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Monitoring of Stem Water Content of Native and Invasive Trees in Arid Environments Using GS3 Soil Moisture Sensors

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Dielectric soil moisture sensors have the potential for nondestructive and real-time monitoring of the stem water content (θst) of living trees. This study was conducted to investigate the water use characteristics of trees in drylands through monitoring of θst using newly developed capacitance sensors (GS3). The plants used for data collection were Prosopis juliflora (Sw.) DC. (mesquite, invasive) in Sudan and Tamarix ramosissima Ledeb. (tamarisk, invasive) and Prosopis pubescens Benth.(screwbean mesquite, native) in the United States. The GS3 probes were installed into the trunks of two trees for each species. Stem-specific calibration equations and temperature calibration equations were derived through laboratory experiments and analysis of field observation data. The temperature calibration equations reduced inappropriate variations of θst caused by daily fluctuations in stem temperature, suggesting that these are essential for correct interpretation of monitoring data of θst in arid environments. The θst of the mesquite trees in Sudan clearly increased after heavy rainfall events and started decreasing when the soil water content became close to the wilting point. These findings indicate that mesquite trees use soil water in rainy seasons, even though they are generally considered to use groundwater through deep tap roots. The θst of neither species in the United States responded clearly to rainfall events, indicating that they depend on shallow saline groundwater. The θst of the tamarisk decreased monotonically throughout the monitoring period, apparently in response to feeding damage caused by the tamarisk leaf beetle (Diohabda sp.), which had been released for biological control of tamarisk. © Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved.
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Vadose Zone Journal | Advancing Critical Zone Science
Monitoring of Stem Water
Content of Native and Invasive
Trees in Arid Environments Using
GS3 Soil Moisture Sensors
Tadaomi Saito,* Hiroshi Yasuda, Miyu Sakurai, Kumud
Acharya, Sachiko Sueki, Koji Inosako, Kiyotsugu Yoda,
Haruyuki Fujimaki, Mohamed A.M. Abd Elbasit, Ahmed
M. Eldoma, and Hiroshi Nawata
Dielectric soil moisture sensors have the potential for nondestructive and
real-time monitoring of the stem water content (qst) of living trees. This study
was conducted to investigate the water use characteristics of trees in dry-
lands through monitoring of qst using newly developed capacitance sensors
(GS3). The plants used for data collection were Prosopis juliflora (Sw.) DC.
(mesquite, invasive) in Sudan and Tamarix ramosissima Ledeb. (tamarisk,
invasive) and Prosopis pubescens Benth.(screwbean mesquite, native) in
the United States. The GS3 probes were installed into the trunks of two trees
for each species. Stem-specific calibration equations and temperature
calibration equations were derived through laborator y experiments and
analysis of field observation data. The temperature calibration equations
reduced inappropriate variations of qst caused by daily fluctuations in stem
temperature, suggesting that these are essential for correct interpretation
of monitoring data of qst in arid environments. The qst of the mesquite trees
in Sudan clearly increased after heavy rainfall events and started decreas-
ing when the soil water content became close to the wilting point. These
findings indicate that mesquite trees use soil water in rainy seasons, even
though they are generally considered to use groundwater through deep
tap roots. The qst of neither species in the United States responded clearly to
rainfall events, indicating that they depend on shallow saline groundwater.
The qst of the tamarisk decreased monotonically throughout the monitoring
period, apparently in response to feeding damage caused by the tamarisk
leaf beetle (Diohabda sp.), which had been released for biological control
of tamarisk.
Nondestructive methods of soil water monitoring have long been desired for
environmental evaluation and optimum agricultural water management. In recent years,
dielectric soil moisture sensors such as time domain reflectometry (TDR), amplitude
domain reflectometry (ADR), and capacitance probes have been developed that take
advantage of the relatively high permittivity of water to estimate the volumetric water
content (e.g., Topp et al., 1980). All types of dielectric moisture sensors output an elec-
trical signal that varies depending on the apparent dielectric permittivity of the sensing
volume of porous materials. Because wood is also a porous material, dielectric moisture
sensors have the potential for measurement of volumetric water content in wood. If the
water content of the trunk (xylem) of living trees can be monitored by dielectric moisture
sensors, these sensors can be used to enable suitable irrigation through real-time detection
of plant water stress, clarif y water use mechanisms of natural trees, and evaluate the water
storage ability of tree stems.
