire temperatures and that common hackberry seed buried in the soil could survive fire, but field studies are needed to determine if mature trees or buried seeds are indeed likely to survive fire. In plantations and Natural Areas in Illinois, researchers evaluated the physical and protective characteristics of common hackberry bark. Using a technique designed to mimic conditions produced by a low-severity surface fire (750 °F (400 °C) for 120 s), researchers found that cambium temperatures for 4 out of 10 common hackberry trees exceeded the thermal cell death threshold temperature of 140 °F (60 °C). For the study trees, DBH averaged 15.2 inches (38.6 cm), bark thickness averaged 0.5 inch (13.7 mm), and the specific gravity and moisture content of bark averaged 0.91 g/cm³ and 28.4%, respectively. Using linear modeling, researchers calculated that common hackberry trees needed at least a 3.3-inch (8.5 cm) DBH to develop the 0.3-inch (8.57 mm) thick bark necessary to keep vascular cambium temperatures below lethal levels during exposure to experimental flaming [196]. While this experiment suggests that large common hackberry trees could survive fire, common hackberry is reported as "highly susceptible to fire" in a review of southern, fruit-producing, woody plants. Burned trees were considered susceptible to colonization and injury from wood-decaying organisms [78]. Given the following studies and claims, it seems that long-term postfire studies are needed to improve the understanding of common hackberry's survival of fire and potential for delayed mortality after fire.

In controlled experiments, common hackberry seeds survived and germinated after undergoing fermentation that produced temperatures of 95 to 110 °F (35-43 °C) [185], suggesting that viable common hackberry seeds in the soil could survive a fire that failed to raise soil temperatures substantially. Studies are needed to determine the temperature at which common hackberry seeds are killed and to better evaluate common hackberry's potential to establish from the seed bank on burned sites. Experimental fire studies that determine the survival of a known quantity of common hackberry seeds buried at increasing soil depths would greatly improve understanding of postfire regeneration.

Although field studies suggest that common hackberry has established on burned sites [121,183], it is unclear whether or not seedlings established from on- or off-site seed sources. Common hackberry was reported 10 years after a May 1966 wildfire that top-killed nearly all trees in a 23-year-old oak-hickory stand in Iron County, Missouri. The frequency and density of common hackberry were 2% and 52 trees/ha, respectively. Common hackberry did not occur in samples of the prefire community, which was dominated by oak saplings that had developed after an earlier fire in 1943 [121]. In dry, oak-dominated, sandstone barrens in southern Illinois, common hackberry emerged but did not persist during prescribed fire management of the area. Fire management involved a moderate-intensity, fast-moving fire in late November 1989 and another fire in mid-March 1994 that produced variable flame heights and spread rates. Common hackberry was not present on the burned site before the 1st fire and was no longer present the year after the 2nd fire [183].

Plant response to fire: Typically, there are fewer saplings and larger-sized common hackberry trees on burned than unburned sites. There can be more seedling-sized common hackberry trees on burned than unburned sites when there is prolific postfire sprouting of small, top-killed common hackberry trees [1,3]. Common hackberry has been reported on repeatedly burned sites [22,28,184]; however, abundance is typically least on frequently burned sites [28,184]. Seedling establishment has been reported on burned sites [121,183], but it does not appear that fire encourages germination from a seed bank or that burned sites provide ideal common hackberry seedling establishment or survival conditions.

Because common hackberry is rarely a dominant [150] and is often reported at low frequency and importance levels [108,127], comparisons of its abundance in burned vs. unburned sites provide much less reliable information than comparisons of prefire vs. postfire abundance, since a species with low frequency can easily be present on a site but missed in sampling. Many of the following studies lack prefire comparisons, and their descriptions of fire effects on common hackberry may or may not be accurate.

The cover of common hackberry was reduced after a prescribed fire in a bur oak savanna in Iowa's Loess Hills, which was "recently" invaded by woody vegetation. The fire occurred on 1 December when the air temperature was 43 to 60 °F (6.1-15.5 °C), relative humidity was 22% to 48%, and winds were 5 to 13 miles (8-21 km)/hour. The author suggested that the lack of bur oak litter, which was blown off site, left primarily eastern hophornbeam (Ostrya virginiana) and common hackberry litter to fuel the fire, which burned patchily. Common hackberry cover in the understory was reduced by about half on burned and logged sites; reductions were slightly greater on sites that were only burned. Common hackberry's response to fire and fire with logging is summarized below [133].

