Increase in carbon sink in a protected tropical seasonal rainforest in southwestern China over 20 years

Tropical forests play a pivotal role in the global carbon cycle, but the lack of long-term in-situ datasets renders our understanding of the specific carbon dynamics in tropical forests uncertain. In this study, we analyzed two decades (2003–2022) of eddy-covariance measurements from a primary tropical seasonal rainforest reserve in Xishuangbanna, southwest China, to characterize long-term trends in gross primary productivity (GPP), ecosystem respiration (Reco), and net ecosystem productivity (NEP). The protected rainforest functioned as a modest but steadily strengthening carbon sink (annual mean NEP = 157.9 ± 56.7 g C m⁻² year⁻¹, growth rate = 3.4% year⁻¹), consistent with an observed increase in carbon use efficiency (CUE) (annual mean CUE = 5.9% ± 1.8%, growth rate = 2.4% year^-1), which reflects increasingly efficient carbon utilization and aligns with rising aboveground biomass. The enhancement of the interannual carbon sink was mainly driven by increasing GPP (mean = 2658.1 ± 254.5 g C m⁻² year⁻¹, growth rate = 1.0% year⁻¹). With the same 6-month duration, the tropical seasonal rainforest exhibited a stronger carbon sink during the dry season (148.3 g C m^-1 season^-1) than during the rainy season, with the dry season accounting for 93.9% of the annual carbon sink. The enhanced dry season radiation and precipitation throughout the two decades positively affected the upward trend of the carbon sink. Notably, the annual carbon sink showed a temporary decline approximately two years after droughts, suggesting a lagged ecosystem response to climatic disturbances. Overall, these findings underscore the long-term carbon sequestration potential of well-preserved tropical rainforests and provide critical empirical evidence for improving carbon budget assessments in tropical regions under ongoing climate change.