How wrong are we in estimating the abundance of mistletoe occurring on Scots pine? – a case study from Central Europe

This paper presents data on the occurrence of the semi-parasitic mistletoe plant (Viscum album L.) in a pine stand in central Poland. The number of mistletoe bushes, the percentage of mistletoe in the crown volume, and the location of its occurrence were determined from the ground level on 193 pine trees growing in a 212c subcompartment in the Głuchów Forestry, Rogów Forest Experimental Station. In addition, 15 trees were analyzed in detail after felling to verify the previously obtained results. The age, diameter, and location (top, middle, and bottom of the tree crown) of the shrubs were measured. The study showed that the pine stand analyzed was strongly infested by mistletoe. The number of mistletoe bushes recorded on standing trees from the ground level was significantly lower than that found on trees after felling. An in-depth visibility analysis showed that in some cases, an operator assessing mistletoe bush abundance from the ground level may miss a very large number of juvenile individuals, as well as mistletoe shrubs with large diameters. This is probably related to the clumping of perennial individuals growing in proximity to a single shrub that is visible from the ground. V. album mainly colonizes the apical zone (observations from the ground) and middle zone of the host crown (detailed analysis of mistletoe abundance on felled trees). Older shrubs were most abundant at the apex of the crown, while younger shrubs were most likely to colonize the middle and bottom of the host crown. In a study of felled trees, 289 mistletoe individuals were found on the most infested trees. It was also found that the most numerous shrubs, V. album were young, aged between three and six years. The increase in mistletoe infestations in forests observed in recent years may pose a serious threat to pine stands weakened by abiotic factors.