Water availability affects species interactions and species diversity in a semi-arid grassland community

Elise C. Elwood, Evolution and Ecology, University of California, Davis, Davis, CA, Margaret M. Moore, School of Forestry, Northern Arizona University, Flagstaff, AZ, Daniel Laughlin, Botany Department, University of Wyoming, Laramie, WY and Jennifer R. Gremer, Center for Population Biology, University of California, Davis, Davis, CA

Background/Question/Methods
Species interactions are fundamental processes driving community assembly. As the climate changes, altered precipitation patterns are expected to affect both the direction and magnitude of species interactions. In drylands, positive species interactions (facilitation) influence community composition and diversity, for example when nurse plants ameliorate harsh conditions and improve establishment for other species. Yet, it is unclear if and how facilitation influences seedling establishment and diversity in semi-arid grassland communities. Interspecific trait variation in potential facilitators may determine which graminoids act as nurse plants. Developing a better understanding of the mechanisms associated with species establishment will assist in predicting the future of grassland communities.
Here, we used detailed maps of grassland community plots exposed to control, drought, or water addition treatments in northern Arizona. We evaluated the nature of species interactions from spatial patterns consistent with aggregation (positive interactions) and dispersion (negative interactions) among individuals. Further, we tested whether those patterns varied with environmental conditions, namely water availability. Further, we compared microenvironmental conditions within and outside of the canopy of perennial graminoids. Finally, using NMDS and linear models, we assessed if functional traits of species predict the direction or magnitude of interactions.
Results/Conclusions
Our spatial analyses reveal patterns of spatial aggregation that are consistent with facilitation, though patterns vary according to water availability, species, and their interaction. Neighborhoods surrounding Festuca arizonica and Muhlenbergia montana have high metrics of diversity, while abundance of species is higher near Blepheroneuron tricholepsis. However, these patterns vary with water availability and by species. Local richness around graminoids is less variable under water stress. While positive interactions increased with water addition for some species, other species interactions were unchanged under water stress.
Microenvironmental climate at the base of perennial graminoids varies by species, with Festuca arizonica and Elymus elymoides having lower light availability within than outside their canopies, reducing light by 82% and 32% respectively, potentially alleviating stress from evapotranspiration or photorespiration. Different suites of functional traits in graminoids may impact local diversity and local abundance. Developing our understanding of how spatial patterns of establishment and species diversity are shifting within communities exposed to new climatic extremes and what mechanisms may be affecting these shifts is a critical step in protecting and managing our grassland systems.