Grassland resilience during the warm drought of the early 21st century
Grasslands across the United States play a key role in regional livelihood and national food security. Yet, it is still unclear how this important resource will respond to the prolonged warm droughts and more intense rainfall events predicted with climate change. The early 21st century drought in the southwestern U.S. resulted in hydroclimatic conditions that are similar to those expected with future climate change. For six US grasslands from years 2000-present, we compiled in situ measurements of surface soil moisture and precipitation, and estimates of above-ground net primary production (ANPP) derived from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI). This study addressed the impact of the early 21st century drought on ANPP of Desert and Plains grasslands dominated by C4 (warm-season) grasses, in terms of significant deviations between observed and expected ANPP.
In Desert grasslands, drought-induced grass mortality led to shifts in the functional response to annual total precipitation (PT), and in some cases, new species assemblages occurred that included invasive species. In contrast, the ANPP in Plains grasslands exhibited a strong linear function of the current-year PT and the previous-year ANPP, despite prolonged warm drought. The intra-annual precipitation pattern (larger storms and longer inter-storm dry periods) had a secondary impact on ANPP, and that impact was greater for Desert than for Plains grasslands. Results also showed a fundamental difference in the response of average annual soil moisture (at 5-cm depth) to intra-annual precipitation patterns between semiarid and mesic grasslands. That is, in mesic grasslands, soil moisture decreased with an increase in high-intensity storms; whereas in semiarid grasslands, soil moisture decreased with longer dry periods. We used these results to disentangle the impacts of inter-annual total precipitation, intra-annual precipitation patterns, and grassland abundance on ANPP, and thus, to generalize the functional response of C4 grasslands to predicted climate change. Our results will help managers to plan for predictable shifts in resources associated with climate change.