Needle surfaces of Pinus sylvestris L. were examined at three sites, two in clean air (SO2 < 10 μg m−3) at Saltoun, (altitude 200 m) and Cairngorm (400 m) and one in polluted air (SO2 c 40 μg m−3) at Gorple (300 m). Needles one, two and (where available) three years old were sampled during the period July 1981 to March 1982. Needle longevity was greater at the two clean air sites where 50 % of two year old needles were present compared with only 10% at the polluted site.
The physical structure of epicuticular wax was assessed using scanning electron microscopy. The wax tubes shortened, thickened and fused into an amorphous surface ‘crust’ with time. The rate of this‘weathering’ declined logarithmically. A constant (k) was denned as the weathering constant for this process. Values of/? (months−1) differed significantly between sites (Saltoun 0.061 ±0.01, Cairngorm 0.084 + 0.012 and Gorple 0.154±0.013).
In addition to faster weathering, needles at the polluted site were heavily contaminated with particulate debris, much of which had the physical characteristics of pulverized fuel ash that is produced by large coal combustion sources. During‘weathering'the surface layer of tubular wax crystals show a marked increase in thickness from ∼ 130 nm for newly exposed needles to 220 nm after 30 months old. No significant inter-site differences in the rate of change in tube width were noted.