The effects of European mistletoe (Viscum album L.) on urban tree health

Background. Urban forests play a crucial role in making the urban environment and provide for sustainable development of urban areas. At the same time, urban habitats are stressful for trees therefore the latter become more susceptive to a range of disturbances, in particular to biotic agents such as the European mistletoe. The paper aims to evaluate the detrimental impacts of this hemiparasitic plant on urban trees. Materials and Methods. The study was conducted in an urban watershed of central and northwestern parts of the city of Kharkiv, Ukraine. Urban tree health survey took place in different stand types along 4 transects of the total length of 6.54 km. Totally, 956 individual trees were surveyed during the vegetation season of 2019. The health status of individual trees was assessed applying the 7-grade scale after O. D. Maslov, modified by the Ukrainian Forestry Research Institute. The mistletoe presence on individual trees was quantified using a semi-logarithmic index. The statistical analysis of the data collected involved the comparison of two empirical distributions, the normality test of the initial data, and the determination of the relationship between two independent samples. Results and Discussion. The analysis of the collected data revealed that the studied trees divided into two groups – “non-infested” and “mistletoe-infested” – grew in quite similar conditions and showed quite similar distribution by stand types, classes of soil conditions and age classes. We found differences in the mean health status between non-infested (transient from “weakened” to “dry-top”), and mistletoe-infested trees (“dry-top”): the tree health index of 2.43±0.03 for the former vs. 2.98±0.08 for the latter. Empirical distributions of the number of trees by the tree health classes in both groups were different too (the Kolmogorov–Smirnov criterion λ = 3.14 at Р >0.999). We found a significant positive correlation between both mistletoe abundance and the age of individual infested trees (Spearman’s R = 0.36 at Р >0.999), as well as the health index of individual infested trees (Spearman’s R = 0.23 at Р >0.95). Conclusions. The study has proven a negative effect of mistletoe’s infestation on the tree crown foliage assessed with the tree health index. Such an effect increases with the age of infested trees. The findings of this study suggest that in urban forests the mistletoe acts as a trigger towards acceleration of tree stand health deterioration.