Print PageThe influence of biodiversity on ecosystem functioning is now well established but our ability to predict the consequences of biodiversity changes remains limited by our poor understanding of the mechanisms underlying biodiversity effects. Light partitioning has been proposed as an example of complementarity among plant species that can promote overyielding, i.e. increase biomass production in mixtures compared to that expected from monocultures. Yet, experimental evidence for a better distribution of photosynthetic tissues in space and time in diverse communities remains scarce and is restricted to temperate grasslands. Moreover, previous studies were not able to control for the effect of plant size although overyielding is bound to enhance light capture in mixtures whether or not light partitioning occurs. Here, we investigated whether, and though what mechanisms, tree diversity enhanced light capture in a biodiversity experiment that established a synthetic gradient of diversity (plots with 1, 3 and 6 species) by planting in 2001 more than 5000 trees over ca. 6 ha in central Panama.
Results/Conclusions
We first tested for diversity effects at one point in time, when all species were fully foliated. Using light availability measurements along with a spatially explicit light interception model, mixtures were found to capture more light than their ‘best’ constituent monocultures. Thanks to independent tests for each mechanism, we showed that this result was explained by positive diversity effects on tree growth, by plastic changes in crown shape in one species but not by architectural differences among species. We then investigated mixture effects on light capture over time. Set in a seasonally dry environment, the experiment included species with contrasted leaf phenology with one brevi-deciduous species and 5 deciduous species. We reconstructed species-specific leaf phenology from sapflux data, direct foliage observation and litter trap data to carry out a virtual biodiversity experiment with our calibrated light model. Over a complete calendar year, light capture increased with the degree of phenological differences among trees but this trend was partly driven by the fact that the species with the most distinct phenology also reduced light most. At the community level, the effects of temporal niche complementarity were thus confounded with selection effects. At the individual scale, differences in phenology alleviated interspecific competition for light, thereby contributing to overyielding. Highlighting the role of temporal niche differences and phenotypic plasticity, our work sheds new light on the mechanisms underlying biodiversity effects on ecosystem functioning.
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