CHARACTERISTICS OF WATER AND ENERGY EXCHANGE IN A BLACK SPRUCE FOREST OVER PERMAFROST IN INTERIOR ALASKA

In the high latitudes, ecosystem development is influenced by low precipitation, the supply of snowmelt water, and limited water storage capacity due to the presence of permafrost. The ecosystem development, in turn, affects the regional climate through surface fluxes. Thus, to predict climate change, it is necessary to quantify the water and energy budgets. Using data observed over six years in a black spruce forest that stands on discontinuous permafrost in Interior Alaska (64°52N, 147°51W), we investigated the water and energy exchange in order to clarify the response of water balance in a boreal forest to climate variation. The black spruce forest is 120 years old, and the forest floor is covered with mosses, sedges, and shrubs. In this forest, fluxes of energy, CO2, and water were observed using a sonic anemometer and open-path gas analyzer. Micrometeorology such as net radiation, PAR, temperature, rainfall, and soil heat flux were also observed at and around the observation tower. Energy Partitioning After the snow melted, a larger fraction of available energy was partitioned into sensible heat flux due to low transpiration limited by shallow soil thawing. The Bowen ratio (BR) was approximately 2.0 in this period. With increased thaw depth, more energy partitioned into latent heat (LE) flux during the latter half of the growing season (BR was approximately 0.7). Energy balance of this site during the growing season was close to 90% when heat storage in the soil above the soil heat flux plates was accounted for. It is also important to consider LE absorption during the snowmelt period for closure of energy balance. This energy ranged from 15 to 20% of available energy. Water balance During the first half of the growing season, water was supplied first by snowmelt (50-130mm) and subsequently by soil thawing. As a result, the evapotranspiration (ET) exceeded precipitation. Some of the snowmelt was used to saturate the shallow layer of thawed soil, but most was lost as runoff because of the shallow thawed layer (up to 80mm in this season). Peak ET was observed around DOY 200 with a maximum of 2.5mm/day. Thereafter, ET decreased due to leaf senescence of annual plants despite an increase of precipitation. The excess water supply was preserved in the frozen ground to support vegetation growth in the next early growing season. Total precipitation and ET were 327mm and 246mm, respectively, for hydrological year 2007 and 290mm and 225mm, respectively, for 2008. Consequently, the water balance at the site remained in a sustainable condition under the current climate.