Decoupling the integrated microwave signal originating from soil
and vegetation remains a challenge for all microwave remote sensing
applications. To improve satellite and airborne microwave data products in
forest environments, a precise and reliable estimation of the relative
permittivity (
The current generation of L-band satellite-based radiometers launched over the last decade – the NASA Soil Moisture Active Passive mission (SMAP; Entekhabi et al., 2010), the European Space Agency Soil Moisture Ocean Salinity mission (SMOS; Kerr et al., 2010) and the NASA/CONAE (Comisión Nacional de Actividades Espaciales) joint Aquarius mission (Le Vine et al., 2010) – offer global coverage of L-band (1–2 GHz) observations with a revisit time of only a few days. Although microwave measurements at L-band wavelengths are mostly used to detect soil moisture, ocean salinity and land surface freeze–thaw, these wavelengths are also sensitive to vegetation water content (VWC) since the microwave vegetation optical depth (VOD) is proportional to the VWC of the aboveground biomass (Konings and Gentine, 2017; Wigneron et al., 2017; Ulaby and Long, 2014; Jackson and Schmugge, 1991). However, decoupling the integrated signal originating from soil and vegetation remains challenging for all microwave remote sensing applications (Kerr et al., 2012; Roy et al., 2012, 2014) because vegetation contributes to microwave brightness temperature measurements and thus scatters and attenuates ground surface emissions (Wang et al., 1980; Wigneron et al., 2007).
Vegetation canopy radiative transfer modeling at L band remains a challenge
when it comes to quantifying the non-negligible scattering and emission
effects of vegetation (Wigneron et al., 2017; Kurum et al., 2012). The
relative permittivity (
Recent studies examining the impact of vegetation on L-band microwave
passive remote sensing have focused mainly on tropical regions and
grasslands (Konings et al., 2017a, b). In this paper, we
focus on boreal forests, which encompass
Freeze–thaw state detection at L band is based on the dielectric contrast
between water and ice at these frequencies (Fig. 1). At L band, permittivity
drops when water freezes. For oblique incidence angles (
Real (
During the growing season the tree contributions to the L-band signal are
related to water fluxes and storage in tree sapwood in accordance with the
demonstrated relationship between
To our knowledge, only a small number of measurements of tree permittivity
at microwave frequencies have been reported (e.g., El-Rayes and Ulaby, 1987;
Way et al., 1990; McDonald et al., 2002; Franchois et al., 1998). Most of
these studies report limited details on the accuracy and validation of the
instruments used to measure permittivity and are restricted in scope. For
example, El-Rayes and Ulaby (1987) focused on the measurement of leaf
permittivity in a laboratory. The trunks of only two species of tree, balsam
fir (
The goal of this study is to introduce and validate a newly developed open-ended coaxial probe (OECP) for in situ measurements of tree trunk permittivity at L band. The OECP measures a reflection coefficient at the interface between the probe and the medium of interest. Subsequently, the permittivity is inferred from its empirical relationship with the reflection coefficient. Section 2 describes the theory behind OECP measurements and their calibration. Section 3 explains the OECP measurement methodology, the study sites and the tree species. In Sects. 4 and 5, the performance, capabilities and limitations of the probe are presented and discussed. Furthermore, vegetation permittivity results during frozen and thawed periods for various species are presented.
Permittivity (
The general principle when dealing with permittivity measurements is to calculate the medium response to an applied electric field. Five main techniques are used for measuring this response in the microwave domain: waveguide (Demontoux et al., 2008), transmitting and receiving antenna (Ghodgaonkar et al., 1990), resonant cavity (Hakki and Coleman, 1960; Bircher et al., 2016), transmission line (Topp et al., 1980) and OECP (Stuchly et al., 1982; Demontoux et al., 2016). The first three techniques are more difficult to adapt for in situ field measurements on trees due to their constraints on the geometry of the samples and the size and fragility of the measuring devices (e.g., network analyzer required, sample of a specific shape required to fit in a container).
