From Colonization to Collapse: a Mechanistic Perspective on Tree Mortality by Bark Beetles

Purpose of Review: Bark beetle outbreaks are intensifying under climate change, yet the links among drought stress, beetle attack and colonization, rapid tree mortality, and tree physiology, remain poorly integrated across disciplines. This review provides a comprehensive, mechanistic synthesis of recent advances in tree physiology, bark beetle ecology, and microbial interactions to examine how drought alters xylem–phloem function, carbon allocation, and defense capacity, how these changes affect beetle host selection and reproductive success, and how associated microbes accelerate host decline. Recent Findings: Drought-driven declines in xylem water potential constrain phloem transport through reduced turgor and increased sap viscosity, reshaping carbon distribution and defense deployment. Moderate drought may transiently enhance some defenses, whereas severe or prolonged drought depletes non-structural carbohydrates, impairs resin flow, and increases phloem nutritional suitability for beetles. Beetle attacks cause spatially complex phloem disruption and local carbon depletion together with their associated fungi, which also contribute to pit membrane degradation, increasing vulnerability to embolism. The altered bark and phloem microenvironments, including gas exchange and moisture conditions within bark beetle galleries, may further influence microbial activity and host responses. Summary: We propose a cross-disciplinary, integrated mechanistic framework in which drought-induced physiological destabilization predisposes trees to beetle attack, after which phloem disruption and microbial activity amplify carbon limitation and hydraulic dysfunction. Hydraulic failure may represent a convergence point among interacting stressors, underscoring the complex multi-trophic feedbacks driving infestation expansion. Key gaps remain in quantifying phloem dynamics, bark permeability, and subcortical microclimates during attack, limiting predictions of forest vulnerability under increasing drought and bark beetle pressure.