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Environmental regulation of water flux in young and old growth Douglas-fir trees over five years.
George, Kate1, Phillips, Nathan2, Pypker, Thomas1, Schauer, Andrew3, Bond, Barbara1, 1 Oregon State University, Corvallis, OR2 Boston University, Boston, MA3 University of Utah, Salt Lake City, UT
ABSTRACT- In order to predict vegetation responses to climate change in the future we need to understand how dominant species at different developmental stages will respond to variations in environmental conditions. We analyzed the extent to which year-to-year variations in water flux could be explained by differences in vapor pressure deficit (VPD) and soil moisture. We measured daily water flux per unit sapwood area in young (approximately 25 years old) and old growth (approximately 450 years old) Douglas-fir (Pseudotsuga menziesii) trees over five years (1998-2002) using constant heat sap flow sensors. Water flux of old growth Douglas-fir became asymptotic at higher values of VPD, whereas young Douglas-fir did not reach an asymptote. At higher values of VPD water flux in young Douglas-fir was approximately 50% greater than old growth trees. The water flux of young and old growth trees was significantly different between years at high values of VPD. The difference in water flux between years in a high VPD environment can be explained in part by soil moisture. When soil moisture was 30% or greater for part or all of the growing season maximum water flux was higher compared to drier years for both young and old growth trees. When water flux is scaled to the water use of all Douglas-fir present within a stand, young Douglas-fir use approximately four times more water than old-growth Douglas-fir on a ground area basis. On a stand level basis the water use of young Douglas-fir stands are constrained by soil moisture, whereas water use of old growth Douglas-fir stands are not altered by seasonal and annual changes in soil moisture, even though there are no differences between soil moisture at both sites. The increasing dominance of young Douglas-fir stands and reduction of old growth stands in the Pacific Northwest may significantly alter the hydrology of these ecosystems.
Key words: sap flow, sapwood area, sap flux, Granier