STOMATAL CONDUCTANCE PATTERNS AND ENVIRONMENT IN HIGH ELEVATION PHREATOPHYTES OF WYOMING

High-elevation (2305 m) phreatophytes (cottonwood, Populus angustifolia James, and willows, Salix monticola Nutt. and Salix exigua Bebb) in southeastern Wyoming commonly showed a pattern of increasing stomatal conductance (g1) throughout the day, despite minimum leaf water potentials (psi-1) as low as -2.7 MPa. Diurnally, light and leaf temperature were the most important variables influencing g1, but there were also threshold values of psi-1 and leaf-to-air vapor pressure deficit for stomatal closure. Seasonally, maximum daily g1 (g(max)) was similar between species, was positively related to minimum air temperature the previous night, and showed a threshold response to soil temperature. Two types of evidence existed for large soil-to-leaf conductances (g(s-1)) and for liquid water being available to the roots. The first was no change in psi-1 despite increasing transpiration during the afternoon. The second was rapid evening recovery of psi-1 often to values higher than mean soil water potential to 60-cm depth. The combination of low atmospheric pressures at high elevation, large g(s-1) and access to a water table may explain how these phreatophytes attained g(max) of 0.8-0.9 mol m-2 s-1, much higher than g(max) of low-elevation phreatophytes or high-elevation nonphreatophytes.