Biomass allocation of trees in response to mono- and heterospecific neighbourhoods

Carbon sequestration by trees is crucial to mitigate the effects of the current climate crisis. The extent to trees sequester and allocate carbon to above- or belowground structures in turn is mediated by neighbouring species. Although many studies have demonstrated positive effects of diverse neighbourhoods on a tree’s productivity, little is known about biomass allocation responses to mono-vs. heterospecific neighbourhoods. In the present study we quantified above- and belowground biomass production and root-to-shoot ratios (RSR) of trees grown in mono- and heterospecific neighbourhoods. To this end we analysed growth of mono- and heterospecific tree species pairs (TSPs) established in a greenhouse and a field experiment. In the greenhouse experiment response variables were measured after one year of growth after sapling harvest. In the field experiment, conducted in the context of a forest biodiversity experiment in subtropical China, we analysed biomass density and RSR over three years using terrestrial laser scanner and minirhizotrons. RSR of trees in heterospecific TSPs were significantly higher than in monospecific TSPs. In the greenhouse experiment, this was related to a stronger below-than aboveground overyielding in heterospecific TSPs. In the field experiment, trees in heterospecific TSPs showed a stronger increase in aboveground investments over time than in monospecific TSPs, indicating that positive diversity effects became stronger for aboveground structures with progressing tree development. Our findings are consistent with the optimal biomass partitioning theory and highlight the importance of tree-tree interactions on biomass allocation. Higher RSR in mixtures further suggest a higher resistance or resilience of tree saplings against environmental stressors related to climate change (drought, heat waves).