Are Mosses Required to Accurately Predict Upland Black Spruce Forest Soil Carbon in National-Scale Forest C Accounting Models?

The boreal forest plays a key role in the global carbon (C) cycle, and black spruce (Picea mariana (Mill.) BSP) forests are the dominant coniferous forest type in the Canadian boreal forest. National-scale forest C models currently do not account for the contribution of moss-derived organic matter that we hypothesize to be significant in the C budget of black spruce ecosystems. One such model, the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3), is designed to meet Canada's forest-related greenhouse gas reporting requirements. In this study our goal was to determine if black spruce forest soil C stocks are significantly underestimated by the CBM-CFS3, and if so, to determine if estimates could be improved by adding moss-derived C. We conclude that in black spruce sites, organic layer C is significantly underestimated by CBM-CFS3 compared to sites with all other leading tree species analyzed. We compiled and used published moss net primary productivity rates for upland forest systems, with decomposition rates, in mass-balance calculations to estimate mean moss-derived C in black spruce forests for feather mosses at 64 Mg C ha(-1), and for sphagnum mosses at 103 Mg C ha(-1). These C pools are similar to the CBM-CFS3 mean underestimation of black spruce soil organic layers (63 Mg C ha(-1)). We conclude that the contribution of mosses is sufficiently large that a moss C pool should be added to national-scale models including the CBM-CFS3, to reduce uncertainties in boreal forest C budget estimation. Feather and sphagnum mosses should be parameterized separately.