Clone 1	Clone 2
Root collection date	9/4	10/7
Mean age (years)	8.5	-
# root segments	30	60
Sucker emergence (days)	32	30
Segments suckering (%)	60	90
Suckers/segment	16.9	13.2
Segments with surface suckers (%)	100	93
Surface suckers/segment	4.9	6.0
Height of tallest sucker (mm)	-	66

SITE CHARACTERISTICS:

Soils: Hansen and others [60] state that black cottonwood mature stands "occur on a wide range of soil textures but are most numerous on medium- to coarse-textured, well-drained soils [60]. Soils are Mollisols in older communities and Fluvents in younger communities [62]. Black cottonwood has high nutrient requirements, particularly for calcium and magnesium and is, for this reason, not common on soils characterized by acid humus [57]. Optimal soil pH for black cottonwood growth is between 5 and 7 [19,57]. On the Blaeberry River, British Columbia, black cottonwood grew on stable coarse-textured islands, never in silty depression areas; these were dominated by sedges (Carex spp.) [46].

Climate: Black cottonwood grows in a wide variety of climates, including coastal areas receiving 140 inches (3,500 mm) of annual precipitation and arid areas receiving 6 to 8 inches (150 to 200 mm), though in the latter climate the species is more restricted to wet habitats [11].  Black cottonwood is not present in the most humid areas of British Columbia (immediately along the coast); it is more common in "marginally subalpine climates" [57].

Elevation ranges of black cottonwood are summarized below:

State	Elevation range	Reference
AK	sea level-2,000 feet (0-600 m)	[35]
CA	0-10,000 feet in the southern Sierra Nevada, to 9,200 feet (2,800 m) in the northern part of the state	[52]
ID	4,600-7,400 feet (1,390-2,100 m)	[58]
MT (western)	4,600-7,400 feet (1,390-2,100 m), < 4,600 feet (1,390 m) east of the Continental Divide	[58,59]
UT	4,500-7,700 feet (1,370-2,350 m)	[159]
WA (Cascade Range)	0-5,000 feet (1,500 m)	[35]
WY (Wind River Range)	dominant between 4,500 and 5,000 feet (1,370-1,520 m)	[105]
BC	0-6,900 feet (2,100 m)	[57]
Baja California	4,600 feet (1,400 m) and 4,340 feet (1,325 m)	[98]

Water table: Harrington [63] states that black cottonwood is very tolerant of short-duration flooding, and Hansen and others [60] have found it to be "very tolerant of frequent and prolonged flooding." In any case, the water level is close to the surface in nearly all areas where black cottonwood is dominant [45], though sometimes black cottonwood colonizes avalanche chutes on steep slopes [56]. Because black cottonwood requires water with dissolved oxygen content, it grows better along moving water than near stagnant water [57].

SUCCESSIONAL STATUS:

Black cottonwood is a pioneer and early seral species [28,41,57,58]. It and other members of the Populus genus are some of the fastest growing temperate trees, an adaptation useful for pioneering disturbed sites [41,57]. Any activity that exposes moist, mineral soil in full sunlight creates a favorable habitat for black cottonwood seedlings, particularly when seed trees are nearby [28,58].  Black cottonwood is very shade intolerant [2,57,70]. The species can tolerate flooding and sediment deposition; when young stems are covered by sediment during floods, sprouts grow from the stem towards the current [2].

In wet climates in forest clearings on non-alluvial sites black cottonwood is a common pioneer, but these stands are generally small and soon overtaken by secondary succession conifers such as Douglas-fir, western hemlock, western redcedar, Sitka spruce, Engelmann spruce, white spruce, or grand fir. Similarly, black cottonwood, in moist climates like that of the Pacific Northwest, colonizes agricultural clearings where mineral soil is exposed and light penetration is high [19].

In southeastern Alaska, stages of community development include pioneer, mid-successional willow-alder, and climax white spruce-western hemlock communities. The pioneer community includes mosses and lichens (Rhacomitrium and Sterocaulon) with mountainavens (Dryas spp.). The mid-successional willow-alder stage includes Sitka alder, black cottonwood, Sitka willow (Salix sitchensis), and Alaska bog willow (S. fuscescens). Following the willow-alder stage, in the absence of disturbance, white spruce and western hemlock eventually become dominant [6]. In a generalized model of forest succession following fire in Alaska, Viereck [153] stated that balsam poplar and black cottonwood were not "climax" species but were invaders, along with quaking aspen, after fires on relatively warm sites. Stands 70 to 80 years after fire consist "of white spruce and hardwood trees without a closed overstory" [155].

On the Hoh River of the Olympic Mountains in Washington, 5 successional stages are defined with respect to site age. Red alder and Scouler's willow are dominant on the newest gravel bars, red alder alone dominates 80- to 100-year-old floodplains, an Engelmann spruce-bigleaf maple-black cottonwood association is present on "1^st terraces" that are approximately 400 years old, Sitka spruce and western hemlock dominate on "2^nd terraces" approximately 750 years old, and western hemlock is the dominant on "3^rd terraces" [44].

