Energy balance and canopy conductance of a boreal aspen forest: Partitioning overstory and understory components

The energy balance components were measured throughout most of 1994 in and above a southern boreal aspen (Populus tremuloides Michx.) forest (53.629 degrees N 106.200 degrees W) with a hazelnut (Corylus cornuta Marsh.) understory as part of the Boreal Ecosystem-Atmosphere Study. The turbulent fluxes were measured at both levels using the eddy-covariance technique. After rejection of suspect data due to instationarity or inhomogeneity, occasional erratic behavior in turbulent fluxes and lack of energy balance closure led to a recalculation of the fluxes of sensible and latent heat using their ratio and the available energy. The seasonal development in leaf area was reflected in a strong seasonal pattern of the energy balance. Leaf growth began during the third week of May with a maximum forest leaf area index of 5.6 m(2) m(-2) reached by mid-July. During the full-leaf period, aspen and hazelnut accounted for approximately 40 and 60% of the forest leaf area, respectively. Sensible heat was the dominant consumer of forest net radiation during the preleaf period, while latent heat accounted for the majority of forest net radiation during the leafed period. Hazelnut transpiration accounted for 25% of the forest transpiration during the summer months. During the full-leaf period (June 1 to September 7) daytime dry-canopy mean aspen and hazelnut canopy conductances were 330 mmol m(-2) s(-1) (8.4 mm s(-1)) (70% of the total forest conductance) and 113 mmol m(-2) s(-1) (2.9 mm s(-1)) (24% of the total forest conductance), respectively. Maximum aspen and hazelnut canopy conductances were 1200 mmol m(-2) s(-1) (30 mm s(-1)) and 910 mmol m(-2) s(-1) (23 mm s(-1)), respectively, and maximum stomatal conductances were 490 mmol m(-2) s(-1) (12.5 mm s(-1)) and 280 mmol m(-2) s(-1) (7 mm s(-1)), aspen and hazelnut, respectively. 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 Priestly and Taylor [1972] alpha 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 alpha of 0.91 indicated some stomatal control on transpiration, with a directly proportional relationship between forest leaf area and forest canopy conductance, varying alpha during much of the season through a range very sensitive to regional scale transpiration and surface-convective boundary layer feedbacks.