Bark beetles pose a growing threat to coniferous forests worldwide, with outbreaks increasingly linked to environmental stress and host vulnerability. Most mechanistic models focus on equilibrium states, neglecting the dynamical nature of outbreaks triggered by environmental perturbations. Using Ips typographus as focal species, we present a minimal model capturing key processes such as threshold-driven mass attacks, with parameters adaptable to different environmental conditions. Under current conditions, the model predicts a stable endemic state that is reactive, meaning environmental shocks trigger isolated, transient outbreaks. Under altered conditions, reflecting potential climate change scenarios, the system undergoes a regime shift, producing recurrent outbreaks that arise spontaneously from insect–host feedbacks. Finally, we examine repeated environmental disturbances and forest management, showing that the effectiveness of outbreak mitigation depends on the interplay between management frequency and intensity. Overall, we provide a mechanistic framework for understanding outbreak dynamics and guiding management strategies under changing environmental conditions.

The process of sapwood/heartwood transition in trees is not fully understood. We tested whether the ontogenetically-stable apex-to-base conduit widening generates path length effects limiting the conductance of inner sapwood rings. The axial scaling ( b) of conduit hydraulic diameter ( Dh) was estimated at annual resolution in a spruce and beech tree. We compiled a global dataset of sapwood ring number ( NSWr), their average width ( SWrw), tree height ( H) and stem elongation rate ( ΔH) in conifer and angiosperm trees. A numerical model simulated the effects of H and ΔH on the conductance of each xylem ring ( KRING). b resulted ontogenetically stable. Simulations well predicted the observed patterns of increasing NSWr with H and decreasing NSWr with ΔH, assuming that heartwood forms when the marginal conductance gain of maintaining the functionality of an inner ring becomes negligible. Sapwood/heartwood transition minimizes the C costs associated to allocation to secondary growth and maintenance of living sapwood required to attain a given sapwood conductance. The number of sapwood rings depends on the effects of H and ΔH on the conductance of inner sapwood rings. The width of sapwood rings contributes to compensate for the lower conductance of inner sapwood rings at high ΔH.