Species-specific mediation of stand structural diversity on climatic effects on productivity in Larix plantations across China

Understanding how forest productivity responds to climate and stand structural diversity is critical for sustainable forest management under climate change. However, whether and how the association between structural diversity and productivity varies across climatic conditions and tree species remains poorly understood. In this study, generalized additive mixed models (GAMMs) were used to quantify nonlinear and interactive effects of climate and structural diversity on mean annual increment (MAI) of stand volume, based on 479 permanent sample plots of four Larix species. The final GAMM explained 64.1% of MAI variation. The interactions between annual heat:moisture index (AHM) and Simpson diversity index of stem density across diameter classes (N_Simpson_Div) revealed that the effect of structural diversity on productivity varied along the AHM gradient, and the four Larix species exhibited distinct configurations of climate–structure–productivity responses. Larix principis-rupprechtii exhibited a pronounced nonlinear shift in this association, with higher structural diversity associated with increasingly positive partial effects at intermediate AHM but predominantly neutral to negative effects at higher AHM. L. olgensis showed a similarly complex response surface, but the positive effects associated with low structural diversity weakened with increasing AHM. In contrast, L. kaempferi and L. gmelinii exhibited comparatively limited variation along the AHM gradient but differed in the strength of their responses to structural diversity. Overall, the association between structural diversity and productivity was strongly dependent on both climatic conditions and species identity, indicating that a uniform structural-diversity-productivity relationship cannot adequately characterize Larix plantations across China. These findings highlight the importance of considering species-specific climatic context when evaluating structural conditions and developing adaptive management strategies for maintaining plantation productivity under climate change.