Disentangling the effects of compositional and structural diversity on forest productivity

Abstract

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.

Forest structure, diversity, and primary production in relation to disturbance severity

“…However, numerous unresolved questions center on understanding which vegetation structural features are most closely tied to production and whether they change over time and across ecosystems. The principal vegetation structural determinant of forest production may change as ecosystem development unfolds over decades to centuries (Silva Pedro et al, 2017) and may differ among plant functional types (Scheuermann et al, 2018). Additionally, the successional context of ecosystems—in our study case, a forest in transition from early to middle stages—differentially influences the production response to disturbance.…”

“…Effects of disturbance severity on indexes describing forest tree species diversity and vegetation structure and, separately, on NPP have been long theorized and observed (Clements, 1916; Pickett & White, 1985). However, these effects have rarely been examined together despite evidence of forest structure–function coupling in a number of ecological contexts (Scheuermann, Nave, Fahey, Nadelhoffer, & Gough, 2018; Silva Pedro, Rammer, & Seidl, 2017). Joint investigation of forest structure–function relationships is timely as the range of disturbance severities present on temperate forest landscapes expands and, consequently, broadly reshapes plant species diversity and vegetation structure (Seidl et al, 2017; Turner, 2010) and NPP (Stuart‐Haëntjens, Curtis, Fahey, Vogel, & Gough, 2015), sometimes in surprising ways (Curtis & Gough, 2018).…”

Estimation and Uncertainty of the Mixing Effects on Scots Pine—European Beech Productivity from National Forest Inventories Data

“…Similarly, the difficulty to estimate correctly size distribution from NFIs data (due to the concentric sample plots design and the different plot radius between countries) made us give up the inclusion of size structure in the analysis. However recent studies found that stand productivity in mixed stands is related not only to species composition but to size structure (e.g., [73][74][75]). On the other hand, the main advantage of using data from NFIs is that they provide systematic information and therefore there are data of many species mixtures covering larger gradients of site and stand conditions than those available through other sample plots [21,30].…”

Difference in shade tolerance drives the mixture effect on oak productivity

“…We confirmed results obtained with two-species mixtures by Lu et al (2016) that found an overyielding in mixture of broadleaved and coniferous species which were also distant in their ST. This suggests that functional type and ST are complementary drivers, among other determinants, of the mixture effect on tree species productivity (Forrester, Kohnle, Albrecht, & Bauhus, 2013;Silva Pedro, Rammer, & Seidl, 2017;Yuan et al, 2016).…”