On the McKenzie River of Oregon, 3 early distinct types of early successional vegetation were identified: black cottonwood communities, red alder communities, and willow communities. In the absence of fire or flooding the black cottonwood stage is succeeded by a western hemlock/western sword fern/ Oregon oxalis climax; in the presence of fire, the black cottonwood stage is followed by a Douglas-fir/ beaked hazelnut (Corylus cornuta)/ western sword fern community [64].

In black cottonwood habitat types of southern and eastern Idaho, black cottonwood establishes on recent gravel bars and remains dominant if there is continual flooding and sediment deposition. Without this disturbance the community is gradually replaced by Douglas-fir, Engelmann spruce, subalpine fir, or Rocky Mountain juniper, but occasionally yellow willow or Geyer's willow (Salix geyeriana) will become dominant [58]. Similar succession has been described in California, where it was found that flood control leads to conifer types in California [70].

In the Swan Valley of western Montana, on wet portions of grand fir habitat types, paper birch (Betula papyrifera) and black cottonwood pioneer clearcuts or seed-tree cuts where mineral soil has been exposed; after these partially develop, Douglas-fir, grand fir, western white pine (Pinus monticola), and spruce establish [7]. In Glacier National Park, as well as Oregon, Washington, and British Columbia, black cottonwood is a common, though often short-lived component of early to mid-seral successional stages in western redcedar-western hemlock forests [55]

SEASONAL DEVELOPMENT:

In poplars, shoot elongation continues after bud burst. Leaves mature and expand throughout the growing season which is defined, at least partially, by photoperiod [117].  All temperate members of the genus flower in the spring before leaf growth initiation [41,57], as is typical of plants that depend on wind dispersal [57]. Three or four weeks before flowers develop in the spring, the following year's inflorescences develop at leaf nodes [16].

Seasonal development of black cottonwood has been studied in detail in British Columbia, Yellowstone National Park, and northern Idaho. Black cottonwood flowers in late March or early April until late May in coastal British Columbia, though in the interior flowering may not occur until the middle of June. Leaves appear after flowers. Fruit ripens about a month after flowers appear; seed dispersal occurs between late April and July [57].

Phenology of black cottonwood was studied in Yellowstone National Park (east of continental divide) [125]: 

	Bud burst	Leaves fully grown	Flowers start	Flowers end	Fruit ripe	Leaves start to color or wither	Leaves begin to fall	Leaves fallen and withered
Average date	May 14	June 20	May 11	June 5	July 2	September 10	September 23	October 14
Earliest date	April 28	June 10	April 25	April 10	May 17	July 23	August 10	October 5
Latest date	May 29	July 12	June 4	July 12	July 19	September 19	October 3	October 22
Standard error	3	2	3	10	7	4	4	2
Sample size	14	14	10	10	10	14	14	14

Northern Idaho phenology is described below (west of continental divide) [125]:

	Bud burst	Leaves fully grown	Flowers start	Flowers end	Fruit ripe	Seed fall begins	Leaves start to color or wither	Leaves begin to fall	Leaves fallen and withered
Average date	April 30	May 30	May 19	June 11	August 14	September 19	September 15	September 28	October 14
Earliest date	April 20	May 1	April 1	May 15	June 25	August 18	August 20	September 15	October 2
Latest date	May 14	July 13	June 16	July 2	September 5	October 21	October 25	October 10	October 26
Standard error (days)	2	5	4	3	5	13	5	2	2
Sample size	13	13	15	15	14	5	15	16	14

FIRE ECOLOGY

SPECIES: Populus balsamifera ssp. trichocarpa | Black Cottonwood

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• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

FIRE ECOLOGY OR ADAPTATIONS:

Fire Adaptations: Black cottonwood is frequently damaged by fire with low-severity burns even often causing "considerable injury" [3,57]. Young black cottonwood trees and seedlings are usually killed by fire regardless of severity [24]. Severe fire kills or top-kills even older trees. In low- and moderate-severity fires older trees with thick bark may not be top-killed [58,97]. In members of the Populus genus stem bark remains thin for longer than in other trees [41]. Though old trees have increased fire resistance due to thicker, furrowed bark, they have higher fuel loading and more heartrot, which can increase fire severity [49]. Trunks that are not top-killed may be more susceptible to Cytospora spp., and other fungal pathogens; this was observed near D'Arcy in southern British Columbia [34].

Black cottonwood sprouts from stumps, charred boles, root crowns, or lateral roots following fire [24,28,58,97,109] and because of this has been referred to as a fire "endurer" rather than "resister" [3]. Fire-induced sprouting is more common in the Tacamahaca section than in the Aigeiros section. Rates of sprouting are highest if fire occurs when cottonwoods are dormant [49]; this time also has the highest probability of fire, with late summer and fall, or late winter (in low snow years) most common [58]. In general older trees sprout less than young trees [49], and sprout survival is highest when the water table is close to the surface [58]. In 1986, Coates and Haeussler [28] stated that there was little information regarding black cottonwood sprout vigor after fire, or impact of fire severity on sprouting potential. Though this is still at least partially true, a study of clonal reproduction after fire in Alberta by Gom and Rood [49] provides much useful data that is summarized in the "Fire Effects" section of this species summary.

