| Summary: | 碩士 === 國立臺灣大學 === 森林環境暨資源學研究所 === 106 === Transpiration is a major part of the water cycle in forested ecosystems. Sap flux (F_d) measurement is a well-known technique for investigating transpiration at the individual-tree and forested-stand levels. Although numerous studies have indicated significant spatial and temporal variations among the F_d values of individual trees, few have examined seasonal patterns corresponding to these values across multiple trees. Spatial and temporal variations in F_d and related seasonal affects must be determined to enable estimations of individual and stand transpiration as well as clarification of the transpiration mechanism. To this end, the present study examined 1) azimuthal variations in F_d and the related effects of seasonal changes, and 2) time lags between F_d and meteorological factors such as vapor pressure deficit (VPD) and solar radiation (SR), as well as the related effects of seasonal changes. These investigations were conducted in a Japanese cedar forest in the Tanzawa Mountains, located in the western part of Kanagawa Prefecture, Japan under a humid temperate climate. Additionally, 3) to characterize the results derived from the samples in the Tanzawa Mountains, the data sets were compared with those from previous measurements in a Japanese cedar stand in Xitou, located in central Taiwan. The Xitou stand is characterized by a humid subtropical climate with less distinct seasonality than that of the temperate climate in the Tanzawa Mountains. For objectives 1 and 2, 32 sets of Granier-type sensors were placed on 16 trees to measure F_d in two directions for each tree. Data were collected from August 21, 2017 to April 30,2018.
The main results are summarized as follows. First, azimuthal variations in the F_d of individual trees corresponded with seasonal changes. From autumn to winter, F_d on the southwest sides increased, but F_d on the northeast sides decreased. From winter to spring, however, F_d on the southwest sides decreased, but F_d on the northeast sides increased. These seasonal azimuthal variations may have been related to seasonal changes in crown exposure to sunlight. Second, the time lags between F_d and the meteorological factors also clearly exhibited seasonal changes: the time lags were longer in autumn (122 and 163 min for VPD and SR, respectively) and winter (188 and 214 min for VPD and SR, respectively) than in summer (54 and 116 min for VPD and SR, respectively). Soil drought was not the cause of the longer time lags. Possible causes included the microclimate around branches and the anatomical characteristics of the vascular systems. Third, comparison of the results for the Tanzawa Mountains with those for Xitou in Taiwan revealed that seasonal changes in time lags were not as pronounced in Xitou as in Tanzawa. The inconsistent temporal patterns of azimuthal variation in F_d and indistinct seasonal relationship with time lags in Taiwan may be attributed to the less distinct seasonality of the spatial and temporal variations in F_d in Taiwan compared with Japan. This study indicated that the characteristics of F_d differed between temperate and subtropical climates, even for the same species. Therefore, different strategies should be adopted according to season and region in estimates of transpiration based on F_d.
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