Thinning increases forest ecosystem carbon stocks

Thinning, a major measure of forest management, is known to alter carbon sequestration by affecting the forest microclimate and soil environmental conditions. However, how thinning effect on forest ecosystem carbon stocks have presented inconsistent results. Therefore, we conducted 1776 pairs of thinning experiment observations worldwide to explore how thinning affects forest vegetation and soil carbon stocks. The results showed that overall thinning significantly increased total tree aboveground biomass carbon stocks (AGCtotal, +23.9%, +1.3 Mg ha-1 yr-1), understory vegetation carbon stocks (UVC, +68.3%, 0.12 Mg ha-1 yr-1), and soil organic carbon stocks (SOCstocks, +4.8%, 1.0 Mg ha-1 yr-1) at the global, but exhibited significantly decreased carbon stocks of litterfall (LBC, –9.6%, –0.03 Mg ha-1 yr-1). Belowground biomass carbon stocks remained unchanged under thinning. The sensitivity of carbon sequestration to thinning gradually disappeared after 6 years, except for AGCtotal, as they were jointly subject to stand density and thinning intensity and not just recovery ages. The response to UVC was more positive in medium-density (1500 – 3000 trees ha-1) and moderately thinned (30 – 50%) stands, and the thinning effects on UVC and SOCstocks were more noticeable in the subtropical and temperate zones. In summary, thinning enhanced forest ecosystem carbon sinks (+2.4 Mg ha-1 yr-1) but reduced carbon stocks in low-density stands (≤ 1500 trees ha-1). Soil moisture content (SMC, +6.6%) and bulk density (BD, –1.5%) dynamics after thinning were important factors regulating vegetation growth and soil carbon cycling processes.