Towards a set of national forest inventory indicators to be used for assessing the conservation status of the habitats directive forest habitat types

Since the enactment of the EU Habitats Directive, the conservation status of forest habitat types, habitats of species, and species has become the central concept of the Directive's nature conservation legislation. Despite its role, it has drawn relatively little attention. Within a rather short research period, a few research papers have addressed the existing definitions, indicators for the conservation status assessment, and assessment techniques. This paper attempted to complete the set of measurable indicators available in national forest inventories and connect them with the forest habitat types’ conservation status components (area, function, structure, and prospects). A set of 40 indicators was defined, labelled with one or more of the four conservation status components and assessed with the quality dimensions. The analysis uncovered that five indicators could be used to assess the component of range and area, 20 that of structure, 22 that of function and 27 that of prospects. It also showed that conventional forestry indicators such as tree species, diameter at breast height, and regeneration are less sensitive regarding the data quality. Conversely, some typical biodiversity indicators lacked completeness, timeliness, and precision. In addition to this analysis, the data distributions (data for them were provided by the national forest inventories of Italy, Slovenia, and Spain) of some indicators were analysed. Based on all the results, it was also possible to conclude that there is a shortage of national forest inventory indicators for the assessments of the area and function conservation status components. While the area component should be described with the indicators of forest habitat type fragmentation, mingling and perforation with non-forest and other forest vegetation communities, the functional component is bereft of indicators describing processes such as biomass growth and carbon cycling. Future research should thus search for more indicators to represent all conservation status components in a more balanced way. More efforts should also be expended into the harmonisation of indicators.