Thinning promotes forest carbon stocks by optimizing tree growth and soil properties

Forest thinning is a common silvicultural practice that influences the forest carbon dynamics. However, current research focused on a few thinning management variables and specific regions, and the drivers of carbon stock changes in thinned forests at the global scale remain unclear. Through a global meta-analysis of 197 studies comprising 821 pairs of thinning data, we comprehensively evaluated the effects of thinning on aboveground carbon (AGC) and soil organic carbon stocks (SOCS), and to identify the drivers of them. Thinning generally reduced AGC in the short term, but moderate intensities (20 %–35 %), especially combined with understory plant removal, accelerated AGC recovery. Stem-only harvesting (SOH) increased the recovery rate of AGC by 6.05 % compared with whole-tree harvesting (WTH). Strip thinning impeded AGC recovery due to edge effects. Thinning significantly enhanced SOCS by 3.53 %. However, thinning intensity was not the primary determinant, and the positive effects on SOCS gradually disappeared after 10 years. Moreover, fertilization, pruning, and understory plant removal all contributed to further increased SOCS in thinned forests. Structural equation modeling (SEM) revealed that AGC was driven by thinning intensity and recovery time, and soil organic carbon (SOC) and tree height were main direct determinants. SOCS was indirectly enhanced by increases in soil total nitrogen (TN), whereas it was negatively influenced by diameter at breast height (DBH) growth driven by recovery time. Overall, our findings emphasize the critical role of appropriate thinning practices in facilitating carbon stock recovery, while also elucidating the driving mechanisms of tree growth and soil properties on carbon dynamics in thinned forests. • Moderate thinning and understory plant removal were conducive to the recovery of AGC. • Overall, thinning enhanced SOCS, but effects gradually disappeared after 10 years. • Stem-only harvesting was more favorable for maintaining AGC than whole-tree harvesting. • SOC and tree height were the main drivers of AGC, TN was an indirect driver of SOCS.