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 occidental is at 2865 m were similar throughout the day. Transpiration was reduced before 1000 h as a result of dew on the leaves, but high stomatal conductances to water vapor diffusion (>15 mm s-1 ) and transpiration flux densities (>100 mg m-2 s-1) occurred from 1000 to 1600 h, resulting in a daily water loss per unit leaf area of 4.5, 5.2, and 4.0 Kg m-2, respectively. Seasonal trends revealed a decrease in maximum stomatal conductance after 1 September, occurring 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.
For comparison, a site at lower elevation (2255 m) was examined. Diurnal variations of S. exigua and S. amygdaloides at this site were similar to the high-elevation site, but maximum stomatal conductances were only 51% of the high-elevation Salix species. However, transpiration occurred from sunrise to sunset at lower elevations, resulting in a daily water loss of 3.7 and 3.4 Kg m-2 of leaf area, respectively. A midday depression 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 the onset of senescence. The growth season at the low-elevation site was estimated to be from 15 June to 30 September.
Although differences between the two sites occurred with respect to maximum rates, microclimate and length of growth season, the data suggest that for both sites and all species examined, a stomatal conductance and seasonal water use were most influenced by solar irradiance, dew on leaves, minimum air temperatures in spring and fall, and phenology.
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