Effects of ozone and acid mist on foliar leaching from eastern white pine and sugar maple

Abstract

Upper canopy branches of mature eastern white pine (Pinusstrobus L.) and sugar maple (Acersaccharum Marsh.) were exposed in the field to ozone (25, 70, or 140 ppb) for 5 h, followed by acid mist (pH 5.0 or 3.8) for 1 h, and the exchange of major ions between the mist and the foliage was measured. The two species responded similarly to the pollutants: ozone exposure did not affect canopy exchange of any ions, but the pH 3.8 mist approximately doubled the leaching of Ca2+ and Mg2+ relative to the pH 5.0 mist. Hydrogen ions were released from the branches of both species in the pH 5.0 treatment but taken up from solution in the pH 3.8 treatment. Consideration of ionic balance in the chemical exchange indicates that organic acids were released from the branches of both species. In the white pine, it appears that these acids were deprotonated in the pH 5.0 solution, but not in the pH 3.8 solution. In the sugar maple, some acid deprotonation appears to have occurred under both pH treatments. In a related experiment, sugar maple branches that were moderately damaged by pear thrips (Taeniothripsinconsequens Uzel) did not have canopy exchange rates different from undamaged branches. Canopy exchange in the damaged branches did not respond to ozone treatment. Hydrogen ions were retained at pH 3.8 and released at pH 5.0, as in the undamaged branches. In the damaged branches, leaching of Ca2+ and Mg2+ tended to be higher at pH 3.8, although the results were not statistically significant. Overall, we conclude that short-term ozone exposures that do not produce visible damage do not affect foliar leaching, but that the acidity of rain can have a significant effect on Ca2+, Mg2+, and H+ exchange in white pine and sugar maple canopies. The net effect of those canopies on the acidity of the rain passing through them will depend on the initial acidity of the rain.