Characterization of Vegetation Phenology in Response to Drought in the Middle and High Latitudes of China

Vegetation phenology directly affects its biomass and regulates ecosystem carbon cycling processes. Currently, the impacts of climate change (especially drought) on vegetation phenology at mid- to high-latitudes in China are still unclear. Therefore, based on the GIMMS NDVI3g data, the Start of the Season (SOS) and End of the Season (EOS) parameters for the growing seasons of several vegetation types in the region north of 30° N were extracted. The extracted data were then combined with field observations to verify the effects of climate change on ecosystem carbon cycling. The results were then combined with field observations to verify the reliability of the parameters, and the saturated vapor pressure difference (VPD) and the improved standardized precipitation evapotranspiration index (SPEI) were used to investigate the characteristic patterns of vegetation phenology in response to drought. The results showed that: (1) the vegetation phenology varied significantly in different regions, with the SOS of the first growing season of mono- and bi-coastal vegetation concentrated between the 30th and 180th days of the year, and the SOS of the second growing season of bi-coastal vegetation concentrated between the 200th and 220th days of the year. The EOS spanning the 1st growing season of single-season and double-season vegetation was mainly concentrated between the 180th and 300th day of the year, and the EOS of the 2nd growing season of double-season crops was mainly concentrated between the 260th and 300th day of the year. (2) The rise of forest pre-season VPD leads to advancement of SOS and delay of EOS of vegetation; the rise of grass pre-season VPD leads to lagging of SOS and advancement of EOS of vegetation. (3) SPEI was positively correlated with SOS and EOS of vegetation in most parts of the study area, drought tended to contribute to the advancement of SOS and EOS of vegetation in the area.