Print PageOver the next century, global climate change is expected to include increased mean air temperatures, as well as increased frequency and intensity of climate extremes, such as heat waves and droughts. We examined responses of a tallgrass prairie ecosystem to a simulated two-week heat wave with contrasting soil moisture levels; those reflecting ambient rainfall vs. experimentally imposed drought conditions. We focused on responses of the co-dominant C4 grasses Andropogon gerardii and Sorgastrum nutans, since they comprise the majority of biomass in these ecosystems. Precipitation treatments were imposed from May to October 2010 using rainfall manipulation shelters placed on intact prairie, with the ambient treatment receiving natural rainfall amounts and the drought treatment receiving ~30% of total rainfall. In the context of the rainfall treatments, a two-week heat wave of increasing intensity (0, +2, +4, and +8 °C above ambient) was imposed in late July. Previous studies suggest that the two species may differ in their responses to alterations in temperature and precipitation, and therefore we expected that S. nutans would be more sensitive and respond negatively to drought whereas A. gerardii would respond negatively to increasing intensity of heat waves.
Results/Conclusions
Greater sensitivity to drought was evident for S. nutans with larger reductions in leaf water potential, net photosynthesis, and aboveground net primary productivity (ANPP) relative to A. gerardii, which was less sensitive to heat and drought. The effect of the heat wave on net photosynthesis was strongly contingent on soil moisture for both dominant grasses, however. At ambient soil moisture levels, A. gerardii displayed a negative linear response to increasing intensity of heat wave, while a non-linear response was evident for S. nutans, with reductions only occurring at the highest heat wave intensity. There was no effect of the heat wave treatments on net photosynthesis for either species when soil moisture levels were low due to drought. Total ANPP was reduced overall by the drought treatment. However, because S. nutans ANPP was reduced by drought whereas A. gerardii was not, dominance by A. gerardii increased under drought conditions. Perhaps because the heat wave lasted only two weeks mid-summer after which most plant growth had occurred, there was no effect of increasing intensity of heat wave effect on ANPP. These results suggest that more extreme climates have the potential to alter patterns of productivity and dominance in this mesic grassland.
