Treeline biogeochemistry and dynamics, Noatak National Preserve, northwestern Alaska

The extensive boreal biome is little studied relative to its global importance. Its high soil moisture and low temperatures result in large below-ground reservoirs of carbon (C) and nitrogen (N). Presently, such high-latitude ecosystems are undergoing the largest temperature increases in global warming. Change in soil temperature or moisture in the large pools of soil organic matter could fundamentally change ecosystem C and N budgets. Since 1990, we have conducted treeline studies in a small (800 ha) watershed in Noatak National Preserve, northwestern Alaska. Our objectives were to ( 1) gain an understanding of treeline dynamics, structure, and function; and (2) examine the effects of global climate change, particularly soil temperature, moisture, and N availability, on ecosystem processes. Our intensive site studies show that the treeline has advanced into tundra during the past 150 years. Inplace and laboratory incubations indicate that soil organic-layer mineralization rates increase with a temperature change >5°C. N availability was greatest in soils beneath alder and lowest beneath willow or cottongrass tussocks. Watershed output of inorganic N as NO^-₃ was 70 percent greater than input. The high inorganic-N output likely reflects soil freeze-thaw cycles, shallow flowpaths to the stream, and low seasonal biological retention. Concentrations and flux of dissolved organic carbon (DOC) in stream water increased during spring melt and in autumn, indicating a seasonal accumulation of soil and forest-floor DOC and a shallower flowpath for meltwater to the stream. In sum, our research suggests that treeline transition zone processes are quite sensitive to climate change, especially those functions regulating the C and N cycles.