Richard L. Lindroth
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Chemical Ecology of Aspen: Herbivory and Ecosystem Consequences
Richard L. Lindroth, University of Wisconsin, Madison, WI
Much of the remarkable ecological and evolutionary success of aspen (Populus tremuloides) can be attributed to its secondary chemistry. This chemistry is dominated by phenolic compounds, principally phenolic glycosides (salicylates) and condensed tannins. Aspen exhibits extraordinary intraspecific variation in chemical expression, due to a complex of genetic, environmental, and ontogenetic (development) factors. This variation, in turn, mediates interactions with herbivores, shapes community organization, and influences ecosystem function.
The signature secondary compounds in aspen are the salicylate phenolic glycosides (salicin, salicortin, tremuloidin and tremulacin). Levels of these compounds are strongly genetically determined, but only slightly responsive to resource (e.g., nutrient, light) availability. Phenolic glycosides provide defense against a variety of insects, as well as against browsing mammals (e.g., elk and porcupine). Genetic variation among aspen clones in production of phenolic glycosides is related to a trade-off between growth and defense; well-defended genotypes grow slowly, whereas poorly-defended genotypes grow rapidly. Levels of phenolic glycosides are also influenced by plant development; they are exceptionally high in young trees and much lower in mature trees. This pattern suggests that mammalian herbivores were the driving selective force for the expression of high levels of defense in young aspen.
Levels of tannins in aspen are determined by a combination of genetic, resource, and developmental factors. Tannin concentrations are highly “plastic” (flexible) in response to resource availability. Tannin levels show developmental shifts to opposite those of phenolic glycosides; they are low in young trees and increase with age. To date, however, little evidence supports the role of tannins as defense against either insect or mammalian herbivores.
The efficacy of aspen chemical defense against herbivores is density-dependent. At critical densities of herbivores, aspen is consumed independent of its defense characteristics (poor food is better than none). This is the situation that occurs during major outbreaks of insect defoliators in the Great Lakes Region, and may explain the apparent lack of resistance of aspen to ungulate herbivores in the Intermountain West.