Ecological responses to an extreme climatic event in the tallgrass prairie ecosystem

Background/Question/Methods

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 heat wave under well-watered and severe drought conditions.    The severe drought treatment (33% of ambient rainfall) was imposed using passive rainout shelters from March through September 2011, while the well-watered treatment received ambient rainfall and supplemental irrigation from June to August to maintain soil moisture levels. The severe drought treatment resulted in a 50% reduction in soil moisture when compared to the well-watered treatment.  The heat wave treatments were imposed for two weeks in mid-July at four levels (0, +4, +7 and +11 degrees C above ambient).  The effects of the simulated climate extremes on ecophysiology of the co-dominant C4 grasses (Andropogon gerardii and Sorghastrum nutans) and aboveground net primary productivity (ANPP) were assessed.  Our goals were to evaluate whether the ecosystem experienced and extreme climatic event (ECE) – defined as a statistically rare climate extreme(s) resulting in a statistically rare or unusual ecological response, and to explore the mechanisms underlying the ecological response to the climate extremes.    

Results/Conclusions

Co-dominant C4 grasses showed similar physiological responses to the severe drought and heat wave treatments: 1. mid-day leaf water potential was significantly reduced by both climate extremes, whereas 2. net photosynthesis was significantly reduced only by the severe drought. Although ecophysiology was similarly impacted in the co-dominant grasses, the severe drought treatment significantly reduced biomass in S. nutans by 54% but not in A. gerardii. This along with a 75% reduction in biomass of the dominant forb, Solidago canadensis, resulted in a 60% reduction in total ANPP.   Although the heat wave treatments exceeded Kansas state records, the heat-wave treatments did not significantly impact ANPP, yet a strong trend for an interaction between the heat wave and severe drought treatments was evident.  Despite this difference in productivity, we were able to impose an ECE with the severe drought treatment but not the heat wave treatments, as growing season rainfall (May-August) for the severe drought treatment (210 mm) was well below the 5^th percentile of rainfall for that period based on a 25 year record, which resulted in a reduction in total ANPP that was also well beyond the normal bounds of variation for the ecosystem.