The long-term effects of simulated acid rain treatment (pH 4 and pH 3) on the physicochemical characteristics of the needle surfaces in a natural stand of Scots pine was investigated in the extreme north of Finland during the period 1985–1988. By the end of this period, the pH 3 treated pines had received a sulphur load of 7.48 g m−2 and a nitrogen load of 2.62 g m−2, and the pH 4 treated pines a sulphur load of 1.23 g m−2 and a nitrogen load of 0.43 g m−2 in addition to ambient deposition. No visible symptoms of acid rain injury were observed in the needles, but their growth was significantly reduced. Delayed development of the epistomatal waxes and four types of deformed stomatal complex, namely narrow, half-formed, occluded, and double-sized, were found by scanning electron microscopy. The total number of deformed stomatal complexes in the pH 4 treated pine needles was over 50% greater than in dry control specimens and over 70% greater than in water controls. The rapid erosion of young, undeveloped waxes increased needle wettability (11%) significantly, but the quantity of chloroform-soluble waxes had decreased only slightly. The effects of acid rain on the epicuticular waxes were accentuated during unfavourable growing seasons with reduced effective temperature sums. Key words: acid rain, Scots pine, epicuticular wax, wettability.
