Abstract
Acidic precipitation, wet or frozen deposition with a hydrogen ion concentration greater than 2.5 μeq/1 is a significant air pollution problem in the United States and Canada. The chief anions accounting for the hydrogen ions in rainfall are nitrate and sulfate. Although visible injury to foliage has been documented in a variety of greenhouse studies, no experimental evidence demonstrates loss of field crop value or reduction in plant productivity due to visible foliar injury. Acidic precipitation can contribute nutrients to vegetation and could also influence leaching rates of nutrients from vegetation. Although these processes occur, there are no data that show changes in nutrient levels in foliage that relate to crop or natural ecosystem productivity. Experimental results show that fertilization of ferns is inhibited by present levels of acidic precipitation in the north-eastern United States. However, the overall impacts of inhibited fertilization on perpetuation of the species or ecosystem productivity have not been evaluated. Simulated acidic precipitation has been shown to affect plant pathogens in greenhouse and field experiments. Simulated acidic precipitation inhibited pathogen activities under some circumstances and promoted pathogen activities under other circumtances. No general conclusions can be drawn about the effects of current levels of precipitation acidity on plant pathogen-host interactions under field conditions. Few experiments have been aimed at determining the impacts of acidic precipitation on crop and forest yields. Most of these studies are inadequate because they are not conducted in nature with adequate randomization of treatments coupled with vigorous statistical analyses. From the above information, it must be concluded that research on the effects of acidity in precipitation on terrestrial vegetation is too meager to draw definitive conclusions about the effects of ambient and/or anticipated levels of acidity.
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