Hydrologic and dielectric properties of woody plant tissue: Implications for remote sensing of canopy water status
The measurement of xylem water potential provides information about the current water available to plants. Furthermore, the knowledge of actual stand transpiration and canopy water conductance provides essential information for estimating canopy carbon, water and energy budgets. Thus, successfully coupling remote sensing measurements directly to canopy water status would close a gap in our ability to estimate these parameters using remote sensing techniques. Connecting canopy water status to variations in observed radar backscatter via the plant dielectric constant will demonstrate the capability of using remotely sensed radar data in ecological studies of canopy water relations. This paper examines in situ observations of hydrologic and dielectric properties of several tree species under different environmental conditions. Characteristics of the dielectric constant measured for the woody plant material are examined together with simultaneous observations of such hydrologic parameters as xylem water potential and xylem water flow. Dielectric constant is monitored with field portable probes or with multichannel measurement systems designed specifically for continuous monitoring of dielectric properties. Several sets of experimental observations are reviewed. Results of these measurements indicate that a direct link exists between the dielectric constant of woody plant tissue and xylem water potential. Further examination of the physiological properties of these trees yields an indication as to the interrelationship between the physiological plant processes and dielectric constant. Moreover, inasmuch as radar is sensitive to dielectric constant, the implications of the observed relationship between dielectric and hydrologic parameters on the utility of radar to remotely detect variations in canopy water status are examined.
