COS 2-7
Does interspecific vertical canopy space partitioning promote productivity of mixed subtropical forest stands?
Most experimental biodiversity studies report higher productivity in more diverse ecosystems. One of the mechanism proposed to explain these effects is interspecific complementary in resource use. However, identifying the relevant resource niches and their complementarity (or conversely: overlap) in species mixtures is extremely difficult, so that the ultimate nature of complementarity generally remains elusive.
We studied vertical canopy space partitioning in two year-old experimental stands of subtropical trees. Species were planted in monocultures and in all two-species combinations, and this design repeated for three separate four species-pools. The experiment further contained a factorial shade treatment. Canopies were harvested in layers of 50cm, separately for leaves and woody biomass. Niche location was quantified as average biomass height. Niche breadth was calculated as standard deviation of height or as Levin's B. Niche overlap between species pairs was calculated as Proportional Similarity (PS).
We tested (1) whether complementarity in vertical space use explains observed stand-level productivity, and (2) whether realized niches in mixed stands shift relative to monocultures, resulting in a "plastic character displacement" that reduces niche overlap and increases complementarity, and (3) whether these effects depended on available light.
Results/Conclusions
Monocultures biomass varied more than tenfold, and so did the biomass of mixtures. These differences were significantly lower in shade than in light. PS of species vertical leaf distribution, calculated from monoculture data, correlated negatively with the net biodiversity effect, explaining 61% of the variance (P=0.0001) in shade but only 20% (P=0.06) in light. Similar predictive effects of PS were found when the complementarity and selection component of the effect, derived using Loreau and Hector's additive partitioning scheme, were analyzed. Niche location and niche breadth significantly depended on light availability but not on species richness.
Differences between PS predicted from monocultures and realized PS in mixtures were species pair-dependent (P<0.05). Species showed reduced PS in mixtures in one pool (P<0.01) but only a marginally significant trend or no effect in the other pools. Shade tended to reduced PS (P<0.1). In shade, shifts in PS negatively correlated with functional distance of species (calculated from eight growth-related traits), indicating that plastic niche displacement increases in pairs of functionally more similar species.
Our analyses suggest that complementarity in canopy space use promotes productivity in mixed stands. Furthermore, realized niches shift in a way reducing overlap with competitors, thus increasing complementarity, at least under light-limited conditions.