Quantity, chemistry, and wettability of epicuticular waxes on needles of red spruce along a fog-acidity gradient

Abstract

Needle wettability and epicuticular wax physicochemical characteristics were examined for red spruce (Picearubens Sarg.) trees growing along a gradient of fog quantity and acidity. Trees at four coastal and one inland site were sampled in November 1988 for three needle age-classes. Needle wettability was assessed by measuring needle–water droplet contact angles. Mean angles per site and age-class ranged from 73 to 42° and decreased significantly with increasing needle age. Needle contact angles were highly correlated with fog acidity (R = 0.99), decreasing with increasing acidity. Angles were lower on needles from declining trees than on needles from visually healthy trees. Epicuticular wax amount was highly correlated with quantity of fog collected at the sites (R = 0.99). Needles from trees at the inland site located outside the coastal fog zone had the smallest wax deposits, while needles from trees exposed to the greatest amount of fog had the largest wax deposits. Epicuticular wax was comprised of secondary alcohols (42%), diols (19%), alkyl esters (16%), primary alcohols (6%), hydroxy fatty acids (2%), and fatty acids (1%). Estolides (14%) in the wax were tentatively identified but not confirmed by mass spectrometry. Needles from trees exposed to the most fog had the greatest amounts of secondary alcohols and alkyl esters. Needles from trees not exposed to fog had greater amounts of diols. There was no obvious relationship between fog acidity and wax chemical composition.