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SEASONAL PATTERNS AND VERTICAL PROFILE OF SOIL WATER UPTAKE AND UTILIZATION BY YOUNG AND OLD DOUGLAS-FIR AND PONDEROSA PINE FORESTS
Brooks, J R., F. C. Meinzer, J. M. Warren, J. C. Domec, AND R. Coulombe. SEASONAL PATTERNS AND VERTICAL PROFILE OF SOIL WATER UPTAKE AND UTILIZATION BY YOUNG AND OLD DOUGLAS-FIR AND PONDEROSA PINE FORESTS. Presented at 4th North American Forest Ecology Workshop, Corvallis, OR, June 16-20, 2003.
Water availability has a strong influence on the distribution of forest tree species across the landscape. However, we do not understand how seasonal patterns of water utilization by tree species are related to their drought tolerance. In the Pacific Northwest, Douglas-fir occupies more mesic habitats than ponderosa pine. We monitored water uptake in the upper two meters of soil in order to determine whether these tree species respond differently to changes in water content and water potential over the seasonal drying cycle (May through October). At the beginning of the 2002 growing season, soil water storage in the upper 2 m was almost twice as great in the Douglas-fir stands (580 mm) as in the ponderosa pine stands (320 mm). Douglas-fir used more water per day and maintained water uptake rates throughout the summer compared to ponderosa pine. Both species used water from the upper soil layers (0-60 cm) first, and then relied more heavily on the deeper layers later in the season. For both species, soil water utilization was similar for a given soil water potential, indicating that they did not differ in their ability to extract water, but the dryer ponderosa pine systems reached limiting soil water potentials sooner than the Douglas-fir system. The landscape distributions of the two species does not appear to be related to their ability to utilize water within the top two meters of soil; however, access to deeper soil water is a key in the success of these species to resist summer droughts.