Sap flux measurements in the field tracked expected variations in vapor pressure deficit and tree phenology. In a 174-d field installation of 12 devices, batteries (eight rechargeable Ni-MH AA) needed to be replaced every 14 days. Device accuracy was confirmed by laboratory estimates of sap flow made on excised branches of Acer saccharum and Tsuga canadensis (adjusted R 2 = 0.96). Energy savings were achieved by reducing the voltage of heat pulses and using an internal clock to completely shut down the device between pulses. We developed and tested a cost-effective (<$150), simple to construct, and energy efficient sap flux device based on the heat pulse method. However, the cost and power consumption of commercial systems based on sap flow methods still limit their usage. Accurate estimation of transpiration in individual trees is important for understanding plant responses to environmental drivers, closing the water balance in forest stands and catchments, and calibrating earth system models, among other applications.
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