Natural regeneration of low-elevation mountain forests along a stand composition gradient

Ecosystems of higher latitudes and elevations are more vulnerable to environmental changes. In the case of retreat of forest-forming tree species, recognition of natural regeneration patterns is essential for evidence-based forest management and nature conservation. We aimed to assess relationships transition from spruce to beech and sycamore-ash low-elevation mountain forests, and to determine drivers of their natural regeneration. We hypothesized that retreat of ash and spruce will promote beech natural regeneration and that stand species composition, together with soil pH and light availability will limit the abundance of natural regeneration. We assessed stand structures and natural regeneration within 32 plots systematically distributed along streams in Stołowe Mountains National Park (SW Poland). We used ordination methods to assess gradients of stand and natural regeneration species composition and we developed generalized linear mixed-effects models of natural regeneration density. We revealed a gradient of stand and natural regeneration species composition from artificial spruce stands through beech forests to the ravine and riparian ash-sycamore-maple forests. We also showed that regeneration of beech, ash, and spruce is determined by humus layer thickness, light availability, soil pH, total deadwood mass, and stand composition. The present continuum of forest vegetation is already threatened by the retreat of two important tree species, typical of both ends of compositional gradient – spruce and ash. Beech regeneration in spruce forests will help in the restoration of mixed forests, typical of the low-elevation mountain forest belt. Therefore, such stands will require fewer conservation efforts. Ash can survive and its conservation in low-elevation mountain forests will require monitoring of both optimal and suboptimal sites. Here encroachment of beech can lead to biotic homogenization of broadleaved forests, however a simultaneous increase in proportion of sycamore maple will differentiate stands with different geomorphological characteristics.