Active management promotes plant diversity in lowland forests: A landscape-scale experiment with two types of clearings

Forestry intensification in Central European lowland forests leads to a substantial increase in canopy closure, which causes biodiversity decline and functional homogenization across multiple trophic levels. Small-scale canopy interventions such as local clearings with few left-over adult trees are proposed to halt such a decline but the outcomes can be highly scale- and site-dependent. Here two types of experimental clearings were applied - clearings connected to alluvial meadows distributed along migration river corridors versus clearings inside the forest - in an attempt to restore thermophilous oak forest diversity at six woodland sites in Central Europe. Within each clearing, we studied vegetation changes at two spatial scales (1600 m(2) and 4 m(2)) for six years in relation to composition of surrounding habitats (closed forests, relict open forests, meadows, forest edges), soil seed bank and plant functional traits in order to assess the relative role of species pool, dispersal limitation and niche-based competition processes for forest diversity restoration. In both types of forest clearings, plant species diversity substantially increased, peaking in the second and third years after cutting the trees when the vegetation was still sparse. Afterwards, when the vegetation was becoming dense and plant competition increased, the species richness in clearings decreased but was still higher compared to other forest habitats. Species composition changed from thin-leafed, wind dispersible early colonizers represented by light-demanding shortlived species towards shade-tolerant perennial species. Connected clearings were more species-rich than isolated clearings at the larger scale, but less diverse at the fine scale, indicating larger within-site heterogeneity caused by lower dispersal limitation but stronger competition compared to isolated clearings. Light and nutrient-demanding plants were more diverse and abundant in connected than isolated clearings while threatened species of relict open forests tended to establish more in isolated clearings. Our results show the important role of clearing location within the landscape context for restoring plant biodiversity of lowland forests.