Canopy cover (%) of common hackberry in the understory of burned and burned and logged bur oak savannas in Loess Hills of Iowa [133]
 
Control
Burned only
Burned and thinned*
Burned and cleared**
Western site
Prefire 25 19 64 57
1st postfire growing season 21 8 34 24
Eastern site
Prefire 3 22 17 2
1st postfire growing season 4 5 9 1
*All trees except bur oak with >1 cm DBH were removed.
**All trees including bur oak with >1 cm DBH were removed.

There were fewer small common hackberry stems (<2 inches (5 cm) DBH) after fire when pre-and postfire sites were compared in an upland forest in Ogle County, northern Illinois after both a dormant-season fire in early March and a growing-season fire in early May; however, common hackberry was rare on the sites burned in May. Researchers reported that, in general, "most woody saplings sprouted" and experienced "very little mortality". The March fire was considered low to moderate "intensity", burned when the air temperature was 62 °F (17 °C) and relative humidity was 70%, and was set 8 days after the last precipitation. Flame heights were 6 to 39 inches (15-100 cm), and the fire spread rate was 4.3 feet (1.3 m)/minute. The May fire was considered moderate to intermediate "intensity", burned when the air temperature was 78 °F (26 °C) and relative humidity was 29%, and was set 9 days after the last precipitation. Flame heights were 4 to 29 inches (10-75 cm), and the fire spread rate was 5.6 feet (1.7 m)/minute. Details regarding common hackberry are summarized below [165].

Density (stems/ha) of common hackberry before and after dormant-season and growing-season fires and on unburned sites in upland forests of northern Illinois [165]
Burn status March fire May fire Unburned
Stem DBH <2 cm 2-5 cm <2 cm 2-5 cm <2 cm 2-5 cm
Prefire 15 9 2 1 12 0
Postfire (1-2 yrs) 8 7 1 1 12 0

In the WK Kellogg Experimental Forest in Kalamazoo County, Michigan, large common hackberry seedlings (>3.2 feet (1 m) tall, 0.7 inch (1.9 cm) DBH) did not occur on burned sites, although there were 111 large common hackberry seedlings/ha on unburned sites dominated by mature red pine (Pinus resinosa) and eastern white pine (P. strobus). Small common hackberry seedlings (<3.2 feet tall) were not reported on burned or unburned sites surveyed in the study area [142]. For more about the fire conditions and fire effects on other species, see the Research Project Summary of this study.

The density of seedling-size common hackberry stems (<5 feet (1.5 m) tall) increased while sapling-size stems (>5 feet tall) decreased with prescribed fires in bur oak woodlands on the Konza Prairie in northeastern Kansas. Before the fire, there were 100 sapling-size stems/ha and 550 seedling-size stems/ha. After the 1st spring fire, there were no sapling-size common hackberry stems but 650 seedling-size stems/ha. After the 2nd spring fire, there were still no sapling-size stems, but there were 900 seedling-size stems/ha. The researcher reported that multiple basal sprouts per root crown were common. Spring fire conditions were: 61 to 70 °F (16-21 °C) temperatures, 21% to 44% relative humidities, 7.2 to 10.8 feet (2.2-3.3 m)/s wind speeds, slow fire spread rates (up to 6.6 feet (2 m)/min), and less than 1.6-foot (0.5 m) flame heights. More than 90% of the study area burned. Surface fuels were primarily bur oak foliage, scattered branches, and a few downed trunks [1,3].

The density and frequency of understory common hackberry stems were generally greater on wildfire-burned than unburned plots in an even-aged, 35-year-old loblolly pine (P. taeda) stand in Orange County, North Carolina. Unburned plots were compared to plots that burned in a mixed-severity fire in November. In areas burned by surface fires, the overstory tree density was not reduced. In areas burned by crown fires, all but 4% of overstory trees were killed. Common hackberry stems in the small (>1-foot (0.3 m) tall) and very small (<1-foot tall) size classes were poorly represented in unburned plots but had frequencies of 20% and 30%, respectively, in burned plots regardless of fire behavior. Understory common hackberry stems between the canopy and small stem size class did not occur in plots burned by the crown fire but occurred in unburned plots and plots burned in the surface fire. Study findings are summarized below [146]:

The density (stems/10 m²) and frequency (%) of common hackberry stems by size class in unburned, surface-burned, and crown-burned plots [146]
 
