Developing National Forest Inventory-based indicators for monitoring minority ravine forests

Ravine forests represent a priority habitat type of the European Natura 2000 network for which empirical data are limited, particularly regarding the influence of stand structure on biodiversity. Assessment of forest habitats can largely be supported by National Forest Inventory (NFI) data, which enable frequent and spatially dense monitoring of the stand conditions and potential vulnerability of forest habitat types. In this study, we established an independent, nationwide classification system of close-to-nature managed ravine forests dominated by different characteristic broadleaf trees, based on stratifying NFI data into homogeneous subtypes. On the basis of tree species composition, which is a basic component in forest habitat types, we identified three subtypes of ravine forests, dominated by Acer pseudoplatanus , Fraxinus excelsior , and Tilia spp. We examined these subtypes using structural, compositional, deadwood, and diversity-related indicators. The Tilia -dominated subtype was more common in the lower altitudinal belt (≤ 502 m), while the Acer -dominated subtype was more prominent in the higher belt (> 502 m). The Acer -dominated subtype predominated in stands with SDI lower than 432, while the Tilia -dominated subtype was relatively more common in stands with higher SDI. In stands with Evenness values lower than 0.3, the Acer -dominated subtype predominated, while in stands with higher Evenness index values, the Fraxinus -dominated subtype was more common. In the Fraxinus- dominated subtype, the volume of standing dead trees was statistically significantly higher than in the other two subtypes (14 m 3 /ha compared to 8 m 3 /ha) due to the high mortality rate of trees caused by ash dieback. In all three subtypes of ravine forests, we observed a lack of natural regeneration of key tree species, which is crucial for maintaining the favorable conservation status of the habitat type. The observed ranges of structural and compositional attributes, deadwood components, and diversity indices provide empirical reference conditions that reflect the current nationwide variability of ravine forests. • Based on tree species composition, we identified three ravine forest subtypes. • We examined subtypes using structural, compositional, deadwood, and diversity indicators. • We observed a lack of natural regeneration of key tree species. • Higher standing deadwood in Fraxinus-dominated subtype due to ash dieback mortality.