Year‐round observations of the energy and water vapour fluxes above a boreal black spruce forest

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Abstract
Fluxes of energy and water vapour were measured continuously over an ∼120‐year‐old black spruce (Picea mariana (Mill.) B.S.P.) forest in northern Saskatchewan, Canada, from 6 May 1999 to 5 May 2001 using the eddy covariance technique as part of the Boreal Ecosystem Research and Monitoring Sites (BERMS) program. These results demonstrate that long‐term eddy covariance fluxes can reliably be measured during the harsh boreal winter. Typical daytime bulk surface conductance values were about 5–8 mm s−1 during the growing season. Surface conductance sharply declined after midday in response to increasing vapour pressure deficit. The monthly mean values of energy balance components showed that, in spring (March–June), partitioning of available energy flux was mainly in the form of sensible heat, which changed to almost equal proportions of sensible and latent heat in the summer (July–October). In winter (November–February), most of the net radiation was balanced by sensible heat flux. The monthly mean values of net radiation, sensible heat and latent heat flux were about −20 to 20 W m−2, −10 to 25 W m−2 and 5 to 10 W m−2 respectively in winter. The average mid‐day Bowen ratios were approximately 3·5, 1·7 and 5·2 in the spring, summer and winter seasons respectively. The maximum daily evaporation was about 3·5 mm day−1 in summer and 0·1–0·25 mm day−1 in winter. Over the 2 year period, the accumulated precipitation was 835 mm; this compared with 711 ± 70 mm of evapotranspiration, which showed that more than 85% of water was returned to the atmosphere through evapotranspiration. This study reports the first complete annual cycles of energy and water vapour fluxes at this black spruce site. Since black spruce is the dominant tree species in the North American boreal forest, these results have significance for regional and global energy and water cycles. Copyright © 2003 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.