Mosses are a dominant ground cover in the boreal biome. They insulate the ground, thus partly controlling the ground thermal regime. Mosses also contribute to carbon (C) sequestration and biological nitrogen fixation (BNF). However, mosses are not consistently represented in terrestrial biosphere models such as CLASSIC. Here, we implemented a feather moss plant functional type (PFT) in CLASSIC including BNF by cyanobacteria associated with feather mosses. First, we compared simulated gross primary productivity (GPP) against observation-derived GPP at eight black spruce-dominated boreal forest stands. Ecological realism in CLASSIC is improved, as mosses can now contribute to total GPP both directly through photosynthesis and indirectly through a fertilizing effect on vascular PFTs (once in the soil pools, the nitrogen input from BNF is not accessible by feather mosses in CLASSIC due to mosses’ lack of roots). Total GPP is increased by the addition of a feather moss ground cover, increasing mean absolute error and mean bias error between simulated and observation-derived GPP at most forest stands. Earlier versions of CLASSIC were initially tuned to reproduce observation-derived GPP with vascular PFTs exclusively. Model parameters need to be recalibrated to account for contributions of both vascular and moss PFTs to total GPP. Next, we will run CLASSIC across Canada’s boreal biome to examine the spatial variability in changes in simulated GPP introduced by this additional complexity.