Long-Term Effects of Historical Disturbances on Mountain Forest Structural Diversity Assessed from Tree-Level Metrics

Forest structural diversity is a critical determinant of ecosystem function, yet the long-term influence of disturbance history on stand architecture remains complex. Here, we evaluate how historical disturbances shape structural and compositional diversity across 59 fully mapped plots in temperate mountain forests. Using field-based measurements, we quantified a suite of spatial, size-based, and compositional metrics, including Shannon entropy, diameter/height differentiation, species intermingling, and composite indices (SDI, Arten–Profill). Plots were categorized by time since disturbance (Recent, Intermediate, and Historical). Our results reveal a decoupling of structural recovery trajectories: biomass and tree size metrics (QMD, height, volume) follow a predictable, linear path of accumulation with disturbance age. In contrast, complex spatial indicators—such as SDI, species intermingling, and Clark–Evans aggregation—exhibit a threshold-based recovery pattern, increasing rapidly during early succession before plateauing as stands mature. Multivariate analyses (PCA and hierarchical clustering) indicate that forest maturity is not a monolithic structural endpoint; rather, late-successional stands diverge into multiple successional trajectories, each defined by unique spatial and size configurations. Compositional metrics showed lower sensitivity to disturbance history than structural indices, suggesting that structural heterogeneity is a more persistent "memory" of stand development than species composition alone. These findings suggest that structural complexity is a "built-in" feature of early stand development. Consequently, management strategies should shift from volume-centric goals toward trajectory-based approaches, employing early-successional interventions (e.g., variable-density thinning) to steer uniform stands toward complex, resilient structural configurations before developmental thresholds stabilize.