Photosynthetic and Water Use Efficiency Responses to Diffuse Radiation by an Aspen-Dominated Northern Hardwood Forest

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Clouds can exert strong effects on ecosystem CO2 and water vapor fluxes and may be important determinants of terrestrial primary production. We used three years of eddy-covariance and meteorological data from an aspen-dominated northern hardwood forest in Michigan to investigate how canopy photosynthesis (P), evapotranspiration (E), and water use efficiency (WUE) responded to changes in cloud cover, or the proportion of diffuse (I f) to total (I t) photosynthetically active radiation (PAR). Canopy quantum efficiency (the initial slope of the P versus PAR response curve) increased with increasing cloud cover, but both midday canopy P and canopy photosynthetic potential (the asymptote of the P versus PAR response curve) were greatest under partly cloudy skies (I f/I t = ∼0.57). Midday canopy E decreased and midday canopy WUE increased with increasing I f/I t. The relationship between canopy P and cloud cover varied with soil moisture. Canopy P was insensitive to changes in soil moisture under overcast skies (I f/I t > 0.70), whereas under clearer skies, canopy P was lower during periods of low compared to high soil moisture. These results further our understanding of cloud cover effects on canopy physiological processes and will aid in more accurate assessments of forest responses to climatic change. FOR. SCI. 50(6):793–801.

Keywords: Canopy photosynthesis; clouds; eddy covariance; environmental management; evapotranspiration; forest; forest management; forest resources; forestry; forestry research; forestry science; natural resource management; natural resources; net ecosystem CO2 exchange

Document Type: Regular Article

Affiliations: 1: The Ohio State University Columbus OH 43210 Present Address: Department of Earth Science University of California Irvine CA 92697, Email: 2: East Carolina University Greenville NC 27858, Email: 3: University of Michigan Biological Station Pellston MI 49769, Email: 4: Indiana University Bloomington IN 47405, Email: 5: Corresponding Author Department of Evolution, Ecology, and Organismal Biology The Ohio State University 318 W. 12th Avenue Columbus OH 43210-1293 Phone: (614) 292-0835;, Fax: (614) 292-2030, Email:

Publication date: December 1, 2004

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