Acid Rain

Few studies have demonstrated injury to terrestrial plants caused by an exposure to ambient acid rain. Although many experiments have demonstrated injury to plants after treatment with artificial "acid rain" solutions, the toxic thresholds are usually at substantially more acidic pHs than normally occur in nature.

For example, some Norwegian experiments involved the treating of young forests with simulated acid rain. Lodgepole pine watered for three years grew 15-20% more quickly at pHs 4 and 3, compared with a "control" treatment of pH 5.6-6.1. The height growth of spruce was not affected over the pH range 5.6 to 2.5, while Scotch pine was stimulated by up to 15% at pHs of 2.5 to 3.0, compared with pH 5.6-6.1. Birch trees were also stimulated by the acid treatments. However, the feather mosses that dominated the ground vegetation were negatively affected by acid treatments.

Because laboratory experiments can be well controlled, they are useful for the determination of dose-response effects of acidic solutions on plants. In general, growth reductions are not observed unless treatment pHs are more acidic than about 3.0, and some species are stimulated by more acidic pHs than this. In one experiment, the growth of white pine seedlings was greater after treatment at pHs of 2.3 to 4.0 than at pH 5.6. In another experiment, seedlings of 11 tree species were treated over the pH range 2.6 to 5.6. Injuries to foliage occurred at pH 2.6, but only after a week of treatment with this very acidic pH.

Overall, it appears that trees and other vascular plants are rather tolerant of acidic rain, and they may not be at risk of suffering direct, short-term injury from ambient acidic precipitation. It remains possible, however, that even in the absence of obvious injuries, stresses associated with acid rain could decrease plant growth. Because acid rain is regional in character, these yield decreases could occur over large areas, and this would have important economic implications. This potential problem is most relevant to forests and other natural vegetation. This is because agricultural land is regularly treated with liming agents to reduce soil acidity, and because acid production by cropping and fertilization is much larger than that caused by atmospheric depositions.

Studies in western Europe and eastern North America have examined the possible effects of acid rain on forest productivity. Recent decreases in productivity have been shown for various tree species and in various areas. However, progressive decreases in productivity are natural as the canopy closes and competition intensifies in developing forests. So far, research has not separated clear effects of regional acid rain from those caused by ecological succession, insect defoliation, or climate change.