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 volumet-ric 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 pre-viously 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 con-tents estimated monthly during 1994 and 1995 with a time-do-main 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) de-creased from May to August, reached a minimum in September (0.29 to 0.37 g cm−3), slightly increased in October and No-vember, 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 Septem-ber and again in DecemSeptem-ber. Stem water contents estimated with a TDR broadly agreed with gravimetric analyses of excised stem segments and increment cores, although there was evi-dence 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, capaci-tance, Nyssa sylvatica, Quercus alba, Quercus prunus, stem water storage, TDR.
Introduction
Water stored within the woody tissues of forest trees has been viewed as a reservoir from which water could be withdrawn to buffer the evaporative demands of a transpiring plant canopy (Reynolds 1965, Turner and Waggoner 1968). This pool of available water, whether it is drawn from intracellular or ex-tracellular storage (Ewers and Cruiziat 1991, Holbrook 1995), may be of sufficient volume to influence the whole-plant water balance of some species. Studies in Scots pine (Pinus sylvestris L.) showed that 30 to 50% of the water transpired by a stand could be supplied over short periods of time from water stored within the sapwood (Waring et al. 1979). This agrees closely with the results of Waring and Running (1978) who reported that water stored in the sapwood of old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) was capa-ble of supplying up to 50% of the tree’s short-term water requirements. These estimates, however, far exceed those ob-tained in an earlier study with Scots pine (Roberts 1976) and a more recent modeling exercise with Thuja occidentalis L. (Tyree and Yang 1990), where stem water storage was esti-mated to contribute little to daily water use. Water stored within stems of deciduous hardwoods and the use of these supplies to offset whole-plant water requirements have been less well studied, perhaps because there is some expectation that stem water storage in hardwoods is less important than in conifers (Hinckley et al. 1978, Chaney 1981).
Despite the need to quantify better the contribution of stem water storage to whole-plant water balance, Holbrook (1995) points out that this requires more than simply establishing the presence of water in the stem. Attention must be directed toward improved estimates of whole-tree water uptake from soils and water loss by the entire canopy, and improved meth-ods for in situ monitoring of stem water content. Attempts to visualize better the spatial distribution of water in woody tissues have led to the promising use of gamma-ray attenuation (Edwards and Jarvis 1983), nuclear magnetic resonance (Byrne et al. 1986), and computer tomography (Raschi et al. 1995), and to the application of methods such as stem capaci-tance (Holbrook et al. 1992), electric resiscapaci-tance (Borchert 1994), and time-domain reflectometry (Constantz and Murphy
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
1990, Holbrook et al. 1992) for the study of stem water con-tent. Time-domain reflectometry (TDR), has been used for years to estimate soil water content (Topp et al. 1980) and is based on the speed at which an electromagnetic wave is propa-gated through a water-bearing material and the dielectric con-stant of that material (Fellner-Feldegg 1969). Concon-stantz and Murphy (1990) used the concepts of TDR to follow changes in stem water content over time for various forest tree species and noted that the technology provided a rapid and convenient means of measuring stem water content and potentially esti-mating water storage. Holbrook and Sinclair (1992) built on these studies and used TDR to monitor both stem water content and water storage in an arborescent palm, concluding that the technology readily detected changes in stem water content and that up to 60% of the total water transpired by Sabal palmetto ((Walt.) Lodd, ex J.A. and J.H. Schult.) during an imposed period of drought was drawn from stem storage.
Given the initial success with which TDR has been applied to trees, our objective was to use this technology to monitor seasonal changes in woody tissue water content of four decidu-ous hardwoods. Curves relating the apparent dielectric con-stant (Ka) to volumetric water content (g cm−3) were developed for both ring-porous and diffuse-porous species, and a pro-posed ‘‘universal’’ calibration equation was derived. This equa-tion was then used to estimate monthly stem water content for 160 trees growing in an upland oak--hickory forest during 1994 and 1995. Stem water contents were compared among species, across a range of stem diameters, and against estimates of water content determined on excised stem segments and on increment cores. We conclude that TDR is a reliable technique for measuring stem water content, but note that as applied in our study it has some shortcomings.
