Abstract High-elevation phreatophytes were studied to identify environmental parameters influencing daily and seasonal water consumption. The water relations of Salix planifolia, S. wolfii and Betula occidentalis at 2865 m were similar throughout the day. Transpiration was reduced before 1000 hr as a result of dew on the leaves, but high stomatal conductances to water vapor (>15 mm s-1) and transpiration rates (>100 mg m02 s-1) occurred from 1000 to 1600 hr, resulting in daily water losses per unit leaf area of 4.5, 5.2 and 4.0 kg m-2 respectively. Maximum stomatal conductances declined after 1 September in conjunction with minimum air temperatures consistently below 0 C and increased leaf senescence. The period of significant water consumption was from 10 July through 20 September.
Diurnal variations in stomatal conductance of Salix exigua and S. amygdaloides at 2255 m were similar to those at the high-elevation site, but maximum stomatal conductances were only 51% as great. However, transpiration occurred from sunrise to sunset at the lower elevation, resulting in daily water losses of 3.7 and 3.4 kg m-2 of leaf area, respectively. Midday depressions in leaf conductance at the lower site may have been in response to xylem pressure potentials below -1.7 MPa. Seasonally, conductances were constant until a decrease occurred after 15 September, probably due to low temperatures and leaf senescence. The growth season at the low-elevation site was estimated to be from 15 June-30 September.
Although there were differences between the two sites in maximum transpiration rates, microclimate and length of growth season, the data suggest that for both sites and all species examined, stomatal conductance and seasonal water use were most influenced by solar irradiance, dew on leaves, minimum air temperatures and phenology.
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