Evaluation of actual transpiration from a forest by means of a long-term monitoring system

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In the study, a long-term monitoring system was established for evaluating the actual transpiration from a forest. An improved thermal technique called Granier method was applied for monitoring the amount of sap flow. Sap flux measured by the Granier method corresponds well to that by the heat pulse method, and single tree transpiration estimated from the sap flux by the Granier method also corresponds to the value reported in the previous study.To clarify the characteristics of transpiration process and water balance of a tree, continuous measurement of sap flow was conducted in a Japanese red pine forest located at Terrestrial Environment Research Center, University of Tsukuba. The results of the study indicate that the daily maximum transpiration was observed in the early morning and transpiration showed small rates in the afternoon under fine weather condition in the summer. This diurnal change of transpiration was regarded as a midday depression caused by the water stress due to the water up take shortage. The ratio of forest transpiration to evapotranspiration fluctuates seasonally, the maximum and minimum value were found in winter and in summer, respectively. This phenomenon may be attributed to the much amount of shrubs containing deciduous trees which exist in understory. Furthermore, in the study, not only summarize the hydraulic characteristics of stem water in Japanese red pine and oak trees but also summarize the measuring method for stem water storage by the TDR method and the role of forest in the hydrological cycle of a basin.