Factors Supporting a High Level of Understorey Plant Diversity in Ravine Forests (EU Priority Habitat Type)

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 ground vegetation were studied in the ravine forests of a Natura 2000 site in eastern Slovenia. A high species richness of 218 plant species was observed in the herb layer, including some woody species. On average, 52.8 different plant species were recorded per plot. Species richness was significantly associated with topographic and forest stand factors, rather than soil characteristics. It was positively associated with altitude and the amount of deadwood and negatively associated with tree height. However, the main predictors for the species composition of the ground vegetation were tree layer cover and soil pH. Among the studied ravine forests, Tilia-dominated stands are characterised by the highest species diversity and the lowest herb-layer cover, indicating a composition of less competitive, site-specific species inhabiting sites with high resource heterogeneity and diverse microhabitats. To preserve the high level of biodiversity of heterogeneous ravine forests and to maintain their favourable conservation status, it is crucial to sustain the natural state of forest soils and stands by implementing appropriate management measures. Such measures may include close-to-nature forest management, which is already being implemented in the studied ravine forests.