Disentangling the effects of compositional and structural diversity on forest productivity

Questions: Tree species diversity is widely reported to positively influence forest productivity. Yet, a consistent attribution of productivity effects is complicated by the fact that compositional and structural diversity are often related in forest ecosystems. Here, our objective was to disentangle the effects of diversity in species and structures on forest productivity. We further assessed whether the influence of structure and composition on productivity changes over the course of forest development. Location: Hainich National Park, central Germany. Methods: We conducted a factorial simulation experiment in which 63 unique combinations of six different tree species were studied over 500 yr of forest development. The model used was iLand, a process-based simulator operating at individual tree resolution. The indicators of compositional diversity considered included species richness, entropy, evenness and identity, while structural diversity was characterized by indicators describing vertical and horizontal stand structure. Net primary production (NPP) was studied as the response variable, and random forest analysis was used to synthesize simulation output. Results: We found positive effects of both compositional and structural diversity on productivity, but their influence changed distinctly over the course of forest development. In early-seral stages, diversity effects on NPP were dominated by aspects of tree species composition, and displayed a strong positive selection effect for European beech (Fagus sylvatica L.). In later stages of forest development, diversity effects on NPP were dominated by structural diversity, with productivity increasing with increased variation in tree diameter and canopy complexity. Conclusion: To better understand the effects of diversity on ecosystem functioning, both the compositional and structural dimensions of diversity in forest ecosystems (and their changes over time) need to be considered. In the context of ecosystem management our results suggest that the reduction in productivity associated with the loss of a canopy tree species (e.g. due to the invasion of an alien pest species) can to some degree be compensated through increased structural diversity. Fostering both compositional and structural diversity are important means to increase the robustness of forest ecosystem functioning.