Effects of forest stand structure on population of endangered orchid species Cypripedium calceolus L.

Lady's slipper orchid (Cypripedium calceolus L.) has suffered a dramatic decline and is one of the most endangered orchid species in Europe. It grows mainly on calcareous soils in deciduous and mixed forests. Although numerous studies have contributed to our understanding of the causes of C. calceolus decline, surprisingly, little is known about the effects of the forest stand structure on the spatial distribution and morphometric characteristics of C. calceolus ramets. In this study, we analyzed how the spatial distribution of tree species and several structural characteristics of the forest stand affect the distribution and morphology of ramets of C. calceolus. For this, we used a remnant population of this species located in extensively managed forests in the Krowiarki Range (the Sudetes, SW Poland). The alarming decline in C. calceolus in this region over the last century is commonly attributed to forest management and land-use changes (abandonment of pastoralism in forests). We analyzed the morphometric characteristics and spatial distribution of C. calceolus ramets in this population in relation to the spatial distributions of trees, canopy gaps, saplings and structural characteristics of the forest stand. For this, we combined precise measurements of the forest stand structure, spatial analyzes in GIS and multivariate logistic regression modeling. The probability of the occurrence of C. calceolus was best predicted by its proximity to silver fir trees. We attributed this mainly to a positive effect by silver fir on the topsoil moisture, which can be especially beneficial to C. calceolus as this species is sensitive to drought. The occurrence of C. calceolus ramets was negatively affected by the basal area and positively affected by the diameter at the breast height of trees growing in a 5 m buffer zone. The structural attributes of the forest stand structure also affected the orchid's morphology. Ramets growing close to beech or sycamore had lower leaf dimensions than ramets close to fir, and a pattern of decreasing leaf size with proximity to beech or sycamore was visible. This study spotlights the relationship between forest stand structure and the population of the endangered orchid C. calceolus. Based on the results of this and previous research, we have formulated several recommendations for the protection of the remnant population of this species in managed forests. These include maintaining mixed beech-fir forests rather than pure beech forest stands, supporting forest regeneration by species other than European beech, cutting young beech trees to prevent overgrowing the orchid ramets, fencing remnant populations of C. calceolus to exclude browsing pressure and maintaining spatial heterogeneity to support wild pollinators.