The fate of canopy water in the Findley Lake basin, WA

Abstract

This dissertation examined the fate of canopy water in the Findley Lake Basin, WA (47°04' N, 121°25'W) during the growing seasons of 1994 and 1995. Field measurements were taken at three sites: Lake (1170 m), Midslope (1230 m) and Ridge (1320 m). Field data included stand and canopy structure, canopy wetness, fog, and throughfall. Water uptake by Abies amabilis and Tsuga heterophylla shoots was measured in the laboratory. In 1995, the forest canopy was wet one third of all hours. During 1994 and 1995, throughfall was 31.3 and 27.9% less than apparent above-canopy precipitation at the Lake, 14.5 and 8.6% less at the Midslope, and 16.8 and 10.0% less at the Ridge. 1995 fog interception at the Ridge was three times greater than at the Lake (n = 6). Fog was related to the percent interception loss of precipitation (r2 = 0.64, p < 0.01), but not to the quantity of throughfall. Within-plot throughfall distribution was related to profiles of "shielding" and "collecting" trees (Ridge, r2 = 0.34), and to indices of near-collector leaf area (Lake, r2 = 0.39). Foliar absorption was assessed with a new technique based on shoot drying curves. The method indicated that shoots absorb water (p < 0.01). Uptake was apparently constant for at least 6 hours. Surprisingly, cuticular conductance to water influx ( gcin , 3.5--7.2 m s-1) was an order of magnitude greater than cuticular conductance to water efflux ( gcout , 3.0--3.9 x 10-5 m s-1 ). Irrespective of the direction of water flux, shade-adapted shoots were more conductive than sun-adapted shoots, and Tsuga heterophylla was more conductive than Abies amabilis. A transcuticular flux model was proposed, where water influx occurs through hydrated micropores, while efflux is by diffusion. Estimated 1995 canopy uptake was 2.2 (Ridge and Midslope) to 2.6 (Lake) mm of precipitation, or 2--3% of transpiration. Fog sedimentation onto the canopy was the best explanation for the difference in interception between the Lake and the two upper sites. Canopy uptake appears too small to have hydrological significance, but may have physiological importance, especially to plants or plant parts under water stress.