Energy and water vapour exchanges over a mixedwood boreal forest in Ontario, Canada

Abstract Measurements of energy, carbon dioxide and water vapour fluxes and supporting meteorological variables were made from 1 September 2003 to 31 August 2004 in a 74‐year‐old mixedwood forest in northern Ontario, Canada, using the eddy covariance technique. Land surface cover data analysis showed that although this forest is comprised of deciduous and coniferous species, their proportion throughout the forest was almost equal, resulting in a homogeneously mixedwood stand. The seasonal pattern of albedo showed a distinct pattern as a result of mixed deciduous and coniferous species with the first low radiation reflection period in early spring (between snowmelt and deciduous leaf‐out), and the second low reflection period in late autumn (between senescence and snowfall). Root zone soil water content remained above 0·32 throughout the growing season, which is characteristic of wet soil conditions. The evaporative flux exceeded the sensible heat flux during most of the growing season. Daily mean evapotranspiration values during the peak growing season were about 2–2·5 mm day −1 with maximum values reaching up to 4–5 mm day −1 on sunny days. The total water loss over the 12‐month measurement period was 480 ± 30 mm, while the total precipitation was 835 mm. Despite high soil water content, bulk surface conductance calculated using the inverted Penman–Monteith formulation, showed a strong correlation with vapour pressure deficit (VPD), indicating stomatal control on water loss during the afternoons with high atmospheric demand and during occasional dry periods. Bulk surface conductance values in this mixedwood forest were relatively higher than those in other boreal coniferous forests and were more similar to values observed in the southern boreal deciduous forests. The Priestley–Taylor α values showed a wide range but during most of the growing season its values were close to unity, indicating that water stress does not play a major role in the overall water loss from this boreal forest ecosystem, while energy supply has a strong control on evapotranspiration. These results will help parameterize accurately energy and water exchanges for the moist mixedwood forests in land surface‐atmosphere interaction and hydrologic models. Copyright © 2006 John Wiley & Sons, Ltd.