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

Tropical forests play a significant 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. This study investigated the long-term trends (from 2003 to 2022) in gross primary productivity (GPP), ecosystem respiration (Reco), and net ecosystem productivity (NEP) at a primary tropical rainforest reserve in Xishuangbanna, southwest China based on the eddy covariance technique. Our study found this protected tropical seasonal rainforest to be a modest carbon sink (annual mean NEP = 157.9 ± 56.7 g C m⁻² year⁻¹), with a NEP growth rate of 3.4 % year⁻¹ and a similar upward trend of annual mean carbon use efficiency (CUE) (annual mean CUE = 5.9 % ± 1.8 %, growth rate = 2.4 % year⁻¹). The increase in NEP was mainly due to the rising trend in GPP, which averaged 2658.1 ± 254.5 g C m⁻² year⁻¹ and grew at 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⁻² 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. These findings underscore the potential of well-protected primary tropical rainforests to act as carbon sinks in the long run, contributing to future carbon budget predictions for the tropical region.