Black cottonwood is not only a fire "endurer" but also a fire "invader." Fire can improve seedling establishment by increasing light penetration and exposing mineral soil to allow seedling establishment if moisture is available [3,24]. Increases in light penetration following fire also aid establishment [24]. Black cottonwood seedlings have been observed 1 year after stand-replacing fire in upland ponderosa pine/ Rocky Mountain Douglas-fir habitat on the Bitterroot National Forest, Montana. The seedlings grew in cavities in mineral soil after the root systems of large trees had burned [131]. A study of seedling establishment (artificially seeded) of balsam poplar after experimental fires in a black spruce habitat demonstrated the dependency of the species on mineral soil exposure for germination and survival. Fire was prescribed on 1 m² plots. On moderately burned plots 17 black cottonwood germinated, 1 survived 1 year, and 0 survived 3 years. On heavily burned plots 71 germinated, 39 survived 1 year, and 39 survived 3 years [158].

Fire is infrequent on recently formed gravel bars, but when it does occur, damage to cottonwoods is greatest because their root systems have not developed [58].

Fire Regimes: Historic fire regimes have not been explicitly studied in black cottonwood communities [10]. It has been speculated that because of high moisture content and rapid decomposition of litter in riparian forests, frequency of fire is less than it is in adjacent areas [132]. These factors could protect forested riparian sites when severe fires occur on adjacent uplands. Conversely, wind-driven fires beginning in adjacent upland communities may spread to riparian forests, particularly when fuel accumulation on upland sites has increased as a result of fire suppression [132]. Arno [10] states that black cottonwood forests along major rivers of the Pacific Northwest likely burned frequently, as they were historically surrounded by communities characterized by high fire frequency, such as ponderosa pine savannas or sagebrush steppes. Fires could have easily spread from adjacent communities; prior to widespread livestock grazing and irrigation, dry grassy fuels were more continuous than they are currently [10]. When fires do occur, they are most severe in older cottonwood stands where fuel accumulation is highest [49,132].

Fire regimes for plant communities and ecosystems in which black cottonwood is dominant are summarized below. For further information regarding fire regimes and fire ecology of communities and ecosystems where black cottonwood is found, see the "Fire Ecology and Adaptations" section of the FEIS species summary for the plant community or ecosystem dominants listed below.

Community or Ecosystem	Dominant Species	Fire Return Interval Range (years)
grand fir	Abies grandis	35-200 [9]
sagebrush steppe	Artemisia tridentata/Pseudoroegneria spicata	20-70
Rocky Mountain juniper	Juniperus scopulorum	< 35 [104]
western larch	Larix occidentalis	25-100
Engelmann spruce-subalpine fir	Picea engelmannii-Abies lasiocarpa	35 to > 200
blue spruce*	P. pungens	35-200 [9]
Rocky Mountain lodgepole pine*	Pinus contorta var. latifolia	25-300+ [8,9,118]
western white pine*	P. monticola	50-200
interior ponderosa pine*	P. ponderosa var. scopulorum	2-10 [9]
eastern cottonwood	Populus deltoides	< 35 to 200 [104]
quaking aspen (west of the Great Plains)	P. tremuloides	7-120 [9,53,96]
Rocky Mountain Douglas-fir*	Pseudotsuga menziesii var. glauca	25-100 [9]
coastal Douglas-fir*	P. menziesii var. menziesii	40-240 [9,100,116]
western redcedar-western hemlock	Thuja plicata-Tsuga heterophylla	> 200 [9]
western hemlock-Sitka spruce	Tsuga heterophylla-Picea sitchensis	> 200 [9,107]
elm-ash-cottonwood	Ulmus-Fraxinus-Populus spp.	< 35 to 200 [39,154]

*fire return interval varies widely; trends in variation are noted in the species summary

POSTFIRE REGENERATION STRATEGY [140]:

Tree with adventitious bud/root crown/soboliferous species root sucker
Geophyte, growing points deep in soil
Initial off-site colonizer (off-site, initial community)
Secondary colonizer (on-site or off-site seed sources)

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FIRE EFFECTS

SPECIES: Populus balsamifera ssp. trichocarpa | Black Cottonwood

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• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

IMMEDIATE FIRE EFFECT ON PLANT:

Black cottonwood sprouts from the lateral roots, root crown, and bole after top-kill by fire [49,58]. It also establishes from seed [3,24,131,158]. Gom and Rood [49] studied the effects of severe fires along the Oldman River near Lethbridge, Alberta, which occurred in early and mid-April before cottonwoods' spring bud burst. The fires burned during the afternoon; strong Chinook winds increased fire severity. Black cottonwood, narrowleaf cottonwood, plains cottonwood, and hybrids among them dominated these riparian areas. All cottonwoods observed were girdled by the fires. Fire damage to trunks varied from "slight bark charring to complete trunk incineration." This caused 100% top-kill in trees even though not all canopies were severely damaged, demonstrating the susceptibility of these cottonwood species to severe fires. The exact amount of mortality was not determined (because it is difficult to tell from which tree's lateral root system suckers originate), but mortality was definitely less than 25%, as 75% of trunks (including all species) produced coppice sprouts. The most severe fires were in older stands, likely the result of higher fuel accumulation, and there were many pits in the ground on these sites where root systems were burned. Young stands close to the river (without as much fuel loading) experienced less canopy damage but were top-killed via girdling.

