Harmonized decay classification for dead wood in Nordic national forest inventories

Dead wood quality is recorded as a biodiversity indicator and in estimating forest ecosystem carbon storage, using decay classification systems. In large-scale national forest inventories (NFIs), these systems are typically slightly different among countries, but harmonizing them would allows analyses over much broader scales and the use of larger data sets. Here, we developed a harmonized decay classification for the NFIs of Finland, Norway, and Sweden, using wood density as the harmonizing criterion. We sampled 441 pine, spruce, birch and 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 dead wood). 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 some losses of information, especially in the advanced decay stages in Finnish and Norwegian classifications. Importantly, the harmonized classes included slightly different decay classes from the national classifications, 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.