WWRC 84-06
Water Relations of High-Elevation Salix Phreatophytes in Wyoming
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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