報告書番号

U36

タイトル（和文）

スギの生育におよぼす酸性雨とオゾンの影響

タイトル（英文）

Effect of acid rain and ozone on the growth of Cryptomeria japonica

概要 （図表や脚注は「報告書全文」に掲載しております）

人工酸性雨をスギ，ヒノキ，サワラに暴露して生長におよぼす影響について検討した結果，pH3.0以上では生長抑制が認められなかったことから，現状程度の降雨の酸性度がスギの衰退要因となる可能性は考えにくいことが明らかとなった。オゾンと人工酸性雨の複合暴露実験を行った結果，オゾン濃度が高くなると地上部の乾物重量の割合が増加し，根の割合が減少すること，人工酸性雨のpHが3.0の場合には，その傾向がより明瞭となることが明らかとなった。この結果は，オゾン濃度が高くなると根の重量が相対的に少なくなり，降雨の酸性度が増すとその傾向が加速され，樹木が乾燥ストレスを受けやすい状態になることを示唆している。スギとヒノキを比較した場合，スギの方が乾燥ストレスの影響を受けやすいことから，都市周辺域のスギはオゾンと硝酸沈着量増加の複合影響によって水ストレス感受性が相乗的に増大することが衰退の原因であると考えられた。

概要 （英文）

Cryptomeria japonica, Chamaecyparis obtusa and Chamaecyparis pisifera were exposed to SAR of pH 5.6 to 2.0 for 23 months. Plant height, stem diameter and total dry weight of three conifers supplied with fertilizer were greater than those of plants grown without fertilizer. These conifers reduced total dry weights due to visible injuries such as reddish brown necrosis in large part of tops after the exposure to SAR at pH 2.0. Any growth reduction did not occur in any conifers exposed to SAR at pH 3.0 and 4.0 compared with those exposed to pH 5.6. Increased acidity of precipitation associated with increasing input of S and N may accelerate plant growth in the asymptomatic range of pH. Soil pH exposed to SAR of pH 4.0 did not show any significant differences from those exposed to pH 5.6. Exposure to SAR of pH 2.0 for 23 months lowered soil pH to 4.0 from 4.8 of the initial value. Exposure to 5450 mm of pH2.0 for 23 months significantly decreased concentrations of water- and 1M ammonium acetate-soluble Ca and Mg in soils, while Al concentration increased. Thus, molar ratio of (K+Ca+Mg)/Al reduced significantly at pH2.0. However, conifer root growth did not change at its ratio of about 0.4. Water culture experiment of C. japonica and C. obtusa demonstrated that both species showed best growth performance at the pH level of 3.5-4.0, however, decreased at the higher pH level. The results suggested that they have an adaptability to such a low pH condition. Concentration of Al at or below 1mM had no effect on the mortality of these species at the pH range of 3.5 to 4.0. However, Al concentration higher than 2mM increased the mortality and induced significant growth reduction. These results suggested that soil acidification stress associated with the exposure to SAR did not appear to cause significant growth reduction in Japanese conifers. C. japonica, C. obtusa, and C. pisifera were exposed to ozone of of 0, 60, 120 or 180 ppb for 6 hours in combination with 4300mm of SAR at pH 3.0 or 5.6 for 23 months in the controlled chambers under the natural light conditions. Dry weight of the plants exposed to SAR at pH 3.0 was greater than that of the plants at pH 5.6. Ozone exposure did not modify the total dry weight of the plants, however, leaf dry weight in plants exposed to higher ozone concentrations was significantly greater than that of plants exposed to clean air. In contrast, high ozone exposure significantly reduced root dry weights. These results suggested that higher ozone concentration in combination with increased nitrate input interfered with carbon allocation. This may lead to increase susceptibility to drought stress in these species. As C. japonica is more sensitive to drought stress than C. obtusa, this may support that high concentration of ozone and high input of nitrate in suburban areas may have a linkage with the decline of C. japonica.

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