According to the U. S. Forest Service, quaking aspen (Populus tremuloides) has the largest natural growing range of all North American native trees. It is found in woodlands from Alaska to Newfoundland and south to the Ohio River, and scattered widely in pockets in the Rocky Mountains from Canada south into Mexico. Numerous insect pests, fungi and other disease pathogens can harm quaking aspens, many animals eat seedlings, and the thin bark of the trees is readily damaged by deer, moose, and woodpeckers.
Overall, quaking aspen grows in cool summer regions that also have pronounced chilly to bitterly cold winters. The trees do not grow well where the summers are long and hot, and heat is the limiting factor why this plant species doesn't grow much farther south than 40 degrees north latitude, except in high montane elevations. Quaking aspen is hardy to U.S. Department of Agriculture zones 1 through 6, where annual winter low temperatures range between 0 and minus 50 degrees F. The American Horticultural Society lists it for heat zones 1 through 6, where there is no more than 60 days of temperatures above 86 degrees F in summertime. Interestingly, the U.S. Forest Service notes that in cold regions like Alaska, the aspen is most often growing on the warmer southern slopes of mountains, whereas in the mountains of the American Southwest and Mexico, they occur on the cooler north-facing slopes.
Quaking aspen trees grow in a variety of soils across their native range. They grow best in fertile, moist loams or well-draining silts or clay loams, according to the J.S. Earl Rook Online website. They also manage to prosper in shallow and rocky soils since the fallen leaves decay quickly to replenish nutrients. In fact, the Rook website mentions that where aspen trees have grown for decades, the organic matter deposited causes a higher pH soil when compared to surrounding areas of woodlands that are filled with conifers. Aspens do not cope with dry soils well, and grow quickly where the water table is between 18 to 60 inches below the surface. Conversely, aspen trees do not grow in soggy, flooding soils.
Quaking aspen grows in mixed composition in forests all across its native range, most often occurring alongside conifer trees. The identity of these associated conifers is diverse and varies across the North American continent, according the U.S. Forest Service. Some conifers include jack pine, Engelmann spruce, white spruce, red spruce, white fir, and numerous pines. Maples, birches, oaks, larch and basswood deciduous trees also grow alongside aspens. Often, huge groves of quaking aspen result when other forest cover trees decline, caused by both germination of aspen seeds but more so from the suckering roots of aspens that radiate outward from the original, old tree. As other hardwood trees mature and cast shade, quaking aspen trees die off. In sunny openings around aspens, a wide array of small shrubs and herbaceous plants grow.
Quaking aspen readily hybridizes with other closely related trees across its native range, including those in garden settings. Its flowers and pollen are compatible only with other species in the genus Populus. Ambiguous populations result when quaking aspen creates seedlings sharing genes with bigtooth aspen, European aspen, white poplar, curly poplar, narrowleaf cottonwood, balsam poplar and the eastern cottonwood.
Response to Fire
While the thin bark of the quaking aspen tree fails to protect it from occasional forest or prairie fires, the root system underground remains unscathed. As the above-ground biomass is burned, becoming a source of nutrients in the soil, quaking aspen tree roots respond quickly to create suckering sprouts. These sprouts are dense and numerous, choking out and readily out-competing other tree species' seedlings. According to the U.S. Forest Service, a large, mono-cultured stand of quaking aspens can dominate a landscape that was cleared by fire only 10 years in the past. In fact, seeds of quaking aspens may not germinate well until exposed to the heat of fire, especially in the coldest reaches of interior northern North America.
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