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| Photo © Dennis Kalma, 2006. |
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.
United States:
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| Photo © 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: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: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 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. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".
| 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. | ||||||||||
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| 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) |
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| 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) |
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| 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]. |
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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 m² 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.
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: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].
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].
1. Amlin, Nadine A.; Rood, Stewart B. 2001. Inundation tolerances of riparian willows and cottonwoods. Journal of the American Water Resources Association. 37(6): 1709-1720. [3600]
2. Argus, George W. 1957. The willows of Wyoming. University of Wyoming Publications in Science. Laramie, WY: University of Wyoming. 21(1): 1-63. [4962]
3. Argus, George W. 1974. An experimental study of hybridization and pollination in Salix (willow). Canadian Journal of Botany. 52(7): 1613-1619. [67515]
4. Beals, E. W.; Cottam, G.; Vogl, R. J. 1960. Influence of deer on vegetation of the Apostle Islands, Wisconsin. Journal of Wildlife Management. 24(1): 68-80. [67570]
5. Beck, Alan M.; Vogl, Richard J. 1972. The effects of spring burning on rodent populations in a brush prairie savanna. Journal of Mammalogy. 53(2): 336-346. [196]
6. Begin, Yves. 1990. The effects of shoreline transgression on woody plants, upper St. Lawrence estuary, Quebec. Journal of Coastal Research. 6(4): 815-827. [17681]
7. Bergerud, Arthur T. 1972. Food habits of Newfoundland caribou. Journal of Wildlife Management. 36(3): 913-923. [14760]
8. Bird, Ralph D. 1927. A preliminary ecological survey of the district surrounding the entomological station at Treesbank, Manitoba. Ecology. 8(2): 207-220. [63548]
9. Bird, Ralph D. 1961. Ecology of the aspen parkland of western Canada in relation to land use. Contribution No. 27. Ottawa: Canada Department of Agriculture, Research Branch. 153 p. [15620]
10. Booth, W. E.; Wright, J. C. 1962. [Revised]. Flora of Montana: Part II--Dicotyledons. Bozeman, MT: Montana State College, Department of Botany and Bacteriology. 280 p. [47286]
11. Braun, E. Lucy. 1961. The woody plants of Ohio. Columbus, OH: Ohio State University Press. 362 p. [12914]
12. Brown, Charles P. 1946. Food of Maine ruffed grouse by seasons and cover types. Journal of Wildlife Management. 10(1): 17-28. [67563]
13. Brown, David T.; Doucet, G. Jean. 1991. Temporal changes in winter diet selection by white-tailed deer in a northern deer yard. Journal of Wildlife Management. 55(3): 361-376. [15406]
14. Brumelis, G.; Carleton, T. J. 1989. The vegetation of post-logged black spruce lowlands in central Canada. II. Understory vegetation. Journal of Applied Ecology. 26: 321-339. [7864]
15. Catling, Paul M.; Brownell, Vivian R. 1998. Importance of fire in alvar ecosystems--evidence from the Burnt Lands, eastern Ontario. The Canadian Field Naturalist. 112(4): 661-667. [30338]