Materials and methods Site description
This study was conducted on a south-facing slope within the Walker Branch Watershed, a part of the U.S. Department of Energy’s Oak Ridge Reservation in Anderson County, Tennes-see (35°58′ N and 84°17′ W). The vegetation is typical of an upland oak--hickory (Quercus--Carya spp.) forest, consisting primarily of chestnut oak (Q. prinus L.), white oak (Q. alba L.), black gum (Nyssa sylvatica Marsh.) and red ma-ple (Acer rubrum L.). Although 17 species occupy the study site, these four comprise almost 75% (15.2 out of 21.1 m2 ha−1 total) of the site’s basal area. Mean annual rainfall (30-year average) is 134 cm and median temperature is 14.4 °C. Com-pared with the 30-year average, 1994 was a wet year and 1995 was a dry year (Table 1). Soils at the study site are primarily Typic paleudults. A comprehensive description of the climate, vegetation, soils, and land use history of the Walker Branch Watershed can be found in Johnson (1989).
Calibration procedures
Although a universal calibration curve has been proposed for use in determining soil water content (Topp et al. 1980), it is doubtful that such a curve developed for soils could be used to
estimate accurately the water content of woody tissues (Con-stantz and Murphy 1990, Holbrook and Sinclair 1992). There-fore, curves relating apparent dielectric constant (Ka) to volumetric water content (g cm−3) were specifically con-structed for large-diameter stems of red maple, white oak, chestnut oak, and black gum. One tree of each species was felled early on December 2, 1993 and the boles sectioned into segments of about 30 to 40 cm in length. Water loss from fresh-cut surfaces was minimized with a coating of paraffin (Gulf Lite and Wizard Inc., Memphis, TN). Four segments ranging in diameter from 16 to 22 cm were selected for each species and two stainless-steel wave guides (15 cm long, 2.5 cm spacing) were inserted radially into pre-drilled holes. Wave guides were also inserted into two additional segments of each species and estimates of stem water contents for these samples were used to verify the accuracy of the calibration curve. Wave-guide length was considered 13 cm with 2 cm of the stainless-steel rods exposed for sensor head attachment.
All calibration segments (16 total) and segments used to verify the calibration curve (8 total) were weighed and then allowed to dry on a greenhouse bench for 2 to 4 months. At 3-week intervals, each segment was weighed and the Ka meas-ured with a time-domain reflectometer (Model 6050X1, Soil Moisture Equipment Corp., Santa Barbara, CA). At comple-tion, all segments were placed in an oven and dried at 60 °C to constant weight. Segment volumes were determined by the water displacement technique and then, knowing the wet weight of each segment at any sampling date and the final dry mass of each segment, wood densities (g cm−3) and volumetric stem water contents (g cm−3) were calculated. A single calibra-tion curve was developed from these data and those published previously for sapwood blocks of Pinus radiata D. Don (P. in-signis Dougl. ex Loud.) (Constantz and Murphy 1990). The accuracy of this calibration equation was verified by compar-ing stem water contents estimated by TDR with those deter-mined by gravimetric analyses on additional stem segments Table 1. Monthly air temperatures and rainfall for 1994 and 1995 at the Walker Branch Watershed study site. Mean air temperatures for each month were calculated from hourly observations.
and on increment cores taken from randomly chosen trees at the time of wave guide installation in the field.
Estimates of stem and soil water content
Seasonal and species-specific variations in stem water content were examined for red maple, white oak, chestnut oak, and black gum trees into which stainless-steel wave guides had been installed at about breast height during mid-March 1994. A total of 160 trees were examined, 35 red maples, 46 white oaks, 49 chestnut oaks, and 30 black gum. These trees were measured monthly during both 1994 and 1995 beginning in early April and continuing until early December. Multiple sets of wave guides were installed into a few trees (four of each species) to investigate whether the time since wave guide installation influenced estimates of stem water content.
