From canopy complementarity to asymmetric competition: The negative relationship between structural diversity and productivity during succession

Positive relationships between structural diversity and forest productivity have been documented in controlled experiments and early secondary forests, however, negative relationships have also been observed in late successional forests. The mechanisms causing observed relationships between structural diversity and productivity are not well‐established, but complementarity among crowns and asymmetric competition have been suggested. We used LiDAR and repeated census data to examine the relationship between canopy structural diversity and productivity in nine 1‐ha subtropical forest plots along a disturbance gradient in southeastern China. We quantified the relative importance of community composition, species diversity, canopy structural diversity, leaf area index (LAI) and disturbance regime on productivity using piecewise structural equation modelling. We also tested how vertical leaf area distribution affected productivity. Contrary to many prior observations, we found a negative relationship between canopy structural diversity and forest productivity. The negative effect may stem from asymmetric competition between overstorey and understorey leaves, leading to a lower leaf area efficiency (i.e. wood production per leaf area). Asymmetric competition was suggested by a negative relationship between understorey leaf area and total productivity. Changes in community composition over the disturbance gradient, but not species diversity, had a significant effect on productivity. Synthesis. Our study suggests that leaf area and canopy structural diversity have contrasting effects on productivity in this subtropical forest, and this needs to be considered when estimating rates of carbon sequestration in secondary forests. The negative effect of asymmetric competition on productivity is comparable to that of the shift in species composition over succession, highlighting the role of canopy structural diversity in shaping forest productivity.