Unburned
Surface fire
Crown fire
Understory stems (between overstory and >1 foot tall size classes)
Density 0.1 0.4 0
Frequency 10 30 0
Basal area (number/10 m²) 0.09 0.06 0
Small stems (>1 foot tall)
Density 0 0.2 0.3
Frequency 0 20 20
Very small stems (<1 foot tall)
Density 0 1.1 0.2
Frequency 0 30 30

Effects of repeated fires: Common hackberry has been reported on sites burned as often as annually [28]; however, studies evaluating the long-term survival of common hackberry on repeatedly burned sites are lacking. Many studies report that common hackberry was normally absent from repeatedly burned prairies and savannas until fire frequencies were dramatically reduced after European settlement (see Succession in the absence of fire). The long-term fate is largely unknown for common hackberry that has sprouted repeatedly after top-kill by fires.

On the Konza Prairie, common hackberry was most common on sites that burned least often. April prescribed fires burned at frequencies ranging from annually to once in 20 years. Common hackberry was reported at a density of 3.1 stems/ha on annually burned plots that were also grazed but did not occur on plots that had burned every 3 to 5 years. On plots burned once in 15 years, the density of common hackberry was 2.8 stems/ha on grazed plots and ranged from 0.8 to 7.3 stems/ha on ungrazed plots [28]. In a fire-managed, oak forest in east-central Missouri, common hackberry trees larger than 4-inch (10 cm) DBH persisted on sites burned 2 to 4 times, with the last fire occurring 1 year before sampling. There was an average of 3 to 4 large common hackberry trees on the burned plots and 1 large common hackberry tree on the unburned plots [22].

Understory common hackberry stems were dramatically reduced after 3 prescribed fires in 6 years in post oak (Q. stellata)-dominated flatwoods in south-central Illinois. Fires occurred in February or March. At the time of ignition, air temperatures were 36 to 54 °F (2-12 °C), relative humidities were about 70%, and wind speeds were less than 15 miles (24 km)/hour. Sites were free of precipitation for the 4 days before the fires. More than 95% of the area's vegetation burned. Before the fire, the density of common hackberry in the understory averaged 26.7 stems/ha. One year following the 3rd fire, common hackberry averaged 5 stems/ha in the understory [184].

In a mixed-mesophytic forest in Kentucky's Dinsmore Woods State Nature Preserve, the density of small common hackberry stems decreased over time on both burned and unburned sites. Common hackberry density decreases were much less on lowland sites burned twice than on upland sites burned 3 times. Fire effects were evaluated 2 growing seasons after the last fire on lowland sites and in the 1st postfire growing season after the last fire on upland sites. Fires burned in the fall, were considered moderate severity, and produced flames up to 6 inches (15 cm) tall. The density and percent change of common hackberry on unburned and burned and lowland and upland sites are summarized below [126].

Density (#/ha) of small common hackberry stems (<5 cm in diameter) on burned and unburned plots on lowland and upland sites in Kentucky [126]
 
1995 (prefire)
1998
Change (%)
Unburned lowland site 240 50 -79
Lowland site, burned twice 220 170 -23
Unburned upland site 3,860 690 -82
Upland site, burned 3 times 3,410 270 -92

FUELS AND FIRE REGIMES:

Fuels: In the reviewed literature, little was reported on the flammability and accumulation of common hackberry fuels, which may relate to the extreme rarity of pure common hackberry stands.

Some sources report that common hackberry is relatively difficult to burn. The Virginia Firewise Landscaping Taskforce gave common hackberry a low flammability rating based on a combination of leaf moisture retention, leaf oil or resin content, litter and debris accumulation, foliage and dead branch production, branching architecture, landscape maintenance needs, and/or drought resistance [11]. The patchy burning of a prescribed fire in Iowa's Loess Hills was blamed on low-flammability surface fuels provided by common hackberry and eastern hophornbeam litter, which remained after bur oak litter was blown off the site [133]. In central Illinois, researchers determined the litter accumulation and loss of leaf mass in streamside, transitional forests dominated by common hackberry. From September 1974 and August 1975, leaf litter accumulation was 495.7 m²/year, and accumulation of twigs was 155.8 m²/year. Common hackberry leaf weight decreased slowly until midspring, when it decreased much more rapidly [20].