In this study, we designed, developed and validated an OECP operating at the
L band. The permittivity of the probed medium is inferred from the
reflection coefficient (
A reflectometer (Planar R54; Copper Mountain Technologies, Indianapolis, IN,
USA) was used to measure
To obtain the estimate of the medium's permittivity, the OECP must first be
calibrated with reference media. The calibration process involves obtaining
the calibration parameters necessary to transform
The probe reflection coefficient
In order to obtain the reflection coefficient
Probed depth is an important parameter needed for permittivity measurements
because it indicates the minimum required sample thickness. By measuring the
permittivity of a stack of paper sheets with a metallic plate underneath, it
is possible to experimentally evaluate the probed depth of the OECP
(El-Rayes and Ulaby, 1987). The permittivity of paper is relatively low,
around
To investigate the uncertainty associated with the OECP measurements, liquid and solid standards with well-known permittivities were used to identify any systematic errors and evaluate probe precision. The root mean square error (RMSE) percentage between our measured permittivity and the expected permittivity as found in the scientific literature was calculated to quantify the probe's precision. The four liquid references chosen (ethylene glycol, 1-propanol, 2-propanol and 1-butanol) have well-known frequency and temperature dependencies (Gregory and Clarke, 2012). Furthermore, they do not interact with the probe's materials. For the solid reference, polytetrafluoroethylene (PTFE) was chosen as the permittivity is given by Webster (2003) and it can be easily shaped into an appropriate sample size.
One of the main challenges in taking measurements of vegetation material with an OECP is obtaining a smooth surface to ensure full contact between the probe and the solid sample. Any air gap between the probe and the solid wood sample substantially affects the measurement as the low permittivity of air will systematically produce lower permittivity readings. To obtain accurate measurements in tree trunks, a set of Forstner drill bits (Freud Tools Inc., High Point, NC, USA), typically used in woodworking, was modified to create a flat surface at the desired depth in the tree trunk (Fig. 3a). To ensure proper contact, soft rotary tools were used to polish and clean the wood surface (Fig. 3b). Figure 3b shows a typical tangential cut that has been prepared for measurement. This method for creating a smooth surface in the tree trunk proved to be suitable for obtaining reliable and reproducible permittivity measurements of tree trunks without disturbing the sampling area of the trunk (see Sect. 4.2). Figure 3c demonstrates the process followed to collect the permittivity measurements of the tree. For every depth in a given trunk cavity, two or three measurements were taken. Permittivity results presented in this article are the average of those multiple measurements at the same depth in the examined tree trunks.
Tangential cut of a black cherry trunk for permittivity
measurement.
During the growing season, due to tree transpiration and associated water flow in the sapwood, OECP measurements must be taken shortly after the cut to avoid an accumulation of sap around the sample area. Repeated measurements after the initial cut show that the measured trunk permittivity remains stable for 2 to 5 min depending on the species and season (data not shown). Under frozen conditions, this is not a concern since there is no biological reaction in response to the cut. The sapwood depth was estimated visually using tree cores extracted with an increment borer (Maeglin, 1959). Because it was shown that the OECP calibration is temperature dependent, the OECP was always thermalized to the outdoor temperature before calibrations and measurements were taken.
