SYMP 5-4 - CANCELLED - Differential sensitivity of aboveground and belowground processes to drought in tallgrass prairie

Background/Question/Methods
Climate change will increase the frequency, duration, spatial extent, and intensity of droughts across the world, leading to impaired ecosystem function of most terrestrial ecosystems. Grasslands are particularly vulnerable to drought. Water shortages often impair plant photosynthesis and gas exchange, leading to reduced primary production. Drought can also impact microbial communities, reducing microbial abundance, shifting community composition, and inhibiting microbial activity. Yet few studies have simultaneously examined the impacts of drought on both above- and belowground processes, such that it is often difficult to directly compare which of the two ecosystem components is more drought sensitive. In this experiment, we imposed a severe drought on a tallgrass prairie for two consecutive years. In each year, we measured aboveground (gas exchange, primary production) and belowground (microbial respiration, community composition, abundance, and decomposition) ecosystem processes. Furthermore, the second year was a natural drought year, allowing us to compare our experimental results to a natural drought.

Results/Conclusions
We found that aboveground processes were surprisingly resilient to both natural and experimental drought. During the first year, drought had no impact on leaf-level gas exchange or ecosystem primary production. During the second year, patterns in the natural drought mimicked the experimental drought of the first year. However, the stronger drought treatment in the second year did demonstrate slight reductions in gas exchange for Schizachyrium scoparium, although primary production was not affected. In contrast, belowground processes were significantly impaired by drought. Though microbial composition was unaffected by drought, we did find reduced microbial activity throughout most of the growing season due to drought in both years. The reduced microbial activity lead to lower decomposition rates, but only of refractory material. Our results suggest that belowground processes might therefore experience sharper and more acute changes to function long before any visual impairment of ecosystem function aboveground.