Photosynthetic responses of a dominant C4 grass to an experimental heat wave are mediated by soil moisture
Extreme heat waves and drought are predicted to increase in frequency and magnitude with climate change. These extreme events often co-occur, making it difficult to separate their direct and indirect effects on important ecophysiological and carbon cycling processes such as photosynthesis. Here, we assessed the independent and interactive effects of experimental heat waves and drought on photosynthesis in Andropogon gerardii, a dominant C4 grass in a native mesic grassland. We experimentally imposed a two-week heat wave at four intensity levels under two contrasting soil moisture regimes: a well-watered control and an extreme drought. There were three main findings from this study. First, the soil moisture regimes had large effects on canopy temperature, leading to extremely high temperatures under drought and low temperatures under well-watered conditions. Second, soil moisture mediated the photosynthetic response to heat; heat reduced photosynthesis under the well-watered control, but not under the extreme drought treatment. Third, the effects of heat on photosynthesis appeared to be driven by a direct thermal effect, not indirectly through other environmental or ecophysiological variables. These results suggest that while photosynthesis in this dominant C4 grass is sensitive to heat stress, this sensitivity can be overwhelmed by extreme drought stress.
KeywordsCanopy temperature Climate extremes Drought Ecophysiology Tallgrass prairie
We thank Patrick O’ Neal, Jeff Taylor, Kelsey Duffy, Mellissa Steen, Peter Bridgwater, Qee Qee Gao, and Katie Earixson for help with field and lab work. We would also like to acknowledge the Konza Prairie Biological Station, the Konza Prairie Long-Term Ecological Research program, Kansas State University Division of Biology and Colorado State University for institutional support. This project was funded by the Department of Energy Terrestrial Ecosystems Science Program (DE-FG02-04ER63892).
Author contribution statement
DLH, AKK, and MDS designed the study, analyzed the data, and wrote the manuscript. DLH established the experiment and collected the field data. MDS acquired financial support.
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