ENERGY EXCHANGE FOR A FOREST SITE and A CLEAR‐CUT SITE AT CHALK RIVER, ONTARIO

Abstract

This article reports energy balance measurements which were made over a forest anda clear-cut site in the summer of 1985, and over the forest in 7987 at the Petawawa National Forestry institute (PNFI), Chalk River, Ontario.Evapotranspiration was evaluated by the Bowen ratio1 energy balance method using a model of the Reversing Temperature Difference Measurement System (RTDMS) at each site in each year. The evapotranspiration regimes of the forest and clear-cut sites are interpreted within the resistance framework of the Penman-Monteith combination model. The behaviour of the Priestley and Taylor (1972) [Y term and the McNaughton and Jarvis (1983) R factor are discussed for both sites. The summer of 1987 was exceptionally dry and thus provided an opportunity to examine the forest's evapotranspiration regime under severe water stress. The soil moisture conditions in 1987 are in marked contrast to those in 7985, a wetter year.Dans cette communication, on fait &at des mesures du bilan 6nergCtique prises, a 1985, au-dessus d'une region forestiere et d'une region de terrain debla ye de sa futaie et, en 7987, au-dessus de la m h e region foresitiere 21 I'lnstitut Forestier Nationalde Petawawa, A Chalk River, Ontario. Avec la methode du rapport de Bowenl bilan energetique, on a determine /'evapotranspiration en utilisant un dispositif de mesure de la difference de temperature a renversement dans chaque region pour chacune des annees. On interprete les regimes de I'6vapotranspiration de la region forestiere et de la r6gion de terrain deblaye de la futaie dans le cadre de la resistance du modele combine de Penman et Monteith. On discute la tenue du terme a de Priestley et Taylor ( 7 972) et du factor fl de McNaughton et iarvis (1983) pour toutes les r6gions. Le temps en 6t6 1987 etait exceptionnellement sec, favorisant I'examen d'un regime d'evapotranspiration sous des conditions de penurie deau. Les conditions de secheresse du sol font contraste avec celles de I'an 1985 qui etaient plus humides.Aspects of the energy balance of a mixed forest and a clear-cut area will be presented in this paper. Results are part of the work begun at PNFI in 1981. Comparisons of the energy balance of the forest and clear-cut sites using data from 1981 have been presented previously (McCaughey 1985a). Here, data from both 1985 and 1987 will be discussed. For 1985, data are available from both the forest and a clear-cut site; in 1987 only data from the forest are available. 1985 was a wet season, whereas 1987 was the driest since 1981. The marked contrast in rainfall in the two years provided an opportunity to study the role of water availability in the energy balance of the forest.Evapotranspiration rates were evaluated by the Bowen ratio/energy balance method, using reversing psychrometry and the direct measurement of temperature differences. The energy balance for both sites i s interpreted using the resistance framework of the Penman-Monteith combination model. The behaviour of the [Y term of the Priestley and Taylor (1972) model an...

Carbon sequestration in British Columbian sub-boreal cut blocks

“…Timber harvesting typically results in higher soil temperatures (Lewis 1998;Londo et al 1999) and often a decrease soil moisture (McCaughey 1989;Londo et al 1999). The soil moisture in a clear-cut is frequently reduced because of higher surface temperatures, but the loss can be partially offset by reduced transpiration from plants because of lower plant biomass.…”