Cross-country patterns of compositional and structural forest diversity across European regions

Abstract Forest diversity is a multidimensional concept comprising different components such as species diversity, functional diversity, structural diversity and genetic diversity. These diverse elements are recognised as being connected to the health and functioning of forest ecosystems and human well-being. However, information on forest diversity at broad spatial scales is scarce. Thus, the primary goal of this study is to quantify compositional diversity (i.e., tree species heterogeneity) and structural diversity (i.e., tree size heterogeneity) across a wide climatic gradient in European forest ecosystems, while also examining the influence of forest attributes and climatic variables on these two key components of forest diversity. Using harmonised data from eight European National Forest Inventories (n = 146,235 plots), we calculated Shannon’s Diversity Index as a measure of compositional and structural diversity. Finally, we estimated measures of forest diversity at three spatial scales ( α , β and γ -diversity) for each country. Basal area was positively related to compositional and structural diversity. In contrast, the quadratic mean diameter of the trees in each plot presented both positive and negative relationships with compositional and structural diversity, respectively. Climatic variables played a minor role, with precipitation and temperature showing a positive association with forest compositional and structural diversity. Furthermore, our findings revealed a positive link between compositional and structural diversity. Finally, the compound analyses of α , β , and γ-diversity emerged as key elements in interpreting compositional patterns at landscape scale. Results revealed strong scale dependence (from local to landscape level) in diversity metrics across countries, thereby highlighting the importance of reporting national forest information at multiple spatial scales.