Anthropogenic climate change is altering fundamental abiotic and biotic factors that often put chronic pressures on plant communities. These chronic resource alterations can drive changes in biodiversity, including variation in both the numbers and dominance of species, and shifts in community composition. Consequently, community functional trait composition and diversity may become more similar or dissimilar. Quantifying convergence or divergence of plant communities in trait space may reveal how specific traits mediate the impacts of long-term chronic resource alteration. The objective of this study is to test whether and how functional traits will shift and converge or diverge in response to global change manipulations. We considered both changes in mean trait values between treatment and control plots, as well as changes in variance amongst replicate plots within treatments and controls. Our preliminary results are from an irrigation and fertilization experiment running since 2007 at Cedar Creek Ecosystem Science Reserve, but other long term resource manipulation experiments will be included in subsequent analyses. Community weighted means were calculated for each trait. We investigated shifts in trait composition using resemblance based permutation methods including permutation ANOVA (PERMANOVA), and convergence/divergence of replicate plots by heterogeneity of multivariate dispersion (PERMDISP).
We found that the interaction between the irrigation and fertilization manipulations significantly shifted trait composition relative to control plots; alone, irrigated plots were significantly different from control plots, but fertilized plots were not. Specifically, the manipulated plots were pulled towards higher values for specific leaf area, plant height, photosynthetic rate and towards lower values for leaf nitrogen content. There were no differences in the variation in trait space of manipulated plots compared to the controls. Thus, while mean trait compositoin is effected by the manipulations, there is no convergence to similar trait values compared to control plots. This suggests that under chronic resource alterations, communities are shifting functionally based on their traits but may still have enough variation within the community to be able to respond to other perturbations.