Ribbonized sap flow: an emerging technology for the integration of sap flow sensor components onto a single platform

Trees play an important role in both the water and carbon cycles. Sensors that measure the flow rate of sap in individual trees provide critical insight into water use dynamics, and these data are vital for the understanding of ecosystem functions. Current sap flux sensors typically require the installation of multiple needle-like probes into the sapwood of the tree. Despite the scientific value of high-density measurements, the high cost and complexity of existing methods prevents the deployment of large networks of sensors. This study presents a new sap flow sensor design where all sensor components are on a single integrated circuit board (ribbon) thereby reducing cost and complexity. This system also eliminates misalignment errors, which are a problem inherent in some sap flow sensor designs. The new ribbonized sap flow (RSF) sensor was compared to gravimetric measurements of water flow to quantify the accuracy of the method and the tree wounding response. In a pair of field experiments, we tested the general performance of the sensor design over a growing season, and among several tree species. The laboratory calibration showed that the RSF sensor has similar accuracy and wounding response as traditional three-probe designs. At a flow rate of 15 cm/h, the sensor underreported rates by 68% with a 19-d wounding response, compared to other sensor designs, which underreport rates by 20-60%. Collectively, our field experiments show that the RSF sensor can produce reliable field data in a variety of species across full growing seasons. The RSF sensor platform is compact, robust, and versatile. Ribbonized sap flow sensors can be quickly and easily manufactured at low cost, which makes them ideal for the dense network of observations needed for the study of complex forest ecosystems and their response to global change.