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SPECIES: Populus tremuloides | Quaking Aspen
GENERAL BOTANICAL CHARACTERISTICS : Quaking aspen is a native deciduous tree. It is small- to medium-sized, typically less than 48 feet (15 m) in height and 16 inches (40 cm) dbh [75]. It has spreading branches and a pyramidal or rounded crown [60,75,88,166]. The bark is thin. Leaves are orb- to ovately shaped, with flattened petioles [90]. The fruit is a tufted capsule bearing six to eight seeds. A single female catkin usually bears 70 to 100 capsules [88,166]. The root system is relatively shallow, with widespreading lateral roots and vertical sinker roots descending from the laterals. Laterals may extend over 100 feet (30 m) into open areas [88]. Gifford [59] found that vertical roots of quaking aspen in Utah extended more than 9 feet (2.7 m) down, branching into fine, dense roots at their extremities [88]. Quaking aspen forms clones connected by a common parent root system. It is typically dieocious, with a given clone being either male or female. Some clones produce both stamens and pistils, however [88]. Quaking aspen stands may consist of a single clone or aggregates of clones [166]. Clones can be distinguished by differences in phenology, leaf size and shape, braching habit, bark character, and by electrophoresis [123]. In the West, quaking aspen stands are often even-aged, originating after a single top-killing event. Some stands, resulting from sprouting of a gradually deteriorating stand, may be only broadly even-aged [88]. Clones east of the Rocky Mountains tend to encompass a few acres at most [125], and aboveground stems are short lived. Maximum age of stems in the Great Lakes States is 50 to 60 years. Clones in the West tend to occupy more area, and aboveground stems may live up to 150 years [86]. A male clone in the Wasatch Mountains of Utah occupies 17.2 acres (43 ha) and has more than 47,000 stems. To date, it is the world's most massive known organism. Clone age can be great; the large Utah clone is estimated to be 1 million years old [107]. Seedling morphology: Quaking aspen seedlings can easily be misidentified as cottonwood (Populus spp.) or willow (Salix spp.) seedlings because quaking aspen seedlings bear only a slight resemblance to the adult form. Leaves of quaking aspen seedlings are nearly lanceolate. During the first growing season, vertical flattening of the leaf petioles is not obvious, and there is no lateral branching. By the second growing season, leaves are characterisitically orbicular to ovate, and there is vertical branching. Renkin and others [133] have published photographs of excavated quaking aspen seedlings. Quaking aspen seedlings can be differentiated from root sprouts by leaf morphology, lack of woody tissue, lack of vertical shoots, and presence of a taproot [90,133]. There are a few visual clues that can distinguish seedlings from sprouts without excavation. Seedlings have paired cotyledons or cotyledon scars a few millimeters above the soil surface. The first pair of true leaves is nearly opposite, at right angles to, and directly above the cotyledons. Leaf pattern of sprouts is strongly alternate [133]. Physiology: Quaking aspen is not shade tolerant [123,130]; neither does it tolerate long-term flooding nor waterlogged soils [123]. Even if quaking aspen survives flooding in the short term, stems subjected to prolonged flooding usually develop a fungus infection that greatly reduces stem life (and renders the wood commerically useless) [37,118,126]. Sprouting is hormonally controlled in quaking aspen. Sprouting is suppressed by auxin, which is transported from the stem to the roots. Auxin therefore maintains apical dominace. When stems are killed and apical dominance is removed, cytokinins in the roots initiate root sprouting. Clones with a strong tendency to sprout probably have high cytokinin:auxin ratios [145]. RAUNKIAER LIFE FORM : Phanerophyte Geophyte REGENERATION PROCESSES : Quaking aspen regenerates from seed and by sprouting from the roots [146]. Stump and root crown sprouting is rare in older trees, but saplings sometimes sprout from the stump and root crown as well as the roots [123,145]. Vegetative reproduction: Root sprouting is the most common method of regeneration. Root suckers originate from meristems in the root's cork cambium and can develop anytime during secondary growth [140]. Saplings may begin producing root sprouts at 1 year of age [123]. There are thousands of suppressed shoot primoridia on the roots of most mature quaking aspen clones. Recently initiated meristems or primordia usually sprout and elongate more vigorously than older primorida