Responses of Energy Budget and Evapotranspiration to Climate Change in Eastern Siberia

The structure and functioning of the Arctic terrestrial ecosystems are greatly sensitive to climate change. Evidence continues to mount that warming experienced in the Arctic region during the past few decades has been affecting the structure and functioning of the Arctic terrestrial ecosystems (Oechel et al., 2000; Serreze et al., 2000). Observations indicate an increase in the number of shrubs in tundra regions (Chappin et al., 1995) and an increase in early greening of the Arctic vegetations (Buermann et al., 2003). Changes in the structure and phenology of vegetation act to modify energy and water budgets, because all components in the Arctic are interrelated through a network of linkages, feedbacks, and multi-dependent interactions. In fact, expansion of shrub cover has its own positive feedback on climate because of the lower albedo of shrubs compared to tundra, and consequently earlier snowmelt than snow-covered tundra (Chapin et al., 2005). In this manner, a change in one variable in a part of the system can initiate a cascade of regional effects and have global ramifications. Soil moisture is the most important factor that links climate and vegetation. Climate influences the soil moisture via evapotranspiration (ET). Winter is the period of the lowest temperature in the Arctic, and hence, the low saturation of water vapour leads to less evaporation. Soil-freezing also controls plant water uptake. Snowmelt during the early spring releases the stored ice water causing higher soil moisture, subsequently increasing ET. Conversely, higher levels of ET cause a decrease in soil moisture content, resulting in soil water deficits. This soil water deficit in turn controls ET, stomatal conductance, and photosynthesis. ET rates at high-latitude are lower than at low-latitude. According to a recent review (Park et al., 2008), ET rates in eastern Siberia during the growing season were less than 3 mm day-1 whereas the corresponding rate at mid and low latitudes was 1–6 mm day-1. The low ET rates in high latitudes can cause an extremely large seasonal variability in soil moisture. In Lena and Kolyma watersheds, for example, ET during the summer has often exceeded precipitation, resulting in a negative water balance (Park et al., 2008). The higher rates of ET during the summer subject the vegetation to frequent soil water stress. Unless the stress is settled by an increase in precipitation, summer water balances will