Ion exchange reactions on the stem surface of Chamaecyparis obtusa Sieb. et Zucc.

In order to clarify the sources of soil acidification below stands of hinoki (Chamecyparis obtusa Sieb. et Zucc.), artificial stemflow experiments were performed to separate out the role rainfall acidity has from that of the acidity potentially occurring during stemflow. Various solutions, including distilled water, NaCl, NH4Cl, KCl and CaCl2+MgCl2 solutions, were added in a controlled manner to the upper stem and the solutions systematically collected at breast height. Although the original pH values of the salt solutions were around 5.5, the pH values of stemflow ranged from 3.8 to 4.2. The ion balance [anion (including bicarbonate ions) to cation concentration] decreased with initial stemflow but recovered after ca. 5 l of flow. At this point, pH values continued to decrease and this decrease was accompanied by a decrease in the cation concentration of the stemflow. These results indicate that there are two ways by which stemflow can be acidified. In the initial stage of flow, organic and inorganic acids are dissolved in the stemflow. Following this, ion exchange occurs between cations contained in the stemflow and hydrogen ions adsorbed in the stem. The last step appears most critical in the acidification process.