or suppressed root buds [145]. Root suckering is affected by depth and diameter of parent roots. In Utah and Wyoming, Schier and Campbell [144] found that 25 percent of sprouts came from roots within 1.6 inches (4 cm) of the surface, 70 percent from within 3.2 inches (8 cm), and 92 percent within 4.7 inches (28 cm). Compared with parent roots of quaking aspen in the Great Lakes States, those of quaking aspen in the West were deeper. On a Utah burn site, high-severity fires increased the depth of the parent roots from which sprouts originated. Range in diameter of roots producing sprouts was 0.04 to 3.7 inches (0.1-9 cm). Sixty percent of suckers grew from roots smaller than 0.4 inch (1 cm) in diameter, 88 percent from roots smaller than 0.8 inch (2 cm), and 93 percent from roots smaller than 1.2 inches (3 cm) in diameter. On a Wyoming site, the percentages were 38 percent, 68 percent, and 86 percent, respectively. Sprout development is largely suppressed by apical dominance [145]. Closed stands produce a few inconspicuous sprouts each growing season; the sprouts usually die unless they occur in a canopy gap. When stems are removed by cutting, burning, girdling, or defoliation, suppressed primoridia, buds, and shoots resume growth. Best sucker production follows either a fire that kills all parent trees and brush or other complete clearing [23]. The number of suckers produced can vary markedly among clones [7,159], but the potential for suckering is enormous. Jones [87] indicated that 20,000 to 30,000 sprouts per acre is typical the first year following top-kill. Natural thinning is heavy and effective. The least vigourous suckers die within 1 to 2 years. After 5 to 10 years, most sucker clumps reduce to a single stem [87]. Most stems are overtopped by more vigorous neighbors. Diseases, insects, browsing mammals, and snow damage also reduce sprout density [35,87,108]. Bella and De Franceschi [17] reported that in Alberta and Saskatchewan, stem density averaged 280,000 per hectare at age 2; 190,000 per hectare at age 3; and 80,000 per hectare at age 5. Seedling establishment: Quaking aspen commonly establishes from seed in Alaska, northern Canada, and eastern North America. Seedling establishment is less common in the West, where rainfall is often followed by dry periods that kill newly germinated seedlings [90]. Even in the West, however, quaking aspen may establish from seed more frequently than previously thought. Studies on frequency of seedling establishment in the West are conflicting. Some researchers found absolutely no quaking aspen seedling establishment despite diligent searching [4,5,16]; others reported the presence of only one [51] or a few [52] seedlings, while still other researchers documented the presence of hundreds of seedlings [7,90,97,167]. Only since the stand-replacement fires of the late 1980's have researchers used permanent plots to monitor quaking aspen seedling establishment and survival in the West. Data from one such study are summarized after the following discussion of sexual reproduction in quaking aspen. Sexual reproduction: The staminate-pistillate ratio of adult clones is 1:1 in most localities, although it may be as high as 3:1 or more [117]. Some clones alternate between staminate and pistillate forms in different years, or produce various combinations of perfect, staminate, and pistillate flowers [50]. Quaking aspen first flowers at 2 to 3 years. Minimum tree age for production of large seed crops is 10 to 20 years, and maximum seed production occurs at about 50 years of age. In Utah, one 23-year-old tree produced an estimated 1.6 million seeds in one spring [123]. There are 3- to 5-year intervals between heavy seed crops [55,102,110,148]. Seeds disperse a few days after they ripen. Dispersal lasts 2 to 3 weeks [123]. The plumose seeds are dispersed by wind for distances of 1,600 feet (500 m) to several miles with heavy winds. Seeds also disperse by water, and can germinate while floating or submerged [54]. Viability of fresh seed is good; germination of 80 to 95 percent is reported under laboratory conditions [103,109,142]. Viability lasts 2 to 4 weeks under favorable conditions of low temperature and humidity [123], but seed loses viability rapidly under less than optimum conditions [54,171]. Optimum conditions for germination and seedling survival include a moist mineral seedbed with adequate drainage, moderate temperature, and freedom from competition [104]. In various collections, seeds have germinated at temperatures from 32 to 102 degrees Fahrenheit (0-39 deg C), with germination sharply reduced from 35 to 41 degrees Fahrenheit (2-5 deg C) and progressively