Forest Resistance to Biotic Stressors Under Climate Change: Lessons from the European Ash System

Forests are increasingly exposed to interacting biotic and abiotic stressors, including invasive insects, emerging pathogens, and climate-driven disturbances. Understanding how trees resist or tolerate these stressors is therefore essential for sustaining forest health and ecosystem services. This review synthesizes current knowledge on forest resistance to insects and pathogens, revisiting classical concepts of resistance, tolerance, and resilience and examining how they operate under contemporary environmental change. We integrate insights from plant–insect–pathogen interactions, phenological dynamics, and multitrophic ecological processes to develop a conceptual framework describing how host traits, environmental conditions, and biotic antagonists jointly determine resistance outcomes. European ash (Fraxinus excelsior) is used as a model system to illustrate these processes in the context of two major invasive threats: ash dieback caused by Hymenoscyphus fraxineus and the emerald ash borer (Agrilus planipennis). Evidence from genetic, physiological, and chemical studies indicates that resistance in ash is polygenic and often expressed as reduced susceptibility or tolerance rather than complete immunity. Recent findings further suggest the existence of cross-resistance mechanisms that may simultaneously influence responses to fungal pathogens and insect pests. Climate change, environmental stress, and host-associated microbiomes may further modify these interactions and shape long-term resistance dynamics. Finally, we discuss implications for resistance breeding and adaptive forest management, emphasizing the importance of maintaining genetic diversity and ecosystem heterogeneity to enhance forest resilience under accelerating global change.