Hardwood saplings (cottonwood, alder, willow) less than 2 inches (5 cm) in diameter experienced 100% mortality in 168 and 42 BTU/second/foot intensity fires in a white spruce, quaking aspen, and balsam poplar (likely including some black cottonwood and hybrids) stand northeast of Edmonton, Alberta [80]. 

Hall and Hansen [58] stated that in low and moderate severity fires older black cottonwood trees with thick bark may not be top-killed, but little quantitative data examining the relations between fire severity, tree age, and top-kill or mortality is available.

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:

No entry

PLANT RESPONSE TO FIRE:

One year after the fires along the Oldman River in Alberta (described above), approximately 75% of the top-killed cottonwoods (including all narrowleaf and black cottonwoods, and balsam poplar) produced shoots from remnants of trunks. Sprouting was not observed on control  transects (on adjacent unburned areas). Of the trunks producing sprouts, 90% were from either narrowleaf or black cottonwood (the 2 species' sprouts were not easily differentiated). The average number of coppice sprouts was 17 per trunk with a range of 1 to 60. Height of the tallest sprout on each tree averaged 16 inches (41 cm) and ranged from 3.5 to 30 inches (9-77 cm). The authors speculated that the high rate of sprouting was in part due to the occurrence of the fires while the cottonwoods were dormant. The degree of canopy or trunk damage did not have much impact on coppice sprouting frequency or vigor. Sprouts were even observed from trunks that had burned to 10 inches (25 cm) below ground. Of trees that experienced "heavy" canopy damage, 60% produced shoots from stumps; 80% of those with "light" damage produced coppice sprouts. The average height of coppice sprouts was not significantly (p>0.05) affected by the degree of trunk damage, and trunk diameter had no significant (p>0.05) effect on either the number of coppice sprouts or the height of sprouts. Over 1,000 root suckers were observed on the study transects, 80% of which were either narrowleaf or black cottonwood. The density of root suckers (including all species) was about 1/ 3 m²). Suckers were more common near trees that had many coppice sprouts. After 1 growing season the average height of root suckers was approximately 3 feet (1 m) [49].

Transects on the Oldman River were surveyed again 5 years after fire. At this time there was an average of 4 coppice sprouts per trunk on the 30% of the cottonwoods that still had living sprouts. Average sprout height was approximately 10 feet (3 m) with a range of 5 to 16 feet (1.5 to 5 m). Of the 34 trunks with surviving coppice sprouts 33 were either narrowleaf or black cottonwood and 1 was plains cottonwood. Survival of root suckers over the 5 years was about 50%. A density of 1 sprout per  7 m² was observed as 550 sprouts still survived on the transects. The average height of root suckers increased from about 3 feet (1 m) to 8 feet (2.4 m). Seven root suckers were present on unburned control sites; the authors stated that these might have been induced by flooding damage 2 years prior to the survey [49].

Gom and Rood [49] summarized the response of several cottonwood species to fire, showing the high sprouting/suckering ability of balsam poplar and black and narrowleaf cottonwoods relative to Fremont and eastern cottonwoods:

Year of fire	Location	Age when studied (years)	Cottonwoods affected by fire	Success of asexual regeneration
about 1988	Peace River near Fort St. John, BC	~10	Balsam poplar	Very good (vigorous sucker sprouts)
about 1992	Red Deer River at Dinosaur Provincial Park, Brooks, AB	~5	Eastern cottonwood	Very poor
1990	Belly River at Lavern, AB	7	Narrowleaf cottonwood, black cottonwood	Very good
spring 1992	Oldman River at Lethbridge, AB	5	Narrowleaf cottonwood, balsam poplar, black cottonwood, eastern cottonwood	Good (sucker and coppice sprouts)
about 1986	South Saskatchewan River, Police Point Park, Medicine hat, AB	~10	Eastern cottonwood	Very poor
summer 1973	Milk River near Manyberries, AB	24	Eastern cottonwood	Very poor
about 1995	Lower Truckee River, near Nixon, NV	~2	Fremont cottonwood	Poor
about 1994	Kane Springs Creek, south of Moab, UT	~3	Fremont cottonwood	Poor (sparse coppice sprouts)

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:

No entry

FIRE MANAGEMENT CONSIDERATIONS:

To increase the longevity of cottonwood stands managers may choose to prescribe fire when the stand is in the "pole" stage, as sprouting is high at this point [58]. Dormant season, particularly fall, fires induce the most sprouting [49]. Gom and Rood [49] suggest that fire- induced sprouting, particularly in old stands, may be used to regenerate stands much the way flooding disturbance allows development of new stands. They also state that regeneration may be more rapid after fire than after flooding because the growth rate of sprouts is about 3 times greater than that of seedlings.

If the goal of prescribed fire is to maintain a cottonwood stand, 5 years of grazing exclusion after fire are recommended [58]. Herbivory and fire interactions have been well documented for quaking aspen stands. In the Rocky Mountains (study sites ranged from Jasper National Park, Canada to Rocky Mountain National Park, Colorado), White and others [157] found that prolific quaking aspen sprouting after fire could not overcome the effects of elk grazing, and, with a short fire return interval, quaking aspen stands would decline.