curtailed above 77 degrees Fahrenheit (25 deg C) [54,172]. Quaking aspen seed from northern Utah showed optimal germination between 59 and 68 degrees Fahrenheit (15-20 deg C), and had no light requirement. Seeds germinated best on the soil surface, with emergence decreased by shallow burial [104]. Burned or scarified soil is an excellect seedbed [61]; litter provides the poorest seedbed. The primary root grows slowly the first few days following germination, and during this critical period the seedling depends upon a brush of hairs to absorb water and anchor the plant [123]. Minor disturbances can uproot surface-germinated seedlings, and a drying seedbed can rapidly desiccate seedlings [104]. Seedlings may reach 6 to 24 inches (15-61 cm) in height by the end of their first year, and roots may extend 6 to 10 inches (15-25 cm) in depth and up to 16 inches (41 cm) laterally. Roots grow more rapidly than shoots; some seedlings show little top-growth until about their third year [23]. During the first several years, natural seedlings grow faster than planted seedlings but not as fast as sprouts. High mortality characterizes young quaking aspen stands regardless of origin. In both seedling and sprout stands natural thinning is rapid. Stems that occur below a canopy die within a few years [123]. Seedling study: Kay [90] documented postfire quaking aspen seedling establishment following 1986 and 1988 fires in Grand Teton and Yellowstone National Parks, respectively. He found seedlings were concentrated in kettles and other topographic depressions, seeps, springs, lake margins, and burnt-out riparian zones. A few seedlings were widely scattered throughout the burns. In Grand Teton National Park, establishment was greatest (950-2,700 seedlings/ha) in 1989, a wet year, but hundreds to thousands of seedlings established each year despite drought conditions in 1986-1988 and 1990-1991. Seedlings surviving past one season occurred almost exclusively on severely burned surfaces. In Grand Teton National Park, where seedlings were monitored for several years, surviving seedlings were associated with bare mineral soil, ash, and the absence of competing vegetation. In both Parks, 100 percent of seedlings were browsed, and mean heights of seedlings at postfire year 5 (Grand Teton) and postfire year 3 (Yellowstone) were nearly equal to mean heights at postfire year 1. During the same period, 0 percent of lodgepole pine seedlings were browsed. Kay predicted that long-term survival of quaking aspen seedlings will be low. Most seedlings established on depressions that are subject to spring flooding. Since quaking aspen does not tolerate standing water, seedlings on depressions such as kettles and lake margins will probably die in the first prolonged flood. At postfire year 5, quaking aspen seedlings in Grand Teton National Park attained only 5 percent more height growth than attained in the first postfire year. In contrast, lodgepole pine seedlings had increased in height by an average of 176 percent. SITE CHARACTERISTICS : Quaking aspen occurs on a wide variety of sites [40,111]. It grows on moist upland woods, dry mountainsides, high plateaus, mesas, avalanche chutes, talus, parklands, gentle slopes near valley bottoms, alluvial terraces, and along watercourses [40,109,158,166]. Climate: Climatic conditions vary widely over the range of quaking aspen, especially minimum winter temperatures and annual precipitation. Generally, quaking aspen occurs where annual precipitation exceeds evapotranspiration. In Alaska and northwestern Canada, quaking aspen is common in the boreal zone and extends into the warmest, frost-free sites of the permafrost zone. At the eastern edge of quaking aspen's range, climate is humid, with snowfall exceeding 120 inches (3,050 mm) per year. The southern limit of quaking aspen distribution in the East is roughly delinated by the 75 degree Fahrenheit (24 deg C) mean July temperature isotherm. In the central Rocky Mountains, altitude plays an important role in quaking aspen distribution. The lower limit of its range coincides with a mean annual temperature of 45 degrees Fahrenheit (7 deg C) [123]. Soils: Quaking aspen grows on soils ranging from shallow and rocky to deep loamy sands and heavy clays. Good quaking aspen sites are usually well drained, loamy, and high in organic matter and nutrients [123]. Cryer and Murray [36] stated that stable quaking aspen stands are found on only one soil order - mollisols - and a few soil subgroups of which Agric Pachic Cryoborolls and Pachic Cryoborolls are dominant. The best stands in the Rocky Mountains and Great Basin are on soils derived from basic igneous rock such as