Scattered CaSO₄-crystallites on needle surfaces after simulated acid rain as an indicator of nutrient leaching

Two acid rain experiments were conducted, one at Oulu using transplanted seedlings of Picea abies and Pinus sylvestris, and the other with adult pines at Kevo where test plots were designated in a natural mixed stand of Pinus sylvestris and Betula tortuosa. The acid solution was made by adding H₂SO₄ and HNO3 (vol:vol 2:1, pH adjusted to 3 and 4) to clean water.The Oulu experiment was carried out from 1986 to 1989, and Kevo from 1985 to 1989. No visible injuries related directly to the acid rain treatment could be observed after 3 yr of the experiments, but microscopic examination revealed significant changes in the structure and morphology of the needles. The conifer seedlings treated at pH 3 had significant amounts of scattered CaSO₄ crystallites and characteristic piles and bows on the needle surfaces, as detected with EDS. The number of CaSO₄ crystallites was higher on the surfaces of the spruce needles than those of the pine needles. The current needles had more crystallites than the previous year's needles.