Changes and driving factors of carbon stock in the tree layer of old-growth forests in Taiyang River Provincial Nature Reserve, Yunnan, China.

The analysis of carbon dynamics in old-growth forests helps us understand forest conservation, restoration, and regional carbon sequestration. There is still controversy over whether old-growth forests are carbon sources or sinks. Studying the carbon storage and dynamics of old-growth forests is of great significance for evaluating their carbon source and sink functions, as well as quantifying forest carbon fixation at the regional scale. Based on the dynamic monitoring data of the old-growth forest in the Taiyanghe River Provincial Nature Reserve in Yunnan Pro-vince in 2014 and 2024, we investigated the biomass and carbon storage of tree layer in the old-growth forest, as well as their distribution characteristics in different diameter classes and organs by using principal component analysis and random forest model. We also analyzed the dynamics of carbon storage and their influencing factors. The results showed that the total biomass of the tree layer in 2014 and 2024 were 359.72 and 449.44 t·hm, respectively, and the total carbon storage was 179.86 and 224.72 t·hm, respectively, showing an upward trend and demonstrating good carbon sequestration function. The carbon storage of large trees (diameter at breast height≥22.5 cm) in 2024 (188.96 t·hm) was significantly higher than that in 2014 (143.69 t·hm), and the proportion of total carbon storage in the tree layer increased from 79.9% to 84.1%. In both surveys, carbon in the tree layer was mainly distributed in the trunk, followed by branches and roots, with leaves having the lowest carbon content. There was a positive correlation between species diversity and the carbon storage of the tree layer. The biomass of large trees, as the main driving factor for changes in carbon storage in the tree layer of aging forests, had variance explanatory rates of 10.1% and 13.6% in 2014 and 2024, respectively. During the monitoring period, the relative importance of driving factors for carbon storage in the old-growth forest tree layer changed: the explanatory power of the coefficient of variation of large tree diameter at breast height and altitude increased, while the explanatory power of species diversity significantly decreased from 13.3% to 2.6%. Overall, the tree layer of the old-growth forest in the study area serves as a carbon sink, with large trees dominating the dynamic changes in carbon storage.