Northern Arizona University Flagstaff, Arizona, United States
Abstract: High-latitude ecosystems are experiencing rapid change due to climate warming, with temperatures rising 2-3 times faster than the global average. The magnitude of carbon(C ) stored in these perennially frozen soils, known as permafrost, is nearly 3 times that of the atmosphere with the potential for release as warming leaves these soils vulnerable to thaw and microbial decomposition. Historically, tundra ecosystems have nutrient poor soils and slow rates of elemental cycling due to low annual temperatures and short growing seasons. However, permafrost thaw deepens the seasonally thawed active layer releasing large amounts of deep soil nitrogen, which has generally stimulated plant productivity and increased dwarf shrub abundance in tundra ecosystems over the last two decades. However, this response is highly variable across the landscape, and the environmental drivers of these changes are poorly understood. This study investigated changes in productivity and species composition in the Carbon in Permafrost Experimental Heating Research project (CiPEHR) located in moist acidic tundra underlain by permafrost. Direct biomass measurements collected in 2022 were compared across air and/or soil warming treatments and plant functional types. Indirect measurements of biomass, collected between 2009-2021 using the point-intercept method, were used to analyze changes in species composition over time. Changes in biomass and composition were then related to environmental variables such as water table, active layer depth, subsidence, etc. collected over the course of the experiment.
After 14 years, soil warming resulted slightly higher biomass than in control plots. Although this response was due to highly variable soil moisture conditions across replicate fences. Shrub biomass, especially deciduous shrubs, increased under soil warming relative to sedges. After 13 years, indirect estimates of biomass used to determine species compositions, revealed more rapid species composition change in soil warmed plots than controls. These changes were largely driven by increasing water tables and thaw depths in soil warmed plots, though the magnitude of change varied greatly by snow fence. This suggests heterogeneity in soil moisture as an increasingly important driver of vegetation change as Arctic temperatures rise.
