Drought timing and primary productivity in a semiarid grassland

While most studies account for the effect of total annual precipitation on aboveground net primary production (ANPP), few studies have demonstrated how decreased precipitation within specific periods of growing season affects ANPP. Using a precipitation manipulation experiment, we assessed the response patterns and underlying mechanisms of ANPP to decreased precipitation in the early (DEP) and late (DLP) growing seasons on a temperate steppe in northern China. The results showed that both DEP and DLP decreased ANPP by averages of 28.3 and 39.4 g m(-2), respectively. The decline of ANPP in DEP was primarily attributed to a 20.7 g m(-2) decrease in grass and 4.9 g m(-2) decrease in sub-shrubs, whereas the decrease in ANPP in DLP was due to a 36.6 g m(-2) decrease in perennial forbs and an 8.7 g m(-2) decrease in sub-shrubs. The reduction in grass ANPP under DEP was likely due to decreased soil moisture and increased specific leaf area of grass during the early growing season, whereas the reduction in perennial forbs ANPP under DLP was mainly caused by decreased soil moisture during late growing season. The decline of sub-shrubs ANPP under DEP and DLP could be explained by shallow root length of sub-shrubs in DEP and by high soil temperature in DLP. Our findings demonstrated how droughts timing affects ANPP at both community and functional group levels. Different responses of functional groups to drought timing indicate that future studies should not neglect the role of precipitation timing in regulating ecosystem productivity.