During the Dust Bowl drought, central US grasslands responded unexpectedly to a decade of hot, dry conditions. Grass species adapted to high temperatures with higher water use efficiency (C₄ grasses) decreased while those preferring cooler climates (C₃ grasses) increased. We reproduced this surprising response by experimentally imposing extreme drought in two native grasslands. Analysis of historical climate records revealed that during extreme drought years, the proportion of annual precipitation that occurs during cooler months increases. This previously unidentified shift in seasonal precipitation patterns during extreme drought years provides a mechanism for C₃ grasses to increase despite overall hot, dry conditions. Thus, alterations in precipitation seasonality may be as important as reduced precipitation amount when forecasting ecosystem responses to extreme drought.

Drought was experimentally imposed at each site for four years using large rainfall exclusion shelters. At each site, 30 plots, each 36-m², were established across a topographically uniform area. Each of these were hydrologically isolated from the surrounding soil using aluminum flashing to a depth of 20 cm and 6-mil plastic barriers installed to a depth of 50 cm. Drought was imposed in 20 plots per site by installing large shelters (10x10 m²) that were used to block 66% of incoming rainfall during each growing season – this is roughly equivalent to a 50% reduction in annual precipitation given that 60-75% of MAP falls during the growing season in these ecosystems. The remaining ten plots per site were trenched and hydrologically isolated as well, serving as controls that received ambient rainfall. At the end of each growing season, all aboveground biomass was harvested in three quadrats (0.1 m²) placed randomly in new locations each year in two 2x2 m subplots designated for destructive measurements. A different subplot was harvested each year. Biomass was sorted to remove the previous year’s growth, dried for 48 hours at 60°C, and weighed to estimate total aboveground net primary production (ANPP), as well as ANPP of dominant plant growth forms (C₄ grasses, and C₃ grasses and C₃ forbs).