Acid fog removes calcium and boron from fir tree: One of the possible causes of forest decline

We have observed acid fog in Mt. Oyama adjacent to the Kanto Plains in Japan, where acid fog under pH 3 has been frequently observed and natural fir forest (Abies firma) has been declining. We applied the simulated acid fog to the needles of fir twigs and the treatment removed calcium, boron, and cell-wall acidic sugars from the needles. We also observed high contents of calcium, boron, and acidic sugars in throughfall collected under the declining forest canopy. Ca⁺⁺ and Mg⁺⁺ ion leach via ion-exchange with major cations of fog-water and the amount of leached boron increased with the increase in the concentration of leached Ca⁺⁺. The deficiency of calcium and boron by leaching could be one of the main causes of decline of the fir forest.