Integrating fine-scale topography and stand structure to explain herb-layer diversity in forest ecosystems

Herb-layer diversity in temperate forests emerges from the interaction between stand structure and fine-scale environmental variation. We studied eight mixed beech–hornbeam and oak–hornbeam plots distributed along a short altitudinal gradient (170–310 m) and contrasting slope aspects in East Europe. For each plot, we measured tree diameter, height and crown-base height, and derived stand structural indices (tree density, basal area, Gini coefficients for DBH diameter at breast height and total height, and median crown base height). Herb-layer communities were characterized using standard diversity indices and Ellenberg indicator values. We then used Jaccard similarity to explore tree–herb cooccurrence and Spearman correlations to relate stand structure to herb-layer diversity. Stands differed markedly in structure along the combined elevation–aspect gradient. Basal area peaked at middle elevation on the north-facing slope and at high elevation on the south-facing slope, whereas size inequality and crown-base height showed contrasting patterns among plots, reflecting a mosaic of dense, structurally heterogeneous stands and more open, even-sized canopies. Herb-layer richness and evenness increased with higher median crown-base height and lower size inequality, but declined with increasing basal area and stem density. Ellenberg values confirmed that open mixed stands supported more light-demanding herbs, while dense, beech-dominated stands were characterized by a few shade specialists. Tree–herb cooccurrence patterns were generally weak, indicating mostly generalist herbs with a few consistent associations. Overall, our results show that simple stand structural indices, combined with fine-scale topography, can effectively predict herb-layer diversity and provide useful indicators for biodiversity-oriented forest management.