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Wildlife, Animals, and Plants |
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BOTANICAL AND ECOLOGICAL CHARACTERISTICS
GENERAL BOTANICAL CHARACTERISTICS:Black cottonwood typically has a straight, branch-free trunk for more than half its length and a broad, open crown [11]. In closed stands, the crown tends to be narrow, with few branches growing into the lower 2/3rds of the trunk [117]. Black cottonwood develops extensive forests along major rivers, narrow forests along smaller streams, and sometimes irregular populations on gravel bars, floodplains, and terraces [58]. Establishment by seed is episodic, often creating stands of several well-defined age groups [89]. Black cottonwood is a fast growing, native deciduous tree, growing to 100 feet (30 m) high [36,159] though occasionally as high as 160 feet (50 m) [159]. Basal diameter is commonly 3.5 to 5 feet (1-1.5 m) [36,159]. Black cottonwood commonly lives 100 to 200 (or occasionally more) years [19]. Flowers grow in catkins. Staminate catkins are usually about 1 inch (2-3 cm) long, though occasionally up to 2 inches (5 cm) long. Pistillate catkins are 3 to 8 inches (8-20 cm) long [68]. Capsules are subsessile and 0.2 to 0.32 inch (5-8 mm) long; each capsule contains 3 (occasionally 2 or 4) carpels [156]. Seeds bear long, white, "cotton" fibers that aid in dispersal via wind and/or water [57]. Each cottonwood seed weighs approximately 0.3 to 0.6 mg [19]. Much genetic variation occurs in black cottonwood with respect to flooding tolerance [130] and cold tolerance [92]; responses to these extremes are influenced by the site from which experimental cuttings are gathered [92,130]. Roots: Black cottonwood develops a "shallow, spreading root system" [11]. In the Tacamahaca section rooting depth is commonly 10 to 16 feet (3-5 m), occasionally deeper [19]. Roots of the Populus genus have well-developed laterals. Eighty percent of the roots of a black cottonwood × plains cottonwood hybrid were horizontal roots (oriented between 0° and 30°). Horizontal roots grow between approximately 2 and 8 inches (5 to 20 cm) below the soil surface. Some roots observed in this study, termed "sinker" roots, originated as horizontal roots, extended laterally a short distance, and then grew downward much like a taproot [113]. Fine root production in black cottonwood is episodic and much more variable than the production and survival of leaves [113]. Mycorrhizal colonization of cottonwoods may be influenced by stand development stage and site characteristics. Members of the Populus genus are typically ectomycorrhizal [38,113,148], but may be colonized by arbuscular mycorrhizal fungi early in stand development or when the site is flooded [38]. RAUNKIAER [114] LIFE FORM:Phanerophyte Geophyte REGENERATION PROCESSES:Breeding system: Flowers are unisexual and all members of the genus are dioecious [16,41,60], though a small number of hermaphroditic or male flowers have been observed on female trees in black cottonwood [139]. In some black cottonwood populations sex ratio is skewed, with male trees dominant on warm, dry sites. In other populations the relationship between site and sex ratio is apparently random [19]. Pollination: Black cottonwood is wind pollinated [41]. Seed production: Black cottonwood is typically produces seed after 7 to 10 years of age [19,57]. Seed production is "copious" [2], with about 30 to 50 seeds per capsule [16]. Seed production is consistent, with large crops every year [57,126]. Seed dispersal: Seeds are lightweight; this characteristic, as well as the "cotton" from the seed pod, allow seeds to be carried by water or wind [57]. These are the general means of dispersal for all cottonwoods; seeds travel "several hundred meters", in some cases up to "several kilometers" (distances are estimates). Seed dispersal occurs when river flows begin to decline after spring peak flows, sometimes as late as mid-summer. If flows are too high seeds may be carried for so long that they lose viability before they reach a favorable microsite [19]. Seed banking: As is common in plants that colonize disturbed habitats, initial viability is high, but propagules are short-lived [2,57]. Seed viability under natural conditions is estimated to be 1 to 2 weeks, and once a seed is wetted it needs to be deposited within 2 to 3 days to establish [19]. Germination: When moist mineral soil is exposed germination rates are high, particularly when moist conditions persist for a month after seed deposition [57]. The process of germination is about 24 hours long. The high initial mortality of seedlings is "clearly a limiting factor in the life cycle of cottonwoods" [19]. Seedling establishment/growth: Seedling mortality is very high because of specific moisture requirements after germination; root growth in members of the Populus genus during the first 12 hours after germination is suppressed by low moisture [113]. Mortality in seedlings most commonly arises when root growth rates are not as fast as the recession of the water table [57]. Root growth rates for black cottonwood seedlings are some of the highest among cottonwoods: up to 0.48 inch per day (12 mm/day) [19]. Thus moisture conditions in the 1st month