San Francisco State University
Department of Geography
Geography 316:  Biogeography


Biogeography of
Quaking Aspen (Populus tremuloides)

Douglas W. Johnson



Aspen grove

Aspen stand near Lake Tahoe, October, 1999.  Mature trees in center, with young sprouts at left. Photo by author.

 

DESCRIPTION
    The golden shimmer of aspen leaves fluttering in an autumn breeze, brilliant against a deep green background of conifer… The sight is one of those that defines home in the mountain West.  Quaking aspen are even considered an “aesthetic resource,” as well as important for wildlife habitat and valuable as a wood resource (DeByle 1981).

    In the eastern US and Canada, aspen is one of hundreds of hardwood trees, but in the arid West the aspen is one of a few that thrive (Peattie 1953). This gives it special value in the West, and is the reason this article focuses on the aspen in this region.  Some National Forests in Colorado and Utah have 15 – 35% aspen cover (Gruell and Loope 1974), and these stands provide unique ecological, recreational, and timber resources.

    Aspens stand 40-70 feet in height, with a smooth white trunk 1-2 feet in diameter. The tree is deciduous, with leaves  that are rounded and shine bright green until they turn yellow in the fall.  Two-inch catkins flower in very early spring, producing small (0.25 inch) narrow cones that split to release copious amounts of tiny, cottony seeds that are dispersed by the wind (Little 1980).  Reproduction, however, is almost entirely vegetative, with suckers sprouting from existing root systems—the aspen is a clone.

    Aspens tend to grow in pure stands as a result of this clonal reproductive strategy. This makes them visually cohesive in the landscape, and also provides particular habitat that make them an important tree ecologically (discussed in Natural History).  Combined with their ability to exploit rare opportunities for sexual reproduction, this  two-pronged reproductive strategy has enabled aspen to maintain a broad range spatially and temporally (Mitton and Grant 1996) (see Distribution).

     The aspen is a tree of paradoxes. While aspen is typically a successional species in the West, dependent upon disturbance (primarily fire) for regeneration, it also forms climax communities in some locations (DeByle and Winokur 1985). While individual aspen ramets (trunks sprouted vegetatively) are among the shortest-lived trees in western forests, a continuous clone can be an incredibly long-lived organism—some conjecture that well-established clones date back 1 million years (Mitton and Grant 1996). Their clonal nature also makes a fairly diminutive tree take on huge proportions when considered en masse. And although a clone connotes genetic homogeneity, the aspen may be the most genetically diverse plant species studied to date (Cheliak and Dancik 1982) (see Natural History).

    Aspens were long considered a weed species, and yet they are a major tree crop in the Great Lakes region and in western Canada. They are the most widely distributed tree on the continent, yet they are not well know by many residents. They are also a species whose overall health—and the health of those creatures who depend on it—is deteriorating from the very practices that we employ to preserve the landscape (see Management).
 

NATURAL HISTORY
Leaves
The characteristic flutter of aspen leaves is the result of stems that are flat in cross-section rather than round. This adaptation gives them strength in the vertical direction while allowing them to twist flexibly in the wind. In a high wind, the leaves clump together in a manner that reduces air drag, the horizontal force that can break trunks.  This feature may help aspen survive storms (Vogel 1993).

Clones
    In the eastern part of their range, with its greater moisture, aspen may grow somewhat more frequently via sexual reproduction, since the exacting requirements of the seeds are more often met. But in the arid west, reproduction is almost exclusively due to vegetative, or clonal, reproduction.

     Clonal roots send up suckers following a disturbance that damages some of the ramets and clears space for sunlight. Fire is the chief agent, though avalanches, logging, and other disturbances are also part of the mix.  A mature root system can put out 400,000 to 1 million shoots per acre, and the sprouts can grow a meter per growing season initially (Mitton and Grant 1996; Madsen 1996).  This easily out-competes other tree forest tree species which must regenerate from seed.  The density of sprouts decreases as the canopy begins to shade out smaller seedlings, since aspen need full sun (Alban 1991). Individual aspen trees may live to 150 years in the West (only 70 in the Great Lakes region), and by this time shade-tolerant conifers have grown taller than the aspen and begin to shade them out. (Little 1980).  Thus, aspen clones depend on periodic disturbance in this time frame in order to maintain themselves.

