In most of the studies, no effects of simulated acid rain on the growth of agricultural crops and tree seedlings were reported when the pH of the applied solution exceeded a value of 3.0. Therefore, significant effects on agricultural crops and tree seedlings at current ambient levels of acid rain (pH 4.0-5.0) are unlikely. However, acid rain potentially could affect trees in natural forests in a variety of ways including the increase of soil acidity, increase of the solubility of phytotoxic aluminum ion, acceleration of acidic leaching of foliar nutrients and soil nutrients, and inhibition of activity of mycorrhizae. Therefore, acid rain may be one of the many environmental stresses contributing to forest decline. However, the cause of forest decline in Japan as well as in Europe and North America has not yet been clearly and completely determined.