The vertical form of the common ivy Hedera helix L. is associated with diverse and semi-natural forests in Central European highlands

Understanding the role of epiphytic vegetation and other microhabitats created by plants (hereafter lianas) developing on the surrounding ones is crucial for understanding the functioning of forest communities. Despite the observed increase in the biomass of lianas, the factors connected with forest plant communities determining their occurrence have not been investigated in a temperate zone. This research concerns the determinants of vertical form of the common ivy Hedera helix L. occurrence in highland forests in Central Europe. We established 69 pairs of sites, which consisted of areas inhabited by the vertical form of ivy and control areas (hereafter non -ivy plots). The sites with ivy were characterized by a higher volume (488 m3 ha-1 +/- 251 SD vs. 411 m3 ha-1 +/- 186 SD), a higher amount of deadwood (34 m3 ha-1 +/- 39 SD vs. 23 m3 ha-1 +/- 29 SD), and a higher tree species richness (8.0 species/ha +/- 2.2 SD vs. 6.8 species/ha +/- 2.14 SD) than non-ivy plots. The presence of vertical form of ivy was mainly influenced by a large share of alder in the stand and a higher diversity of trees and shrubs. Moreover, the number of large ivy vines increased with the presence of veteran trees. The presence of gaps was not conducive to the occurrence of large vines. The northern exposure and the presence of deadwood had a positive effect on the number of vines per tree. Our results contribute to the knowledge about variables influ-encing the occurrence and development of ivy vertical form in forests. The obtained results indicate that ivy requires diverse forests with a mix of trees and numerous veteran ones, which makes this species a good candidate as an indicator of forest habitat quality. We suggest that the common ivy should be protected within its native range and forest patches containing dense ivy vines should be excluded from silviculture.