Fire can be used to maintain black cottonwood stands, but it may cause negative impacts such as erosion and sedimentation. These impacts both on the site of fire and on sites downstream can be increased by heavy postfire rainfall [132].

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MANAGEMENT CONSIDERATIONS, VALUE AND USE

SPECIES: Populus balsamifera ssp. trichocarpa | Black Cottonwood

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• WOOD PRODUCTS VALUE
• IMPORTANCE TO LIVESTOCK AND WILDLIFE
• PALATABILITY
• NUTRITIONAL VALUE
• COVER VALUE
• VALUE FOR REHABILITATION OF DISTURBED SITES
• OTHER MANAGEMENT CONSIDERATIONS

WOOD PRODUCTS VALUE:

The wood of cottonwood is light-colored, lightweight and uniform in texture, but it is easily decayed and not very strong [35,151]. Wood properties of black cottonwood were described by Overholser [106]: weight per cubic foot is 24 pounds (when moisture is 12%), specific gravity when oven dry is 0.37, volumetric shrinkage is 12.4%, radial shrinkage is 3.6%, and tangential shrinkage is 8.6%.

Uses of black cottonwood include high-grade book and magazine paper [11,35], biomass production for alternative energy [65,66], pallets, boxes, and crates [35], veneer and panelling [106,151], and fiberboard and flakeboard for concealed parts of furniture [35]. Because of their rapid growth rates and propensity for sprouting from stumps after harvest, black cottonwood and its hybrids have been studied for use in intensive culture [65,66,115]. Dickmann and Stuart [36] provide much information about hybridization and genetics, planting considerations, and plantation management of cottonwoods.

IMPORTANCE TO LIVESTOCK AND WILDLIFE:

Streamside black cottonwoods contribute to favorable fish habitat by providing streambank stability and reduced siltation, maintaining low water temperatures through shading, increasing debris recruitment for variable stream habitats, and providing nutrient-rich litter for aquatic food webs [57,61,62]. Black cottonwood is an important source of cover and forage (to a lesser extent) for wildlife and livestock. Use of black cottonwood by bird species for nesting and perching is summarized below:

Bird species/genus	Use of black cottonwood	Other comments	Reference
Bald eagle, osprey, great blue heron, Canada geese	common nesting and/or perching site	Bald eagles have been observed spending 44% of perching time in black cottonwood	[57,58]
Woodpeckers, great-horned owls, wood ducks	cavity nesting		[11,58]
Owls, hummingbirds, starlings, sapsuckers, flickers, veeries, orioles, grosbeaks, and vireos	nesting		[57]
Common merganser, red-naped sapsucker, hairy woodpecker, three-toed woodpecker, northern flicker, tree swallow, red-breasted nuthatch, and mountain bluebird	cavity nesting	These species were observed using black cottonwood in Glacier National Park; black cottonwood had the 2nd highest selectivity index (% used/% available), behind quaking aspen	[26]
Pileated woodpecker	roosting, cavity nesting	All nests were in snags with broken tops	[93,95]
Red-tailed hawk	nesting	Black cottonwood and red alder (to a lesser extent) were preferred over western hemlock and Douglas-fir	[134]
Holarctic harlequin ducks	cavity nesting	This was observed on the Elwha River, Idaho; it is probably rare	[25]
Marbled murrelet	nesting	Black cottonwood is not widely used; Douglas-firs or coastal redwoods are usually selected	[14]
Ruffed grouse	foraging	Ruffed grouse use the buds as winter forage	[57]

Black cottonwood is most important as a source of cover rather than forage for big game species. Elk and deer use is high in black cottonwood stands in Montana, particularly when a shrub canopy layer is well developed [58]. White-tailed deer in the Umatilla River drainage of northeastern Oregon used mature forest structural types during winter. These sites provide hiding cover which is needed in areas that are otherwise rather open. The deer used ponderosa pine, Douglas-fir, Englemann spruce, or grand fir stands during the summer [14]. A study of habitat partitioning among elk, white-tailed deer, and moose near Glacier National Park, Montana found a significant (p<0.1) preference of white-tailed deer for later-successional spruce-black cottonwood communities over early or mid- successional communities. Elk and moose had significantly preferred mid-successional black-cottonwood communities. None of the 3 ungulates was observed to prefer early successional communities [75]. Black cottonwood is used by beavers for forage and dam building [60].

In the Kenai Peninsula of Alaska, many plants are summer moose forage; winter forage consisted of willows (38% of diet in winter), Kenai birch (45%), alder (9%), black cottonwood and quaking aspen (5%) and huckleberries (Vaccinium) (2%) [155]. In British Columbia, Roosevelt elk and white-tailed deer forage on buds during winter [57]. A study of black cottonwood use by Roosevelt elk in Olympic National Park found it to be 9% of diet volume in both winter and autumn [127]. Rocky Mountain mule deer generally do not consume black cottonwood (<1% of diet) [85].

Rabbits and hares also consume black cottonwood inner bark and sometimes girdle stem bases. Black cottonwood is the most palatable species for beavers and is also a frequent source of building materials [57]. Flying squirrels nest in trunk cavities [11,58].

