Examining the Effects of Traits on Early-Establishment Tree Communities Under Different Soil Water Conditions

Abstract

Studies have demonstrated the existence of a positive relationship between species richness and productivity in forests around the world. This positive relationship is thought to be explained by two mechanisms: complementarity effects and selection effects. Both mechanisms 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 clearly established that species richness increases productivity, few studies have tested the implicit assumption that traits underpin the established species richness-productivity relationship. To address this gap, this study examined which traits are responsible for the species-productivity relationship in temperate tree communities during the early stages of stand development, and whether they act via complementarity effects or selection effects. In addition, this study aims to determine how the observed relationship and its underlying traits change in environments with different water availabilities by assessing which traits are associated with productivity in each environment.

These questions were answered using an experimental plantation of temperate tree communities in the early stages of stand development, in which water exclusion treatments were implemented. Six temperate tree species were planted in this experiment, either in a monoculture or in a three-species mixed community. To separate the effects of species and trait diversity, the mixed tree communities had a fixed species richness but varied in trait diversity. The species compositions of the mixes were selected to create a trait diversity gradient. 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 biomass and as belowground biomass.

The most consistent pattern of selection in the press treatment was for leaf area, as it was under selection in combination with leaf mass per area and with stomatal density. Pairs of traits including specific root length were also under selection in the pulse and the control treatments. Selection for these traits resulted in three to fivefold increases in aboveground biomass. These results demonstrate that selection effects are the driving force of community productivity during stand establishment. Here, selection acted upon traits related to tree architecture, thermoregulation, photosynthetic rate, and water use, which illustrate the mechanisms by which selection effects drive increases in the productivity of early-establishment communities across varying soil water conditions.