Sap-flow sensors are used to quantify the amount of water flowing through a tree stem. Based on these data, we can estimate the water use of trees and their transpiration via the stomata in their leaves and needles.
There are about a dozen different sap-flow measurement methods known. In the Granier-method used here, two thin metal needles are drilled into the water-conducting wood. Both needles measure the temperature. In addition, the upper needle is heated constantly. The varying temperature difference between the two needles allows us to infer the amount of water transported.
Sap-flow data reflect the upward water flow driven by transpiration in the tree stem. The similar daily cycles of the different trees are clearly recognizable despite the varying amplitudes (Fig. 3).