Variation in soil water content (0 to 35 cm depth) was determined for the study site with a TDR and 310 pairs of stainless-steel wave guides. These were measured at least once a month during 1994 and 1995. Soil water contents were adjusted for percent coarse fragment (Drungil et al. 1987) and soil matrix potentials (MPa) were calculated based on soil-moisture release curves generated for the A and B horizons of these Typic paleudult soils (Peters et al. 1970).
Statistical analyses
Species-specific differences in stem water content throughout the season were identified by a repeated measures analysis of variance (Moser et al. 1990), with species as the between-sub-jects factor and sampling date as the within-subbetween-sub-jects factor. A one-way analysis of variance was used to test whether the length of time since wave guide installation influenced esti-mates of stem water content, whereas a two-way analysis of
variance was used to test whether species differences in stem water content were consistent across stem diameter classes. Duncan’s multiple range test was used for mean separation when differences between species were significant.
Results
Segments of stem used to construct calibration curves for the four species had midwinter volumetric water contents that ranged from 0.46 g cm−3 for white oak to 0.56 g cm−3 for red maple (Figure 1). Over the 2- to 4-month period of drying, these values decreased to about 0.12 g cm−3 for all species. Apparent dielectric constants over this period ranged from 20.1 in red maple to 7.4 in black gum, and there was a consistent, positive relationship between Ka and stem water content for each of the four species. When these data were combined with those of Constantz and Murphy (1990), a curvilinear relationship was observed (Figure 2). A second-or-der quadratic was fitted to the pooled data, yielding the expres-sion,
θ=−0.251 + 4.66×10−2Ka−4.93 ×10−4Ka2, (1)
where θ is stem water content (g cm−3).
Stem water contents estimated with Equation 1 were in close agreement with gravimetric analyses on independent stem segments and on increment cores from the four species (Fig-ure 3). There was, however, evidence early in the study (1994) that overestimation of stem water content was possible with TDR as a result of wounding following wave guide installation (Table 2). In the case of chestnut oak, stem water contents measured by TDR within 1 week of wave guide installation
were higher than those determined gravimetrically on incre-ment cores. For red maple, white oak, and black gum, differ-ences due to the method of estimating stem water content were apparent 12 weeks after wave guide installation (Table 2).
Little additional effect was observed on stem water content when wave guides were left in place for up to 30 weeks.
Stem water contents estimated within 2 weeks of installing the stainless-steel wave guides 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 (Ta-ble 3). There was an indication, however, that water content differed (P < 0.01) among species. Stem water content aver-aged across stem diameters was 0.28 g cm−3 for red maple, 0.39 g cm−3 for white oak, whereas chestnut oak and black gum values averaged 0.42 g cm−3 and above (Table 3).
Seasonal changes in stem water content were observed dur-ing 1994 and 1995, although the pattern of change was quali-tatively different in the two years. Stem water contents measured during 1994 (a wet year) increased from May to July, reached maximum values in midsummer (0.41 to 0.50 g cm−3), and then decreased gradually to a minimum in November (Figure 4). Over the year, stem water contents averaged 0.36 g cm−3 for red maple, 0.47 g cm−3 for white oak, 0.49 g cm−3 for chestnut oak, and 0.46 g cm−3 for black gum. 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 value in September (0.29 to 0.37 g cm−3), increased slightly in October and November, and then de-creased in December (Figure 5). A somewhat different trend was observed in white oak and chestnut oak (two ring-porous species), with stem water contents being stable from May to August, but then decreasing abruptly in September and again in December. During 1995, water contents averaged 0.38 g cm−3 for red maple, 0.50 g cm−3 for white oak, 0.53 g cm−3 for chestnut oak, and 0.48 g cm−3 for black gum.
