Resistance strain gages as physiological transducers on trees

A tree responds to its environment in several ways. One important way is through changes in the rate of transfer of water through the stems of the tree as it transpires. These changes induce extremely small variations in internal water stress and, therefore, in the size of the capillaries through which the water moves. The overall effect is minute changes in the diameter of the stems. By monitoring these changes, it is possible to observe an immediate response of the tree to its environment. This paper describes how electrical-resistance strain gages, bonded directly to the bark of a living-tree trunk, were used to measure these changes. It discusses bonding techniques, circuit design, instrumentation and the response of a red-maple tree to diurnal variations in its environment. Substantial changes in the circumference of the trunk, as large as 1900 microstrains, were observed on a 4-cm-diam tree during a daily cycle when the tree was under physiological stress. The overall results indicated good correlation with classical botanical theory and prior experimental data on the subject. In addition to its primary purpose, the paper illustrates the benefits possible from interdisciplinary cocperation and interaction between the fields of experimental mechanics and physical botany as related to forestry. The method described has potential importance in many aspects of tree husbandry. These include capabilities for remote sensing of forest-tree growth, maximizing seedling growth in large nursery operations, and efficient water management in orchards for maximum productivity with minimum irrigation. It may also provide a helpful new technique for use in basic studies in plant and tree physiology.