Scale-Dependent Persistence and Density-Dependent Regulation of Insect-Induced Leaf Damage in Alder Forests

Long-term monitoring provides a unique opportunity to distinguish persistent ecological processes from short-term fluctuations in forest insect dynamics. However, the extent to which local insect populations persist through time and the mechanisms regulating their occupancy remain poorly understood. Using a 13-year dataset from the Polish ICP Forests network, we investigated temporal and spatial dynamics of insect occurrence in alder stands. We quantified persistence at tree and plot levels, evaluated signatures of density-dependent regulation, and assessed spatial structure using generalized additive models, logistic regression, transition analyses, and spatial autocorrelation metrics. Insect occurrence exhibited exceptionally strong temporal persistence. Occurrence in the previous year was the dominant predictor of current occurrence across all modeling approaches. Persistence patterns differed between plot and tree levels, with transition analyses revealing greater long-term stability at the level of monitoring plots, consistent with the long-term stability of local populations despite turnover among host trees. Occupancy dynamics were consistent with negative density-dependent regulation, with local populations fluctuating around a stable equilibrium occupancy of approximately 75%. Although year-to-year changes occurred regularly, most fluctuations were relatively small and rarely resulted in complete disappearance of populations from occupied locations. Insect occurrence also exhibited pronounced spatial structure. Occupancy remained significantly clustered throughout the study period, indicating persistent regional differences in local population abundance. In contrast, annual changes in occupancy showed only weak spatial autocorrelation, suggesting that short-term dynamics were driven primarily by local ecological processes rather than by highly synchronized regional fluctuations. Our results indicate that infestation patterns in alder stands are consistent with persistent and self-regulating local herbivore populations that remain associated with the same forest stands over extended periods. Long-term dynamics are governed primarily by temporal persistence, density-dependent feedback, and stable spatial structure rather than by repeated cycles of colonization and extinction. These findings highlight the importance of long-term monitoring for understanding population persistence and provide new insight into the processes maintaining herbivore populations in forest ecosystems.