The contact angle method was used to determine the wettability of red spruce (Picea rubens Sarg.) and balsam fir [Abies balsamea (L.) Mill.] foliage in subalpine sites (1010–1400 m above sea level) at three locations in northern New England, USA (Camels Hump in Vermont and Mt. Moosilauke and Mt. Washington in New Hampshire).
The dependence of contact angle (θ) on foliage age (t, in months) could be expressed in the form θ=θ1+θ2 e−kt. The value of the decay constant (k) ranged from 0.02 to 0.12 (month−1), depending upon location, but was independent of elevation, canopy position, and, in most cases, species. The asymptotic value of the contact angle (θ1) i.e. that approached by very old foliage, did not differ between species at some sites but was greater in fir than in spruce at others. The value of θ1 decreased with elevation in both species and tended to increase with an increase in height within the canopy. The value of the component which affected contact angle variation with age (θ2) was always greater in fir than in spruce, but was unaffected by elevation; in fir, it appeared to increase with height in the canopy. Thus, the estimated value of the initial (t= 0) contact angle (θ1+θ2) was greater in fir, increased with height in the canopy, and decreased with elevation. The overall pattern found was that (1) young foliage of spruce is more wettable than young foliage of fir, but the difference becomes smaller in older age classes; (2) young foliage near the bottom of the canopy is more wettable than near the top; and (3) foliage from a given age class and species is more wettable at higher elevations.
The treatment of foliage with xylene to remove epicuticular wax decreased contact angles of fir but increased them in spruce. Scanning electron microscopy revealed that wax removal by xylene did not mimic natural wax removal. Contact angles obtained with acidified water did not vary significantly from those obtained with deionized water.
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