Livestock: Because of proximity to water, livestock use of black cottonwood sites is often heavy, even though the species is of poor to fair palatability. Grazing of black cottonwood community types in Idaho and Montana can lead to the decline of shrubs and maintenance of an open stand with Kentucky bluegrass, timothy, smooth brome (Bromus inermis), and "weedy" forbs in the understory. Ranchers sometimes use these sites as winter ranges for livestock (early season rest is recommended for continued grass productivity) [58].

PALATABILITY:

Black cottonwood's palatability to livestock and wildlife generally fair or poor; ratings for each state are summarized below [37]:

	Montana	North Dakota	Wyoming
Cattle	fair	poor	----
Domestic sheep	fair	fair	----
Horse	fair	poor	----
Elk	good	----	fair
Mule deer	poor	fair	fair
White-tailed deer	poor	fair	fair
Pronghorn	poor	poor	poor
Upland game birds	poor	poor	poor
Waterfowl	poor	poor	poor
Nongame birds	----	----	fair
Small mammals	----	----	good

NUTRITIONAL VALUE:

The nutritional quality of black cottonwood on the Kenai Peninsula was analyzed by Weixelman and others [155]. Mean (and standard error) dry matter percentages of acid detergent fiber (ADF), neutral detergent fiber (NDF), cellulose, and hemicellulose are presented below (n=8):

Age	ADF	NDF	Cellulose	Hemicellulose
7-10 years	38.6 (2.3)	43.9 (2.3)	16.6 (2.8)	24.2 (2.0)
20-30 years	34.9 (1.9)	43.3 (3.1)	14.2 (0.7)	18.8 (0.9)
70-80 years	32.9 (2.7)	39.9 (2.8)	15.0 (1.0)	21.5 (0.5)

McCracken and others [87] observed the nutritional composition of moose diets on the Copper River Delta, Alaska. Here, black cottonwood leaves and twigs were found to be 6.3 % (Standard error= 1.7%) digestible protein, 54.1% (S.E.= 0.5%) digestible dry matter, and 5.1% (S.E.= 1.5%) ash. Cowan and others [29] analyzed the nutritional content of black cottonwood collected in winter in British Columbia: protein was 12.7% dry weight, ether extract was 13.2%, crude fiber was 24.2%, and nitrogen free extract was 43.1%. The energy and protein value of black cottonwood have been rated as "fair" [37].

COVER VALUE:

Black cottonwood's cover value is generally fair to good depending on location [37]:

	Montana	North Dakota	Wyoming
Elk	fair	----	good
Mule deer	----	fair	fair
White-tailed deer	good	fair	good
Pronghorn	poor	poor	poor
Upland game birds	----	----	good
Waterfowl	----	----	poor
Small mammals	----	----	good
Nongame birds	----	----	good

VALUE FOR REHABILITATION OF DISTURBED SITES:

Black cottonwood provides thermal cover, debris recruitment, and bank stability for streams [58,60]. Hansen and others [60] suggest that managers  "give serious consideration to maintaining a buffer strip of the black cottonwood dominance type immediately adjacent to rivers and streams.". The longevity of cottonwood riparian forests makes decline in populations gradual; changes in management may take decades to restore mature cottonwood stands [122]. Protection of a Fremont cottonwood-velvet ash (Fraxinus velutina)-Goodding willow (Salix gooddingii) association in Prescott National Forest, Arizona showed little recovery after 4 years [145].

Plummer [112] evaluated the potential of intermountain plant species for rehabilitation efforts. Black cottonwood was classified as "good" for establishment via transplanting, growth rate, soil stabilization, and adaptation to disturbance, "fair" for natural spread by seed or vegetative reproduction, and "poor" for seed handling and artificial seedling establishment.

Ground scarification is critical for planting to establish a microsite favorable to light requirements. Where a living black cottonwood root system is already present, scarification may also be used to induce sprouting from roots [19].

The following guidelines have been provided for collecting and planting black cottonwood cuttings [58,59]:

• avoid saline or alkaline sites and clay soils
• make cuttings from young trees in open stands
• remove the side branches of the cutting (leave the tip and the two highest side branches)
• cut from trees when they are dormant
• soak the cuttings for 10 to 14 days
• dig the planting holes to the lowest expected groundwater level
• plant the cuttings the day they are removed from soaking
• select cuttings long enough so that they are 3 to 6 feet (1-2 m) high when planted
• back fill the holes and avoid air pockets
• exclude livestock and big game for 2 or 3 growing seasons

McCamant and Black [92] tested black cottonwood clones from different regions for differential cold hardiness. They found considerable variation between sites, and suggested that source of material for hybrid programs or rehabilitation is an important consideration [92]. Variation between populations and even within populations has been observed with respect to flooding tolerance as well. Plants may be monitored for flooding tolerance to select appropriate cutting sources [130]. Similarly, race differentiation in photoperiodicity has also been observed [117]. Regardless of how the selection is chosen, a variety of genotypes is recommended to increase likelihood of population survival [19].

The University of Washington continues experimentation with genetic engineering of members of the Populus genus to produce hybrids that can hyperaccumulate or detoxify industrial pollutants. These species are well suited to environmental rehabilitation of this sort because they have a high transpiration rate and large, spreading root systems. A black cottonwood × plains cottonwood hybrid has been shown to absorb and metabolize trichloroethylene from soil. More trials are needed to test the effectiveness and environmental impacts of such hybrids [38]. 