    In some places, aspen have established themselves as a climax community, where conifers have been kept at bay by regular burning.  The giant clone named Pando (Latin for “I spread”) in south-central Utah stretches over 43 ha and contains more than 47,000 individual stems, with an estimated weight of 6 million kg (Mitton and Grant 1996).

    The age of such clones is not known, but it is commonly assumed that they go back to the last glaciation period about 10,000 years ago. Those in the Great Basin have been estimated at 8,000 years old. Thus the aspen clone is in the same class as other clones, such as creosote (some individuals of which are estimated at 11,000 years old) and huckleberry (13,000 years old). It’s also conceivable that modern clones may be only a few sexual generations from million-year-old ancestors, whose fossilized leaves look identical (Madsen 1996).

Aspen sprouting from a root exposed by a roadcut in eastern Utah, August,
1999. Note aspen stand in background. Photo by author.

    A stand of aspen typically consists of a mosaic of clones. A study of random amplified polymorphic DNA (RAPD) from specimens from several populations found high genetic diversity between the clones within a single population, and low diversity between different populations.  This is thought to be due to the occasional sexual reproduction by seeds that can be carried long distance on the wind, making for a large ‘genetic neighborhood’(Yeh et al.1995).  Long-lived clones themselves may acquire genetic diversity by accumulating somatic mutations.  A study of aspen heterozygosity found that aspen have two to six times the genetic diversity of commonly reported sexually reproducing species of plants and animals (Cheliak and Dancik 1982).

Wildlife Habitat
    A stand of aspen provides habitat for lots of other organisms.  Mitton and Grant (1996) suggest that in the arid West aspen stands are second only in habitat importance to riparian zones.  Compared to coniferous forests, aspen stands have a rich understory of shrubs and herbaceous species (Gruell and Loope 1974).  An aspen canopy typically allows more sunlight to reach the forest floor than do conifers, and stands are renowned for the wildflowers found within them (Alban 1991).  Aspens offer more structural habitat diversity than conifers, like lodgepole pine or spruce (aspen stands are often islands in seas of these trees). The forage in a stand of aspen can be up to 6 times as rich as that under coniferous forests. (DeByle 1981). For instance, in eastern California’s White Mountains, where P. tremuloides account for 64% of the coverage at 2,900m, aspen provide the most productive woodland in the range (Vasek and Thomas 1988).  An aspen thicket has 3–4 layers of vegetation, from small trees like chokecherry and juniper, to shrubs like serviceberry and snowberry, to wildflowers, grasses, and sedges.  Aspens play an important role in the lives of an estimated 500 species, from bears to fungi (Madsen 1996).

    The leaves, twigs and bark are highly nutritious, and deer and elk use them for overwintering., since it’s food they don’t have to dig out of the snow (Madsen 1996). Black bears, cottontails, porcupine, and snowshoe hares feed on bark, buds, and foliage (Peattie 1953), and grouse and quail eat the winter buds (Little 1980).  Small mammals, such as shrews, mice, and voles abound (Alban 1991).  And, of course, aspen is a  favorite food and building material for Castor canadensis, the North American beaver (Hall 1960).

    The layered structure of an aspen grove is popular with birds,. Snags provide perches for birds of prey, and sites for cavity nesters.  Flack (1976) counted some forty bird species in stands ranging across the West, including canopy nesters like the warbling vireo, shrub nesters like the flycatcher, cavity nesters like the mountain bluebird, and ground nesters like the hermit thrush, as well as hummingbirds and birds of prey.  Increasing stand size increases diversity of insectivorous birds (Mitton and Grant 1996).  A good example of aspen’s importance as a food source is the sympatric range of the ruffed grouse, which feeds extensively on aspen buds (DeByle and Winokur 1985).

