Assessing grassland sensitivity to global change

Abstract

Intensification of the global hydrological cycle with atmospheric warming is expected to substantially alter precipitation regimes, and due to the tight functional relationship between precipitation and net primary productivity (NPP), these changes in climate will have large impacts on multiple NPP-linked ecosystem services such as forage production and carbon storage. At regional scales, the sensitivity of aboveground NPP (ANPP) to variation in annual precipitation increases with decreasing site-level ANPP, with this variation in sensitivity tied to turnover of plant communities over the precipitation gradient. Site-level ANPP responses are not expected to conform to regional patterns until plant communities shift, resulting in differential short- vs. long-term ANPP responses to chronically altered precipitation amounts. Although studies in grasslands have shown site-level sensitivities of ANPP to altered precipitation regimes, we lack equivalent knowledge for responses of belowground net primary productivity (BNPP) and total NPP. This will be especially important as simultaneous global change factors occur (e.g., increased fire frequency) and interact with climate change drivers to determine how the sensitivity of NPP will influence ecosystem services. My dissertation examines how plant community structure controls ecosystem sensitivity to altered precipitation amounts and patterns, and how this impacts various ecosystem services by addressing the following questions: (1) How do plant species and functional compositions control ecosystem sensitivity to altered precipitation regimes? (2) Does belowground sensitivity mirror that aboveground? And (3) What are the consequences of differential ANPP and BNPP sensitivity on biogeochemical processes in the presence of annual fire regimes? In my second chapter, I show how functional types (C₃ versus C₄ graminoids) can alter regional patterns of sensitivity to annual precipitation through differences in the timing of growth. I also show that ANPP and BNPP sensitivities can differ, but that it likely depends on vegetation and/or other attributes of an ecosystem. In chapter three, I focus on how shifts in plant species abundances, even within the same functional type, can alter sensitivity to extreme, chronic increases in precipitation. The shift in sensitivity was, again, not in agreement with regional patterns of sensitivity. Lastly, chapter four shows that the differential sensitivity of ANPP and BNPP to long term increases in precipitation can destabilize the carbon and nitrogen sequestration ability of ecosystems in the presence of extreme disturbance regimes also likely to occur in the future. Overall, my dissertation calls into question the predictive ability of regional models of NPP sensitivity under chronic shifts in precipitation amount, at least on short to moderate time scales, and I suggest that incorporation of plant community controls on above- and belowground sensitivity will be better predictors of ecosystem service responses under novel environmental conditions likely to occur in the future.