As recently reported, European mistletoe has become an important agent of damage to forest trees in Central Europe. To document the extent of its impact, host and environmental preferences, as well as spatial and temporal dynamics, we have analyzed the results of 10 years of annual assessments of tree damage in level 1 plots of country-wide national forest monitoring. Mistletoe was found on 12 forest tree species, most frequently on silver fir and Scots pine, much less often on birches and deciduous admixture species. Common beech, pedunculate, and sessile oaks, as well as coniferous admixture species, were found to be free from the parasite. Mistletoe was more common on larger trees, mainly those in the 1st and 2nd Kraft classes, characterized by a higher DBH value than trees without V. album. Infestation was also age-dependent with low numbers of infested trees in young age classes and a much higher proportion of infested trees being over 80–100 years in age. Defoliation of trees colonized by mistletoe was found to be significantly higher than in trees free from the parasite, confirming the adverse effect of infestation on tree health. The increase of relative incidence of tree infestation by mistletoe in forests in Poland was high throughout the entire period 2008–2018 and significant for the most threatened host species: silver fir, Scots pine, and birches. It was also found that spread direction was from west to east and that the occurrence of V. album shows clear regionalization. The most endangered forests were located within a 200 km wide zone ranging through lowland Central Poland from the border with Germany down to the border with Ukraine and Belarus. The highest concentration occurred in the Western part of the country. Outside this zone (to the north and south) the mistletoe was much less abundant as expressed by the amount of injuries caused by the parasite or infested trees per plot.

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.