Increasing importance of precipitation in spring phenology with decreasing latitudes in subtropical forest area in China

Climate warming has significantly advanced plant spring phenology in temperate and boreal biomes in the northern hemisphere. However, the response of subtropical forest phenology to climate change remains largely unclear. This study aimed to determine the spatiotemporal patterns of spring photosynthetic phenology in subtropical forests in China over the period 2002-2017 and explore its underlying mechanism in response to the changes of different climate variables. We applied four methods to extract the start of the photosynthetic period (SOP) from a solar–induced chlorophyll fluorescence (SIF) data set during the period 2002 to 2017, and determined correlations between SOP and environmental factors using partial correlation analyses. Overall, the SOP was advanced by 6.8 days. Furthermore, we found that the SIF-based SOP is highly correlated with the flux data–based photosynthetic onset dates, demonstrating that SIF can be a useful index in characterizing the photosynthetic phenology in subtropical forests. Interestingly, based on partial correlation analysation temperature dominated the SOP in the northern subtropical forest, but the importance of precipitation increased with decreasing latitudes, and the primary climatic control of SOP in southern monsoon evergreen forests is precipitation. These results suggested that the predicted increase in temperature and shift in precipitation regimes under ongoing climate change might potentially largely affect the photosynthetic phenology, and thus affect the carbon and water cycles in subtropical forests.