Since the 1990s, several trial studies have been conducted to measure stem water content
using TDR methodology (Constantz and Murphy, 1990). Holbrook and Sinclair (1992)
Core Ideas
•Stem water content of trees was
successfully monitored using GS3
capacitance sensors.
•Temperature calibration was
essential for correct interpretation of
monitoring data.
•The monitoring clarified water use
characteristics of the trees in arid
environments.
T. Saito and K. Inosako, Faculty
of Agriculture, Tottori Univ., 4-101
Koyama- Minami, Tottor i 68 0-8553,
Japan; H. Yasuda and H. Fujimaki,
Arid Land Research Center, Tottori
Univ., 1390 Hamasaka, Tottor i 680-
0001, Japan; M. Sakurai, G raduate
School of Agriculture, Tottori Univ.,
4-101 Koyama-Minami, Tottori 680-
8553, Japan; K. Achar ya and S. Sueki,
Center for Environmental Remedia -
tion and Monitoring, Desert Research
Institute, 755 E. Flamingo Road, Las
Vegas, NV 89119; K. Yoda, Faculty of
Science and Engineering, Ishinomaki
Senshu Univ., 1, Shinmito Minami-
sakai, Ishinomaki, Miyagi 986-8580,
Japan; M.A. M. Abd Elbasit, Agricultu-
ral Research Council, Institute for Soil,
Climate & Water, Private Bag X79, Pre-
toria 0001, South Africa; A.M. Eldoma,
College of Forestr y and Range
Science, Sudan Univ. of Science and
Technology, Khar toum, Sudan; and
H. Nawata, Faculty of Inter national
Resource Sciences, Akita Univ., 1-1
Tegatagakuen-mach i, Akita 010-
8502, Japan. *Cor responding author
(tadaomi@muses.tottori-u.ac.jp).
Vadose Zone J.
doi:10.2136/vzj2015.04.0061
Received 22 Apr. 2015.
Accepted 5 Jan. 2016.
Open access
Original Research
Vol. 15, Iss. 3, 2016
© Soil Science Society of Amer ica. This is
an open a ccess article distributed under
the CC BY-NC-ND license (http://creative-
commons.org/licenses/by-nc-nd/4.0/).
Published March 23, 2016
VZJ | Advancing Critical Zone Science p. 2 of 9
estimated the contribution of stem water storage to the water bal-
ance of palm trees [Sabal palmetto (Walter) Lodd. ex Schult. &
Schult. f.] using TDR in Florida. Nadler et al. (2003) monitored
the stem water content of lemon trees [Citrus limon (L.) Burman
f.] under different irrigation management programs in Israel
with three-rod (70 mm) TDR probes and found that water stress
was reflected in the TDR-measured stem water changes but that
these changes were too small for routine irrigation control. Irvine
and Grace (1997) measured the stem water content of Scots pine
(Pinus sylvestris L. ) using short (50-mm) TDR probes. Nadler
et al. (2006) also monitored the stem water content of a mango
tree (Mangifera indica L.) in Israel using short (29–70-mm) TDR
probes and found that the response of the stem water content to
root zone applied salinity and water stress were negative and posi-
tive, respectively. Sparks et al. (2001) monitored the stem water
content, ice fraction, and losses in xylem conductivity of lodgepole
pine (Pinus contorta Douglas ex Loudon) using TDR probes in
Idaho. Hernández-Santana et al. (2008) measured the stem water
content of two Mediterranean Quercus species in Spain using TDR
and reported seasonal variation of the stem water content associ-
ated with decreases in water available in the soil. Kumagai et al.
(2009) measured the sap flow and stem water content of Japanese
cedar and cypress using constant-heat sap flow probes and ADR
probes and confirmed that stem water storage has impacts on the
transpiration stream. Development of species-specific equations
describing the relationship between probe output (apparent dielec-
tric permittivity) and stem water content are also important to the
accurate determination of stem water content (Wullschleger et al.,
1996; Hernández-Santana and Martínez-Fernández, 2008). Zhou
et al. (2015) developed a new frequency domain inner fringing
capacitor sensor for measuring the stem water content of crops.