of establishment are a defining factor in survival [126]. The rate of root development is much greater than that of the shoot in cottonwoods; after 46 days of growth a group of balsam poplar × eastern cottonwood hybrid seedlings had a mean leaf area of 0.16 inch2 (1 cm2) and a root system that was 7 inches (17 cm) in length (in a "simulated riparian environment") [113]. Seedling density on new point bars can range from 2 to 40/foot2 (23- 460/m2). The seeds have little endosperm: optimal sunlight and moisture are required for carbohydrate synthesis early in establishment, and, for this reason, seedling establishment in existing vegetation is very low. Though floods often distribute seeds widely, few seedlings survive where vegetation is developed [121]. Seedlings are able to withstand inundation and sediment deposition; this process aids cottonwood establishment as it removes possible competitors [19]. Because of differential soil moisture conditions, "banding" of seedlings with respect to elevation rather than distance from the river has been observed at Oldman River near Lethbridge, Alberta. Following flood on the Oldman River, seedling growth was rapid. Mean height was 0.8 inch (2 cm) with a range of 0 to 6 inches (15 cm) after 1 growing season. After 2 growing seasons mean height was 3 inches (8 cm), with a maximum of 20 inches (51 cm) [121]. Black cottonwoods that had established from seed near Camas, Washington grew in 9 years to 50 feet (15 m) tall and 7 inches (17 cm) in diameter [11]. At another site in Washington average height in a pure stand of 4-year-old trees was 33 feet (10 m) [57]. Though floods can initiate seedling establishment, ice jams and turbulent melting associated with spring runoff often scour seedlings established in preceding years [121]. Cottonwood establishment by seed is episodic, depending on seed viability at time of deposition (which depends on the magnitude of flows) and on moisture conditions in the first month of growth [89]. Suitable conditions for seedling establishment sometimes occur (erratically) in approximately 5 to 10 year intervals [19]. Scott and others [128] studied patterns of cottonwood recruitment with respect to different fluvial processes. They found that studying recruitment is often biased by dating trees from growth rings at ground surface; this method was found to underestimate by an average of 5.1 years (range was 0 to 34 years) because of sediment deposition after establishment. Because of light requirements seedling establishment seldom occurs on sites where cottonwood is already dominant and other vegetation is established [19]. Most regeneration in established forests is by "root suckers" or coppice sprouts. On "older islands" studied in southern British Columbia, root suckers were the most common means of regeneration. On more recently exposed sites seedlings were 95% of dense stands; the other 5% were derived from twigs [47]. Scott and others [128] summarized how different geomorphic processes influence the type of cottonwood stand that will be established:
Asexual regeneration: Cottonwood recruitment is commonly asexual, via root suckering [2,19,41,57], coppice sprouting [57,59], or cladoptosis (the physiological abscission of twigs with leaves still attached) [19,47]. The means of regeneration depends site characteristics and whether or not black cottonwood is already present. Cladoptosis is an important means of regeneration of black cottonwood on gravel bars [36]. This means of asexual reproduction is most viable in moist climates; because of hot, dry summers it apparently does not occur in Alberta [50]. In laboratory conditions up to 95% of freshly collected, abscised twigs produced roots (the greatest rooting was in twigs shed in October) [47]. Branches of black cottonwood that are broken off often grow when deposited in fresh alluvium; this was observed following the Mt. St. Helens mudflows [1]. Rood and others [120] studied origins of cottonwood saplings along the Oldman River in Alberta. Where sprouting occurred it was most often the result of flood damage. When floods removed stems, root suckering was induced, and when floods buried stems, shoot sprouting was induced [120]. Hansen and others [59] stated that the sprouting ability of black cottonwood is less than that of narrowleaf cottonwood but greater than that of eastern cottonwood [59]; the findings of Rood and others [120], given below, at least partially refute that.
Root suckering in narrowleaf, plains and balsam poplar (subspecies not specified, but likely P. b. ssp. balsamifera) in pots was studied by Schier and Campbell [124]. Results for balsam poplar are presented below (n= 30 to 90):
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:
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 "1st terraces" that are approximately 400 years old, Sitka spruce and western hemlock dominate on "2nd terraces" approximately 750 years old, and western hemlock is the dominant on "3rd 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]:
Northern Idaho phenology is described below (west of continental divide) [125]:
Related categories for SPECIES: Populus balsamifera ssp. trichocarpa | Black Cottonwood |
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