Selection Effects Associated with Leaf Area and Stomatal Density Drive Productivity of Young Temperate Tree Communities Under Different Water Exclusion Treatments

Magalie Galarneau, Christian Messier, Benoit Gendreau-Berthiaume, M. Luke McCormack, Laura J Williams, Julie Messier

1. A positive relationship between species richness and productivity has been observed in forests around the world. The two classes of mechanisms thought to underlie this relationship, complementarity effects and selection effects, both rely on species’ traits, whereby the diversity of traits (under complementarity effects) or the presence of specific trait values (under selection effects) can increase productivity. However, while it has been established that species richness increases productivity, few studies have tested which traits underpin this relationship.
2. To address this gap, we examined which traits underpin the diversity-productivity relationship in early-establishment temperate tree communities, and whether they act via complementarity or selection effects. In addition, we tested how the observed relationship and its underlying traits change among environments with different water availabilities.
3. These questions were answered using an experimental plantation of young temperate tree communities with water exclusion treatments. Six temperate tree species were planted in this experiment, either in a monoculture or in three-species mixed communities. To separate the effects of species and trait diversity, the mixed tree communities had a fixed species richness but varied in trait diversity. Leaf area, leaf mass per area, stomatal density, chlorophyll concentration, specific root length, root tissue density, and root diameter were measured in each community. Community productivity was measured both as aboveground and belowground biomass. We compared our results from trait-based analyses of selection and complementarity effects to results from classical additive partitioning analyses.
4. Using trait-based analyses, neither selection nor complementarity effects were detected with belowground biomass as a productivity metric. Complementarity effects were not detected with aboveground biomass, but the trait combinations of leaf area and leaf mass per area, leaf area and stomatal density, and of stomatal density and specific root length were associated with the selection effect in different environments. Similarly, classical additive partitioning analyses found a larger selection effect but also revealed some complementarity effects across all treatments that were not associated with any of the measured traits. These findings suggest that biodiversity–productivity relationships in young temperate tree communities arise mainly through selection on trait combinations associated with carbon gain and water loss.