The mountain pine beetle in a marginal boreal landscape: Cross-scale collapse triggered by population removal

The mountain pine beetle (MPB) exhibits all the hallmarks of a cross-scale disturbance agent, with nonlinear population dynamics driven by positive density-dependent feedbacks that link fast behavioural processes to slow, landscape-level environmental change. Although this feedback loop typically fuels rapid irruption, it can in principle be reversed through sustained population removal. We document such a reversal in Alberta’s commercial pine forest over 2006–2023. No single causal variable effectively captures the collapse mechanism; instead, collapse reflects a syndrome involving two sets of drivers operating at different spatial scales. Intrinsic density-dependent factors include beetle pressure measured at breast-height on the stem, attacks up the height of the stem, and the density of attacked trees in the surrounding cluster. Extrinsic environmental drivers operating at the landscape scale include winter temperatures, drought, pine volumes, and pine ancestry. Despite several million freshly attacked trees detected annually from 2009 to 2012, outbreak expansion did not occur. We explain why the removal of freshly attacked trees is the most plausible explanation for the observed decoupling between r (seasonal recruitment) and R (interannual expansion), which were uncharacteristically uncorrelated throughout 2006–2019. This decoupling indicates that the positive feedback loop linking brood productivity to landscape-level spread was disrupted during the years when it would normally have been strongest. Cold winters in 2018–2020 contributed to the final collapse, but the decisive factor was the earlier disruption of density-dependent feedback through green-tree removal, which limited population growth and tree mortality from 2010 to 2017 and prevented any rebound after 2020. This case illustrates how cross-scale outbreak collapse can be engineered in systems with strong positive feedbacks, and places MPB within a broader class of species whose persistence depends on remaining above population-viability thresholds set by Allee effects. • MPB brood productivity and outbreak spread were decoupled across 2006–2019. • Population removal broke the positive feedback driving beetle irruption. • Cross-scale collapse occurred despite strong density-dependent recruitment. • Cold winters aided collapse, but feedback disruption was the decisive factor. • Five management stages are identified for suppressing irruptive bark beetle systems.