basalt, and from neutral or calcareous shales and limestones. The poorest stands are on soils derived from granite. In the Great Lakes States, the best stands occur in lime-rich, gray glacial drift [123]. Elevation: Quaking aspen spans an elevational range from sea level on both coasts to 11,500 feet (3,505 m) in northern Colorado. At its northern limit, quaking aspen is found only up to 3,000 feet (910 m). In Baja California, it does not occur below 8,000 feet (2,440 m). In Arizona and New Mexico is is most abundant between 6,500 and 10,000 feet (1,980-3,050 m); in Colorado and Utah, it occurs about 1,000 feet (300 m) higher. At either either of its elevational limits, quaking aspen is stunted. At its lower limit, it grows as a scrubby tree along streambanks; at high elevations, its stems are bent or prostrate [123]. Aspect: In Alaska and western Canada, quaking aspen grows best on south to southwesterly exposures. It is common on all aspects in the West, except in the Southwest, where it is most common on northern aspects. In the prairie provinces of Canada, particularly on the prairie-woodland interface, quaking aspen occurs on cooler north and east slopes, and in depressions [123]. SUCCESSIONAL STATUS : Quaking aspen is shade intolerant and cannot reproduce beneath its own canopy [23,40,98,123,126]. Beyond that, there is no single, generalized pattern of succession in quaking aspen. Quaking aspen is seral to conifers in most of its range in the West, and in some portions of its eastern range. In the East, quaking aspen is also replaced by hardwoods [23,98]. In the Great Lakes States, successional trends are toward northern hardwoods, spruce-fir, ash-elm (Fraxinus-Ulmus spp.), oak (Quercus spp.), swamp conifers, and pine (Pinus spp.) types, in decreasing order of importance [23]. Where it is seral, quaking aspen usually persists as a minor tree in late seral stages [98]. The canopy closes rapidly in young aspen stands [126]. A quaking aspen stand in Ontario closed and reached maximum development (foliage/unit area of soil surface) in 4 years [127,128]. If quaking aspen does not remain stable, rate of succession to other species varies with with soil, site, and invading species [71]. Mueggler [112] stated that succession to conifers may occur in a single generation, or take longer than 1,000 years. Harper [72] found that in central Utah, quaking aspen succeeded to conifers in 75 to 100 years on sandstone soils. On limestone or alluvial soils, succession to conifers took 140 years or more. Quaking aspen is apparently stable on some sites. On some former pine stands in the East, extensive clearcutting of the conifer overstory has removed the pine seed sources. Quaking aspen has formed an apparently stable overstory on many of these sites [24]. Quaking aspen stands are also considered stable in parts of Canada and the western United States [71]. Some stands, however, remain stable for decades but eventually deteriorate. Deteriorating stands are often succeeded by conifers, but shrubs, grasses, and/or forbs gain dominance on some sites [71]. Succession to grasses and forbs is more likely on dry sites and is more common in the West than in the East [126]. Quaking aspen readily colonizes after fire, clearcutting, or other disturbance [123]. In Emigrant Wilderness Area, California, red fir (Abies magnifica) stands on north slopes have converted to quaking aspen after fire [66]. In the Great Lakes States, quaking aspen has regenerated on cut/burned sites through sprouting and seedling establishment, becoming the dominant forest cover type [23]. SEASONAL DEVELOPMENT : Quaking aspen catkins elongate before the leaves expand. In New England, catkins appear in mid-March to April; in the central Rockies, flowering occurs in May to June. Sustained air temperatures above 54 degrees Fahrenheit (12 deg C) for about 6 days apparently trigger flowering [55,123]. At high elevation, trees may flower before snow is off the ground [5]. Female trees generally flower and leaf out before male trees. Local clonal variation produces early- and late-flowering clones of either sex, however. Catkins mature in 4 to 6 weeks (usually in May or June). Branches usually leaf out from early May to June [123]. Seed dispersal in the Great Lakes States occurs from early May to mid-June, beginning earliest on protected sites and in southern portions of the region [23].

Related categories for Species: Populus tremuloides | Quaking Aspen

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Information Courtesy: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. Fire Effects Information System

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