Although several studies have been conducted to evaluate stem
water, there have been few studies to monitor the stem water con-
tent of wild trees in arid environments using dielectric sensors.
Wild trees in arid regions have efficient water use characteristics
to cope with limited water resources so that they can survive harsh
conditions. Additionally, a new dielectric moisture sensor that uses
the capacitance method, the GS3 probe (Decagon Devices Inc.),
has been developed. This probe may be an alternative to TDR and
ADR probes because it is inexpensive, tough, and easy to operate.
Hao et al. (2013) used this probe to monitor the stem water content
of paper birch (Betula papyrifera Marshall) trees in Massachusetts.
Thus, the main objective of this study was to investigate the water
use characteristics of trees in arid environments through monitor-
ing of soil and stem water content using GS3 and other dielectric
moisture probes. In addition, the outputs of dielectric moisture
sensors are usually affected by temperature (e.g., Wraith and Or,
1999; Fares and Polyakov, 2006), and large temperature variations
in arid environments appear to seriously impact probe outputs.
Therefore, temperature calibrations were conducted in addition
to stem-specific calibrations (water content calibrations), and the
importance of temperature calibration was determined.
6Materials and Methods
Plant Materials
The plants used for data collection were Prosopis julif lora, Tamarix
ramosissima, and Prosopis pubescens. Prosopis juliflora, commonly
known as mesquite, is native to South and Central America and
the Caribbean but has been introduced into arid and semiarid
regions worldwide (Gallaher and Merlin, 2010; Pasiecznik et al.,
2001). Mesquite is a typical phreatophyte that extends its roots
rapidly into deep aquifers (Nilsen et al., 1983). Because mesquite
can use groundwater through deep tap roots, this species can sur
-
vive in arid environments and thus efficiently compete with many
native plant species (Pasiecznik et al., 2001). As a result, mesquite is
listed as one of the world’s 100 worst invasive alien species (World
Conservation Union, 2004). In addition to destroying ecological
systems, mesquite depletes subsurface water resources. Yasuda et
al. (2014) observed that changes in the groundwater level (about
the 23-m depth) closely followed root water uptake by mesquite
in Sudan.
Tamarix ramosissima (tamarisk, also known as saltcedar) is native
to southeastern Europe and Asia and is a major invasive species
in southwestern and arid regions in the United States. Tamarisk
is also listed as one of the world’s 100 worst invasive alien species
because it causes reduced biodiversity in riparian zones (Bateman
and Paxton, 2010). Tamarisk is a salt-tolerant species that spreads
rapidly in river systems by using shallow saline groundwater
(Imada et al., 2012). In 2001, the northern tamarisk leaf beetle
(Chrysomelidae: Diorhabda carinulata Desbrocher) was released
in the United States for the biological control of Tamarix spp.
(DeLoach et al., 2003). The beetles feed on the epidermis of stems
and leaves, causing partial or complete defoliation of tamarisk mul-
tiple times throughout a growing season, eventually resulting in
tree morta lity. Prosopis pubescens (screwbean mesquite) is native to
the southwestern United States and therefore has a high tolerance
for hot and dry environments (Zappala et al., 2014). Although
screwbean mesquite has also been introduced to areas in India,
Pakistan, and southern and southwestern Africa, these introduc-
tions have rarely been successful.
Dielectric Moisture Sensors
GS3 dielectric probes (Decagon Devices Inc.) were used to monitor
the stem water content of trees in this study. The GS3 probes use
the capacitance method and are well known as low-cost, commer-
cially available soil moisture sensors. In addition to water content,
GS3 probes can measure bulk electrical conductivity and tem-
perature. The GS3 has three rigid steel needles (5.5 cm in length
and 0.30 cm in diameter) as sensing probes and can therefore be
installed into stems. Before installing the probes, the skins of the
target stems were shaved, after which holes slightly larger (0.32
cm in diameter) than the steel needles were drilled into the stems
using an electric drill. It should be noted that a special drill guide
was made and used to drill the holes in some stems to alleviate
VZJ | Advancing Critical Zone Science p. 3 of 9
the need to train the operator on how to drill the holes without
the guide. The probes were gently hammered until the needles
were completely installed into the stems, after which the slight
gaps between the sensor overmold and the stems were sealed with
silicone caulk to prevent infiltration of rainwater and evaporation
of stem water. The edges of the needles were also sealed in case
the lengths of the needles were longer than the diameter of the
stem. The entire sensors and stems were covered with aluminum
heat-insulating materials to reduce heating by direct sunlight. The
temperature sensor of the GS3 probe is not in the tree needles but
rather in the sensor overmold. The temperature measured at the
sensor overmolds (on the surface of the stems) was considered to
be the stem temperature in this study; however, the actual tempera-
ture in the stems might have been slightly different.
