Effect of Simulated Acid Rain on the Development of Leaf Injury in Tree Seedlings

Abstract

The authors have developed newly designed semi-automatic rain droplet generator system for long-term rain exposure experiments. This system generates 2.5 mm of rainfall intensity in an hour event with droplets of about 1 mm in diameter. Its spatial variation of precipitation was significantly smaller than that of the conventional mist type rain systems.
Four exposure experiments with simulated acid rain (SAR) were conducted during the growing season from 1991 to 1993. Total of 46 young tree seedlings planted in pots were exposed to SAR containing sulfate, nitrate and chloride with the weight percentage ratio of 2: 1: 1 for 3 or 4 months. SAR stock solution was volumetrically diluted. Designated pH values were 2.0, 2.5, 3.0, and 4.0. Deionized water of pH 5.6 was used as the control and a diluent of SAR stock solution.
All seedlings exposed to SAR of pH 2.0 showed necrotic visible foliar injuries or defoliation. Since Japanese larch, which is only one deciduous conifer species in the 11 conifers examined in this report, was defoliated at pH 2.5 and lower, it was classified as the most sensitive conifer species to low pH of SAR. Other conifers except this species did not show any visible symptoms at pH 3.0 or higher.
Seven of 14 evergreen broad leaf tree seedlings and 14 of 21 deciduous broad leaf tree seedlings showed foliar injuries at pH 3.0. Among deciduous broad leaf tree species exposed to SAR of pH 2.0, 7 species were almost defoliated, and 3 species were dead at the end of the exposure experiment. However, evergreen broad leaf trees did not show such a severity of the symptoms at pH 2.0. Exposure of SAR of pH 3.0 caused premature defoliation and shot hole symptom in Prunus species, especially P. x yedoensis and P. armeniaca. Any species did not show visible injury symptoms after the exposure to SAR of pH 4.0.
The results obtained in the experiment suggested as follows. Conifer trees were relatively tolerant to acidic rain exposure than broad leaf trees. Deciduous broad leaf trees were relatively more sensitive than the evergreen broad leaf trees. Current acidic rain monitored in Japan, where the lowest acidity of ambient precipitation was around pH 4.0 with quite a few frequencies and volume, could not induce any direct and significant effect on the tree species.