Quaking aspen

The quaking aspen is the most widely distributed tree species in North America. Common characteristics include the trembling leaves and the white bark. Most aspens are clonal species that share a extensive root system and are genetically identical. They are resistant to fire and grow rapidly in areas devastated by forest fires. Quaking aspens are loved for their brilliant fall colors and beautiful appearance.

Anatomy
Quaking aspens grow upright from 20-100 ft. tall with trunks 4-36 in. in diameter. The alternating, deciduous leaves are rounded and heart-shaped with small grooves or teeth on the edges. The leaves rangin from 1.5-3 in. in length, and tend to be dark-green on top and lighter green beneath. In the fall, they turn yellow, orange, gold, or red. The leaf petiole is flattened and extends all the way to the tip of the leaf, allowing it to flutter or "quake" in the wind. 

The slender stems are reddish-brown, but gray over the years. The branches grow up and out, creating a crowned top. The characteristic bark of a quaking aspen is a smooth, creamy white or pale green that becomes thick and lined as the tree ages. It can become 1-2 in. deep on older trees. The roots are widespread and numerous, some reaching as far as 100 ft. away and others deeper than 5 ft.  Flowers are contained in catkins (a long hanging furry cluster of tiny leaves and flowers without petals)  1-3 in. long. The seeds have long seed hairs attached to light and pear-shaped capsules. The fruits are two-valved brown or light-green pods. 

Reproduction
Quaking aspens reproduce via sexual and asexual reproduction. Sexual reproduction is rare because conditions have to be just right for the seeds to germinate and survive. Aspens are dioecious, meaning that "male and female flowers [are] on different plants of the same species." Quaking aspens first flower when they are 10-20 years old and reach a production peak around 50 years old. The flowers are contained in catkins (see Anatomy) and are pollinated by the wind. The seeds are dispersed by the wind, some up to several miles, and they can also be scattered by water in aquatic environments. A single quaking aspen can produce 1.6 million seeds every year, and larger crops are produced in 2-3 or 4-5 year intervals. The seeds can only survive with suitable temperatures (39-98 degrees Fahrenheit), nutrient-rich soil, and plenty of water. Seedlings produced by sexual reproduction provide diversity in an otherwise genetically identical or similar colony of trees. 

Asexual reproduction occurs through cloning. The extensive root system of a quaking aspen sends up shoots or suckers where there is an area of sunlight. A mature system of roots can produce 400,000 to 1 million suckers per acre. Aspen clones rely on occasional habitat disturbances to provide them with enough sunlight to survive. These clone shoots are genetically identical to the original aspen, but are individual trees. A grove of quaking aspens is commonly composed of a variety of widespread, interconnected clones. Most clones grow within 25 or even 80-100 ft. of the parent tree. Clonal colonies survive for many years, some estimated to be hundreds or even thousands of years old! Quaking aspens also sprout from root collars and stumps in asexual reproduction. 

Ecology
Quaking aspens grow in many types of soil, but prefer mineral-rich, moist soil. Over the years, they develop their own "aspen soil" from decayed leaves. Quaking aspens are extremely intolerant of any shade, requiring mass amounts of sunlight. They are usually replaced by hardwoods or conifers. They can tolerate colder temperatures, but do poorly in hotter climates. Aspens are most commonly found at elevations from 6,000 to 10,000 ft. They are also intolerant of soils that are water-logged and areas of long-term flooding. Quaking aspens are a perennial, native species that flower from April-June and fruit ripens May-July. 

Aspen stands provide ideal habitats for many organisms with a balanced amount of sunlight and shade. The forage beneath quaking aspens can be up to six times richer than that of a coniferous forest! Wildflowers, shrubs, small trees, and grasses thrive beneath aspen canopies. Quaking aspens are also important to around 500 species of organisms and animals. Leaves, bark, buds, and twigs are all food sources for bears, deer, elk, birds, and small rodents or mammals, especially in the winter months. Stands of aspens are a popular nesting places for birds, from cavity-nesters to ground nesters and canopy-nesters. Aspens are susceptible to rot-producing fungi such as Shoestring Root that travels through the extensive root system and Aspen Trunk Rot. Cankers, like Sooty Bark Canker, can also infect and even kill quaking aspens. Leaf rollers, Leaf Miner Beetle, and Western Tent Caterpillars are some of the many insects that prey on aspen trees. Grazing animals like deer eat young aspens and can damage saplings by rubbing antlers against the bark. Small mammals such as mice also harm quaking aspens by stripping off the lower bark of the tree. 

Forest Fire
Forest fires have both beneficial and detrimental effects on quaking aspens. A small fire may damage the thin bark and allow fungus to enter the tree, and a larger fire may kill the tree completely. Fortunately, aspens are not easily burned and are sometimes called "asbestos trees." Wildfires clear the land and put nutrients back into the soil. This is very beneficial to quaking aspens because the sunlight and mineral-rich soil is exposed for growth. Their recovery from fire is very rapid, having the advantage of being a clonal species. The surviving root systems send up many new sprouts for several years following a fire. The sprouts feed off of the larger root system, and an entire stand of quaking aspens can be produced within a decade following a fire.

Forest fires remove shady canopies that hinder aspen growth, as well as blackening the soil, which increases the heat absorption and aids the production of sprouts. Quaking aspens are also self-thinning, meaning that they don't grow too densely after a fire. Even though aspens have thin bark, they resist fire because they tend to be surrounded by moist fuel compared to nearby conifers. Quaking aspen groves act as natural firebreakers, and fire will sometimes pass them by in favor of drier trees and fuel. When an aspen is damaged by wildfire, it can recover and survive due to nutrients from the interconnected root system of healthy, untouched aspens. Forest fires are necessary to quaking aspen growth because they allow them to outcompete taller and shadier trees, which would block out the sunlight and kill young aspen shoots.