在水文循環中，蒸發散量一直是許多研究所探討之重點，但以往之測量方式無法完全區分土壤蒸發量與植物蒸散量的值。本研究利用樹液探針和風洞實驗，研究風速、淨輻射量和土壤含水量等環境參數對馬拉巴栗樹液流速和蒸散率的影響。研究結果顯示風速增加，樹液流速會上升，當風速大於8 m/s，植物樹液流速之改變量會趨於一定值不再增加，並以秤重法所量得之蒸散量與樹液探針求得之流速成正比關係。而當輻射量變大，植物之樹液流速會隨之升高，淨輻射量Rn大於263 W/m2後，樹液流速則會趨於平緩，維持在3.5 m/s左右。土壤含水量由接近凋萎點突然增加至飽和時，植物之樹液流速也會突然增加。另外，本研究根據Phillips et al. (2004)所提出之時間函數求得植物受外在環境變化影響所需要的反應時間，及其時間常數，得知本實驗使用的馬拉巴栗其時間常數約為10 ~ 30 min。 The transient responses of sap flow to the changes of environmental parameters, such as wind speed, radiation and soil water content, were experimentally investigated in this study. A Granier-type sap flow sensor was calibrated and tested in a porous tube for its transient time response. The results showed that the time constant of the sensor is smaller than the response time of sap flow to the wind speed. Then the sensor was installed in a plant Pachira Macrocarp, which was placed in a wind tunnel. The transient response of sap flow to wind speed and radiation can be described by the resistance-capacitance model suggested by Phillips et al. (2004). The values of steady sap flow rate increases as the wind speed increases until wind speed is larger than 8.0 m/s. Once the wind speed is larger than 8.0 m/s (net radiation larger than 260 W/m2), the steady sap flow rate does not increase anymore. The transpiration rate measured by the weighting method showed similar phenomenon. Furthermore, the nocturnal sap flow and transpiration rate were measured and compared with the results of day time. Under the same wind speed, the nocturnal response time is longer and steady sap flow rate is smaller than that of day time.