Discussion
Techniques for measuring stem water content in trees range from simple correlations with electrical resistance (Dixon et al. 1978, Davis et al. 1979) to rather complex associations be-tween the absorption of ionizing radiation and the chemical composition of the material being studied (Raschi et al. 1995). In most cases, however, it appears that reliable spatial and temporal estimates of water content are difficult to obtain on woody tissues. Invasive sampling of stem water content, such as the analysis of stem cores, is quick and repeatable, but has been criticized because of the possible bias introduced as water is forced out of the sample during extraction (Whitehead and Jarvis 1981). More complicated methods, such as estimating water content with the attenuation of gamma radiation (Ed-Figure 2. Calibration curve of stem water content and apparent
dielec-tric constant obtained from data for the four hardwood species used in this study and from data of Constantz and Murphy (1990).
Figure 3. Stem water contents estimated by time-domain reflec-tometry and by gravimetric analysis of increment cores. The 1/1 line represents an essential agreement between the techniques for monitor-ing stem water content. Data were collected durmonitor-ing the 1994 growmonitor-ing season.
Table 2. Stem water contents (g cm--3) (mean ± SD) of four deciduous hardwoods as affected by the time since wave guide installation. Values within a row followed by the same letter are not significantly different.
Species Time since wave guide installation (weeks) Gravimetric
30 12 1
Red maple 0.44 ± 0.09 a 0.45 ± 0.08 a 0.27 ± 0.03 b 0.29 ± 0.04 b
White oak 0.57 ± 0.01 a 0.58 ± 0.06 a 0.52 ± 0.01 b 0.47 ± 0.01 b
Chestnut oak 0.51 ± 0.04 a 0.51 ± 0.03 a 0.51 ± 0.01 a 0.41 ± 0.02 b
wards and Jarvis 1983) and nuclear magnetic resonance (Byrne et al. 1986), may provide improved spatial and tempo-ral estimates of stem water content, but these techniques suffer from problems of expense, portability, and calibration.
Recent advancements in the use of TDR for measuring in situ water content of soils have led some to consider whether such methods can be applied to other porous media, such as the woody tissue of forest trees. Time-domain reflectometry was used by Constantz and Murphy (1990) to monitor daily and
seasonal changes in water content for a range of tree species and it was concluded that TDR offered a rapid and convenient technique for measuring sapwood water content. Short-term (days) and long-term (months) studies indicated that stem water content was responsive to flood irrigation in the genus Juglans, and that water content in the genera Aesculus, Euca-lyptus, Pinus, Quercus, and Sequoia exhibited annual vari-ations that ranged from 0.61 to 0.52 g cm−3 (a 15% change) in the ring-porous species Quercus argifolia Née (Q. oxyadenia Table 3. Stem water contents (g cm--3) (mean ± SD) of four deciduous hardwoods as related to stem diameter. A diameter tape was used to measure stem diameters during late March, whereas water contents were estimated by time-domain reflectometry in early April. Average values of stem water content for the four species followed by the same letter are not significantly different.
Species Diameter class (cm) Average
20--30 30--40 40--50 > 50
Red maple (n = 35) 0.26 ± 0.06 0.30 ± 0.06 0.31 ± 0.01 0.32 ± 0.05 0.28 ± 0.06 c White oak (n = 46) 0.39 ± 0.03 0.40 ± 0.05 0.37 ± 0.05 0.39 ± 0.02 0.39 ± 0.04 b Chestnut oak (n = 49) 0.42 ± 0.05 0.43 ± 0.04 0.41 ± 0.05 0.45 ± 0.03 0.42 ± 0.05 a Black gum (n = 30) 0.43 ± 0.06 0.42 ± 0.07 0.47 ± 0.04 nd 1 0.43 ± 0.06 a
1 No black gum trees of this diameter class were measured.
Figure 4. Seasonal estimates of stem water content during 1994 for (A) red maple, (B) white oak, (C) chestnut oak and (D) black gum. Each point represents the mean ± SD for 30 to 49 trees.
