Wind Erosion of Leaf Surface Wax in Alpine Timberline Conifers

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Abstract
Loss of leaf surface wax during winter could contribute to the extreme desiccation and leaf mortality previously observed for timberline conifers. For Picea engelmannii, leaf surface wax declined most rapidly for leaves above the snowpack as winter progressed, until there was approximately 60% less surface wax than for leaves protected by snow cover. Near the upper limit of the timberline ecotone (3400 m), leaves of P. engelmannii had less surface wax compared to individuals near the lower ecotone limit (3200 m), as well as smaller differences between windward versus leeward sides of the same shoot. Abies lasiocarpa leaves at 3400 m had the least amount of wax, while leaves of Pinus contorta at a lower, wind-exposed site (3100 m) had the greatest amount. Only small differences in wax quantities occurred on windward versus leeward leaves on P. contorta saplings, despite much higher mortality for windward leaves near the snow surface. From measurements on artificial wax cylinders placed in the field, an increase from 20 cm to 4 m in height above the snowpack would result in an estimated 90% decrease in surface wax erosion. Also, wax erosion as a function of height above the snow surface was similar to a typical vertical profile of the kinetic energy flux of blowing snow. A similar estimate using wax erosion from actual leaves showed that a similar decrease in cuticle erosion would result for an approximate decrease in elevation of about 230 m. Leaf surface wax erosion via blowing snow may be a primary force influencing the distribution of evergreen plants in the timberline ecotone of the central Rocky Mountains [North America].