COS 30-1 - Catastrophic loss of plant species and ecosystem function with a Dust Bowl-type drought

Abstract: With climate change, a well-supported expectation is that drought events will increase in magnitude and duration. However, because such extreme events are by definition rare, our understanding of the impacts of extreme drought on ecosystem structure and function is primarily anecdotal. We conducted a five-year, year-round drought experiment to assess how a drought of this level of extremity and prolonged duration would affect semi-arid shortgrass steppe ecosystem structure and function. Our goal was to simulate experimentally a similarly profound drought as that which occurred during the 1930’s Dust Bowl. We accomplished this goal using rainout shelters established above intact shortgrass steppe. The shelters were established in 2018 and had roofs that passively reduced each rainfall event by 66%. The roofs were in place year-round for five years, and during this time we measured plant composition and aboveground net primary production (ANPP) each year, and in the final year of the experiment we also measured soil CO₂ efflux biweekly during the growing season and installed root ingrowth cores (0-30 cm) to estimate belowground net primary production (BNPP).

During the five-year drought, we observed a significant loss in ANPP, with the greatest loss (~70% reduction) occurring by end of the second year of drought. By the fifth year of drought, ANPP was >90% lower in the drought treatment when compared to plots that received ambient precipitation. The large loss of ANPP was accompanied by significant loss in plant species, total abundance (cover) and a dramatic change in species composition. In particular, the dominant grasses, Bouteloua gracilis and B. dactyloides, were almost entirely lost from the community by the fifth year of drought, with only a few forb and cactus species remaining. During the final year of drought, soil CO₂ efflux was significantly reduced, as was BNPP, with the greatest loss observed in the top 10 cm of the soil profile (77%), with lesser reductions at 10-20 cm and 20-30 cm depths (50%). Thus, both losses of carbon and inputs via above and belowground production were dramatically impacted by the multi-year drought, with a collapse in the plant community attributable to the loss of the dominant grasses. These results suggest that extreme drought – comparable to that observed during the Dust Bowl and forecast with climate change – can lead to catastrophic changes in structure and function of shortgrass steppe, with important consequences for carbon cycling and potentially long-lasting impacts after drought ends.