Foliar nutrient composition, fructification and previous years’ climatic conditions explain changes in defoliation of temperate tree species

The main threat to Central European forests today is climate change, which increases defoliation and reduces tree vitality. Despite extensive monitoring, the physiological mechanisms and their link to site factors like nutrient availability remain poorly understood.Therefore, our study focuses on defoliation and its dependence on foliar nutrition in comparison to climate of Fagus sylvatica, Quercus robur/petraea, Picea abies and Pinus sylvestris in Rhineland-Palatinate, Germany - a region representing typical climatic gradients from lowlands to mid-elevation mountains. Using annually resolved data from a 23-year monitoring period, defoliation was modelled either with climatic variables only, or with foliar nutrient contents and ratios in addition to climate. The modelling approaches were evaluated by simulating defoliation and comparing it to observations at sites assessed for foliar nutrients every six years. As we could only indirectly account for the impact of biotic damage through climatic conditions – despite the often-reported relationship between the two - this limits the interpretation of our results. For F. sylvatica and P. sylvestris, including foliar nutrition improved defoliation simulations compared to models based on climate variables alone. In F. sylvatica, this improvement was associated with increased foliar C content at warmer sites in the year prior to measurement, suggesting an adaptation towards more sclerophyll in preformed leaves. In P. sylvestris and Q. robur/petraea, foliar K nutrition played an important but species-specific role in explaining defoliation, suggesting differing strategies for osmotic adjustment or nutrient retranslocation. For P. abies, the close match between observed and climate-only simulated defoliation affirms the widely reported decline in vitality linked to drought stress. Our study shows that species-specific foliar nutrition, alongside water availability and fructification, is key to tree defoliation. Tree species selection should therefore account for soil nutrient availability to boost forest resilience.