MEASURING STEM WATER-CONTENT IN 4 DECIDUOUS HARDWOODS WITH A TIME-DOMAIN REFLECTOMETER

New technologies in time-domain reflectometry offer a reliable means o f measuring soil water content. Whether these same technologies can be used or adapted to estimate the water content of other porous media, such as the woody tissue of forest trees, has not been thoroughly addr essed. Therefore, curves relating the apparent dielectric constant (K- a) to volumetric water content (g cm(-3)) were constructed for large-d iameter stems of red maple (Acer rubrum L.), white oak (Quercus alba L .), chestnut oak (e. prinus L.), and black gum (Nyssa sylvatica Marsh. ). This information was combined with previously published data and a proposed ''universal'' calibration equation for wood was derived. Stai nless-steel rods (15-cm wave guides) were inserted into 160 trees (30 to 49 per species) growing in an upland oak-hickory forest and stem wa ter contents estimated monthly during 1994 and 1995 with a time-domain reflectometer (TDR). Volumetric water contents in April ranged from 0 .28 g cm(-3) for red maple to 0.43 g cm(-3) for black gum, with no evi dence that water content changed as a function of stem diameter. Stem water contents estimated during 1994 (a wet year) increased from May t o July, reached a maximum in midsummer (0.41 to 0.50 g cm(-3)), and th en decreased in November. During 1995 (a dry year), stem water content s for red maple and black gum (two diffuse-porous species) decreased f rom May to August, reached a minimum in September (0.29 to 0.37 g cm(- 3)), slightly increased in October and November, and then decreased in December. A different trend was observed during 1995 for white oak an d chestnut oak (two ring-porous species), with water contents remainin g fairly stable from May to August, but decreasing abruptly in Septemb er and again in December. Stem water contents estimated with a TDR bro adly agreed with gravimetric analyses of excised stem segments and inc rement cores, although there was evidence that overestimation of water content was possible with TDR as a result of wounding following wave guide installation. Nonetheless our results hold promise for the appli cation of TDR to the study of stem water content and to the study of w hole-plant water storage.