OTHER MANAGEMENT CONSIDERATIONS:

The exact amount of decline in riparian cottonwood stands is unknown. In the Southwest, an estimated 70 to 95% of riparian vegetation has been lost or disturbed; in the Sacramento Valley of California the area of riparian forests has been reduced by 98.5% since 1850; in Idaho, Wyoming, Montana, and Canada, less has been lost. Causes of decline of riparian cottonwood stands in western North America were outlined by Braatne and others [19] as follows:

Factor	Impacts
1. Livestock grazing	consumption and trampling of seedlings - death of older trees causes gradual decline
2. Water diversion	drought stress
3. Domestic settlement	clearing area for home development
4. Exotic plants	saltcedar (Tamarix ssp.) and Russian-olive (Elaeagnus angustifolia), among others, can reduce recruitment
5. Stream reservoirs	many stands were lost when dams were built in the West
6. Channelization	this reduces meandering, a process critical to cottonwood establishment
7. Agricultural clearing	this was greatest prior to 1950, there has likely been little net change since then
8. Gravel mining	excavated areas and associated infrastructure require forest clearing, but cottonwoods often regenerate asexually
9. Direct harvesting	during early European-American settlement the wood was used more than it is currently
10. Beavers	populations of beavers are unnaturally high in many areas due to a lack of predators

Rood and Mahoney [122] examined decline of cottonwood forests in southern Alberta. They listed many of the above-mentioned activities. Causes of decline were, from largest to smallest impact, 1) livestock grazing, 2) agricultural clearing, 3) water diversion, 4) domestic settlement, 5) stream reservoirs, 6) increase in beaver population, 7) gravel mining, 8) direct harvesting, 9) channelization, and 10) herbicide spraying.

Major causes of decline of black cottonwood stands in eastern Oregon include: conversion of stands for pasture, farmland, or urbanization, conversion of streams from multiple to single channel systems, restriction of lateral movement of streams across floodplains, and control of flooding with dams. Overbrowsing by livestock, elk, and deer, reduced fire frequency, and logging for firewood, lumber, and pulp have also had impacts [31].

Livestock management: Though they require flooding disturbance for establishment, black cottonwood seedlings are generally not tolerant of much grazing disturbance. In southern Alberta, stress from livestock use, including foraging and trampling, is "the greatest overall impact on riparian cottonwood" stands according to Rood and others [121]. In the Wallowa Mountains of eastern Oregon, an exclosure study showed that livestock use slowed the development of black cottonwood stands and delayed succession from shrub-dominated to black cottonwood dominated communities [78]. Gravel bar vegetation surveys in the Catherine Creek watershed of eastern Oregon also highlighted the sensitivity of black cottonwood to livestock pressure. In exclosure plots mean height increased from 6 to 40 inches (15 to 101 cm) in 10 years; on plots with grazing (1.3- 1.8 ha/AUM), mean height increase was only from 5 to 10 inches (12-26 cm) in the same time period. During this study shrub (mountain alder, Bebb willow, and Missouri River willow (Salix eriocephala)) density increased significantly (p<0.01) on sites excluded from grazing, but the density of black cottonwood was not significantly different between grazed and ungrazed sites [51]. Management has sought to control livestock foraging and trampling effects on black cottonwood via short-rotation grazing and exclosure; this allows young trees approximately 5 years to establish [19]. 

Prolonged heavy grazing of black cottonwood community types can cause decline in shrub canopy cover and thus big game use of the site. Shrub cover can sometimes be restored with a "dramatic change" in management including elimination of grazing. The success of this depends most heavily on the level of the water table. Whether or not the goal is to restore shrub cover, early season rest from grazing is beneficial for forage production alone [58].

Timber management: Though black cottonwood has beneficial effects on bank stability, black cottonwood can have detrimental competitive effects on more economically valuable trees. This effect is greatest during the first 5 years after clearing; self-thinning at this point reduces black cottonwood abundance but many persist throughout rotation times [57]. Logging mature cottonwood produces abundant rapidly growing stump sprouts. Sprouts grow whenever the tree is cut, but cutting in November through February produces the most.  Even repeated cutting (up to 5 times in 10 years) does not harm the ability to sprout from stumps. In addition twigs or roots that are covered with soil during logging often take root.  When mature trees are still present, seeding is rapid. Because of these characteristics manual treatment is not effective in reducing density or cover of black cottonwood [28].

To increase habitat for nesting birds, habitat management plans are best developed for each individual species, but some general considerations include: within a 1,000 acre (400 ha) area about 50 to 100 acres (20 to 40 ha) with an old-growth structure of ponderosa pine, western larch, or black cottonwood should meet most nesting needs. This 50- to 100-acre area is most effective if scattered throughout the larger area rather than isolated, and in the remaining 900 acres (360 ha) snags are retained. Also suggested for bird habitat maintenance is that the collection of ponderosa pine, western larch, or black cottonwood for firewood be discouraged, particularly collection of snags greater than 15 inches (38 cm) in diameter [94].

