Common mistletoe effects on Scots pine growth in northern Poland: a case study

The spread of common mistletoe (Viscum album L.) and its increasingly frequent occurrence in forest stands have become important research and economic issues. Increased mistletoe occurrence has been observed in many European countries, and its effects on infested stands include slower growth, reduced needle biomass, and deteriorated wood quality. Empirical data were collected from an 80-year-old Scots pine stand, where diameter at breast height and tree height were measured for 500 trees. At the same time, the degree of mistletoe infestation was assessed and trees were assigned to three groups: uninfested trees (p0), trees with up to 50% crown infestation (p1), and trees with more than 50% crown infestation (p2). Mistletoe infestation was observed in 36.4% of all trees. From each infestation group, two sample trees were selected from each of five diameter classes using Urich’s method, and increment cores were collected at a height of 1.3 m using a Pressler increment borer. Annual radial increment for the period 2000–2022 was determined for each core using WinDendro software. Statistical analyses comparing p0, p1, and p2 trees, including ANOVA and the Kruskal–Wallis test, revealed statistically significant differences in radial increment in the final year of the study (2022). Pairwise comparisons (Student’s t-test, Welch’s t-test, and the Mann–Whitney U test) indicated statistically significant differences in annual radial increment at breast height between p0 and p2 trees over the last three years of the study (2020–2022). The strongest effect was observed in 2022, when the mean annual radial increment of trees with crown infestation exceeding 50% (p2) was more than 60% lower than that of uninfested trees (p0). These results indicate that the mere presence of mistletoe does not necessarily limit tree growth; rather, growth reduction depends on the degree of crown infestation. A marked reduction in radial increment occurs mainly at high infestation levels and is likely exacerbated by environmental stress.