Dynamics of net primary productivity on the Mongolian Plateau: Joint regulations of phenology and drought

Vegetation phenology has long been recognized as an effective indicator of ecosystem function and plays a significant role in the dynamics of plant productivity. Using the 30-year NDVI and meteorological data (1982-2011), we quantified the spatiotemporal dynamics of net primary productivity (NPP), start (SOS), end (EOS) and length (LOS) of growing season and summer drought index as standardized precipitation index (SPI) for the Mongolian Plateau. The independent and interactive contributions of phenological changes and summer drought on annual NPP were analyzed to explore the potential regulatory mechanisms of phenology on plant productivity. Results showed that NPP, SOS, EOS and LOS averaged at 265.4 g C/m(2), 123, 272 and 149 days, respectively, on the plateau and appeared stable during 1982-2011, but with high spatial variations. However, the summer droughts were significantly intensified from 1982 to 2011 (R-2 = 0.21, P = 0.01), with more pronounced drought during 1999-2011. More importantly, summer droughts played a very significant role in determining annual NPP dynamics (R-2 = 0.47, P < 0.001) due to the highest proportion of summer NPP to annual NPP (70%). The SOS and EOS had stronger contributions to NPP in the spring (R-2 = 0.26, P = 0.004 for April) and autumn (R-2 = 0.44, P < 0.001 for September; and R-2 = 0.25, P =0.005 for October) than that to the annual NPP. Due to the stronger influences of EOS on NPP compared to SOS and the larger proportion of autumn NPP (15%) to annual NPP compared to spring (13%), we emphasize the importance of future studies on the climatic extremes (e.g., droughts) during the spring and autumn.