Tage der Biodiversität 2027

Tree and shrub species diversity is a key component of forest biodiversity and an important indicator of forest ecological condition. Its conservation and restoration are central to EU biodiversity policy, from the European Green Deal and Biodiversity Strategy for 2030 to the the Forest Strategy for 2030. Most recently, the EU Nature Restoration Regulation (Regulation (EU) 2024/1991) established legally binding restoration objectives and explicitly identifies tree species diversity as one of seven indicators for assessing progress in enhancing forest biodiversity. However, biodiversity patterns depend on how diversity is measured and on the environmental and forest conditions under which species occur. Using nationwide data from the Austrian National Forest Inventory (period 2016-2021), we assessed the relative importance of forest structure, productive capacity, climate, and forest management as drivers of tree and shrub species diversity across three Hill numbers (⁰D, ¹D, ²D), capturing species richness, effective species diversity and dominant species diversity.

Climate and forest structure variables emerged as the dominant predictor groups, but their relative importance varied substantially among forest types and, to a lesser extent, among diversity dimensions ⁰D, ¹D, and ²D. Climate, particularly temperature seasonality and warming rate, was most influential in several conifer-dominated forest types, while productivity contributed more strongly to broadleaf-dominated forests. Forest structure was consistently important, with mean tree height and diameter, their variability, and vertical layering among the most influential variables. Forest management generally had a comparatively small independent contribution but was more important in spruce and spruce-fir forests.

Our results demonstrate that tree and shrub diversity is strongly forest type-dependent and cannot be adequately characterized by a single set of drivers. For biodiversity assessment and monitoring, this highlights the importance of considering both, the dimension of diversity being measured and the forest type and climatic context in which it occurs. The study illustrates the potential of consistent, nationwide forest inventory data to support forest biodiversity monitoring and reporting under the EU Nature Restoration Regulation and to provide a basis for interpreting biodiversity patterns in the context of changing environmental conditions and forest management.