© 1996 Heron Publishing—Victoria, Canada
Measuring stem water content in four deciduous hardwoods with a time-domain reflectometer
Stan D. Wullschleger, Paul J. Hanson and Donald E. Todd
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422, USA / Received February 28, 1996
Summary
New technologies in time-domain reflectometry offer a reliable means of 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 addressed. Therefore, curves relating the apparent dielectric constant (Ka) to volumetric water content (g cm–3) were constructed for large-diameter stems of red maple (Acer rubrum L.), white oak (Quercus alba L.), chestnut oak (Q. 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. Stainless-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 water 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 evidence that water content changed as a function of stem diameter. Stem water contents estimated
during 1994 (a wet year) increased from May to July, reached a maximum in midsummer (0.41 to 0.50 g cm–3), and then decreased in November. During 1995 (a dry year), stem water contents for red maple and black gum (two diffuse-porous
species) decreased from 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 and chestnut oak (two ring-porous species), with water contents remaining fairly stable from May to August,
but decreasing abruptly in September and again in December. Stem water contents estimated with a TDR broadly agreed with gravimetric
analyses of excised stem segments and increment 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
application of TDR to the study of stem water content and to the study of whole-plant water storage.
Keywords:
Acer rubrum, apparent dielectric constant, capacitance, Nyssa sylvatica, Quercus alba, Quercus prunus, stem water storage, TDR.
