Both species showed a decrease in canopy conductance as the saturation deficit increased and both showed an increase in canopy conductance as the photosynthetic active radiation increased. There was a linear relationship between forest leaf area index and forest canopy conductance. The timing, duration, and maximum leaf area of this deciduous boreal forest was found to be an important control on transpiration at both levels of the canopy. The full-leaf hazelnut daytime mean Priestley and Taylor [1972] a coefficient of 1.22 indicated transpiration was largely energy controlled and the quantity of energy received at the hazelnut surface was a function of aspen leaf area. The full-leaf aspen daytime mean c• of 0.91 indicated some stomatal control on transpiration, with a directly proportional relationship b6tWeen forest leaf area and forest canopy conductance, varying c• during much of the season through a range very sensitive to regional scale transpiration and surface-convective boundary layer feedbacks. IntroductionThe boreal forest represents one of the world's largest yet least understood ecosystems. Of the estimated 48.5 million km 2 total land area of the world's forests, 12.0 (25%) is covered by boreal forest, second only to the 17.0 (35%) covered by tropical rain forest. Boreal forest net primary productivity (ex---1 pressed as dry matter) accounts for an estimated 9.6 Gt yr (13%) of the world's forests 73.9, exceeding both temperate deciduous 8.4 (11%) and temperate coniferous 6.5 (9%) forests [Salisbury and Ross, 1978].•University of British Columbia, Vancouver, Canada. 2Atmospheric Environment Service, Downsview, Ontario, Canada.•Yale University, New Haven, Connecticut. Continuing with the work presented by Black et al. [1996], the objectives of this paper are (1) to describe the diurnal and seasonal patterns of the aspen overstory and hazelnut understory energy balance, (2) to describe the diurnal and seasonal patterns of canopy water vapor conductances for both the aspen and the hazelnut, and (3) to relate the canopy conductances to the ambient meteorological conditions at both the canopy and the regional levels. Site DescriptionThe study site (

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