THE ROLE OF WILLOW BIRCH FOREST IN THE SURFACE-ENERGY BALANCE AT ARCTIC TREELINE

Continuous measurements of the energy balance components were made dur ing the 1991 growing season over a willow-birch forest located near Ch urchill, Manitoba, Canada. On the basis of measurements of leaf area i ndex, the growing season was divided into three distinct periods: grow th, mature, and senescence. Changes in surface albedo were strongly co rrelated with changing leaf area index during the growth period with a lbedo increasing as leaf area increased. The latent heat flux density, Q(E,) represented 74% of net radiation during the mature period compa red to 55 and 54% during the growth and senescence periods, respective ly. The greater Q(E) at plant maturity is due primarily to canopy tran spiration. The sensitivity of Q(E) to net radiation was largest during the growth period. In contrast, the sensitivity of Q(E) to the surfac e resistance and aerodynamic resistance was largest during the mature period. The implications of climatic variability on the timing of leaf development and the surface energy and water balance are discussed.