For continuous measurement and to avoid any oozing or drying issues due to the
tree's biological reaction to the wound, the gap between the probe and the
cavity edges was sealed with plumber's putty. The plumber's putty does not
affect measurements because it is placed around the edge of the probe, away
from the open-ended measuring surface of the OECP probe. The OECP was
inserted into the middle of the sapwood (6 mm of depth) in a red pine (
In this article, the state of the vegetation will be referred to as fully
frozen when the soil, air and tree-skin temperatures are permanently below
the freezing point. It will be referred to as fully thawed when the soil,
air and tree-skin temperatures remain permanently above the freezing point
for several consecutive days. A winter thaw event refers to a period when
the air and tree-skin temperature rise above the freezing point, while the
soil temperature stays below 0
Tree permittivity measurements were taken at three different sites for a
total of seven tree species (five conifers and two hardwoods). For each
species, several measurements were obtained at several depths per tree. All
measurements were taken at breast height (
The Old Black Spruce (OBS) site is located in northern Saskatchewan near
Canada's boreal forest southern limit and is composed mainly of black
spruce (
Study sites and tree measurement details for Old Black Spruce (OBS), Montmorency Forest research site (NEIGE-FM) and Site Interdisciplinaire de Recherche en Environnement Extérieur (SIRENE). DBH stands for the diameter of the tree at breast height.
The permittivity of a stack of paper sheets stabilized at a thickness of 10 mm (Fig. 4). At that thickness, the effect of paper sheet height is too low to be observed given the probe's precision (see Sect. 4.2). Because the probed depth depends on the permittivity of the material, it is expected that samples with higher permittivity will have a shallower probed depth. The permittivity of paper is close to the lower end of the range of permittivity expected for vegetation material, and thus a 10 mm measurement should be seen as the upper limit of the probed depth of the OECP. For this reason, all results shown in this article were taken with samples of thickness greater than 10 mm to ensure there is no measurement disturbance in the probe's effective electrical field.
Real relative permittivity (unitless) at L band (1–2 GHz averaging) of a stack of paper sheets. The permittivity imaginary part is provided in the Supplement (Fig. S6).
A validation of the OECP performance was conducted by using liquid and solid standards (Fig. 5). The results summarized in Table 2 show consensus between our measurements and the reference data. In the L-band wavelength range, the standard deviation between the reference and measured data is under 2.5 % (Table 2). For some liquids, we can observe the beginning of a deviation out of the frequency domain for which the probe was designed (1–2 GHz). Such deviation was to be expected and can be ignored for our L-band measurements since only the 1–2 GHz range is used. As a result, measurements made on the same liquid standard vary less than 0.5 % over 20 independent measurements.
Real
To test contact with the flattened surface and evaluate OECP precision,
solid sample measurements were also conducted (Fig. 6). The consensus
between our measurements and the reference data deteriorates slightly to
3.3 % for the real permittivity while using PTFE solid samples (Table 2).
It should be noted that the 40 % standard deviation in Table 1 for the
imaginary part is mainly due to the fact that
Real (
Two patterns can be observed in Fig. 7: (1) the influence of sapwood depth on the radial profile and (2) the effect of the freeze–thaw state on the permittivity of the tree trunks. There are two distinct behaviors of the radial profiles during the thawed season. In the first several millimeters of the trunk, sapwood permittivity is higher due to a high water content, but permittivity decreases quickly to a lower and well-constrained value in the heartwood (Fig. 7). It has to be noted that since the probed depth can reach up to 10 mm, there is a bias toward lower permittivity near the interface sapwood–heartwood because the probe is measuring some dryer wood behind the actual sapwood.
Trunk radial profile of the real
Root mean square error (RMSE) between the measured L-band relative permittivity value (real and imaginary parts) and the accepted value in the scientific literature for the data presented in Figs. 5 and 6. The percentage of error is in parentheses.
