Effect of Elevation and Foliar Age on Maximum Leaf Resistance to Water Vapor Diffusion in Conifers of the Central Rocky Mountains, U.S.A.
Development of cuticular resistance to water vapor diffusion (rcwv) was measured in wind-protected shoots of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) along the entire elevation gradient of these species in the central Rocky Mountains. Increases in rcwv of new shoots occurred primarily during August. Maximum rcwv was lower at high elevation, particularly in krummholz growth forms, as had been observed in other species. Highest mean rcwv (340 ks m-1) occurred in Engelmann spruce at the lowest elevation (2700 m). At relatively high elevation, erect wind-protected trees showed a slight decline in mean rcwv during September. At lower elevations, this decline did not occur during autumn, but a comparison of different age classes of foliage showed that in open-growth trees, second-year foliage had about 40% lower mean rcwv compared to new foliage sampled in October. Thirdand fourth-year foliage did not show any statistically significant additional decline in rcwv. The results could be explained by a superficial wax layer responsible for maximal rcwv, which is produced in trees at low elevation exposed to full sun, but lost during the first fall or winter after the foliage is produced. This superficial wax layer of new foliage also appears to reduce foliar wettability. Loss of this wax or failure to produce it, particularly in wind-exposed trees at high elevations, may simultaneously increase diffusion of water vapor and other gases, and potential adsorption or absorption of solutes by cuticles in older age classes of conifer foliage.
KeywordsWater Vapor Diffusion Cuticular Transpiration Water Content Range Subalpine Conifer Central Rocky Mountain
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