Assessment of the quality of EU boreal forest habitats of importance on the basis of indicators of natural structures and processes

All European Union countries are obliged to report on the status of EU forest habitats of importance, which needs to rely on assessments of their quality; however, standardised methods to accomplish this goal remain elusive. A survey of EU forest habitats of European importance in Latvia was conducted from 2017 to 2021, which resulted in a large dataset of structures, processes and species within forest patches identified as such. We analysed the quality of five selected boreal forest habitats ranging from moist to dry sites. Principal component analysis revealed the key natural structures and processes related to the quality gradient. These included uneven age structure, dead standing and fallen trees, polypores, old trees and gaps. The quality gradient based on indicators of naturalness was also associated with a nutrient and moisture gradient of site types and with dominant tree species; compared to poor dry sites with pine, rich site types with spruce and deciduous trees had higher quality based on structures and processes. This indicated that poor sites develop structures over a longer period of time, thereby having a lower quality assessment and being neglected for protection. The results of the study were used to rank the quality of EU forest habitats, which served to prioritise sites for protection. Considering the criteria and indicators used for assessing the quality of EU forest habitats, it is highly likely that the stands with highest quality can also be defined as old-growth forest.