Evaporation within and above a boreal aspen forest

As part of the Boreal Ecosystem-Atmosphere Study, water vapour, heat, CO₂ and momentum exchange between the atmosphere and a southern boreal aspen (Populus tremuloides Michx.) forest in central Saskatchewan, Canada (53.629 °N, 106.200 °W) were measured continuously throughout much of 1994 using the eddy-covariance method. Measurements were made both above the c. 21.5-m tall 70 year-old aspen stand and within the leafless trunk space above a lush c. 2-m tall hazelnut (Corylus cornuta Marsh.) understory. This research focused on the measurements of and processes controlling water vapour exchange within and above the aspen canopy. Above-canopy turbulent exchange was dominated by large, slowly rotating eddies whereas in-canopy exchange was dominated by the intermittent, downward penetration of gusts. A constant flux layer redeveloped beneath the aspen canopy making eddy-covariance measurements possible. Nocturnal eddy fluxes were often underestimated at both heights due to spatial heterogeneity in turbulence statistics caused by low wind speeds. These periods were identified from the height-independent similarity function normalized by that expected from Monin-Obukhov theory and were empirically corrected as a function of friction velocity. Erratic daytime flux behaviour was corrected on the basis of conservation of energy and partitioning of the missing energy using the original eddy fluxes of latent and sensible heat. Evapotranspiration from the forest accounted for 82-91% of the annual precipitation. Aspen, hazelnut transpiration and soil water evaporation were 68%, 27% and 5%, respectively, of the total annual evapotranspiration. Over the growing season. there was no net change in the soil water content and there was little drainage beyond the root zone. Understory radiation levels decreased exponentially with increasing aspen leaf area. Surface conductance to water vapour was a linear function of forest leaf area and was dominated by the aspen canopy. Aspen and hazelnut canopy conductances decreased non-linearly with increasing saturation deficit and were best parameterized by net assimilation divided by the product of the mole fractions of leaf-level saturation deficit and CO₂ concentration. The accommodation of the transpiring vegetation by the atmosphere was quantified using the Priestley and Taylor α and the McNaughton and Jarvis Ω parameters.

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