| I compared transpiration among different types of forest stands in the western Cascades of Oregon. The three major questions were: (1) How does transpiration compare between a young and old stand and why? (2) Does diversity of overstory trees affect transpiration? and (3) How is transpiration related to stream flow? Transpiration was quantified using thermal dissipation sap flow measurements scaled to a ground-area basis using sapwood surveys for periods during the summer months of 2000, 2001, and 2002, and in a subset of plots, for a full year. I found that a young, rapidly growing stand (∼40 years since disturbance) used 3.3 times more water during the growing season than an old-growth stand (∼450 years since disturbance) because the young stand had 2.3 times higher sap flow rates per unit sapwood in Douglas-fir, had a 21% greater total sapwood basal area, and had a larger component of hardwoods that use 1.41 times more water than conifers per unit sapwood. In two-species mixtures of Douglas-fir and red alder, I found evidence that mixtures are less productive and have lower annual transpiration than monocultures of these two species. The observed differences were probably due to altered biomass rather than diversity itself, but diversity likely played a role in altering biomass.; Such stand age- and diversity-related differences in transpiration potentially impact stream flow. In a small watershed with a 450-yr-old forest, I examined the role of vegetation in stream flow patterns at hourly, daily, and storm scales. Transpiration apparently controlled stream flow during the summer at hourly scales with lags of at least five hours. In contrast, at daily and storm scales, soil water apparently controlled both stream flow and transpiration during the dry season, but there was no relationship during the wet season. These results indicate that forest management practices that reduce stand age and decrease diversity may lead to increased transpiration and consequently may reduce summer stream flow. |
