Structural and compositional indicators of the conservation status of forest habitats: A case study of ravine forests – EU priority habitat type Tilio-Acerion

Maintaining the conservation status of habitat types such as the ravine forests (Tilio-Acerion) assessed in this study is a priority of the European Natura 2000 network. Ravine forests often occur in smaller, fragmented areas, but are widely distributed throughout European forests. Reliable indicators of the conservation status of Natura 2000 habitats, which support monitoring and reporting under Article 17 of the Habitats Directive, are often not available. Therefore, we tested a set of 161 structural and compositional variables as potential indicators of the conservation status of close-to-nature managed ravine forests in a Natura 2000 site in eastern Slovenia. The studied forests ranged from Acer pseudoplatanus-dominated stands to those dominated by Fraxinus excelsior or Tilia species. Most forests were classified as having either a favourable or inadequate conservation status. The main pressures included game browsing and mortality of the key tree species, primarily caused by invasive alien fungi. Favourable conservation status was associated with less intensively managed Tilia-dominated stands on rocky ridges and steep slopes. It was also linked to higher tree layer cover, particularly of Acer pseudoplatanus, in well-preserved forest stands. Conversely, indicators of bad conservation status were associated with Fraxinus excelsior-dominated stands that had been severely affected by invasive alien fungi, resulting in increased volumes of standing and lying deadwood. The resulting tree mortality created more open stand canopies with increased light availability at the forest floor, as indicated by the higher number of plant species in the herb and shrub layer. The conservation status of ravine forests is likely to be increasingly threatened by the adverse effects of climate change, including pests and disease outbreaks and other disturbances. To ensure the continued favourable conservation status of ravine forests, it is essential to monitor key indicators and apply appropriate forest management measures.