Torr.) and 0.71 to 0.39 g cm−3 (a 45% change) in the diffuse-porous species Aesculus californica (Spach) Nutt. (Constantz and Murphy 1990). From the perspective of whole-plant water balance, the magnitude of this annual variation reflects the degree to which water is moved to and from storage compart-ments in the stem, or more specifically in the sapwood (Hol-brook 1995).
We found little evidence that the magnitude of annual vari-ation in stem water content differed among the four species during 1994 (a wet year). Variation in water content ((max-min)/max) ranged from 15 to 20% for all trees and the direc-tion of change indicated a net movement of water into stem storage. It was clear, however, that during 1995 (a dry year) annual variation in water content was greater for stems of red maple and black gum (two diffuse-porous species) than for stems of white oak and chestnut oak (two ring-porous species). Variation in stem water content was 39% in red maple, 35% in black gum, 16% in white oak, and 19% in chestnut oak. Much of this variation occurred between the early spring (May) when soil water contents were high and late summer (September) when soil water contents were low. In the ring-porous species, there was evidence that stem water content decreased through-out the summer in parallel with soil water availability (data not shown). The direction of change during 1995 suggested a net movement of water out of storage, presumably to offset some portion of the seasonal water requirements of a transpiring plant canopy. Our data are not sufficiently resolved to address changes in stem water content over shorter periods of time, although we suspect that these cycles do occur (Constantz and Murphy 1990).
Previous studies have shown that TDR can be used with success to estimate stem water content in forest trees (Con-stantz and Murphy 1990) and an arborescent palm (Holbrook and Sinclair 1992). Like these authors, we recognize that various shortcomings of the technique must be addressed be-fore fully understanding how data on stem water content are to be interpreted. Perhaps most serious is that estimates of stem water contents obtained with the TDR are averaged or inte-grated throughout the length of the wave guides and for an area of wood roughly twice that of the wave guide separation (2.5 cm in our study). For diffuse-porous species this may not be a problem given the preponderance of sapwood versus heart-wood, but in ring-porous species such averaging of stem water content will likely bias estimates because of the varying con-tributions of sapwood and heartwood. Constantz and Murphy (1990) cautioned that when wave guides are installed radially into a stem, as they were in both our study and theirs, the resulting data must be interpreted with an appreciation for the cross-sectional morphology of wood. Clearly, given the cur-rent wave-guide configuration, stem water contents estimated for white oak and chestnut oak reflect changes not only in sapwood water content, but changes (or lack thereof) in heart-wood water content as well. If stem water storage is confined to sapwood, then the annual variation in stem water content as calculated in our study for the oaks in particular may show less seasonal fluctuation than otherwise suspected. This partially explains why annual variation in stem water content was lower
for the ring-porous species than for the two diffuse-porous species.
Another concern related to the use of TDR to estimate woody-tissue water content is that the relationship between volumetric water content and Ka for trees is quite different from that observed for soils. Samples of sapwood from Pinus ra-diata showed a much smaller change in Ka for a given change in water content than is typical of most organic soils (Con-stantz and Murphy 1990). We also observed that the relation-ship between water content and Ka was apparently unique for wood and agree that the calibration curve for soil is not appro-priate for use in estimating woody-tissue water content. Con-stantz and Murphy (1990) suggested that species-specific calibrations may provide more accurate estimates of stem water content for a given species, but we note that their cali-bration curve was developed based solely on data from Pinus radiata and the resulting equation applied to conifers and hardwoods alike. Based on our analysis, however, which in-cluded data from deciduous hardwoods and the conifer data of Constantz and Murphy (1990), it appears that a single ‘‘univer-sal’’ calibration curve relating stem water content and Ka may be warranted.
Acknowledgments
We thank R. Borchert, J. Constantz, and N.T. Edwards for their helpful reviews of the draft manuscript. This research was sponsored by the Program for Ecosystem Research, Environmental Sciences Division, Office of Health and Environmental Research, U.S. Department of Energy under contract No. DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp. Publication No. 4577, Environmental Sciences Division, Oak Ridge National Laboratory.
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