Dominant plant species can strongly affect ecosystem processes and community dynamics. However, not all individuals within a dominant species are necessarily functionally equivalent, and thus intraspecific variation in functional traits may be an important driver of biodiversity in natural systems. Either because the traits of dominant individuals have a larger effect on their environment than sub-dominants or because heterogeneity in general should promote species diversity. Here, we focus on condensed tannin (CT) concentrations in Populus tremuloides (aspen) as a potential driver of understory plant diversity, mediated through changes in soil nitrogen availability (conversion of mineral nitrogen to dissolved organic nitrogen in the presence of tannins). Specifically, we tested whether understory plant diversity was influenced by (1) leaf tannin concentrations of overstory aspen (average and abundance weighted), (2) intraspecific variation in tannin concentrations (heterogeneity hypothesis) in aspen, and (3) stand characteristics
We measured leaf CT concentrations and size for 1,100 ramets of trembling aspen in 80 plots across 10 sites across the Aspen Parkland ecoregion of central Alberta, Canada. Within each plot, we also surveyed understory species composition, stand density, and measured canopy openness, allowing for testing of our alternative hypotheses through a series of mixed models.
Results/Conclusions
Understory species richness was strongly influenced by aspen leaf chemistry, but not stand structure metrics such as canopy openness or stand density. Specifically, increased leaf tannin concentrations (average and abundance weighted) were associated with increased understory plant diversity. In contrast to expectation, increased variation in CT concentrations was associated with decreased understory diversity. Our study is among the first to address the effects of a plastic chemical trait on plant communities. Our results suggest that aspen chemical driven changes, possibly altering soil nutrients, increase overall plant diversity. Our finding that heterogeneity in aspen leaf chemistry did not increase diversity suggests that the impacts of CTs in the soil are not strongly localised around individual aspen, but spread throughout the adjacent area. This is consistent with the idea that CTs are deposited into the soil via aspen leaf litter, which is mobile. Understanding the dynamics of forest understory communities is vital as the much of the biodiversity and ecosystem services in a forested area are maintained in the understory. Our results suggest that chemical feedbacks from aspen can influence understory plant communities and should be taken into account when modelling or restoring aspen dominated communities.