This study investigates how differences in tree age, stand structure and species composition between an old and young Douglas-fir/western hemlock forest impacts overstory transpiration. We measured sap flow (using constant-heat thermal dissipation sensors in g/m2) during the growing season in samples of trees in Watershed 1 (harvested in the 1960's) and Watershed 2 (last major disturbance approximately 450 ybp) at the HJ Andrews Forest in the Western Cascades of Oregon. The trees selected for these measurements differ somewhat each year but all are PSME. Sap flow was scaled to the whole tree level (per unit sapwood area) from measurements in outer and inner portions of the sapwood. We also surveyed vegetation in plots arrayed along transects in a 100 m swath centered on the streambed in each watershed, determining the species composition as well as the basal area (m2/ha) and sapwood basal area by species (m2/ha) . From the sap flow measurements in the sampled trees and the vegetation surveys, we estimated overstory water use within the 100 m swath. Data collection occurred between 1999-2002 and is now complete.
Barbara J Bond, David A. Post, Fox Sparky Peterson, Georgianne W. Moore, Julia A. Jones, Nathan Phillips
The long-term of this project is to better understand how vegetation age, structure, and species composition affects hydrological patterns in small watersheds at the H.J. Andrews Experimental Forest. We found that differences in water use per unit sapwood area, sapwood area density, and species composition between the old and young forests all result in greater water use by the overstory trees in the younger forest. This may explain some observed differences in stream runoff among forested watersheds and provides some insight into the implications for forest management on water resources.