16. Chapman, William K.; Bessette, Alan E. 1990. Trees and shrubs of the Adirondacks. Utica, NY: North Country Books, Inc. 131 p. [12766]
17. Croxton, W. C. 1939. A study of the tolerance of trees to breakage by ice accumulation. Ecology. 20: 71-73. [5993]
18. Curtis, John T. 1959. Tall shrub communities. In: Curtis, John T. The vegetation of Wisconsin. Madison, WI: The University of Wisconsin Press: 352-358. [60529]
19. Dalby, Richard. 1999. Willows are wonderful. American Bee Journal. 139(3): 220-221. [67517]
20. Dirschl, H. J.; Coupland, R. T. 1972. Vegetation patterns and site relationships in the Saskatchewan River Delta. Canadian Journal of Botany. 50: 647-675. [7449]
21. Dirschl, Herman J.; Dabbs, Don L.; Gentle, Garry C. 1974. Landscape classification and plant successional trends in the Peace-Athabasca Delta. Canadian Wildlife Service Report Series 30. Ottawa, ON: Canadian Wildlife Service. 33 p. [6177]
22. Dorn, Robert D. 1970. The willows of Montana. Bozeman, MT: Montana State University, Department of Botany and Microbiology. 18 p. [6176]
23. Dorn, Robert D. 1977. Flora of the Black Hills. Cheyenne, WY: Robert D. Dorn and Jane L. Dorn. 377 p. [820]
24. Dorn, Robert D. 1984. Vascular plants of Montana. Cheyenne, WY: Mountain West Publishing. 276 p. [819]
25. Dorn, Robert D. 1988. Vascular plants of Wyoming. Cheyenne, WY: Mountain West Publishing. 340 p. [6129]
26. Dorn, Robert D.; Dorn, Jane L. 1997. Rocky Mountain Region willow identification field guide. R2-RR-97-01. Denver, CO: U.S. Department of Agriculture, Forest Service, Renewable Resources. 107 p. [29146]
27. Dowhan, Joseph J.; Rozsa, Ron. 1989. Flora of Fire Island, Suffolk County, New York. Bulletin of the Torrey Botanical Club. 116(3): 265-282. [22041]
28. Duncan, Wilbur H.; Duncan, Marion B. 1987. The Smithsonian guide to seaside plants of the Gulf and Atlantic coasts from Louisiana to Massachusetts, exclusive of lower peninsular Florida. Washington, DC: Smithsonian Institution Press. 409 p. [12906]
29. Egler, Frank E. 1948. 2,4-D effects in Connecticut vegetation, 1947. Ecology. 29(3): 382-386. [60983]
30. Egler, Frank E. 1949. Herbicide effects in Connecticut vegetation, 1948. Ecology. 30(2): 248-256. [60965]
31. Egler, Frank E. 1950. Herbicide effects in Connecticut vegetation, 1949. Botanical Gazette. 112(1): 76-85. [61003]
32. Ewing, J. 1924. Plant successions of the brush-prairie in north-western Minnesota. Journal of Ecology. 12: 238-266. [11122]
33. Farrar, John Laird. 1995. Trees of the northern United States and Canada. Ames, IA: Blackwell Publishing. 502 p. [60614]
34. Froiland, Sven G. 1962. The genus Salix (willows) in the Black Hills of South Dakota. Technical Bulletin No. 1269. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 75 p. [5992]
35. Gates, Frank C. 1930. Aspen association in northern lower Michigan. Botanical Gazette. 40(3): 233-259. [16933]
36. Gleason, Henry A.; Cronquist, Arthur. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. 2nd ed. New York: New York Botanical Garden. 910 p. [20329]
37. Grant, Martin L. 1929. The burn succession in Itasca County, Minnesota. Minneapolis, MN: University of Minnesota. 63 p. Thesis. [36527]
38. Great Plains Flora Association. 1986. Flora of the Great Plains. Lawrence, KS: University Press of Kansas. 1392 p. [1603]
39. Haeussler, Sybille; Bedford, Lorne; Boateng, Jacob O.; MacKinnon, Andy. 1999. Plant community responses to mechanical site preparation in northern interior British Columbia. Canadian Journal of Forest Research. 29: 1084-1100. [38978]