    In addition to providing key habitat for wildlife, aspen in their seral form may be important as a ‘nurse crop’ for  shade-tolerant species that do not become established in full sunlight, such as many coniferous tree species and forbs.  A mature aspen canopy passes more sunlight than a stand of conifers, yet provides partial shade as well.  These conditions may be especially well-suited to the growth requirements of some species, such as Engelmann spruce (DeByle and Winokur 1985).

Fire
    Fire has historically been the disturbance force that enabled aspen to out-compete taller, more shade tolerant tree species, for it is in early seral stages that aspen has the distinct advantage with its clonal reproduction.  Aspen themselves don’t burn easily—some firefighters even called them the ‘asbestos tree’ (Engle 1991). But when they do get burned, they are not hardy. The root mass immediately puts energy into sprouting suckers, which grow quickly in the open sun and renewed soil.

    Obviously, suppression of wildfire and elimination of native burning has a huge effect on aspen regeneration.
In a study of Kootenay National Park in British Columbia, the average fire return interval has gone from 92 to 165 years in Kootenay Valley since the park’s inception, and for the whole park, from 60 years in the period from 1508 to 1778, to 2700 years today (Kay 1997).  These rates are unlikely to sustain aspen.

    One note about aspen ‘bark’—the tree has no cork-like fire protection like many conifers.  Its white outer layer is actually the living phloem layer, and is capable of photosynthesis (and thus it makes good browse).  One side effect of this is that the aspen displays wounds very clearly.  Anything carved into trees (or scratched by bears) heals into black scars, recording the event.  Basque shepherds in the Great Basin are known for having left their mark in this way (Little 1980).

Sexual Reproduction
    Aspen exhibited their capacity for rare sexual reproduction following the 1988 Yellowstone fires. In the spring of 1989, aspen seedlings appeared in burned areas, kilometers away from the nearest extant stands. This reflected the coincidence of factors necessary for seedling survival: (1) wind blew aspen seed, with its limited viability of several weeks, to the site’s bare soils at the right time; (2) the spring was moist and cool, but with enough sun; and (3) the sprouts did not get browsed by elk (Turner et al. 1997). This is an example of the type of event, rare though it is, that we believe has happened occasionally over the last 10,000 years, and is important for spreading aspen to new areas, and injecting new genetic variants into the gene pool (Mitton and Grant 1996).  This may also have helped them colonize bare new sites behind receding glaciers (Madsen 1996).
 

DISTRIBUTION
    The aspen is highly successful at adapting to different habitats, and is the most widely distributed tree in North America (and the second most widely distributed in the world).  A European aspen (P. tremula), quite similar to P. tremuloides, is distributed through Europe, North Africa, western Asia, Siberia, northeastern Asia and China (Everett 1968).  This appears to be a circumboreal distribution.

    In temperate North America, the aspen ranges in a continuous swath from the Atlantic coast as far south as Virginia up through Alaska and the Arctic Circle.  Aspen lives in the western US at higher elevations, primarily between 6,500 and 10,000 feet, in high plateau and alpine habitats. They find their southern limit in Mexico’s Sierra Madre (see map).  In the western US, the distribution is disjunct, based on suitable habitat, dependence on fires, and historical climatic variation.  Glaciers through the Pleistocene pushed tundra and boreal forests down into what is now the US. From there, fire (set by lightning or native Americans) shaped forests (Madson 1996). In California, an interesting population of aspen exists in the San Bernardino Mountains.  Aspen skip from the Sierra Nevada to the mountains in Baja California, with only this single population found along Fish Creek in the San Gorgonio Wilderness. Thorne (1988) calls it the “least known plant community in the southern California mountains.".

  From Mitton and Grant, 1996

    Aspen are dioecious, and Grant and Mitton (1979) found that the sexes are not distributed identically.  Males predominated in the higher altitude, harsher areas, while females were more common in the moister, more protected pockets at lower elevations.  Overall, the sex ratio was 1:1. Also, females showed a higher radial growth rate, counter to the common assumption that the energy costs of reproduction are higher for females.  They also found that growth rate increases with heterozygosity, on the order of 35% (Mitton and Grant 1996).

