Abstract
Water use of individual Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) trees was measured in two plots at a forest site in southern British Columbia, Canada. Average daily early summer water use by trees with diameters of 7.5-70 cm varied from 1.8 to 166 L. Sap flux density (cm3 water/cm2 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 sapwood area was used to estimate average early summer stand transpiration for the two plots of 1.08 and 1.5 mm·d-1. A close curvilinear relationship (r2 = 0.85) was found between stem cross-sectional area increment and sapwood area. The relationship was only slightly better (r2 = 0.89) between area increment and early summer individual tree water use. Stan...
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