The effects of extreme heat and drought on a tallgrass prairie ecosystem

Climate extremes, such as heat waves and drought, are expected to increase in their frequency and intensity over the next century.  We examined the responses of the tallgrass prairie ecosystem to a simulated two-week heat wave at four levels (0, +4, +7 and +11 °C above ambient) under well-watered and severe drought conditions. Our goals were to examine the species-specific responses of the co-dominant C4 grasses (Andropogon gerardii and Sorghastrum nutans) to climatic extremes and to evaluate the independent and interactive effects of drought and heat.  We found clear species-specific responses to intense drought with S. nutans having greater sensitivity than A. gerardii, based on photosynthetic and productivity (ANPP) responses.  In addition, there was a strong interaction between heat and precipitation on photosynthesis.  Surprisingly, the only response to heat was within the wetter, ambient treatments, and not the drought.  While A. gerardii appeared more physiologically sensitive to the heat than S. nutans, there were no significant effects (p > 0.05) on productivity for either species.   At the ecosystem level, there were significant reductions in total ANPP in the severe drought treatment, which was well below the 5^th percentile of growing season rainfall for the site.  Finally, while the simulated heat wave exceeded Kansas state records, it did not significantly impact total ANPP, despite a strong negative trend with increasing heat in the severe drought treatment.