Altitude
    Though aspen grow at lower altitudes in the eastern and northern parts of their range (Flack 1976), in the West they are commonly found between 6 ,000 and 10,000 feet (Mueggler 1984).

Climate
    Aspen prefer cool, relatively dry summers with lots of sun, and winters with abundant snow (precipitation from 15-40 inches/year) that recharges soil for growth during spring and early summer.  They don’t like summer temperatures above 90?F, but are fine with winters below 0?F. They can only grow between 40 and 95?F.  In the central Rockies, their lower elevation limit roughly coincides with the 45?F mean annual temperature line (Mueggler 1984).

Soils
    Aspen are found on a broad array of soils, from shallow skeletal soils on bedrock to deep well-developed, nutrient rich soil (Alban 1991; Mueggler 1984). In the Rocky Mountains and Great Basin, aspen does well on soils derived from basalts, limestone, and neutral-to-calcareous shales. It seems to do poorly on granites, and do best in moist fertile loams with abundant calcium and a water table at 3-6 feet of depth (Mueggler 1984). And of course, they like mineral soils uncovered by a humus-destroying ground fire (Peattie 1953).

    Researchers evaluating different parameters for calculating a “site index” to quantify habitat suitability for aspen determined that the index could best be related to edatope, a combination of soil moisture and texture (Graham et al.1963; Chen et al.1998). They also found latitude to be correlated, because northward sites have a longer, wetter growing season.  At the north end of the range, they found aspen favor warm-aspect growing sites, though it’s the opposite at the southern end of their range, understandably.  They have higher productivity in less acid soils, and nitrogen appears to be the most important growth-limiting factor (Chen et al. 1998).

Associations
    In various latitudinal and climatic areas of their range, aspen associate with different plant communities.  Major associations are listed below, from south to north:

Arizona and New Mexico: Engelmann spruce (Picea engelmannii), firs (Abies spp.),  ponderosa pine (Pinus ponderosa), grasses and forbs (Flack 1976).

Utah (Wasatch and Uinta ranges) and Colorado Rockies: Engelmann and Blue Spruce, White and Subalpine Firs, Douglas Fir, and Lodgepole Pine forest, sagebrush (Artemisia spp.) and meadows (Little 1980; Peattie 1953; Flack 1976).

Western Wyoming and southeastern Idaho: forests of lodgepole pine (Pinus contorta), Engelmann spruce, and cottonwood (Populus augustifolia and P. balsamirfera), flats with willow (Salix spp.) and sagebrush, and meadows of sedges, grasses and forbs (Flack 1976; Gruell and Loope 1974).

Canadian parklands: zone between grasslands on one side and boreal forest on the other (Flack 1976).

Far north: white and black spruces (Peattie 1953).

    Recent work has shown that mycorrhizal members of the genus Inocybe are an important part of the mycoflora of aspen stands studied in Montana. Such associations need more examination to determine if the organisms are primarily associated with the soil or the host. These mycorrhizae are also common with willows in alpine and arctic habitats (Cripps 1997).

Changes in distribution
    Though some species that thrive on disturbance are expanding their range—especially those like broom that can seed cleared ground—aspen are faring worse under modern human control. The primary causes are fire suppression, browser populations, and climate.
    In Saskatchewan, there’s a die-back and break-up of large-scale stands. Up to 40,000 acres of 3.3 million acres in in the province are in decline.  A study suggests that repeated dought and defoliation by tent caterpillars are the culprits. These will both be exacerbated by global warming, it is believed (Hart 1998).  Aspen acreage dropped 40% between 1962 and 1984 in Arizona and New Mexico, probably due to lack of fire (Madsen 1996).  Yellowstone National Park has lost 95% of the aspen cover originally extant in the park when it was created in 1872, and one-third of the clones have died.  Even areas that have burned in the park have for the most part failed to regenerate because of intense ungulate browsing pressure. This pattern is repeated in Kootenay and Yoho National Parks in British Columbia.  Fire suppression and overabundance of ungulates are the main causes, and climatic conditions are less strongly correlated to lack of regeneration (Kay 1997).
 

