Physiological characteristics of Japanese red pine, Pinus densiflora Sieb. et Zucc., in declined forests at Mt. Gokurakuji in Hiroshima Prefecture, Japan

The decline of Japanese red pine trees (Pinus densiflora Sieb. et Zucc.) at Mt. Gokurakuji (693 m a.s.l.), 30 km west of Hiroshima city, west Japan, was studied. The effects of air pollution and acid deposition on the physiological characteristics of the trees, especially those of the needles, were investigated. Ozone concentration was not correlated with the physiological status of the needles and SO₂ concentration was not high in the declined area. NO₂ concentration correlated negatively with needle longevity while it correlated positively with ethylene emission from 1-year-old needles. Average needle longevity was about 2.8 years in non-declined areas; however the longevity was 1.3 years in the most polluted area. The minimal fluorescence at night (F₀) of 1-year-old needles decreased with increasing NO₂ concentration. The maximum stomatal conductance (gl), net photosynthesis (P(n)) and intercellular CO₂ concentration (C(i)) in the declined areas were lower than in the non-declined areas (about 50%, 30% and 20% lower, respectively). The lower C(i) suggested that the major part of the decrease in P(n) can be explained by stomatal restriction. The soil pH, N content and C/N ratio showed no significant difference between the declined and non-declined areas. The physiological disorders of needles were due to the damage by air pollutants, and important roles of NO₂ are suggested. Lowering of P(n) and the shortening of needle longevity appear to be the main causes of the decline in pines in the forest decline area.