Tree Neighbourhood Diversity Has Negligible Effects on Drought Resilience of European Beech, Silver Fir and Norway Spruce

Promoting tree species diversity is commonly advocated in the anticipation of predicted increases in drought frequency and severity. However, mixing effects on drought responses vary substantially with site conditions and species identity. We combined annually resolved tree-ring data and repeated forest inventory data spanning the last 90 years to examine the effect of species-specific neighbourhood competition on the drought response (resistance, recovery and resilience) of European beech ( Fagus sylvatica ), silver fir ( Abies alba ) and Norway spruce ( Picea abies ) for six drought events that occurred since the 1970s at three sites in Switzerland. We found predominantly weak neighbourhood competition and tree species diversity effects, with significant interspecific influences only for resistance and recovery of beech. These minor neighbourhood effects were outweighed by tree age and size effects. Although age effects depended on species identity and components of resilience, tree size consistently negatively affected all species. Our results emphasize that diversity effects may vary for each given species combination which makes broader conclusions challenging. This is because species interact through their specific set of traits and interactions vary in space and time. Adaptive management strategies are likely to be more effective when they promote more drought-tolerant species and reductions in stand density. Despite the absence of an unequivocal advantage of tree diversity on drought resilience, striving towards species-rich forests nonetheless allows for a risk spreading among multiple species and the reinforced provision of numerous ecosystem services.