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The impact of heterogeneous forest cover on water fluxes at the tree, stand, and regional scales.
Ewers, Brent1, Mackay, David1, Ahl, Doug1, Burrows, Sean1, Samanta, Sudeep1, Gower, Stith1, 1
ABSTRACT- Land use changes over the last century have resulted in a mosaic of forest types in northern Wisconsin. We chose the following four cover types to measure transpiration and calculate canopy average stomatal conductance: red pine, sugar maple/basswood, quaking aspen/balsam fir, and northern white-cedar/balsam fir/alder. The four cover types represent 90 percent of the basal area and 80 percent of the ground area. The other 20 percent of the ground area consists of mostly non-forested areas, including grassland, shrubland and water. In each cover type, stand transpiration was scaled from sap flux measurements in eight trees of each species. Granier-type sap flux measurements were made in trees over 4 cm in diameter and Kucera-type sap flux measurements were made in trees under 4 cm in diameter. Canopy average stomatal conductance was calculated from sap flux and environmental measurements by solving the Penman-Monteith equation for stomatal conductance. We investigated the interaction between species and environmental controls on canopy average stomatal conductance and transpiration including vapor pressure deficit, radiation, temperature and soil moisture. We hypothesize that species and cover types with high maximum transpiration and canopy average stomatal conductance will have a larger response to environmental factors than species with low maximum transpiration and canopy average stomatal conductance. Our hypothesis implies that under optimal and poor environmental conditions there are large differences in stand transpiration among the four cover types. The differences are minimal under average environmental conditions. Thus, the impact of the heterogeneous forest cover in northern Wisconsin on regional water flux will depend on the environmental conditions.
KEY WORDS: water flux, hydrology, leaf area