Droughts and Deluges in Semiarid Grassland Ecosystems: Implications of Co-Occurring Extremes for Carbon Cycling

Authors

Melinda D. Smith¹* (melinda.smith@colostate.edu), Anping Chen¹, Daniela F. Cusack², Alan K. Knapp¹, Andrew J. Felton³, David L. Hoover⁴, Seth M. Munson⁵, Anthony P. Walker⁶

Institutions

¹Department of Biology, Colorado State University, Fort Collins, CO; ²Department of Ecosystem Science and Sustainability, Colorado State University, Fort Collins, CO; ³Department of Land Resources and Environmental Sciences, Montana State University–Bozeman, MT; ⁴Crops Research Laboratory, USDA-ARS, Fort Collins, CO; ⁵Southwest Biological Science Center, U.S. Geological Survey, Flagstaff, AZ; ⁶Oak Ridge National Laboratory, Oak Ridge, TN

Abstract

The overall research project goal is to assess how co-occurring drought and deluge climate extremes will impact key carbon (C)-cycling processes known to be important for carbon-climate feedbacks. The team will address this goal via research in the 280,000 km² semiarid shortgrass steppe ecoregion located at the western edge of the U.S. Great Plains. Semiarid regions such as this one respond strongly to precipitation extremes and play a dominant role in interannual variability of the global land carbon dioxide sink. The key hypothesis being tested is that when a compound climate perturbation of an extreme deluge occurs within the backdrop of extreme drought, a combination of conditions converge (e.g., warm temperatures, abundant soil moisture, and increased soil nitrogen availability) to strongly stimulate C-cycle processes, potentially resulting in “hot moments” or landscape-level “hot spots” (i.e., increases in biogeochemical processes in time or space that far exceed background levels). To test this hypothesis, researchers are planning to conduct a field experiment designed to quantify the magnitude of C-cycling responses to drought and deluge events (independently and combined) and identify the underlying mechanisms resulting in positive drought-deluge interactions that can lead to hot moments of C cycling. Both above- and belowground C-cycle responses to climate extremes will be quantified during this 3-year experiment. To scale up from the plot-level experiment to the shortgrass steppe ecoregion, the team will use historical climate data to quantify the regional frequency of potential drought-deluge interactions and remote sensing products (e.g., normalized difference vegetation index and solar-induced fluorescence) to estimate C-cycling sensitivity to droughts, deluges, and their combined effects and to identify hot spots in C cycling regionally. Concurrent with these research activities, extreme drought, deluge, and drought-deluge perturbations will be simulated with DOE’s E3SM Land Model (ELM). Researchers will explicitly compare the experimental results and remotely sensed observations of drought-deluge compound climate perturbations to ELM simulations, with the expectation that the process-level understanding gained from field experiment and remote sensing analyses can be used to constrain process representation and parameterization in ELM and to improve Earth system projections of ecosystem C-cycling responses to droughts and deluges at the ecoregion scale.