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 & PLUSMN; 1.15 Mg ha? 1 year? 1 than that of natural forest (2.56 & PLUSMN; 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 pre-cipitation 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.

Automated summary

Reforested areas in China's mountainous regions play a crucial role in carbon sequestration, however, their carbon storage is influenced by forest age, type, and environmental factors. This study analyzed data from forests in Sichuan province and found that young plantation forests have a higher rate of aboveground biomass increase compared to natural forests. Environmental factors were not the main predictors of aboveground biomass, instead, forest stand age, tree species diversity, and tree density were more influential. Accounting for stand age, there were significant increases in aboveground biomass with increasing soil depth, decreasing longitude and altitude. The study emphasizes the importance of promoting species-rich, unevenly-aged, climate-adapted forest stands for sustainable aboveground biomass gains in the future.