Higher levels of multiple ecosystem services are found in forests with more tree species

Abstract

Forests are of major importance to human society, contributing several crucial ecosystem services. Biodiversity is suggested to positively influence multiple services but evidence from natural systems at scales relevant to management is scarce. Here, across a scale of 400,000 km(2), we report that tree species richness in production forests shows positive to positively hump-shaped relationships with multiple ecosystem services. These include production of tree biomass, soil carbon storage, berry production and game production potential. For example, biomass production was approximately 50% greater with five than with one tree species. In addition, we show positive relationships between tree species richness and proxies for other biodiversity components. Importantly, no single tree species was able to promote all services, and some services were negatively correlated to each other. Management of production forests will therefore benefit from considering multiple tree species to sustain the full range of benefits that the society obtains from forests.

Figure 1. Relationships between tree species richness and ecosystem services.

(a) Tree biomass production; (b) soil carbon storage; (c) bilberry production; (d) game production potential; (e) understory plant species richness; (f) occurrence of dead wood. We show mean relationships (black) and 95% Bayesian confidence intervals for the relationships excluding (dark green) and including the residual variation (light green). Other model explanatory variables were kept at mean levels.

Figure 2. Strength of the relationships between tree biomasses and ecosystem services.

The lengths of the ‘petals' in these flower diagrams (one diagram for each tree species) reflect transformed effect sizes for the relationships in the main models. Specifically, the effect sizes (β_n in equation (1)) have been transformed to range between 0 and 1, so that for each ecosystem service the effect size for the tree species with the greatest effect equals the absolute value of 1. Blue are positive relationships, red are negative relationships, and the outer circles represent the maximum value of |1|. The six ecosystem services are labelled by their respective symbols.

Figure 3. Pairwise relationships between ecosystem services.

Blue are positive and red are negative relationships. Grey reflects relationships with no established inter-relationship. The six ecosystem services are labelled by their respective symbols.

Figure 4. Parameter estimates for the main model for each ecosystem service.

The modes (vertical lines) and the 95% Bayesian confidence intervals (thin horizontal lines) for the parameters (β_n in equation (1) and ρ_m in equation (3)) are shown for each service (tree biomass production, soil carbon storage, bilberry production, game production potential, understory plant species richness and occurrence of dead wood). Variables or interactions in parentheses were not included in the final main model. Dashed lines are for visual aid.