Fire regimes: In presettlement time and in the early phases of European settlement, common hackberry was restricted to rarely burned floodplains, but with fire exclusion, common hackberry quickly spread from these less fire-prone floodplains and bottomlands into savannas and prairies. This topic is discussed in more detail in the earlier section on Succession in the absence of fire.

In the Flint Hills and Konza Prairie of eastern Kansas, fires burned in the tallgrass prairies every 2 to 3 years. Many fires in the gallery forests likely originated in the prairie. The extent and intensity of prairie fires were likely reduced once they reached the gallery forests. In gallery forests, surface fuel accumulations are generally low because of rapid decomposition, fuels are often less combustible because of high humidity levels and moisture contents, and fires are likely to spread slowly as they burn downhill. The fire-return interval for gallery forests was estimated at 11 to 20 years before active fire exclusion. Without fire, common hackberry may become the "sole dominant" of the gallery forests on mesic sites. Long-term repeated fire is considered necessary to limit common hackberry recruitment into the oak-dominated canopy. In the Flint Hills region, reduced fire frequency, fire extent, and/or intensity of prairie fires beginning in about 1925 has allowed common hackberry to establish and persist in gallery forests and adjacent prairie sites ([4], original research and personal communications cited in [1]). In the Konza Prairie, fire temperatures at ground level ranged from 100 to 270 °F (38-132 °C) during a mid-April prescribed fire in gallery forests dominated by bur oak, chinkapin oak, common hackberry, and eastern redcedar (Juniperus virginiana) [67].

See the Fire Regime Table for additional information on the fire regimes in vegetation communities where common hackberry may be an associated species.

FIRE MANAGEMENT CONSIDERATIONS:
The use of fire to manage common hackberry populations was not reported in the reviewed literature (as of 2011). However, fire use was recommended to restore prairies and oak savannas invaded by common hackberry and other late-seral deciduous species. Frequent fire is likely necessary to eliminate common hackberry from these habitats. Without sustained frequent fire, common hackberry may persist in the understory (see Succession in the absence of fire). Additional fire studies are needed to understand common hackberry's potential to establish on burned sites, sprout repeatedly on frequently burned sites, and survive for the long term on infrequently as well as frequently burned sites.

MANAGEMENT CONSIDERATIONS

SPECIES: Celtis occidentalis
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:
Common hackberry provides food for deer, small mammals, birds, and is sometimes consumed by cattle.

Deer: Common hackberry use by deer can be heavy. In the Sylamore Ranger District of the Ozark National Forest, the summer utilization frequency of common hackberry by white-tailed deer was 20%, while the frequency of common hackberry in the available vegetation was only 4% [158]. In McLean County, Illinois, common hackberry was considered a preferred browse species for white-tailed deer, although there were other species more highly preferred than common hackberry. Thirty percent of available common hackberry twigs were browsed (P<0.05) [175]. On the Manassas National Battlefield in Virginia, where white-tailed deer population levels exceeded the estimated carrying capacity by about 42 deer/km², common hackberry seedling survival was better on protected than unprotected plots [160].

Small mammals: Rodents, rabbits, and northern raccoons utilize common hackberry. In central Indiana, common hackberry trees were often used for nesting by eastern fox squirrels [163]. In eastern South Dakota, more than 45% of 1- to 2-year-old common hackberry stems received heavy browsing when not protected from rodents and rabbits [12]. On the Fort Hays Kansas State College campus, eastern fox squirrels fed on common hackberry fruits and nipple galls (Pachyslla mamma) on common hackberry leaves [31]. In Connecticut, researchers observed woodchucks feeding on common hackberry leaves in the field. In captive feeding trials, woodchucks ate 51.2% of common hackberry leaves provided. Of the 9 species presented to woodchucks, common hackberry was the 5th most eaten [181]. In a review of southern, fruit-producing woody plants, common hackberry was reported as a principal northern raccoon food [78].

