TEMPORAL AND SPATIAL VARIATIONS IN DIELECTRIC-CONSTANT AND WATER STATUS OF DOMINANT FOREST SPECIES FROM NEW-ENGLAND

Temporal and spatial characteristics of microwave dielectric propertie s and water status of several forest species were investigated during the 1990 and 1991 growing seasons as part of the NASA FED MAC. Data pr esented 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 st atus of the trees studied was determined either by use of a Scholander pressure bomb on branch samples or by fresh weight/dry weight assessm ent of wood core samples. Results indicate the following: 1) Radial di electric profiles matched the regions of the functional sapwood such t hat the sapwood was characterized by a higher dielectric than the bark and heartwood tissues. 2) A hysteresis exists between diurnal variati ons in branch water potential and trunk sapwood dielectric. 3) The die lectric properties were positively correlated with wood core moisture content, while the electrical resistance was poorly correlated with mo isture content. 4) Using categories of electrical resistance measureme nts as a qualitative assessment of relative ion concentrations the die lectric measurements were not sensitive to the different ion concentra tions within the xylem and phloem exudate. These results support the v iew that dielectric properties are strongly correlated with moisture s tatus in trunk wood and that diurnal variations in dielectric are rela ted to diurnal fluctuations in water potential. The lag between change s in branch water potential and trunk dielectric varies, depending on the structure, evaporative demand, and water storage capacity of the s apwood.