Mechanism of the declining process of forest caused by the deposition of acid fog on forest canopies and the regeneration of the forest.

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In the mountain slope of Mt.Oyama, fog is frequently formed and pH of fogwater is low for nitric acid formed from NOx. The concentration of fog components becomes high when LWC is low, and acid fog events with fog water pH lower than 3.5 occurs for about 20% of fog duration there every year. It has been observed that the throughfall amount increased with the altitude for the contribution of fog, and the fog deposition may be dominated by the fog formation frequency and the deposition rate of fog droplets depending on the wind velocity. Deposited hydrogen ion via fog droplets is exchanged with metal ions in the cells in the leaves, and the concentrations of metal ions, calcium, magnesium, and potassium ions, in the throughfall is large for the leaching. In order to clarify the influence of acid fog on fir and beech, their seedlings were treated with simulated acid fog (SAF) at pH 3 and pH 5 prepared with a mixed solution of nitric acid, ammonium sulfate, sodium chloride with a molar ratio of 2:1:1 in a mobile fog chamber. The growth of the seedlings treated with the SAF at pH 3 had been reduced in plant height, stem diameter, the number of leaves, and dry matter production. Furthermore, the exposure to the SAF at pH 3 during growth phase induced early falling of leaves, and the falling rate were nearly 2 times greater than that at control. The laboratory experiment of the application of the simulated acid fog on their leaves also showed a large leaching of metal ions, and the leaching amount increased with the decreasing pH of the simulated fog solution. Not only the field observation but also laboratory experiments show the large effect of acid depositions on the trees.