An overview of stand-scale transpiration measurements using the sap flow technique for evaluating the effects of forest management practices on transpiration

Recently, securing water resources has become one of the major purposes of forest management in Japan; this is done by thinning in coniferous plantations, replacing coniferous plantations with broadleaved forests, and conducting clearing or thinning operations in bamboo forests to reduce water loss to evapotranspiration. However, the effects of forest management practices on water resources have not been fully clarified because of a lack of evapotranspiration data from Japanese forests. Transpiration is a primary component of evapotranspiration in forested areas. We presented an overview of transpiration measurements based on the sap flow technique, a typical method used for measuring transpiration, to contribute to the accumulation of transpiration data for Japanese forests. Firstly, we compared the sap flow technique with other transpiration measurement methods and showed the usefulness of the sap flow technique for examining the effect of forest management practices on transpiration. Secondly, we explained the scaling-up procedures needed to up-scale from measures of sap flux densities to individual tree-scale transpiration and from individual tree-scale transpiration to stand-scale transpiration. Finally, we summarized stand-scale transpiration measurements based on the sap flow techniques as used in Japan, and recommend the types of forests which should be measured to help land and water managers have an informed discussion related to modifying forest management with the goal of securing water resources by forest practices. Measurements of transpiration are needed from larch plantations, broadleaved forests, and bamboo forests in Japan. Transpiration data measured in Japanese cedar and Japanese cypress plantations at ages of more than 60 years and less than 20 years would also be useful for the discussion.