Estimating disturbance impacts on forest carrying capacity, with a case study of a psyllid outbreak in a Eucalyptus regnans forest

Background: Insect outbreaks can cause severe forest mortality, but whether impacts represent a permanent reduction in carrying capacity or a temporary shift followed by recovery is unclear. Carrying capacity is rarely quantified directly in disturbance studies, which rely on measures such as mortality rates or biomass loss. Aim: We developed a framework to quantify how disturbances affect stand carrying capacity and subsequent recovery, demonstrated using a psyllid outbreak in a Eucalyptus regnans forest in Victoria’s Central Highlands, Australia. Methods: We integrated disturbance effects into a dynamic self-thinning model to test four competing hypotheses about the persistence of disturbance impacts, using 422 plot-level observations from 27 permanent plots over 39 years (1962–2001) and Bayesian hierarchical models. Results: Psyllid outbreaks were associated with a temporary increase in mortality, equivalent to an apparent 47% reduction in the self-thinning boundary at onset when modelled as a shift in carrying capacity. However, model comparison favoured a direct, density-independent mortality mechanism with gradual recovery over a half-life of approximately 20 years (95% CrI: 7–73 years), rather than a true shift in carrying capacity. Outbreak probability increased with stand density, and excess mortality preferentially affected suppressed trees ( k = 0.729, 95% CI: 0.68–0.78), consistent with pre-outbreak mortality patterns ( k = 0.693, 95% CI: 0.66–0.73). Conclusion: Sap-sucking insect outbreaks impose substantial but reversible mortality, more consistent with a direct mechanism than a lasting shift in carrying capacity. Outbreak establishment is density-dependent and mortality follows existing competitive hierarchies, suggesting density management may reduce risk.