To investigate the extent to which the energy balance of a globally important ecosystem is controlled by biological and environmental processes, measurements of water vapor flux were made on individual black spruce (Picea mariana [Mill.] B.S.P.) shoots, branches, and a whole canopy at the BOREAS Southern Study Area Old Black Spruce (SSA OBS) site. These measurements were used to estimate stomatal, branch boundary layer and canopy boundary layer conductances to water vapor. On a projected needle area basis, stomatal conductances varied between 14 and 92 mmol m(-2) s(-1), and total branch conductance varied seasonally between zero and about 35 mmol m(-2) s(-1). On a ground area basis, total canopy conductance varied between 24 and 105 mmol m(-2) s(-1) Total canopy conductance was partitioned into aerodynamic and physiological components by using shoot-scale measurements scaled by leaf area index. Good agreement was found with an independent estimate of aerodynamic conductance measured when the canopy was wet. Compared with most coniferous forests, the canopy was relatively uncoupled from the atmosphere, and at the ecosystem scale, the control of water vapor flux was approximately equally divided between physiological and abiotic conductances. Two widely used steady-state models of stomatal conductance were parameterized from the shoot and branch measurements. Parameters varied considerably throughout the growing season. A time-constant term was added to these static models to construct dynamic models of stomatal conductance under naturally varying environmental conditions. The dynamic versions of the models outperformed the static versions in explaining stomatal response to rapidly changing environmental conditions. The length of the time-constant term, derived using the dynamic models, suggested that stomata were slow to respond to changing environmental conditions, and that the speed of the response was strongly temperature-dependent.