EVOLUTIONARY HISTORY

Taxonomy
    Kingdom: Plantae
        Phylum/Division: Magnoliophyta
            Class: Magnoloiopsida (Dicots)
            Sub-class: Dilleniidae
                Order: Salicales
                    Family: Salicaceae
                        Genus: Populus
                            Species: P. tremuloides Michx.

 [After Cronquist (Wilson, 1999)]
 

    The family Salicaceae consists of some 30-40 spp of genera Populus (including the aspens and cottonwoods) and approximately 300 spp of genera Salix, the willows (Lawrence 1951).  All are riparian species, and together form the dominant vegetation along streams in the arid western US.
    While early taxonomists (namely Engler) classified Salicaceae as a primitive order, more recent accounts believe it to be an advanced family (Hutchinson 1969). The current Jepson Manual for California Plants (Hickman 1993) uses the classification system developed by Cronquist (and so have I in this report), though it warns that taxonomy is in perpetual flux, especially today with ever-developing genetics methods.  There are multiple current taxonomic classifications, including those of Cronquist, Thorne, and Takhtajan (Wilson, online).  In addition, the Angiosperm Phylogeny Group (APG) is constucting new classifications based on DNA sequences.  Researchers hope that this method will help clarify relationships in cases where interpretations of  morphology, anatomy and palynology conflict (Soltis et al, 1999). Though the classifications of Cronquist, Thorne and Takhtajan are quite similar, the APG system results in some reorganization, placing the family Salicaceae within the order Malpighiales (Soltis et al, 1999; Patterson 1999).
    Library research and on-line communication with several plant taxonomists produced no cladogram more current than one based on the 1966 Takhtajan system, with the sub-order Salicales under Violales (Stace 1980).  Certainly a more up-to-date cladogram must exist, or could be sketched by a knowledgable taxonomist, but I have not found it to be readily available. Probably this reflects both the complexity and ever-changing nature of plant systematics.
    Currently, angiosperms—flowering plants—are thought to have originated in the early Cretaceous. The earliest conclusive angiosperm fossils are approximately 130 million years old.  However, several new analyses indicate an earlier evolution, as far back as the Triassic (some 208 mya) though these conjectures await substantiation (Taylor and Hickey, 1996).  Pollen grains and leaf fragments of angiosperms from fossils dating to 70-135 myr ago contain evidence that certain arctic willows comprised a large proportion of the flora at that time. Apparent links between primitive members of Populus and the small genus Chosenia indicate that Populus probably originated in what is now eastern Asia (Newsholme 1992).
    Deciduous angiosperms, with their well-developed dormancy mechanisms, seem to have been more able to weather the K-T boundary 65 mya than the previously dominant broad-leafed evergreens (Wolfe 1997).  By the Eocene (38-54 mya), taxa of conifer-deciduous hardwood and sub-alpine forests had reached near-modern morphology.  By contrast, lowlands forests have evolved much more, some species going extinct, others changing significantly.  Today’s sub-alpine forest species, including trees similar to today’s quaking aspen, were widespread east of the Sierra in the Miocene (5-22.5 mya) when the climate there was mild with more summer rain (Axelrod 1988).  (Such determinations are made through fossilized leaves rather than through pollen grains, since Populus pollen is too delicate to have been preserved.)   As boreal forest and tundra expanded in the middle Miocene, Salixes diversified to fill new niches (Wolfe 1997).  Populus, probably more formed and less actively hybridizing (Hutchinson 1969), might be assumed to have expanded as well.
 

