Land-use legacies and drought exposure affect aboveground carbon dynamics along elevational gradients in tropical forests of China

Tropical forests play a critical role in regulating the global carbon balance, yet the dynamics of aboveground carbon (AGC) often vary with land-use legacies and drought, which in turn often covary with elevation. Our understanding of how land-use legacies and drought exposure are associated with AGC dynamics and their underlying drivers along elevational gradients remains limited. We used data from 22 one-hectare plots spanning degraded tropical forests with varying land-use histories (shifting cultivation, clear-cutting, and selective logging) and old-growth tropical forests along an elevational gradient in Hainan, China. The woody stems in each plot were measured in three censuses spanning a humid period (2010–2015) and a period with greater drought exposure (2015–2021). We asked (i) whether AGC stocks and fluxes differ between degraded and old-growth forests across elevational and land-use legacy gradients over the two periods; (ii) how they are further associated with tree diversity, community structure and soil characteristics; and (iii) how tree growth, mortality, and recruitment contribute to changes in AGC. Along the elevational gradient, old-growth forests exhibited lower AGC growth, recruitment, and net change, but higher AGC loss compared to degraded forests. The rate of AGC accumulation tended to be lower during the period with greater drought exposure in both degraded and old-growth forests. Tree mortality dominated net AGC change in old-growth forests, while tree growth was the primary contributor in degraded forests. Piecewise structural equation modeling revealed that elevation mainly affected AGC dynamics indirectly through land-use legacy, forest structure, functional dispersion and traits with resource-conservative strategy, while land-use legacy and forest structure had significant direct effects. Under drought exposure, direct positive effects of elevation on AGC growth and net AGC change emerged. This study highlights the joint importance of land-use legacy, elevation, and drought exposure for AGC dynamics in both degraded and old-growth forests. Despite an overall net carbon gain in both forest types, the underlying mechanisms differ markedly, not only in the relative contributions of tree demographic rates, but also in how these processes vary with land-use legacy and drought exposure.