Birds: Many bird species feed on common hackberry fruits and use common hackberry trees for foraging and nesting. Hunter [93] reports that cedar waxwings, mockingbirds, American robins, bluebirds, yellow-bellied sapsuckers, northern flickers, wild turkeys, quail, and about 20 other bird species eat common hackberry seeds. Common hackberry was found in the stomachs of American robins and eastern bluebirds [15]. In studies conducted from 1976 to 1978 and 1983 to 1984 in Obion County, Tennessee, common hackberry was 6% of the winter plant foods consumed by eastern bluebirds [151]. In the South, common hackberry is reported as a preferred fall and winter wild turkey food [78]. The frequency of common hackberry was 6.7% in 30 wild turkey crops collected over 2 years in Gregory County, South Dakota [113]. In Illinois and Missouri, common hackberry was used for foraging by a variety of birds. In the Brownfield Woods near Urbana, Illinois, common hackberry was 6% of the woodland composition but 8% and 10% of utilization by red-headed woodpeckers and red-bellied woodpeckers, respectively [155]. Use of common hackberry and common hackberry habitats by woodpeckers was also reported by Jackson [95] and Graber and others [71]. When feeding behaviors of birds were compared by season in the Brownfield Woods, utilization of common hackberry by species and season was [207]: In the Illinois River Valley of central Illinois, black-capped chickadees foraged more than expected in common hackberry based on its relative availability [81]. In the oak-hickory forest type in central Missouri, cavity-nesting birds preferred white oak, green ash, and common hackberry snags most for foraging. The following birds were observed: northern flickers, yellow-bellied sapsuckers, other woodpeckers, white-breasted nuthatches, tufted titmice, and black-capped chickadees [27].

On South Bass Island in Lake Erie, 6% of cedar waxwing nests were built in common hackberry trees [152].

Reptiles: In Missouri, the occurrence of common hackberry seed in fecal samples from captured eastern box turtles was 2.8% [177].

Cattle: Heavy browsing of common hackberry by cattle may occur in the winter on poor rangeland sites [41].

Palatability and nutritional value: Several studies have reported the nutritional value of common hackberry fruits, but in the literature reviewed to date (2011), nutrient content of common hackberry foliage was only reported for trees from Wisconsin and Quebec in a review by Blinn and Becker. Average levels of nitrogen, phosphorus, potassium, calcium, and magnesium reported were 2.6 to 2.8%, 0.2 to 0.3%, 1.6 to 1.7%, 1.1 to 7.8%, and 0.5%, respectively [23]. A study in Pennsylvania found that common hackberry fruits were high in calcium and magnesium, low in lignin and cellulose, and had moderately high crude protein levels. For more detail on this study, see Wainio and Forbes [201]. For common hackberry fruits collected near Urbana, Illinois, the protein content averaged 3.5%, lipid content averaged 0.4%, and calcium content averaged 0.4%. During feeding trials with American robins, veeries, and hermit thrushes, researchers found that the digestive efficiency of the birds on common hackberry fruits was moderate (0.74-0.78, where maximum and minimum efficiencies reported were 0.90 and 0.40, respectively) [98]. Stiles [176] reported that fleshy fruits and seeds of common hackberry averaged 4.37% crude fat in eastern deciduous forests. Based on a study of birds and common hackberry fruits on trees in Kansas, a researcher calculated that the fleshy energy/common hackberry fruit was 295.7 calories (1,237.9 J) and seed energy/fruit was 537.8 calories (2,251.8 J) [173].

VALUE FOR REHABILITATION OF DISTURBED SITES:
Common hackberry has been used successfully to rehabilitate mined sites. Two to 3 years after planting on coal surface-mined sites, common hackberry survival was 25% on a mined grassland site in Wyoming and 100% on a mined, mixed-shrubland site in Colorado [91]. Common hackberry occurred, although not commonly, on lead and zinc mine spoils in northeastern Oklahoma. Mine sites had been abandoned 18 years or more, and common hackberry occupied sites with high concentrations of heavy metals [66]. Although not intentionally planted, common hackberry averaged 110 stems/ha on 30-year-old, surface-mined, revegetation sites in Missouri, Kansas, and/or Oklahoma [198].

OTHER USES:
Common hackberry was used by American Indians of the Missouri River region. The Dakota ground common hackberry fruits and seeds into a powder to season meat, and the Pawnee made a meal out of fat, corn, and pounded common hackberry fruits [68].

OTHER MANAGEMENT CONSIDERATIONS:
Several topics, which could impact the present or future management of common hackberry, were discussed in the reviewed literature.

Allelopathy: Herbaceous vegetation was more abundant beneath American elm than common hackberry and other dominant canopy species in bottomland forests of St Louis County, Missouri, even though light intensity and soil characteristics were similar beneath all tree species. In controlled studies, radicle growth and germination of Japanese brome (Bromus japonicus) and Canada wildrye (Elymus canadensis) were lower for common hackberry-treated than untreated control seeds. Treatments included exposure to decaying common hackberry leaves, common hackberry leaf leachate, and soil collected beneath common hackberry trees [119].

