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

1. 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. 2. We used LiDAR and repeated census data to examine
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 effected
productivity. 3. 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 overstory
and understory leaves, leading to a lower leaf area efficiency (i.e., wood
production per leaf area). Asymmetric competition was suggested by a
negative relationship between understory leaf area and total productivity.
Changes in community composition over the disturbance gradient, but not
species diversity, had a significant effect on productivity. 4. Synthesis.
Our study suggests that leaf area and canopy structural diversity have
contrasting effects on productivity in this subtropical forest, and this
need 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.