Watercourse damming and water diversion: Often listed as a major detriment to cottonwoods, damming and water diversion have a multi-faceted impact on establishment, growth, and mortality. These factors were summarized by Rood and Mahoney [122] as follows: 

Possible cause of decline		Effects
Hydrologic changes	A. Reduced water availability	Diversion of water offstream or well pumping creates a water deficit, resulting in drought stress, slow growth, and increased mortality
	B. Reduced flooding	Spring flooding is essential to create moist seedbeds for seedling establishment
	C. Stabilized flows	Dynamic flows are essential for seedling establishment
Geomorphic changes	A. Reduced meandering and channelization	With reduced flooding, channel migration is reduced and suitable seedbeds are reduced

Impacts of watercourse damming on cottonwoods are generally negative, but also dependent on the species present, as reported by Rood and Mahoney [122]:

River	State/province	Species	Effects	Reference
Missouri	ND	plains cottonwood	Reduced tree growth and less seedling establishment	[76]
several	AZ	Fremont cottonwood, narrowleaf cottonwood	Reduced forest abundance	[23]
Colorado	CA	Fremont cottonwood	Reduced forest abundance, absence of seedlings	[102]
South Platte	CO	plains cottonwood	Reduced forest abundance	[30]
Missouri	MT	plains cottonwood	Reduced forest abundance, absence of seedlings	[103]
Owens	CA	Fremont cottonwood	Reduced forest abundance	[22]
Rush Creek	CA	balsam poplar	Reduced tree abundance	[141]
Sacramento	CA	Fremont cottonwood	Fewer seedlings	[142]
Salt	AZ	Fremont cottonwood	Conditions unsuitable for seedling establishment	[43]
Milk	MT, AB	plains cottonwood	Reduced forest abundance, fewer saplings	[20]
Bighorn	WY	plains cottonwood	Reduced forest abundance	[5]
St. Mary, Waterton, Belly	AB	plains cottonwood, balsam poplar, narrowleaf cottonwood	Reduced forest abundance	[119]
Rio Grande	NM	Fremont cottonwood	Absence of seedlings	[72]
Bishop Creek	CA	Fremont cottonwood, balsam poplar	Smaller leaves, reduced transpiration and water potential	[150]
Arkansas	CO	plains cottonwood	Reduced forest abundance	[133]

The effects of altered flood magnitude, frequency, and duration on the establishment and success of black cottonwood are thought to be less severe than those effects on other cottonwoods [83]. This is because black cottonwood is adapted to flow regimes of smaller streams with more variable flood magnitude and duration than large rivers. Stromberg and Patten [143] noted "subtle" effects of diversion on black cottonwood in the eastern Sierra Nevada, California. On Bishop Creek, where water diversion is nearly complete in normal and dry winters, black cottonwood had higher mortality (and thus lower age and size), lower density, and lower canopy foliage density than on Pine Creek, a similar size watershed with no diversion. Means (standard errors in parentheses) from the two watersheds are presented below [143]: 

Site	Elevation (m)	Canopy foliage density1	Tree mortality2 (%)	Tree density (#/ 0.1 ha)	Juvenile density3 (#/ 0.1 ha)
Bishop 2	2,020	1.73 (0.16)	27	28	98
Bishop 3	1,800	0.51 (0.12)	14	33	93
Bishop 4	1,470	0.35 (0.12)	33	6	42
Pine 2	2,100	2.52 (0.25)	9	41	199
Pine 3	1,850	2.14 (0.19)	0	45	170
Pine 4	1,580	3.13 (0.37)	11	38	125

¹ Leaf area index (m²/ m²)
² Dead trees as a percentage of the total
³ Juveniles are plants < 10 cm in diameter

Chemical treatments: Black cottonwood is "extremely sensitive" to glyphosate. In southern Alberta this herbicide and picloram are believed to have caused unintended mortality in even large trees [122]. The 2,4-D ester can damage the canopy but does not cause much mortality [28]. High black cottonwood mortality was observed following summer and fall applications of glyphosate. A mid-May spot treatment with hexazinone produced 80% to 95% defoliation and little sprouting in the Nelson Forest region of British Columbia. Broadcast treatments with 2,4-D amine have been observed to cause moderate or severe damage [57]. Balsam poplar in Alberta were susceptible to metsulfuron and 2,4-D, alone or in combination [18].

Disease and other management considerations: Black cottonwood is susceptible to Cytospora canker, particularly in fire-damaged stands or in young trees or cuttings in nurseries. Wood-decaying fungi also grow on black cottonwood; the most significant parasites of this sort include Polyporus delectans and Philota destruens [117]. Young seedlings are particularly susceptible to pathogenic fungal infection [113]. Trees, particularly young saplings, may be injured or killed by late frost either directly or indirectly via increased susceptibility to fungal decay. High winds sometimes cause damage on sites where adjacent vegetation is much shorter [35], and scouring by ice during spring runoff sometimes kills or induces sprouting from young trees [19].

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Populus balsamifera ssp. trichocarpa: References

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Populus balsamifera ssp. trichocarpa Index

Related categories for SPECIES: Populus balsamifera ssp. trichocarpa | Black Cottonwood

• Introductory
• Distribution and occurrence
• Botanical and ecological characteristics
• Fire ecology
• Fire effects
• Management considerations, value and use
• References
• All pages together [for printing]