PS 5-42
Impact of prolonged soil moisture deficit on water use efficiency in US grasslands
Climate change is predicted to cause an increase in the frequency and duration of droughts over the next century, which will result in prolonged periods of below-normal soil moisture. Prolonged soil moisture deficit impacts biomass production for current and subsequent growing seasons and has been associated with recent regional mortality in shrublands and woodlands across the United States (US). This study investigated the effects of prolonged soil moisture deficit on the response of grassland annual net primary production (ANPP) to precipitation. It has been postulated that the quick ecological response of grasslands to droughts can provide insight to large scale functional responses of regions to predicted climate change. For six grasslands across the US from 2000-present, we compiled in situ measurements of soil moisture and precipitation, and estimates of grassland production from the NASA MODIS Enhanced Vegetation Index (EVI). In addition, for 4 regional grasslands in Southwest US, Peru, Australia and Burkina Faso, we compiled equivalent satellite data including soil moisture from ESA SMOS and precipitation from NASA TRMM. We developed a soil moisture index to determine periods of prolonged soil moisture deficit, and monitored ANPP before, during and after such periods to quantify significant shifts in water use efficiency.
Results/Conclusions
Preliminary results indicated that when altered climatic conditions in grasslands led to an increase in the duration of soil water deficit, then the precipitation-to-ANPP relation became non-linear, disrupting the water use efficiency (WUE) in grassland sites. Changes in WUE were due to anomalous lags between precipitation and ANPP, extreme grassland dieback, and changes in the historic species assemblage including invasive species. The magnitude of change was related to the precipitation regime, where grasslands in hyper-arid and humid regimes were least likely to be affected by prolonged soil moisture deficit, and semiarid and mesic grasslands were most likely to be impacted, depending on the duration of the deficit. These results advance our understanding of herbaceous responses to complex climate variability and demonstrate the value of new global observations of surface soil moisture with the orbiting ESA SMOS and upcoming NASA SMAP missions.