Species interactions are fundamental processes driving community assembly and are expected to shift with environmental conditions. While facilitation is recognized as a universally present process, it is unclear when and how facilitation is important to maintaining diverse grassland communities. Thus, understanding the role of facilitation, and how these interactions may shift with climate change, is critical for predicting the future of grassland communities. Further, understanding the mechanisms associated with these patterns will provide a more comprehensive view of drivers.
In this study, we used 1-meter squared maps of plant communities to look for evidence of species interactions in a semi-arid grassland community in northern Arizona. In order to assess spatial aggregation and dispersion as evidence of facilitation or competition respectively, we used a null model of expected minimum distance observed between a target individual and neighbors. Further, we utilized drought, control, and water rainfall manipulation treatments to see how these patterns compared across environmental conditions. Finally, we explored the mechanisms driving facilitation among species, by comparing environmental conditions, within and outside of the canopy of bunchgrass species, and compared functional trait distributions among interacting species.
Results/Conclusions
Our preliminary analyses reveal patterns of spatial aggregation that are consistent with facilitation. Surprisingly, these patterns were strongest under the water addition treatments suggesting that shade amelioration is only helpful if there is sufficient water available. Festuca arizonica and Elymus elymoides tend to be the strongest facilitators, each was aggregated with a different set of forb species. These species vary in their functional traits, as well as overlap with interacting species, and their impacts on the local environment. While soil moisture was highly heterogeneous, both Elymus elymoides and Festuca arizonica had impacts on soil moisture availability. Both species have lower light availability within their canopies than outside their canopies, however Festuca arizonica reduced light by up to 82% while Elymus elymoides reduced light by only 32%. Thus, both species may alleviate stress by reducing light, which may provide benefits of reduced evapotranspiration or photorespiration. Understanding how interactions shift with climate change, and the mechanisms driving them, is important for predicting and managing systems confronted with climate change.