40. Hann, Wendel; Havlina, Doug; Shlisky, Ayn; [and others]. 2005. Interagency fire regime condition class guidebook. Version 1.2, [Online]. In: Interagency fire regime condition class website. U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior; The Nature Conservancy; Systems for Environmental Management (Producer). Variously paginated [+ appendices]. Available: http://www.frcc.gov/docs/1.2.2.2/Complete_Guidebook_V1.2.pdf [2007, May 23]. [66734]
41. Heinrich, Bernd. 1975. Bee flowers: a hypothesis on flower variety and blooming times. Evolution. 29(2): 325-334. [67540]
42. Hewsholme, Christopher. 1992. Willows: The genus Salix. Portland, OR: Timber Press, Inc. 224 p. [20106]
43. Howard, Lauren Davis. 1983. A simple system for relating physiognomy, floristic composition and relative importance in woody vegetation. Bulletin of the Torrey Botanical Club. 110(3): 360-365. [67532]
44. Hunt, Robley W.; Mangus, Lloyd M. 1954. Deer management study: Mud Lake National Wildlife Refuge, Holt, Minnesota. Journal of Wildlife Management. 18(4): 482-495. [67535]
45. Irwin, Larry L. 1985. Foods of moose, Alces alces, and white-tailed deer, Odocoileus virginianus, on a burn in boreal forest. The Canadian Field-Naturalist. 99(2): 240-245. [4513]
46. Isaak, Daniel; Marshall, William H.; Buell, Murray F. 1959. A record of reverse plant succession in a tamarack bog. Ecology. 40(2): 317-320. [10551]
47. Jeglum, John K. 1971. Plant indicators of pH and water level in peatlands at Candle Lake, Saskatchewan. Canadian Journal of Botany. 49: 1661-1676. [7450]
48. Johnston, Carol A. 2003. Shrub species as indicators of wetland sedimentation. Wetlands. 23(4): 911-920. [67520]
49. Kartesz, John T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. 1st ed. In: Kartesz, John T.; Meacham, Christopher A. Synthesis of the North American flora (Windows Version 1.0), [CD-ROM]. Chapel Hill, NC: North Carolina Botanical Garden (Producer). In cooperation with: The Nature Conservancy; U.S. Department of Agriculture, Natural Resources Conservation Service; U.S. Department of the Interior, Fish and Wildlife Service. [36715]
50. Kudish, Michael. 1992. Adirondack upland flora: an ecological perspective. Saranac, NY: The Chauncy Press. 320 p. [19376]
51. LANDFIRE Rapid Assessment. 2005. Reference condition modeling manual (Version 2.1), [Online]. In: LANDFIRE. Cooperative Agreement 04-CA-11132543-189. Boulder, CO: The Nature Conservancy; U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior (Producers). 72 p. Available: http://www.landfire.gov/downloadfile.php?file=RA_Modeling_Manual_v2_1.pdf [2007, May 24]. [66741]
52. LANDFIRE Rapid Assessment. 2007. Rapid assessment reference condition models. In: LANDFIRE. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab; U.S. Geological Survey; The Nature Conservancy (Producers). Available: http://www.landfire.gov/models_EW.php [66533]
53. Larson, Gary E. 1993. Aquatic and wetland vascular plants of the Northern Great Plains. Gen. Tech. Rep. RM-238. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 681 p. Jamestown, ND: Northern Prairie Wildlife Research Center (Producer). Available: http://www.npwrc.usgs.gov/resource/plants/vascplnt/vascplnt.htm [2006, February 11]. [22534]
54. Little, Elbert L., Jr. 1977. Atlas of United States trees. Volume 4. Minor eastern hardwoods. Misc. Pub. No. 1342. Washington, DC: U.S. Department of Agriculture, Forest Service. 17 p. [21683]
55. Locky, David A.; Bayley, Suzanne E.; Vitt, Dale H. 2005. The vegetational ecology of black spruce swamps, fens, and bogs in southern boreal Manitoba, Canada. Wetlands. 25(3): 564-582. [67524]
56. Martin, J. Lynton. 1956. An ecological survey of burned-over forest land in southwestern Nova Scotia. Forestry Chronicle. 32: 313-336. [8932]
