Diagnosing the impacts of climate extremes on the interannual variations of carbon fluxes of a subtropical evergreen mixed forest

Abstract Climate extremes are expected to have severe impacts on carbon cycle balance and ecosystem services, because of the highly nonlinear dependence of many carbon processes on climate variability. These impacts, such as the European heatwave in 2003, have been well documented for some temperate forests. However, few studies have been conducted on subtropical forests, which play a significant role in the global carbon cycle. Here, we analyzed continuous carbon flux measurements of a subtropical forest in South China from 2003 to 2016, during which the forest experienced extreme cold and hot spells and a prolonged drought. We found that the forest was a strong carbon sink, taking up about 400 to 740 g C m−2 yr−1 in most years, and that the forest became a much weaker sink or even a carbon source during two extreme periods. This large interannual variation of net carbon uptake by this forest resulted from the different responses of gross primary production (GPP) and terrestrial ecosystem respiration (TER) to climate extremes. The observed reduction in GPP during extreme years was well simulated by lowering maximum leaf carboxylation rate (vcmax) in a process-based model. Parameter optimization suggests that the reduced vcmax in the model recovered quickly after hot and cold spells but remained low after the prolonged drought. During the prolonged drought, the observed tree mortality and litter fall significantly increased, which increased TER in the following year. This lagged response in TER significantly contributed to the weakened carbon sinks of the forest during the two extremes. Our study highlights the importance of community dynamics and lagged response of the terrestrial carbon cycle to climate extremes.