Ecology

Aspen

Scientific name: Populus tremuloides

 Aspen, September 27, 2013

Aspen, September 27, 2013

Unlike common names, scientific names always hold clues to the identification of a species. For aspen, the genus name Populus refers to the ability and tendency for aspen to reproduce asexually, and the species name tremuloides refers to a specific adaptation of the leaf petiole that causes the leaves to tremble, hence one of aspen’s common names, quaking aspen.

Aspen trees frequently reproduce asexually. Asexual reproduction occurs when a ramet (a single stem of the whole organism, the genet) sends up a new shoot from the root system. Asexual reproduction in aspen results in entire stands of genetically identical ramets. When looking at an aspen grove, you are actually observing one organism.

Asexual reproduction is regulated by hormones. Auxin is a hormone produced by the stem above ground, and it is transported to the roots. Auxin functions as a sprouting suppressant: when auxin is present, roots will not sprout new ramets. When individual ramets die, the roots no longer receive auxin, and sprouting is no longer suppressed. The roots then produce cytokinin, a hormone that stimulates and initiates sprouting.

Although asexual reproduction is a crucial life history trait of aspen, these trees would not flourish without sexual reproduction. Sexual reproduction allows for genetic recombination through the exchange of gametes, which results in genetic variation. Genetic variation is crucial for survival and adaptation of a species.

Aspen are dioecious, meaning male and female gametes are housed on separate trees. Altough aspen produce seeds every year, the seedlings have very specific environmental requirements (regeneration niche) and often do not become successfully established. Therefore, sexual reproduction is crucial for the species, but it is also a rare event.

Aspen is a pioneering species, which means that after disturbance such as fire, aspen seedlings are the first to colonize the area. Aspen seedlings require high levels of sunlight and water, and fire disturbance opens up prime habitat for seedlings. Often, seedlings that establish are replaced by later successional species; however, sometimes the seedlings will flourish as a climax community and produce a stable colony. It is hypothesized that some aspen colonies are centuries old.

Aspen are shade intolerant, meaning the trees require direct sunlight. The leaves are adapted for full sun exposure with a petiole (the stem that attaches the leaf to the tree) that is flat and oriented perpendicular to the leaf. This petiole orientation allows the leaves to tremble or quake in the wind. As the leaf trembles, air is circulated around the surface of the leaf, cooling the leaf down and reducing transpiration (water loss) in the hottest parts of the day.

Aspen leaf, September 27, 2013

Aspen leaf, September 27, 2013

Although aspen lose their leaves in the fall, photosynthesis continues in the bark of the tree throughout the winter, helping the species survive in a climate where the growing season is particularly short.

Fall color, September 27, 2013

Fall color, September 27, 2013

Human uses: The bark of aspen contains salicylic acid, which is the chemical used in Aspirin. After rain, yeast collects on the bark and can be used as a sunscreen substitute.

Aspen bark, September 27, 2013

Aspen bark, September 27, 2013

Wildlife uses: Aspen are a crucial wildlife tree. Yellow-bellied sapsuckers (a type of woodpecker) drill holes in the aspen, which causes the tree to exude sap. The sapsuckers will then return to the tree to eat insects that become trapped in the sticky sap. Elk will eat the foliage and will also sample the bark when food is scarce. Cavity-nesting birds also find crucial habitat in the soft wood of the tree. Finally, the filtered light provided by the aspen canopy provides shelter for larger animals on hot days.

Sudden Aspen Decline (SAD):

Due to various stressors, aspen populations are on the decline. Sudden Aspen Decline (SAD) is due to major droughts which weaken the trees to the point where additional stressors (browsing, insect infestation) cause the trees to die. These droughts are related to climate change.

Gradual Aspen Decline is also occurring and is due to browsing, leaf minor infestations (insects that attack the leaves), disease, parasites, and fire suppression. As discussed above, aspen are an early successional species that rely on disturbance for sexual reproduction. With fire suppression, there are fewer disturbance events. Without fire disturbance, the conditions required for seedlings are not present, and thus there are fewer new genotypes produced by sexual reproduction. Although existing aspen stands can reproduce asexually, this does not produce new genotypes, which are crucial for the long-term viability of a species.

Field notes, September 10, 2013

Field notes, September 10, 2013

References: Dr. Kevin Krasnow

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