In boreal forests, feather mosses dominate the understory of black spruce (Picea mariana) forests, while broadleaf deciduous stands lack this moss layer. Leaf litter accumulation is a hypothesized mechanism for this difference, but has rarely been tested. Mosses influence important processes in plant-soil feedbacks, such as soil organic layer accumulation, nutrient cycling, and soil temperature regulation. Our goal is to quantify the impact of deciduous leaf litter on feather moss (Hylocomium splendens) biomass accumulation, moss species composition, and soil temperature using field-based experiments near Fairbanks (64°N), Alaska. In 2012, we established 30 study plots across 3 blocks consisting of adjacent Alaskan paper birch (Betula neoalaskana) and black spruce stands. In each plot, we identified one procedural control area with a high abundance of H. splendens. Ninety H. splendenscores (30 cm diameter, 30 cores per block) were harvested in the black spruce plots and transplanted randomly within the plots and assigned to a treatment: birch leaf litter exclusion, birch leaf litter addition, and ambient birch leaf litter deposition. A total of 1200 individual moss shoots were marked and measured annually, species composition was assessed in 2012 and 2015, and soil temperature under the transplants was recorded.
Our results show that mosses in procedural controls were 26% larger in spruce stands than in birch stands (p=0.02). After the first year, mosses from transplants residing in spruce stands were larger than the ones in birch stands (p<0.001). After the second year, experimental and ambient leaf litter inputs significantly hindered moss growth (p=0.01). A threshold of 80% in leaf litter cover was associated with a higher incidence of fungal infection and of dying moss, which suggests that biotic interactions may mediate the response of moss to leaf litter. Soil temperature was not affected by leaf litter, but was 1.5°C lower under the transplants than in the mineral soil in birch stands (p=0.001). We estimated the natural yearly H. splendens carbon biomass accumulation to be about 1.4 g m‑2 yr‑1 in birch stands, and 18.4 g m‑2 yr‑1 in spruce stands, but these values were 40% lower with experimental and ambient leaf litter inputs. H. splendens’ contribution to carbon biomass accumulation in spruce stands highlights the important linkage between feather mosses and black spruce forests. Changes to biomass accumulation induced by the physical, chemical, or biological impacts of leaf litter may have cascading impacts on forest structure and composition.