Propagule pressure, lag-phase dynamics and impacts of European fir mistletoe in silver fir forests revealed by multi-year monitoring

European fir mistletoe ( Viscum album subsp. abietis ) has become an increasingly important biotic stress factor in silver fir ( Abies alba ) forests of Central Europe, yet the mechanisms governing its spread remain poorly understood. We analysed long-term observations from the Polish ICP Forests monitoring network, comprising 143 permanent plots and 1461 individually monitored fir trees surveyed between 2009 and 2025. The objectives were to quantify temporal and spatial patterns of mistletoe expansion, identify tree characteristics associated with colonization, evaluate the role of local and spatial infection pressure, and assess the consequences of infestation for host vitality and growth. Mistletoe occurrence increased steadily throughout the study period. Infestation probability was significantly higher for dominant and large-diameter trees, indicating preferential colonization of canopy-dominant individuals. Discrete-time hazard models revealed that colonization probability was strongly associated with local infection pressure and was best explained by infection pressure recorded approximately 5–6 years earlier, indicating a delayed response between establishment of infection sources and subsequent spread. Spatial infection pressure improved model performance but had a substantially weaker effect than local infection pressure, suggesting that transmission is primarily driven by local-scale processes. Mistletoe occurrence was concentrated mainly at lower elevations, with nearly 90% of infested trees occurring below 430 m a.s.l., suggesting climatic limitations on its persistence and spread. That resulted in 521 observations used for statistical modelling. Following colonization, tree condition progressively deteriorated. Infested fir trees exhibited increased crown defoliation and related reduction of diameter growth was statistically confirmed. The observed pattern of slow initial expansion followed by accelerated spread is consistent with lag-phase dynamics commonly described for biological invasions.