During winter, sap flow approaches zero, resulting in consistently low tree
trunk permittivity across the whole range of trunk depths (Fig. 7). With the
warmer days of spring, plant biological activity including sap flow
recommences, resulting in an increase in sapwood thickness and peak
permittivity. A midwinter thaw event occurred at OBS from 15 to 21 February. During those warmer days, the air temperature reached 5
Substantial differences were observed in the frozen permittivity of different tree species (Table 3), ranging from 3.52 to 9.13 for the real part and from 0.36 to 3.23 for the imaginary part. Evaluating thawed tree permittivity is challenging since permittivity changes with depth. However, it should be noted that L-band interaction would be higher with the sapwood because it is the outer layer of the tree and its permittivity is higher than the heartwood. To ensure a representative averaging of the sapwood permittivity, we did not make measurements too close to the interface between sapwood and heartwood to avoid a bias toward lower permittivity. Knowing that the sapwood thickness of the trees used in this study is around 2 cm, the average permittivity reported in Table 3 for different thawed species was estimated by averaging the permittivity through the first centimeter under the bark using a trapezoidal numerical integration over that first centimeter. Again, significant differences were observed in the thawed permittivity of the tree species, ranging from 11.14 to 27.66 for the real part and from 3.05 to 9.33 for the imaginary part. Most of the thawed data date back to the end of spring (i.e., a fully thawed environment) and it should be kept in mind that some of these value patterns are to be expected during the growing season due to water storage in the trees and diurnal fluctuations as a result of tree hydrodynamics, as discussed below. Since all measurements used to calculate the permittivity of the thawed trees were collected in the latter part of the afternoon (between 15:00 and 18:00 local time), the evaluated permittivity corresponds to the daily minimum (see Sect. 4.3.2).
Complex L-band relative permittivity for different tree species in
thawed and frozen states. The standard deviation of the data is in
parentheses when the number of trees probed was relevant for this statistic.
The continuous measurements on the red pine show that the OCEP captures diurnal permittivity cycles (Fig. 8a). Since permittivity is strongly correlated with liquid water content (i.e., tree water storage), the diurnal cycles are mainly linked to the tree's daily use of its water resources (Matheny et al., 2015). Water storage depletes during the day as a result of tree transpiration. During the late afternoon to early morning hours, trees replenish their water storage as a result of lower atmospheric demand and transpiration rates (e.g., Pappas et al., 2018). This is why maximum peak permittivity occurred during the night (between 18:00 and 20:00 local time) and the minima occurred between 15:00 and 17:00 local time. The soil moisture peaks measured by the EC-TM sensors in Fig. 8b correspond to rain events, with the major one occurring on the evening of 27 September. The days after rain events correspond to higher permittivity. After those rain events, there was a substantial amount of water available in the soil, enhancing water content in the sapwood.
This study presents a new instrument for measuring tree trunk permittivity and demonstrates its applicability, precision and reliability for several common temperate and boreal tree species in North America. The OECP system is affordable (total costs are around USD 7000) when compared with other systems used to measure permittivity at microwave frequencies. The Canadian boreal forest is largely dominated by coniferous species, such as black spruce, which are evergreen trees with a small number of relatively thin branches. The influence of needles and branches on radiometric measurements is considered negligible at L band due to their size and quantity (Ferrazzoli et al., 2002). The diameter of branches needs to be a significant fraction of the wavelength to influence the signal. At L band, the wavelength (about 20 cm) is much smaller than the diameter of branches for typical trees (black spruce) in boreal forests. Nevertheless, for deciduous forest, branch permittivity can still be measured using the same technique used for the trunk, but only for branches with a diameter greater than the OECP diameter. Measurements of broad-leaved leaf permittivity could be obtained by stacking a pile of leaves thicker than the probed depth (El-Rayes and Ulaby, 1987).
