Tree diversity does not always improve resistance of forest ecosystems to drought

Climate models predict an increase in the intensity and frequency of drought episodes in the Northern Hemisphere. Among terrestrial ecosystems, forests will be profoundly impacted by drier climatic conditions, with drastic consequences for the functions and services they supply. Simultaneously, biodiversity is known to support a wide range of forest ecosystem functions and services. However, whether biodiversity also improves the resistance of these ecosystems to drought remains unclear. We compared soil drought exposure levels in a total of 160 forest stands within five major forest types across Europe along a gradient of tree species diversity. We assessed soil drought exposure in each forest stand by calculating the stand-level increase in carbon isotope composition of late wood from a wet to a dry year (Δδ¹³C_S). Δδ¹³C_S exhibited a negative linear relationship with tree species diversity in two forest types, suggesting that species interactions in these forests diminished the drought exposure of the ecosystem. However, the other three forest types were unaffected by tree species diversity. We conclude that higher diversity enhances resistance to drought events only in drought-prone environments. Managing forest ecosystems for high tree species diversity does not necessarily assure improved adaptability to the more severe and frequent drought events predicted for the future.Biodiversity plays an important role in ecosystem functioning by promoting a wide range of functions and services (1–3). This beneficial effect of biodiversity is determined by mechanistic processes directly under the influence of species interactions: complementarity among species for resource use through ecological niche partitioning and/or facilitation processes increase ecosystem performance because resources are better shared among neighboring species and are thus potentially more available (4). Previous studies have demonstrated that, apart from enhancing performance, diverse terrestrial ecosystems may also be more resilient and more resistant to biotic stresses such as insect pests or diseases (5, 6). However, it remains unclear whether higher biodiversity also leads to improved resistance of terrestrial ecosystems to the more frequent droughts expected in temperate regions (7). The rare case studies published thus far have shown contrasting results. Two reported that species in more diverse ecosystems could be more resistant to drought stress (8, 9), whereas another suggested that enhanced biodiversity could trigger higher exposure to drought (10). Improving our understanding of how species diversity influences the resistance of terrestrial ecosystems to a fluctuating climate is crucial.More frequent and intense droughts will greatly affect the carbon and water cycles of the terrestrial biosphere (11), in particular in forested ecosystems (12). Many societies around the world rely on forests for essential services such as wood production, hunting, or watershed protection. We therefore urgently need to improve our knowledge of the physiological response of these ecosystems to drier climatic conditions to propose new climate-smart management options. Forests, although influenced by local environmental conditions, play a major role in the global carbon and water balance as they release into and assimilate from the atmosphere huge amounts of CO₂ while losing water vapor through transpiration. Tree species are known to vary widely in the ecological strategies they use to deal with drought stress. It could therefore be expected that in highly diverse forests composed of tree species with contrasting functional traits, limited water resources could be better partitioned among the neighboring species as a result of complementarity and facilitation processes (4). Such forests should be more resistant to deal with drought stress because the trees should be able to maintain better access to diminishing water resources as the drought progresses. In contrast, if the interacting species in a diverse forest have similar functional traits (i.e., functional redundancy), ecological niche overlap (13) may lead to more stressful conditions during drought than in pure stands due to lower water availability for each species.Carbon isotope composition in C₃ plant tissues (δ¹³C) provides an integrated record of the ratio of intercellular to atmospheric CO₂ concentrations during the period when the carbon was fixed and thus reflects the balance between net CO₂ assimilation and stomatal conductance (14). Plants typically react toward drought stress by closing their stomata and reducing carbon assimilation rates. However, leaf stomatal conductance is affected to a greater extent than assimilation, causing a concomitant increase in δ¹³C (14, 15). Therefore, under soil drought conditions, δ¹³C from organic material has been widely accepted as an indicator of the intensity of drought exposure in plants (16, 17) (SI Text). If complementarity for water use is occurring among species, δ¹³C values should increase less between wet and dry soil conditions with increasing tree species diversity (i.e., a negative relationship). Inversely, if tree species occupy redundant ecological niches, δ¹³C values should either have a similar or higher increase between wet and dry conditions with increasing tree species diversity (i.e., a null or positive relationship).In a previous study, we analyzed the influence of drought on the relationship between tree species diversity and the increase in stand-level carbon isotope composition between a wet and dry year (Δδ¹³C_S) in boreal forests (10). Species diverse forests were shown to be more affected by drought stress than less diverse ones (i.e., a positive relationship between Δδ¹³C_S and tree species diversity). In the present study, we extend our research to five major forest types across Europe, which extends from northern hemiboreal forests to southern Mediterranean forests (Table S1). Our objective was to test whether the relationship between Δδ¹³C_S and tree species diversity would be consistent across a large range of climatic and edaphic conditions. At each of the five study sites, we selected a set of representative canopy trees (Table S2) in 21–42 forest stands varying in tree species diversity. For each site, we used a water balance modeling approach to select 1 y within the last 14 y with high drought stress and 1 reference y when no drought occurred (Figs. S1 and S2). We measured the δ¹³C in the tree rings of the selected canopy trees and calculated Δδ¹³C_S for each stand.