The influence of stand structure in submontane acidophilous oak forests on the presence of the wild service tree and sword-leaved helleborine

The objective of this work was to evaluate the occurrence of two strictly protected vascular plant species in managed stands of submontane acidophilous oak forests (Luzulo luzuloidis – Quercetum petraeae Hilitzer 1932 association) in the Sudeten foothills (Lower Silesia, Poland). During the study, the most important stand parameters influencing the occurrence of the wild service tree (Sorbus torminalis (L.) Crantz) and the orchid sword-leaved helleborine (Cephalanthera longifolia (L.) Fritsch) were ascertained. The stands ranged from 50 to 130 years of age and were dominated by sessile oak (Quercus petraea (Matt.) Liebl.), which comprised 5–100% of trees. Both of the protected plant species were observed in 10% of the examined plots with the most favorable type of forest stand for sword-leaved helleborine as well as the wild service tree being clearly dominated by sessile oak (portion of oak above 80%). The analysis showed that the wild service tree was found in pure oak stands, whereas sword-leaved helleborine was also recorded in mixed stands aged 50–80 years. The increased frequency of sword-leaved helleborine was associated with a higher portion of oak in the tree layer. The results suggest that the modern silviculture practices, “close-to-nature” silviculture, transform pure oak forest into mixed forest and allow for natural expansion of hornbeam (Carpinus betulus L.) as well as beech (Fagus sylvatica L.), which can induce the gradual disappearance of the orchid and the wild service tree in submontane acidophilous oak forests. Restoring semi-natural pure oak stands should play a significant role in supporting both protected species in submontane acidophilous oak forests of the Sudeten Region