The period over which the data were collected allows for a better understanding
of seasonal fluctuations in tree permittivity and its dependence on thaw
events. The clear discrepancy between the freeze and thawed trunk
permittivity is due to the water phase change and the limited biological
activity of trees during winter. Tree species have a broad range of
strategies for regulating their internal water storage and the resulting
transpiration rates. Trees growing in an environment with annual freeze–thaw
cycles have to be resistant to freezing-induced cavitation in their
conductive xylem (plant tissues where water transport occurs). Even with
anti-freeze mechanisms that allow them to keep 25 % of their water in
liquid state while air temperatures are below
Tree permittivity measurements can be of particular interest for the
calibration and validation of microwave radiative transfer models for
vegetation canopies. Radiative transfer models use the permittivity of the
different layers (i.e., soil, snow and vegetation) to simulate wave–matter
interactions. Since permittivity is challenging to measure in the field, it
is typically derived from empirical relationships using more easily
measurable parameters such as VWC and temperature. In Ferrazzoli et al. (2002), the dielectric constant of vegetation was computed with the
semi-empirical formula given by Ulaby et al. (1986). The problem is that few
of those relationships exist for vegetation and their applicability is
limited. When the information for such parameterization is not available,
models like
This study showed that tree permittivity is closely linked to tree hydraulic characteristics: during the thawed period, variations in tree permittivity are related to the tree's water storage, while in winter, tree freezing led to a strong decrease in permittivity. Hence, following empirical calibration (e.g., Matheny et al., 2015), the probe could be used to monitor the hydraulic properties of trees including water storage and the amount of frozen water in the tree. Compared to other methods to measure tree water storage, such as inserting soil moisture probes (Matheny et al., 2015), using several OECPs on a tree would make it possible to measure water storage and the amount of frozen water at different depths in the tree. However, for such long-term measurement applications, the biological reaction of the tree to the wound created by the cut made to insert the probe will have to be evaluated. Figure 8a shows a coherent signal over more than a month of measurements, suggesting that the tree's response to wounding is minimal; however, measurements over a longer period of time would be necessary to ensure the possibility of using the probe over full seasonal cycles to monitor the hydraulic functioning of trees. It is difficult to modify the dimensions of the probe without impacting its frequency limit since this is geometry dependent. Nevertheless, it is possible to reduce the dimensions of the probe for less invasive measurements, which will further increase the frequency limit. Moreover, it is possible to produce a series of probes operating up to higher frequency limits by reducing the size of their aperture, given that the limitation of the probed depth is acceptable.
This paper showed that the open-ended coaxial probe (OECP) is a suitable device to monitor the L-band permittivity (real and imaginary parts) of tree trunks. The OECP device that was developed displayed uncertainties under 3.3 % with a solid reference target and under 2.5 % with liquid standards. The permittivity of seven tree species was evaluated in both frozen and thawed states and revealed significant differences in the permittivity of those species. A clear distinction can be made between the dielectric characterization of (1) sapwood, for which the permittivity is high because of the high permittivity of water but decreases with depth, and (2) heartwood, for which the permittivity is low and constant. Our results indicate that the vegetation freeze–thaw state is sensitive to short winter thaw events. The OECP also proved to be precise enough to capture the growing season's diurnal cycle of fluctuations of tree water storage; however, its suitability for long-term continuous measurements requires further testing in order to quantify the impact of wounding effects.
Future work will examine the suitability of the OECP for soil permittivity measurements. Having a single instrument able to measure the L-band permittivity of both soil and vegetation in situ would be a useful tool for calibrating and validating microwave radiative transfer models.
The research data can be accessed by direct request to the author.
The supplement related to this article is available online at:
AM, AR and Alain Royer designed the study. BF and FB designed the probe. AM, AR and CP collected the data. AM processed the data. All authors contributed to editing the paper.
The authors declare that they have no conflict of interest.
This work was made possible thanks to the contributions of the Canadian Space Agency (CSA), Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Foundation for Innovation (CFI). Christoforos Pappas acknowledges the support of the Swiss National Science Foundation (SNSF), the Stavros Niarchos Foundation and the ETH Zurich Foundation (grants P2EZP2_162293 and P300P2_174477). We thank Bertrand Reulet for providing us with equipment for early tests, Atelier Pedro for manufacturing the probe, Warren Helgason for his logistical help, Bruce Johnson, Peter Toose, Joël Lemay and Mariam El-Amine for their help in the field, and Patrick Cliche and Gabriel Diab for their help with technical issues. We would also like to thank the three reviewers for helping us to improve the paper. Edited by: Mehrez Zribi Reviewed by: S. Bircher and one anonymous referee