Dead wood volume-to-carbon conversion factors by decay class for ten tree species in Croatia and eight tree genera globally

Dead wood (DW) is an important forest carbon (C) pool for which the reporting of C stock changes within the National Greenhouse Gas Inventory Report (NIR) is mandatory. The use of DW volume-to-carbon conversion factors by decay classes, i.e. DW basic density (DWBD) and C fraction (CF), or C density (CD) as their product, facilitates more accurate estimates of DW C stocks and C stock changes. We present DW conversion factors, namely DWBD, CF, and CD, by decay classes for six broadleaf (Quercus robur L., Carpinus betulus L., Alnus glutinosa Garnet., Fraxinus angustifolia Vahl., Fagus sylvatica L., and Quercus ilex L.) and four conifer (Abies alba Mill., Picea abies (L.) Karst., Pinus nigra Arnold and Pinus pinaster Aiton) tree species from Continental, Alpine and Mediterranean biogeographical regions of Croatia. For each tree species DW logs (diameter from 5 to 30 cm) were visually categorized into four decay classes and nine discs per decay class were sampled. In addition, three healthy trees of each species were felled down at the same location for estimating the basic density of fresh wood (BWD, decay class 0) and calculation of species-specific relative density reduction patterns. In total, 390 discs were sampled and analysed in the laboratory. Our results show that broadleaves, on average, have higher DWBD than conifers (p < 0.05) for decay classes 0-3, but for decay class 4 no difference (p < 0.05) was observed between these two tree species groups. Unlike for DWBD, CF showed no trend with decay class, but a difference (p < 0.05) in the average CF was found between broadleaves and conifers with the mean (s.e.) of 47.65% (0.12%) and 50.67% (0.23%), respectively. The application of decay class-specific CD by forest land use strata used in the Croatian NIR yielded 11.1% - 26.6% lower DW C pool estimates, compared to the current ones calculated using BWDs and default CF. DWBD data from our study was compared with DWBD data from the literature. We observed good agreement between national species-specific DWBD and global genus-specific DWBD for the great majority of investigated tree genera and decay classes. Therefore, we combined our results with the published data to provide a global DWBD meta-means with confidence intervals by decay classes for eight tree genera. Our results suggest that in the absence of local or national DWBD data, the use of genus-specific DWBD meta-means is justified.