Long-Term Patterns of Silver Birch (Betula pendula) Decline in Poland: Evidence from the ICP Forests Monitoring Network

Silver birch (Betula pendula Roth) is an ecologically important pioneer tree species widely distributed throughout Europe. Despite numerous reports of birch decline, long-term nationwide assessments integrating multiple categories of damage remain limited. The present study investigated temporal patterns of damage occurrence, severity, distribution, and presumed causal agents affecting silver birch in Poland using nationwide ICP Forests Level I monitoring data collected between 2007 and 2025. The analyses were based on 75,609 damage observations recorded for 7039 trees on 945 monitoring plots distributed across the country. Damage frequency increased markedly after 2014 and reached a maximum during 2018–2019, when approximately 90% of monitored trees exhibited at least one damage symptom. Foliage damage represented the most frequent category throughout the study period and was overwhelmingly associated with insect activity. Partially or completely eaten leaves accounted for approximately 80% of all foliage-damage observations, although damage severity was generally low. Stem damage increased continuously during the monitoring period and was dominated by deformations and stem inclination. Decay occurred less frequently but was associated with the highest severity classes and showed a strong relationship with fungal agents. An increasing proportion of trees were affected by multiple groups of damaging factors simultaneously, indicating the growing complexity of decline processes. The results suggest that silver birch decline may be driven by interacting biotic and abiotic stressors rather than by a single causal agent. The observed patterns are consistent with a decline-spiral model in which long-term reductions in water availability and increasing environmental stress may predispose trees to insect damage, root pathogens, stem and branch decay, structural instability, and ultimately mortality. The nationwide scale and long-term character of the ICP Forests dataset provide important insights into contemporary drivers of birch decline under changing environmental conditions.