Background/Question/Methods
The climate in the southern United States (SUS) is warm and wet, with mild winters and high humidity, compared with the rest of the continental United States; however, due to increased water use demands and climate variability, this water-rich region is experiencing water stress. The precipitation in the SUS has a great spatial variability with an increasing gradient of precipitation from the western to the eastern part and a great inter-annual variation ranging from 797 to 1316 mm during the 20^th century. Regional difference in distribution of drought events makes it more complicated to study impacts of drought on terrestrial ecosystems. We found that the timing and frequency of drought events were greatly differed for the western and eastern SUS during 1895-2008. Based on palmer drought index, we regroup three drought-impact classes: extreme/severe drought, moderate drought, and no drought for the western and eastern SUS, respectively. A well-calibrated and validated Dynamic Land Ecosystem Model (DLEM), which includes a set of sophisticated algorithms to relate water stress to plant growth, was used to simulate the impacts of drought on ecosystem net primary productivity (NPP) and carbon sequestration in the terrestrial ecosystems of the southern United States.

Results/Conclusions
The model simulation results indicated that drought has significantly reduced NPP and ecosystem carbon storage during 1895-2008. NPP could reduce over 50% in some regions during extreme drought years. A large amount of carbon was released to the atmosphere from the terrestrial ecosystem during drought periods. Grassland was more vulnerable to the impacts of drought than other plant functional types. Regional difference in drought intensity caused a huge spatial variability in the reduction degree of NPP and net carbon exchange rate throughout the SUS.