Increasing fire severity associated with climate change is resulting in deciduous forests establishing in areas historically dominated by coniferous black spruce (Picea mariana). Deciduous species often display characteristics consistent with more rapid carbon (C) and nutrient cycling compared to coniferous species. Thus, shifts in species composition could have long-term implications for ecosystem function. We quantified C and nutrient cycling in mid-successional stands of black spruce and the deciduous tree, paper birch (Betula neoalaskana), that established following a fire in 1958. We identified three blocks containing adjacent stands of birch and spruce. Within each block, we randomly located five plots in each stand type (15 plots per species) and measured litter inputs, reciprocal litter and moss decomposition, soil nutrient availability using resin bags, and environmental characteristics (air and soil temperature, soil moisture, light availability, and relative humidity) for up to five years. We expected that birch stands would exhibit greater litter inputs, faster litter decomposition and nutrient release, and higher soil nutrient availability, indicative of faster biogeochemical cycling. We used linear mixed effect models to test differences between stand types (and when relevant, litter types) across years.
Results/Conclusions
Data collected during year one indicated that birch stands exhibited characteristics consistent with more rapid C and nutrient cycling as expected, including greater litterfall, higher foliar nutrient concentrations, and higher dissolved inorganic nitrogen (DIN) and phosphate concentrations in soils. However, overall decomposition measured as percent initial mass remaining was similar between birch and spruce stands for spruce litter and moss. Birch litter, on the other hand, lost significantly more mass when decomposed in spruce stands relative to birch stands (p<0.001). Birch litter only lost about 4% more mass than spruce litter in birch stands (p=0.059), but this difference nearly doubled to 7% in spruce stands (p<0.001). Moss litter, however, lost about 30% less mass than birch and spruce litter in both environments, indicating the importance of moss in these ecosystems. Moss creates a nearly continuous mat on the forest floor of black spruce stands, whereas moss cover in deciduous stands is patchy or absent. Our results show that differences in decomposition in these two forest types is largely driven by moss and highlight the importance of considering moss when determining long-term C and nutrient cycling dynamics in boreal ecosystems and feedbacks to future climate.