Phenological Disorder Induced by Atmospheric Nitrogen Deposition: Original Causes of Pine Forest Decline over Japan. Part I. Phenological Disorder,Cold Death of Apical Shoots of Red Pine Subjected to Combined Exposures of Simulated Acid Rain and Soil Acidification,and Implications for Forest Decline

Seeds of red pine (Pinus densiflora Sieb. and Zucc.) were sown in red-yellow soil artificially adjusted to pH (H₂O) 4.10, 4.60 or 5.90 by adding H₂SO₄ solution to the soil (pH 5.90), and the three-month seedlings were exposed to simulated acid rain at pH 2.0, 3.0 or 5.6 for 10 minutes once, 3 times a week, for 12 months from 4 August 1994 to 3 August 1995 alone or in combination. Significant interactive effects between acid rain and soil acidification on growth and whole-plant net photosynthetic rate, and cold death ratio of new apical shoots following a cold snap were observed in a quadratic response pattern. The simulated acid rain increased budburst, new needle spread and elongation, and new apical shoot death percentage following a cold snap, but did not induce visible injury. In the highest soil acidity treatment at a soil pH 4.1, whole-plant net photosynthetic rate and seedling height exhibited a quadratic responses with increasing rain acidities. On the other hand, soil acidification caused leaf yellowing. The death percentage of new apical shoot of seedlings exposed to rain pH 2.0 following a cold snap was linearly enlarged with increasing soil acidities. With increasing soil acidity, height and whole-plant net photosynthesis of the seedlings exposed to rain pH 3.0 exhibited a linear increase response, while height of seedlings exposed to control rain exhibited a quadratic response. It is suggested that the results provide experimental evidency for phenological disturbances and an enhancement of frost risk by direct acid rain and indirect longterm soil acidification which may be significant in forest decline.