Biotic homogenization can decrease landscape-scale forest multifunctionality

Abstract

Many experiments have shown that local biodiversity loss impairs the ability of ecosystems to maintain multiple ecosystem functions at high levels (multifunctionality). In contrast, the role of biodiversity in driving ecosystem multifunctionality at landscape scales remains unresolved. We used a comprehensive pan-European dataset, including 16 ecosystem functions measured in 209 forest plots across six European countries, and performed simulations to investigate how local plot-scale richness of tree species (α-diversity) and their turnover between plots (β-diversity) are related to landscape-scale multifunctionality. After accounting for variation in environmental conditions, we found that relationships between α-diversity and landscape-scale multifunctionality varied from positive to negative depending on the multifunctionality metric used. In contrast, when significant, relationships between β-diversity and landscape-scale multifunctionality were always positive, because a high spatial turnover in species composition was closely related to a high spatial turnover in functions that were supported at high levels. Our findings have major implications for forest management and indicate that biotic homogenization can have previously unrecognized and negative consequences for large-scale ecosystem multifunctionality.

Keywords: FunDivEUROPE; biodiversity; ecosystem functioning; spatial scale; β-diversity.

Fig. 1.

Quantifying biodiversity and multifunctionality across spatial scales. The light yellow areas represent hypothetical landscapes, consisting of (white) local communities. In these communities, some species are present (colored icons in A and C), whereas others are absent (gray icons). Similarly, some functions are performing above a hypothetical threshold of 0.5 (colored icons in B), whereas others are not (gray icons). Diversity and threshold-based multifunctionality are quantified at (i) the local plot (α-) scale as the number of species present (two and three in A) or functions performing above a given threshold (two and three in B), (ii) the β-scale as the turnover in species composition [$=1-\log \left((a+b+2c)/(a+b+c)\right)=1-\log \left(\left(1+2+2\right)/(1+2+1)\right)=0.90$ in A (49)] or functions [$=1-\log ((a+b+2c)/(a+b+c))=1-\log \left(\left(1+2+2\right)/(1+2+1)\right)=0.90$ in B (49)] across plots, and (iii) the landscape (γ-) scale as the number of functions (four in B) present in at least one plot. Sum-based γ-multifunctionality is defined as the sum of all standardized ecosystem values in a landscape (= 0.8 + 0.2 + 0.7 + 0.4 + 0.9 + 1.0 + 0.1 + 0.6 = 4.7). In contrast to threshold-based multifunctionality, sum-based multifunctionality is not analogous to biodiversity (where species are either present or absent), and can therefore not be partitioned into α- or β-components. (C) This framework allows investigation of whether γ-multifunctionality is promoted by α- and/or β-diversity.

Fig. 2.

Scale-dependent effects of biodiversity on forest ecosystem multifunctionality. Bars represent the standardized regression coefficients of α-diversity (light gray) and β-diversity (dark gray) in generalized LMMs explaining α- (A), β- (B), or γ- (C) multifunctionality. Multifunctionality was quantified at different scales using a threshold approach, with thresholds of 40%, 50%, 70%, and 90%. In addition, sum-based γ-multifunctionality was calculated as the sum of scaled (between 0 and 1) individual function values. Diversity measures were calculated based on individuals of target tree species. *P < 0.05; ***P < 0.001; ****P < 0.0001. NS, not significant.

Fig. 3.

Scale-dependent effects of biodiversity on forest ecosystem multifunctionality are generally consistent across countries. Bars represent the standardized regression coefficients of α-diversity (blue) and β-diversity (red) in LMMs explaining α- (A), β- (B), or γ- (C) multifunctionality. Multifunctionality was quantified at different scales using a threshold approach, with thresholds of 40%, 50%, 70%, and 90%. In addition, sum-based γ-multifunctionality was calculated as the sum of scaled (between 0 and 1) individual function values. Diversity measures were calculated based on individuals of target tree species.