Olivia L. Cope, Eric L. Kruger, Kennedy F. Rubert-Nason, and Richard L. Lindroth
Many plants invest tremendous amounts of resources into chemical compounds that protect against herbivore attack. The relative costs and benefits of production of chemical defenses likely change during a plant’s lifetime, associated with variation in resource availability and herbivore exposure. Thus, the magnitude of defense investment, as well as the types of compounds produced, can change dramatically through time. Despite ample evidence of developmental patterns in chemical defense, little is known about whether the patterns themselves are adaptive – that is, whether they vary among plant genotypes and correlate with plant fitness. Plant populations, especially of trees, are typically mosaics of different-aged individuals; understanding how and why chemical defenses change with plant age has implications for plant-herbivore interactions across space and time.
In the longest-running study to date, we documented patterns of chemical defense for twelve genotypes of trembling aspen (Populus tremuloides) over 13 years and related those patterns to tree fitness metrics (growth and survival). In general, investment in chemical defenses decreased substantially as trees matured. Genotypes varied, however, in both their rate of defense decrease and in their overall levels of defense investment over 13 years. Variation in defense patterns among genotypes was associated with variation in fitness, suggesting that the patterns are indeed adaptive. Interestingly, relationships between defense patterns and fitness metrics often depended on tree sex. Female genotypes benefitted from rapid declines in defense investment with age and low investment overall, probably because they allocate more to reproduction than males and are thus more resource-limited. Males, on the other hand, tended to benefit from defense patterns that afforded higher herbivore resistance for longer into their development.
Aspen is an iconic foundation species in forest systems throughout much of North America and is in decline across significant portions of its western range. Results from our work indicate that genotypic, developmental and sexual differences in the costs and benefits of growth versus defense are likely drivers of variation in productivity and pest resistance in aspen populations. Improved understanding of these growth-defense dynamics will contribute to better-informed aspen management for both ecological and economic purposes.