COS 124-6 - Plant defenses and Conspecific Density-Dependence: Examining patterns in secondary metabolite similarity as an indicator of CDD mechanisms along an elevation gradient

Abstract: The interactions between plant defenses and their enemies can have significant impacts on the population regulation of plants. Population regulating processes, such as conspecific density dependence (CDD), which are largely influenced by species-specific enemies, are likely to be weakened or strengthened by changes to plant-enemy interactions. Plants largely influence interactions with enemies by producing an array of secondary metabolites, many of which are used as chemical defenses. Notably, secondary metabolite profiles in plant leaves tend to become less similar between species in lower latitudes- suggesting an association between the specificity of plant defenses and climate. Decreasing similarity in plant secondary metabolites in lower latitudes may be a response to an increase in the specificity of enemies that the plants must defend against. Greater species specificity by plant enemies also indicates stronger negative CDD interactions among plants, which stabilizes populations. The correlation between secondary metabolite similarity and negative CDD strength may provide evidence of specialized foliar enemies as a mechanism driving CDD in plants. However, it is unknown whether this pattern in secondary metabolite similarity exists at smaller spatial scales as it does in CDD. I hypothesize that at the landscape scale, similar to the latitudinal scale, plant secondary metabolites become less similar within and between species as temperature and precipitation increases.

We tested our hypotheses by collecting 143 leaf samples from 8 target species along an elevation gradient at the H.J. Andrews Experimental Forest in the western cascades in Oregon, USA. We did not find support for the hypotheses that secondary metabolite similarity within and between species would decrease in lower elevations- characterized by higher temperature and precipitation. Instead, we found that while species were chemically distinct from one another, similarity between species remained largely consistent across elevations. We also found that the richness of secondary metabolites within species increased with elevation. While not supporting our initial hypotheses, our results provide significant insight into mechanisms of CDD. While studies have demonstrated that negative CDD is stronger at lower elevations at the H.J. Andrews, we cannot conclude that stronger negative CDD is being driven by a greater diversity of foliar enemies at lower elevations. Additionally, preliminary evidence suggests that higher elevation tree seedlings may experience positive CDD. The increase in secondary metabolite richness may therefore be a response to supporting mutualisms in higher elevations, more so than defending against enemies.