57. McVaugh, Rogers. 1947. Establishment of vegetation on sand-flats along the Hudson River, New York. Ecology. 28(2): 189-193. [67550]
58. Middleton, Beth. 2002. Nonequilibrium dynamics of sedge meadows grazed by cattle in southern Wisconsin. Plant Ecology. 161: 89-110. [6628]
59. Miller, B. K.; Moser, B. C.; Johnson, K. D.; Swihart, R. K. 1994. Designs for windbreaks and vegetative filterstrips that increase wildlife habitat and provide income. In: Campbell, Kenneth L.; Graham, Wendy D.; Bottcher, A. B., eds. Environmentally sound agriculture: Proceedings of the 2nd conference; 1994 April 20-22; Orlando, FL. St. Joseph, MI: American Society of Agricultural Engineers: 567-574. [67523]
60. Mohlenbrock, Robert H. 1986. [Revised edition]. Guide to the vascular flora of Illinois. Carbondale, IL: Southern Illinois University Press. 507 p. [17383]
61. Moss, E. H. 1932. The vegetation of Alberta: IV. The poplar association and related vegetation of central Alberta. The Journal of Ecology. 20(2): 380-415. [63588]
62. Moss, E. H. 1938. Longevity of seed and establishment of seedlings in species of Populus. Botanical Gazette. 99: 529-542. [26846]
63. Mosseler, A. 1989. Interspecific pollen-pistil incongruity in Salix. Canadian Journal of Forest Research. 19: 1161-1168. [9348]
64. Mosseler, A. 1990. Hybrid performance and species crossability relationships in willows. Canadian Journal of Botany. 68(11): 2329-2338. [60064]
65. Mosseler, A.; Papadopol, C. S. 1989. Seasonal isolation as a reproductive barrier among sympatric Salix species. Canadian Journal of Botany. 67: 2563-2570. [10066]
66. Nielsen, Etlar L.; Moyle, John B. 1941. Forest invasion and succession on the basins of two catastrophically drained lakes in northern Minnesota. The American Midland Naturalist. 25(3): 564-579. [62782]
67. Ohmann, Lewis F.; Grigal, David F.; Brander, Robert B. 1976. Biomass estimation for five shrubs from northeastern Minnesota. Res. Pap. NC-133. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 11 p. [20786]
68. Peek, J. M. 1974. A review of moose food habits studies in North America. Le Naturaliste Canadien. 101: 195-215. [7420]
69. Peek, James M., Urich, David L.; Mackie, Richard J. 1976. Moose habitat selection and relationships to forest management in northeastern Minnesota. Wildlife Monographs No. 48. Washington, DC: The Wildlife Society. 65 p. [13902]
70. Peek, James M. 1970. Relation of canopy area and volume to production of three woody species. Ecology. 51(6): 1098-1101. [64408]
71. Porter, Glen L. 1990. Willow species of disturbed sites in the sub-boreal spruce zone in north-central British Columbia. FRDA Handbook 004; ISSN-0835-1929. Victoria, BC: Forestry Canada; B.C. Ministry of Forests. 67 p. [28015]
72. Ramovs, B. V.; Roberts, M. R. 2005. Response of plant functional groups within plantations and naturally regenerated forests in southern New Brunswick, Canada. Canadian Journal of Forest Research. 35(6): 1261-1276. [61144]
73. Raunkiaer, C. 1934. The life forms of plants and statistical plant geography. Oxford: Clarendon Press. 632 p. [2843]
74. Rawson, James W. 1974. Willows. In: Gill, John D.; Healy, William M. Shrubs and vines for northeastern wildlife. Gen. Tech. Rep. NE-9. Upper Darby, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station: 147-149. [51835]
75. Robinson, George R.; Handel, Steven N. 1993. Forest restoration on a closed landfill: rapid addition of new species by bird dispersal. Conservation Biology. 7(2): 271-278. [22062]
