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178 WESTERN NoRTH AMERICAN NATURALIST [Volume 65 from the bole using a 1 -cm- diameter soil corer. The depth of 15 cm was justified because (1) these are rocky riparian soils in which repeat- able deeper measurements are difficult, and (2) root distribution in trenches and minirhi- zotron measurements suggest that about half of surface roots are in the first 15 cm (data not shown). Whole-tree Physiology We measured sap flux and transpiration for each study tree (g H2O m-2 sapwood s-I) using the Granier sap flux method at the base of the live crown in each study tree from DOY 214 to 248 during 2002 (Granier 1987, Granier and I,oustau 1994, Granier et al. 1996, Clearwater et al. 1999). The Cranier method uses a heated probe inserted 10 cm above, a nonheated probe in the sapwood. Each probe is 2 cin long with a copper constantan wire thermocouple inserted inside at the midpoint. We calculated sap flux density based on the temperature difference between the heated and nonheated probe by Granier'i empirical equation (Granier 1987, Clearwater et al. 1999). Sensors were placed at tip to 4 depths (0-2 cm, 2-4 cm, 4-6 cm, and 6-8 cm), depending on the diameter of the tree. In all cases we attempted to measure the entire length of the hydroactive xylem from the bark to the heartwood. Sensors were placed at 1 randomly chosen aspect on each tree to randomize over aspect effects. Data were col- lected every 30 seconds and averages stored every 15 minutes using a Campbell Scientific CR10X data logger and a Campbell Scientific AM416 multiplexer (Logan, UT). Whole-tree sap flux was calculated by apportioning sap flux density rates from each probe to its corre- sponding sapwood area and summing data from all sapwood areas. Transpiration was ex- pressed as total daily whole-tree leaf specific transpiration rate (Et; 11 1120 M-2 LA d-1), which was calculated by dividing whole-tree sap flux by whole-tree leaf area (LA, see below). On all trees used for sap flux measurements, we measured predawn and midday plant water potentials with a pressure chamber (PMS Instruments, Corvallis, OR; Ritchie and Hink. ley 1975) 5 times during the last 10 days of the study. Predawn values provide an estimate of the soil water potential in the rooting zone of the tree, while midday water potentials provide an estimate of tree water stress (Ritchie and Hinckley 1975, Koide et al. 1990). Measure- ments were taken on mid-canopy branch tips between 0400 and 0600 hours for predawn water potential estimates ('Pp, and between 1400 and 16M hours from sun-exposed parts of the tree for midday water potential (T.1d) values. Branches from each tree were mea- sured until 2 measurements within 0.1 MPa were obtained, and these were averaged to ob- tain a mean value for the tree. Canopy conductance and whole-tree hydrau- lic conductance were determined for each study tree. Mean leaf-specific, canopy conductance (G,) was calculated over 15-minute periods for each tree with the following model used by Fischer et al. (2002), which substitutes vapor pressure deficit for the difference in water potential between leaf and air (Montieth and ti ns-worth 1990): cc = EOVPD/AI), (1) where C. is canopy conductance, El is leaf specific transpiration rate (L 1120 m-2 LA s-I), VPD is vapor pressure deficit (Wa), and A is average atmospheric pressure for the location of the study (-86.1 kPa for our site). Whole -tree hydraulic conductance was cal- culated in a manner similar to that of Ryan et al. (2000) and Fischer et al. (2002): Kh = E1/(T"p,,7'T".,id), (2) where Kh is whole-tree hydraulic conductance (g H2O m-2 s-L MPa-1). Calculation of Kh was limited to those dates when water potential was measured. We determined projected leaf area and sap- wood area of each study tree. Leaf area was estimated for all trees using an allometric equation based on branch diameter. We devel- oped the equation by removing 3 branches of 3 size classes from each tree at the end of our study. till leaves were removed, dried (772 hours at 700C), and weighed. A subsample of 10 leaves from each branch was used to deter- mine specific leaf area (m2 kg-1) using an Agvis Imaging System (Decagon Devices, Pull- man, WA). To estimate projected leaf area, we multiplied dried leaf weights from each branch by specific leaf area. These data were combined with data from a previous study from other trees at the site (Fischer et al. 2004) to con- struct a more robust equation for estimation of projected leaf area (CM2) based on the diame- ters (cm) of removed branches (r2 = 0.86, P <