Tree species richness induces strong intraspecific variability of beech (Fagus sylvatica) leaf traits and alleviates edaphic stress

Manipulating stand composition is an important management tool that foresters can use to affect the nature of forests and ecosystem processes. In mixed stands, interspecific interactions among trees can cause changes in tree performances. Nevertheless, these interactions are context dependent (cf. stress-gradient hypothesis, SGH). We thus investigated how intraspecific functional changes in leaf trait (19 traits) of European beech (Fagus sylvatica) were influenced by stand composition. We compared pure beech stands with four mixed stands containing from one to three additional tree species along a gradient of edaphic stress (gradient of soil water-holding capacity and rooting depth). First, we demonstrated that stand composition induced strong intraspecific leaf trait variation in beech for LDMC, LMA, phenolic compounds, leaf pH and magnesium concentration, suggesting higher nutrient acquisition by more diverse stands. Nevertheless, these results were modulated by edaphic stress. Mixed stands only conferred an advantage in relatively stressed sites (luvisol and leptosol). Besides, the addition of oak to beech stands had unexpected negative effects in sites with less severe stress (cambisol) as indicated by the null or positive LogRR of LMA, LDMC and phenolics. This study found that stand composition is an important though often-overlooked driver of intraspecific variability in leaf quality, and potentially reflects changes in beech tree physiology and productivity. Our results also suggest that positive interactions prevail in sites with stressful conditions. Such validation of the SGH is rare in natural or managed mature forests. Lastly, we strongly recommend that forest managers consider stand composition and abiotic factors when implementing forest growth models to improve their yield predictions.