Drought- induced decrease in tree productivity mainly mediated by the maximum growth rate and growing-season length in a subtropical forest

It has been well demonstrated that tree growth is remarkably impacted by drought stress, which considerably influences terrestrial carbon (C) cycling and its feedbacks to climate system. However, how drought-induced changes in maximum growth rate and phenology regulate the trunk radial growth remain unclear, limiting our mechanistic understanding of tree growth responses to drought. In this study, a long-term 70 % rainfall exclusion experiment was conducted to examine drought effects on the trunk radial growth, timing of growth onset and cessation, and maximum growth rate of dominant tree species Schima superba in a subtropical forest. Our results showed that extreme drought significantly decreased maximum growth rate and total radial growth of S. superba by 51.00 % and 62.80 %, respectively. In addition, the length of the growing season was decreased by drought via delaying the timing of growth onset by 21 days and advancing the ending by 33 days. The drought-induced decline on maximum growth rate explained a greater proportion (67 %) of reduction in cumulative radial growth of S. superba (P<0.05) relative to phenology shift (e.g., onset and ending of growing season). This study identifies drought as a major potential driver of radial tree growth and highlights the dominant role of maximum growth rate in above-ground biomass accumulation in subtropical forests under drought stress. These findings suggest that short-term regional water management at the drying periods would be an important and feasible way for increasing forest C sink in the future climate.