Mountain forest biomass dynamics and its drivers in southwestern China between 1979 and 2017

Reforested areas can act as important carbon (C) sinks. In China, extensive reforestation has been carried out in mountainous regions, with resulting C storage affected by forest age, forest type and environmental settings. Evaluations of forest C sequestration therefore require a detailed spatio-temporal analysis of C storage dynamics. Here, we used aboveground biomass (AGB) of trees as a proxy for overall forest C storage to investigate spatiotemporal patterns and changes in AGB of 136,988 individual trees distributed over 1399 permanent plots in the forests of Sichuan province, China. Mean AGB of young plantation forests increased more rapidly at 5.25 ± 1.15 Mg ha⁻¹ year⁻¹ than that of natural forest (2.56 ± 0.38 Mg ha⁻¹ year⁻¹). Forest stand age, tree species diversity and tree density were superior predictors of AGB when compared to environmental and climatic factors. Linear Mixed Effect models accounting for stand age showed significant AGB storage increases with increasing soil depth as well as with decreasing longitude and altitude. Stocks in plantation forests also increased with southerly exposition and decreasing slope steepness, while in natural forests, slope steepness showed positive correlations. Warming temperatures depressed AGB increases across all forests, while decreasing annual precipitation negatively affected AGB increases in natural forest, only. Our study highlights that, to sustain forest AGB gains into the future, management especially of forest plantations needs to promote species-rich, unevenly-aged, climate-adapted forests stands.