Abstract: Despite increasing evidence regarding the positive effect of structural diversity on forest functioning, the underlying biotic and abiotic mechanisms remain poorly understood. Critically, how structural diversity in different forest strata affects productivity components, such as growth of surviving trees, recruitment of new individuals and biomass net change incorporating mortality, is largely unknown. Here, we used lidar and repeated census data to examine the relationship between structural diversity and forest productivity in a 9-ha temperate forest plot in Northeast China. We quantified the contributions of species diversity, functional diversity, functional composition, structural diversity and environmental factors to productivity using hierarchical partitioning and structural equation modeling. We found that structural diversity was strongly associated with forest productivity, accounting for 65% of the influence of all variables on net productivity change. We found that structural diversity mediates the effects of other biodiversity attributes (species diversity and functional diversity) and environmental factors on productivity. However, overstorey and understorey structural diversity exert contrasting effects on productivity. Overstorey structural diversity elevates productivity by enhancing canopy space filling and light capture. Conversely, a structurally diverse understorey often signals intense competition for water and soil nutrients, thereby suppressing the growth of large trees. Our study suggests that distinct mechanisms of overstorey and understorey structural diversity on productivity exist, and neglecting this contrast may partly account for the inconsistent observations in structural diversity-productivity relationships. These findings underscore the critical role of structural diversity in shaping forest productivity, providing essential insight for advancing biodiversity-ecosystem functioning (BEF) relationships.

Key words: Biodiversity-ecosystem functioning, Forest productivity, Overstorey and understorey, Species diversity, Structural diversity