Acid Rain

In any forest, leaves and bark are usually the first surfaces encountered by precipitation. Most rainwater penetrates the foliar canopy and then reaches the forest floor as so-called throughfall, while a smaller amount runs down tree trunks as stemflow. Throughfall and stemflow have a different chemistry than the original precipitation. Because potassium is easily leached out of leaves, its concentration is especially changed. In a study of several types of forest in Nova Scotia, the concentration of potassium (K⁺) was about 10 times larger in throughfall and stemflow than in rain, while calcium (Ca²⁺) and magnesium (Mg²⁺) were three to four times more concentrated. There was less of a change in the concentration of H⁺; the rainwater pH was 4.4, but in throughfall and stemflow of hardwood stands pH averaged 4.7, and it was 4.4-4.5 in conifer stands. The decreases in acidity were associated with ion-exchange reactions occurring on foliage and bark surfaces, in which H⁺ is removed from solution in exchange for Ca²⁺, Mg²⁺, and K⁺. Overall, the "consumption" of hydrogen ions accounted for 42-66% of the input of H⁺ by precipitation to these forests. Similarly, H⁺ consumption by the tree canopy was 91% in a hardwood forest at Hubbard Brook, New Hampshire, 21-80% among seven stands in New Brunswick, and 14-43% in stands in upstate New York.

In areas polluted by SO₂ there can be large increases in the sulfate concentration of throughfall and stemflow, compared with ambient precipitation. This is caused by the washoff of SO₂ and SO₄ that had been previously dry-deposited to the canopy. At Hubbard Brook this SO₄ enhancement is about four times larger than ambient precipitation, while in central Germany it is about two to three times greater. These are both regions with relatively large concentrations of particulate SO₄ and gaseous SO₂ in the atmosphere.