Water use of interior Douglas-fir

Water use of individual Douglas-fir (Pseudotsuga menziesii var. glauca (Bei ssn.) Franco) trees was measured in two plots at a forest site in southern British Columbia, Canada. Average daily early summer water use by trees wit h diameters of 7.5-70 cm varied from 1.8 to 166 L. Sap flux density (cm(3) water/cm(2) sapwood per hour) was linearly related to shoot xylem pressure potential and was found to increase with increasing vapour pressure deficit (VPD) and short-wave irradiance (I), reaching maximum rates with VPD > 0.6 kPa and I > 200 W.m(-2). Daily sap flux density varied among trees but was not related to tree diameter, so an average value of 1137.4 L.m(-2) sapwoo d area was used to estimate average early summer stand transpiration for th e two plots of 1.08 and 1.5 mm.d(-1). A close curvilinear relationship (r(2 ) = 0.85) was found between stem cross-sectional area increment and sapwood area. The relationship was only slightly better (r(2) = 0.89) between area increment and early summer individual tree water use. Stand volume growth for 1988-1998 for the two plots was 36-47 m(3).ha(-1). Stem volume relative growth rate over this 10-year period is estimated at 0.027 and 0.029 m(3). m(-3).a(-1).