Recovery Processes and Delayed Density-Dependent Regulation Reduce Long-Term Insect Damage in Alder Stands

Long-term monitoring data provide an opportunity to distinguish between the prevalence of herbivorous insects and the ecological consequences of their activity. Using observations collected in alder stands within the Polish ICP Forests network during 2013–2025, we investigated temporal patterns of insect-related damage, recovery dynamics, and evidence for delayed density-dependent regulation. Damage severity exhibited strong interannual persistence, although transitions toward lower damage classes occurred more frequently than shifts toward higher classes. Ordinal regression analyses identified contrasting effects of infestation prevalence, with positive contemporaneous effects and negative lagged effects on subsequent damage severity. Plot-level analyses revealed negative feedback and convergence towards an equilibrium damage level, supporting the presence of delayed regulatory processes. Infestation prevalence and damage severity remained strongly associated through time, yet damage declined more rapidly following peak infestation, indicating stronger recovery of ecological impact than of occurrence itself. Spatial analyses revealed persistent clustering of damage levels but comparatively weak spatial structure in annual damage change. Overall, the results demonstrate that persistence, recovery, and delayed feedback jointly shape long-term herbivore impacts in alder ecosystems and highlight the value of assessing both infestation prevalence and damage severity in forest monitoring programmes.