Importance of local driving factors for the plant species diversity and composition of Tilio-Acerion ravine forests (Natura 2000)

In addition to being influenced by global drivers, forest herb-layer communities are also shaped by various local factors, such as topography, stand characteristics and soil properties. The responses of plant species richness and composition to these factors were studied in the ravine forests (Natura 2000 priority habitat type of Tilio-Acerion ) in eastern Slovenia, including different stands dominated by different Acer and Tilia species, and by Fraxinus excelsior . Ravine forests with high species richness were mostly found at higher altitudes. The presence of high amounts of deadwood and a high dead-to-living tree ratio, primarily associated with Tilia and Fraxinus excelsior stands, also had a positive impact on species richness. However, species richness was found to be negatively associated with higher values of tree layer cover, tree height and DBH, primarily in well-preserved Acer stands. The most species-diverse Tilia stands had the lowest herb layer cover, indicating a composition of less competitive, site-specific species. Tilia stands with a higher degree of tree clustering were indicated by low Clark and Evans index values. Altitude, soil pH and tree layer cover were among the main factors driving species composition. Dense Acer stands with admixed Castanea sativa were mostly observed on more acidic soils at lower altitudes. To preserve the high level of biodiversity in the ground vegetation of heterogeneous ravine forests, and to maintain their favourable conservation status, it is crucial to sustain the natural state of the forest soil and stand by implementing appropriate management measures. Such measures may include close-to-nature forest management, which is already being implemented in the studied ravine forests.