Mat-forming “ground layers” of mosses and lichens often have functional impacts disproportionate to their biomass, and are responsible for sequestering one-third of the world's terrestrial carbon as they regulate water tables, cool soils and inhibit microbial decomposition. Without reliable assessment tools, the potential effects of climate and land use changes on these functions remain unclear; therefore, we implemented a novel “Ground Layer Indicator” method as part of the U.S.D.A. Forest Inventory and Analysis (FIA) program. Non-destructive depth and cover measurements were used to estimate biomass, carbon and nitrogen content for nine moss and lichen functional groups among eight contrasted habitat types in Pacific Northwest and subarctic U.S.A. (N = 81 sites). Ground layer cover, volume, standing biomass, carbon content and functional group richness were greater in boreal forest and tundra habitats of Alaska compared to Oregon forest and steppe. Biomass of up to 22769 ± 2707 kg ha−1 (mean ± SE) in upland Picea mariana forests was nearly double other reports, likely because our method included viable, non-photosynthetic tissues. Functional group richness, which did not directly correspond with biomass, was greatest in lowland Picea mariana forests (7.1 ± 0.4 functional groups per site). Bootstrap resampling revealed that thirty-two microplots per site were adequate for meeting data quality objectives. Here we present a non-destructive, repeatable and efficient method (sampling time: ca. 60 min per site) for gauging ground layer functions and evaluating responses to ecosystem changes. High biomass and functional distinctiveness in Alaskan ground layers highlight the need for increased attention to currently under-sampled boreal and arctic regions, which are projected to be among the most active responders to climate change.