A Compact, Low-Cost Sensing System to Enable Distributed Measurements of Urban Tree Transpiration

Abstract

Urban trees reduce air temperature and alleviate urban heat island effects through shading and transpiration, making them crucial components of climate adaptation strategies. Understanding their transpiration rates and patterns can give new insights into their response to climate stress, yet traditional sap-flux measurements—which often involve long extension cords and solar panels—are challenging to deploy in urban public areas. Here, we present a compact, low-cost monitoring system designed specifically for the urban deployment of the well-established Granier-style thermal dissipation sensors (TDS). Our design decentralizes heating control and data logging processes to individual trees through a custom printed circuit board (PCB) design that meets the technical requirements of TDS using affordable components. With reduced research footprint and equipment cost, the system is less vulnerable to the risk from and the consequence of vandalism in an urban setting. We validated this system through deployment on 18 ash trees (Fraxinus spp.) across four parks in the City of St. Paul, MN, USA in summer 2023, achieving a 72% data collection success rate. Our results demonstrate the system's capability for long-term urban ecohydrology monitoring while providing practical insights into deployment challenges and solutions. This technical advancement enables wider implementation of urban tree monitoring, supporting both basic research in urban tree physiology and applied management of urban forests under increasing environmental stress.