Effects of stand structural changes on vegetation carbon sequestration capacity during secondary succession in...

In the context of global climate change, stand structure is a critical lever for enhancing forest carbon sink capacity because it is amenable to management. Leveraging inventory data gathered in 2014 and 2019 from forty-two 20 × 20 m permanent plots of evergreen-deciduous broadleaved mixed forest, encompassing three successional phases (secondary stands 20 years and 35 years subsequent to logging, and undisturbed old-growth forest), this research integrated species - specific allometric equations with field measurements to quantify vegetation carbon density and annual carbon sequestration rates. We further disentangled how stand non-spatial structure (plot-level average characteristics such as diameter-class distribution and stem density) and stand spatial structure (distance-dependent indices derived from tree coordinates and mixing patterns) regulate carbon sink capacity. The principal findings are as follows. (1) Secondary succession markedly increased stand-level structural complexity and stability; carbon sequestration rates peaked at the mid-successional stage, with the tree layer emerging as the principal contributor to carbon storage. (2) Across all plots, structural diversity and competition intensity were the most powerful predictors of carbon-sink function. (3) Spatial structure exerted direct effects on canopy-layer sequestration, playing a pivotal role during early succession, while non-spatial structure increasingly regulated understory carbon dynamics and overall carbon accumulation in mature forests. (4) The synergy between spatial and non-spatial structure provides a dual mechanism: In younger stands, competition-oriented target tree management enhances annual carbon sequestration rates over short temporal scales, whereas in mature stands, the maintenance of structural heterogeneity and large-tree cohorts is linked to greater carbon storage capacity and enhanced long-term stability across decadal timescales. Taken together, the study underscores the multi-layered, multi-pathway coupling between stand structure and ecosystem function in subtropical evergreen-deciduous mixed forests, furnishes a science-based framework for structure-oriented carbon enhancement, and offers empirical guidance for sustainable forest management in pursuit of global carbon-neutrality goals.