Temporal and Spatial Variations in Dielectric Constant and Water Status of Dominant Forest Species from New England

Temporal and spatial characteristics of microwave dielectric properties and water status of several forest species were investigated during the 1990 and 1991 growing seasons as part of the NASA FED MAC. Data presented were acquired from Durham, New Hampshire (white pine, eastern hemlock, and American beech), and Howland, Maine (eastern hemlock and red spruce). Dielectric properties of trunk wood were measured using C-band, L-band, and P-band dielectric probes. For the Durham specimens, electrical resistance was measured using a digital ohmmeter. Water status of the trees studied was determined either by use of a Scholander pressure bomb on branch samples or by fresh weight/dry weight assessment of wood core samples. Results indicate the foUowing: 1) Radial dielectric profiles matched the regions of the functional sapwood such that the sapwood was characterized by a higher dielectric than the bark and heartwood tissues. 2) A hysteresis exists between diurnal variotions in branch water potential and trunk sapwood dielectric. 3) The dielectric properties were positively correlated with wood core moisture content, while the electrical resistance was poorly correlated with moisture content. 4) Using categories of electrical resistance measurements as a qualitative assessment of relative ion concentrations, the dielectric measurements were not sensitive to the different ion concentrations within the xylem and phloem exudate. These results support the view that dielectric