Tree diversity and productivity in subtropical secondary evergreen broad-leaved forests under different thinning intensities in Zhejiang Province, China

Understanding how thinning alters multiple dimensions of tree diversity and subsequently affects productivity in subtropical secondary forests remains limited, particularly over short timescales. In a subtropical secondary evergreen broad-leaved forest in Zhejiang, China, we established 30 plots in 2020, subject to three thinning treatments: control, light (10–15% basal area removal), heavy (20–25% basal area removal), with 10 replicates each). Structural, species, and functional diversity (based on eight plant traits) were quantified, and biomass and productivity were measured three years post-treatment. Heavy thinning significantly increased species diversity, functional evenness and divergence, but reduced functional richness by 61.9% relative to the control. Structural heterogeneity, measured as the Gini coefficient of basal area and the standard deviation of tree height, increased in thinned stands. The relative growth rate of total biomass were 66.7% higher under heavy thinning than under the controls. Importantly, the diversity-productivity correlation shifted with thinning intensity: functional diversity shifted from negative correlations in unthinned and lightly thinned stands to significant positive correlations in heavily thinned stands. These three-year responses represent transient dynamics rather than equilibrium states. Our findings demonstrate that thinning intensity fundamentally alters diversity-productivity relationships, with functional diversity showing the strongest treatment-dependent shifts. Long-term monitoring is essential to determine whether these patterns persist or reverse over time.