Harmonized decay classification for dead wood in Nordic national forest inventories

Dead wood quality is typically recorded as a biodiversity indicator and as part of forest carbon estimates, using decay classification systems. In large-scale national forest inventories (NFIs), these systems typically differ among countries. Harmonizing them would allow analyzes over broader scales and the use of larger data sets. Here, we exemplified the development of a harmonized decay classification for the NFIs of Finland, Norway, and Sweden, using wood density as an objective harmonizing criterion. We sampled 441 Scots pine, Norway spruce, birch and trembling aspen in different conditions and decay classes, measured their density and developed the harmonized classification for each species and dead wood type (standing and fallen). The assignments relied on minimizing within-class variance in wood density in the harmonized classes. Assigned into three (standing dead wood) and four (fallen dead wood) classes, the harmonization led to modest losses of information, especially in the advanced decay stages. How the original decay classes were assigned into the harmonized classes differed slightly, depending on the tree species and type. This is an advantage over the expert assessment that typically treat all species and types of dead wood the same way despite clear differences in decomposition pathways.