Climate change: Two studies suggest a northward shift in common hackberry's range with increased temperatures associated with climate change. In the eastern United States, common hackberry's importance is predicted to increase in its ecologically optimum habitats, but a northern shift of its southern distribution boundary is expected with a warming climate and a doubling of current carbon dioxide levels [94]. A northern shift of common hackberry's range was also predicted from climate change models used by McKenny and others [132]. The northward shift was expected to be larger if common hackberry was successful in colonizing all habitats made suitable by climate change [132].

Insect pests: Many sources describe the identification, damage, and potential control of insects and fungi that utilize common hackberry as a host. For common hackberry pest information, see the reviews by Dix and others [48], Krajicek [106], Krajicek and Williams [107], and Riffle and Peterson [156].

Invasive species: In floodplain forests along the Lower Wisconsin State Riverway, common hackberry has not been negatively impacted by nonnative common buckthorn (Rhamnus cathartica) thickets. In these floodplains, the frequency of common hackberry was much greater in 2000 than in 1950, when common buckthorn began to increase in the habitats. While increases in common hackberry frequency could have been the result of successional advancement with decreased flooding and/or successful colonization of canopy gaps created by disease-killed American elms, when sites were compared in 2000, basal area of common hackberry on sites with common buckthorn was more than twice that on sites without common buckthorn. In the understory layer, there were more common hackberry saplings on sites without common buckthorn but more seedlings on sites with common buckthorn [10]. The reasons for the differences in common hackberry abundance associated with common buckthorn were not discussed.

APPENDIX: FIRE REGIME TABLE

SPECIES: Celtis occidentalis
The following table provides fire regime information that may be relevant to common hackberry habitats. Follow the links in the table to documents that provide more detailed information on these fire regimes.

Fire regime information on vegetation communities in which common hackberry may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models [112], 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.
Northern Great Plains Great Lakes Northeast
South-central US Southern Appalachians Southeast
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
Southern mixed-grass prairie Replacement 100% 9 1 10
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
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    
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 Grassland
Mosaic of bluestem prairie and oak-hickory Replacement 79% 5 1 8
Mixed 2% 260    
Surface or low 20% 2   33
Great Lakes Woodland
Northern oak savanna Replacement 4% 110 50 500
Mixed 9% 50 15 150
Surface or low 87% 5 1 20
Great Lakes Forested
Great Lakes floodplain forest Mixed 7% 833    
Surface or low 93% 61    
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    
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    
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
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    
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    
South-central US
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
South-central US Grassland
Southern tallgrass prairie Replacement 91% 5    
Mixed 9% 50    
Oak savanna Replacement 3% 100 5 110
Mixed 5% 60 5 250
Surface or low 93% 3 1 4
South-central US Woodland
Interior Highlands dry oak/bluestem woodland and glade Replacement 16% 25 10 100
Mixed 4% 100 10  
Surface or low 80% 5 2 7
Oak woodland-shrubland-grassland mosaic Replacement 11% 50    
Mixed 56% 10    
Surface or low 33% 17    
Interior Highlands oak-hickory-pine Replacement 3% 150 100 300
Surface or low 97% 4 2 10
South-central US Forested
Interior Highlands dry-mesic forest and woodland Replacement 7% 250 50 300
Mixed 18% 90 20 150
Surface or low 75% 22 5 35
Southern floodplain Replacement 42% 140    
Surface or low 58% 100    
Southern floodplain (rare fire) Replacement 42% >1,000    
Surface or low 58% 714    
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    
Southern Appalachians Woodland
Appalachian shortleaf pine Replacement 4% 125    
Mixed 4% 155    
Surface or low 92% 6    
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 white pine-northern hardwood Replacement 72% 475    
Surface or low 28% >1,000    
Oak (eastern dry-xeric) Replacement 6% 128 50 100
Mixed 16% 50 20 30
Surface or low 78% 10 1 10
Appalachian oak forest (dry-mesic) Replacement 6% 220    
Mixed 15% 90    
Surface or low 79% 17    
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 Grassland
Southeast Gulf Coastal Plain Blackland prairie and woodland Replacement 22% 7    
Mixed 78% 2.2    
Southeast Forested
Coastal Plain pine-oak-hickory Replacement 4% 200    
Mixed 7% 100      
Surface or low 89% 8    
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 [79,111].

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