Components and Controls of Water Flux in an Old-growth Douglas-fir–Western Hemlock Ecosystem

We report measurements of rates of sap flow in dominant trees, changes in soil moisture, and evaporation from coarse woody debris in an old-growth Douglas-fir–western hemlock ecosystem at Wind River, Washington, USA, during dry periods in summer. The measurements are compared with eddy-covariance measurements of water-vapor fluxes above the forest (Ee) and at the forest floor (Eu) to examine the components of ecosystem water loss and the factors controlling them. Daily values of Eu were about 10% of Ee. Evaporation from coarse woody debris was only about 2% of Ee. Transpiration (Et), estimated by scaling sap-flow measurements accounted for about 70% of (Ee− Eu); transpiration from subdominant trees may account for the remainder. The daily total change in soil moisture (Es) in the top 30 cm was larger than the net change, probably because of hydraulic redistribution of soil water by roots. Observed differences between Es and Ee were probably because roots also extract water from greater depth, and/or because the measuring systems sample at different spatial scales. The ratio of Et to Es decreased with decreasing soil water content, suggesting that partitioning in water use between understory and overstory changed during the season. The rate of soil drying exceeded Ee early in the day, probably because water vapor was being stored in canopy air space and condensed or adsorbed on tree stems, lichens, and mosses. The daily variation of Ee with vapor-pressure deficit showed strong hysteresis, most likely associated with transpiration of water stored in tree stems and branches.