Effects of chronic eutrophication on community and ecosystem resistance and resilience to drought

Meghan Avolio, Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD and Sally E. Koerner, Department of Biology, University of North Carolina Greensboro, Greensboro, NC

Background/Question/Methods
Increased nutrient inputs and drought independently affect the composition and ecosystem function of plant communities. While eutrophication is known to change the plant community, it remains relatively unknown if these altered plant communities are less resistant or resilient to drought. Here we addressed whether eight years of chronic nutrient additions impacted the response of a native tallgrass prairie to drought. This study took place at Konza Prairie in Manhattan, KS, within a biennially burned watershed. A three-year drought followed by three years of recovery was nested within two crossed N and P treatments, 0 and 10 g m-2. The nutrient additions started in 2003 and resulted in drastically different plant communities. Over eight years the control and P had similar plant communities, while N additions resulted in greater forb dominance, and N and P together drastically shifted the plant community to one dominated by annual and perennial forbs. In 2010 the drought treatments were imposed on these different plant communities. Plant species composition was surveyed both in the spring and late summer each year, and aboveground biomass was harvested at the end of each summer to estimate aboveground net primary productivity (ANPP).
Results/Conclusions
We found high resistance to drought across all plant community types, with drought not impacting Shannon’s diversity, evenness, grass biomass, forb biomass, ANPP in any of the different eutrophication treatments. We did however, observe an N x drought interaction for species richness and cover of C4 grasses. Drought in plots without N reduced species richness and cover of C4 grasses, but drought with N increased species richness and cover of C4 grasses. The three year drought also impacted the recovery of the communities. The N and P addition plots had significantly higher cover of annual grasses when previously droughted compared to those which received ambient rainfall. High amounts of bare ground early in the growing season in the N and P previously droughted plots likely created conditions where annual grasses could germinate and thrive. During recovery we also found higher grass biomass and ANPP but lower forb biomass in N and P plots that had been previously droughted compared to those that had not. Overall, our results demonstrate that despite strong effects of eutrophication, the altered plant communities were equally able to resist and capable of recovery from drought as native unaltered tallgrass prairie.