USES & MANAGEMENT

 The environment in which P. tremuloides grew to be one of the planet’s most successful trees has changed significantly. Because the value of aspen for lumber, livestock forage, and wildlife habitat is increasingly recognized (Madsen 1996), more attentive management is called for.
    Aspen are an important source of pulpwood in the Great Lakes states (Graham et al. 1963), where it constituted 51% of the pulpwood harvested in the early 1990s (Alban 1991).  In Canada, aspen is big industry. It’s used for bleach kraft pulp, solid wood products, and oriented-strand board (Alberta Research Council 1987). Aspen are the most common tree in western Canada’s grasslands, and dominant in the boreal forest. They have been used for the last 20 years in the multi-billion-dollar Alberta, Saskatchewan and Manitoba pulp and construction board industry. (Hart 1998).  Large conferences are held to address forestry concerns (Poplar Council of Canada 1991). In the U.S., aspen comprise 25% of the commercial forests in Colorado and Utah (Jacobi 1998).
    Though the wood is too weak for dimensional lumber, it is tough, so farmers used it for barn floors, fence posts and siding, and especially horse stalls  where it stands up to kicking and biting without splintering. It was also used for mine props, and it makes good excelsior (shavings) for packing produce, padding furniture, air-conditioner filters, and wallboard (Peattie 1953). It has also been used for construction of pallets (Engle 1991). Finally, it’s biggest use if for paper.  It’s strong pulp, inexpensive, easy to peel, and it bleaches well.   When mixed with stronger fibers from something like spruce, it produces quality paper (Peattie 1953; Engle 1991).
    Another forward-thinking use for the species that is currently under development is in the field of phytoremediation. Populus species, with the strong pumping action of its root system, is finding use in controlling or removing organic contaminants from groundwater or controlling leachate from landfills. An industry report projects a $214$370 million business by 2005 for cleaning up everything from lead to trichloroethylene (Black 1999).
    Several diseases threaten aspen, especially when they are stressed by lack of fire.  Tent caterpillar (Malacosoma disstria) does the most damage of any insect, defoliating aspen over large regions. Also poplar borer (Saperda calcaraia) wounds increase rate of infection by the fungus Hypoxylon mammatum, which causes cankers that weaken trunks to the point of breakage. The primary decaying organism is Phellinus tremulae, which causes white trunk rot (Alban 1991).  Researchers found that a combination of wet winters that flood soil followed by summer droughts make aspens especially vulnerable to canker-inducing fungi. The damp conditions cause a shallow root system that is unable to reach water in summer (Jacobi 1998).
    Managing aspen means looking at how we manage fire and wildlife. The trick to management is to recognize that aspen are dependent on the constant change that has shaped the land historically, like fire (Madsen 1996).  Even if fire is returned to its historical levels, ungulate browsing is more intense than in the past. Elk graze aspen sprouts voraciously. Under current conditions, aspen will decline, as will the many species that depend on it (Kay 1997).  Likewise, in other areas we need to manage deer and beaver to maintain stands (Graham et al. 1963).
    Some ecologists suggest that restoring predators may be an important component of maintaining healthy aspen. In a four-level trophic model with humans, wolves, elk and aspen, humans and wolves keep ungulate populations low, which enables vegetation to flourish. By managing on the assumption that an ecosystem works on a bottom-up, food-limited principle, we ignore crucial top-down aspects, and allow ungulates to repress aspen sprouting. Besides restoring predators, the ecologists recommend using fire in areas with low elk density, and controlling human uses that displace carnivores--even basic activities like hiking can keep them away (White, Olmsted and Kay 1998) .   By following a hands-off management strategy in places like Yellowstone, Kay (1997) maintains that we are supporting an entirely different ecosystem than that supposed as the prior ‘primitive’ state. The resulting ecosystem is then used as the standard for wilderness by which to gauge the health of other, more developed, ecosystems.  Thus it is imperative that we be aware of our implicit choices.
    And in the case of what may be the planet’s largest organism, Utah’s Pando clone is threatened by encroaching development, which increases the likelihood of increased fire suppression (Mitton and Grant 1996).
 
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