76. Schneider, Rick E.; Faber-Langendoen, Don; Crawford, Rex C.; Weakley, Alan S. 1997. The status of biodiversity in the Great Plains: Great Plains vegetation classification. Supplemental Document 1. In: Ostlie, Wayne R.; Schneider, Rick E.; Aldrich, Janette Marie; Faust, Thomas M.; McKim, Robert L. B.; Chaplin, Stephen J., compilers. The status of biodiversity in the Great Plains, [Online]. Arlington, VA: The Nature Conservancy (Producer). 75 p. Available: http://conserveonline.org/docs/2005/02/greatplains_vegclass_97.pdf [2006, May 16]. On file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. [62020]
77. Seymour, Frank Conkling. 1982. The flora of New England. 2nd ed. Phytologia Memoirs 5. Plainfield, NJ: Harold N. Moldenke and Alma L. Moldenke. 611 p. [7604]
78. Shafi, M. I.; Yarranton, G. A. 1973. Vegetational heterogeneity during a secondary (postfire) succession. Canadian Journal of Botany. 51: 73-90. [15191]
79. Stallard, Harvey. 1929. Secondary succession in the climax forest formations of northern Minnesota. Ecology. 10(4): 476-547. [3808]
80. Stephens, H. A. 1973. Woody plants of the North Central Plains. Lawrence, KS: The University Press of Kansas. 530 p. [3804]
81. Stevens, George C.; Perkins, Anjeanette L. 1992. The branching habits and life history of woody plants. The American Naturalist. 139(2): 267-275. [17983]
82. Stevens, O. A. 1921. Plants of Fargo, North Dakota, with dates of flowering. I. The American Midland Naturalist. 7(2): 54-62. [49786]
83. Stickney, Peter F. 1989. Seral origin of species comprising secondary plant succession in Northern Rocky Mountain forests. FEIS workshop: Postfire regeneration. Unpublished draft on file at: U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Fire Sciences Laboratory, Missoula, MT. 10 p. [20090]
84. Strausbaugh, P. D.; Core, Earl L. 1977. Flora of West Virginia. 2nd ed. Morgantown, WV: Seneca Books, Inc. 1079 p. [23213]
85. Takos, Michael J. 1947. A semi-quantitative study of muskrat food habits. Journal of Wildlife Management. 11(4): 331-339. [67566]
86. Tardif, Jacques; Dery, Sephane; Bergeron, Yves. 1994. Sexual regeneration of black ash (Fraxinus nigra Marsh.) in a boreal floodplain. The American Midland Naturalist. 132(1): 124-135. [23927]
87. Tester, John R.; Marshall, William H. 1962. Minnesota prairie management techniques and their wildlife implications. Proceedings, 27th North American Wildlife Conference. 27: 267-287. [14960]
88. U.S. Department of Agriculture, Natural Resources Conservation Service. 2007. PLANTS Database, [Online]. Available: https://plants.usda.gov /. [34262]
89. Vogl, R. J. 1964. The effects of fire on the vegetational composition of bracken-grassland. Wisconsin Academy of Sciences, Arts and Letters. 53: 67-82. [9142]
90. Vogl, Richard J. 1964. Vegetational history of Crex Meadows, a prairie savanna in northwestern Wisconsin. The American Midland Naturalist. 72(1): 157-175. [61264]
91. Vogl, Richard John. 1961. The effects of fire on some upland vegetation types. Madison, WI: University of Wisconsin. 154 p. Dissertation. [52282]
92. Voss, Edward G. 1985. Michigan flora. Part II. Dicots (Saururaceae--Cornaceae). Bull. 59. Bloomfield Hills, MI: Cranbrook Institute of Science; Ann Arbor, MI: University of Michigan Herbarium. 724 p. [11472]
93. White, Keith L. 1965. Shrub-carrs of southeastern Wisconsin. Ecology. 46(3): 286-304. [8858]
94. White, W. W. 1956. Native willows found in Montana. Proceedings of the Montana Academy of Science. 16: 21-35. [6001]
95. Wilhelm, Gerould S. 1991. Implications of changes in floristic composition of the Morton Arboretum's East Woods. In: Burger, George V.; Ebinger, John E.; Wilhelm, Gerould S., eds. Proceedings of the oak woods management workshop; 1988 October 21-22; Peoria, IL. Charleston, IL: Eastern Illinois University: 31-54. [49325]