Another type of dielectric moisture sensor, ECH2O probes
(Models 5TM, 5TE, and 10HS, Decagon Devices Inc.), was
used to monitor the soil water content. All probes, including
the GS3, were connected to EM50 dataloggers (Decagon Devices
Inc.), and data were recorded every 10 to 30 min throughout the
monitoring period.
Study Sites
Two study sites were established in Sudan and the United States
in 2012. The Sudan site was established in a Prosopis juliflora
community in Soba, Khartoum (15°31¢8.3² N, 32°36¢52.2² E),
next to the campus of the College of Forestry and Range Science,
Sudan University of Science and Technology. Annual precipita-
tion at the site is about 150 mm, and the rainy season is from June
to September. The groundwater level at this site was about 25 m.
Two Prosopis juliflora trees were selected for installation of GS3
probes (hereafter PS1 and PS2) to monitor the stem water con-
tent in June 2012. The diameters of the stems of PS1
and PS2 were 7 and 4 cm, respectively. Additionally,
three ECH
2
O 5TM probes were set at soil depths of
5, 15, and 30 cm near the trees (Fig. 1). The moni-
toring data from July 2012 (the beginning of the
rainy season) to October 2012 (the end of the rainy
season) were analyzed in this study.
The site in the United States was established in a
mixed community of Tamarix ramosissima and
Prosopis pubescens in Mesquite, NV (36°42¢8.9² N,
114°15¢29.3² W) near the Virgin River. The study
site has an annual precipitation of about 400 mm
and a groundwater level of about 1 m. In addition,
the salinity of the groundwater was high (electrical
conductivity = 18 dS m−1), and salt accumula-
tion was observed at the soil surface. Two Prosopis
pubescens trees (hereafter PU1 and PU2) and two
Tamarix ramosissima trees (hereafter TU1 and
TU2) were selected for installation of the GS3
probes in April 2012. The diameters of the stems
of PU1, PU2, TU1, and TU2 were 8, 11, 8, and 9 cm, respec-
tively. The soil water content was measured using ECH
2
O 5TE
probes at depths of 5 and 30 cm and an ECH
2
O 10HS probe at
80 cm. Monitoring at this site was conducted from April 2012 to
November 2013; however, the soil water content for PU1 and PU2
are partly lacking because of problems with the probes and datalog-
gers. Meteorological cond itions (precipitation, air temperature and
humidity, wind speed and direction, global solar radiation, and
barometric pressure) were monitored at both the Sudan and US
sites. Pressure-type water level gauges were set in observation wells
(at the 25-m depth in the Sudan and 2.5 m in the United States) to
monitor groundwater levels at both sites.
Water Content and Temperature
Calibration
Laboratory calibration experiments were performed to elucidate
the relationships between probe outputs and the actual water
content in the stems. The stems with GS3 probes at the US site
were cut and brought back to the laboratory for the calibra-
tion experiments. The stems at the Sudan site could not be cut
because the monitoring continued. Therefore, stems with the
same diameters as PS1 and PS2 were cut from other trees and new
GS3 probes were installed into these samples. These sample and
probe combinations were then used as substitutes for PS1 and
PS2 in the calibration experiments.
The output of the GS3 is the volumetric water content calculated
by the default calibration equation provided by the manufacturer
(Decagon Devices, 2011). This probe output is referred to here
as the apparent water content (qp, m3 m−3). Initially, the stem
samples with the probes were placed into water for >2 wk. After
confirming that there had been no change in the probe output (q
p
)
Fig. 1. Setup of the dielectric moisture sensors: (a) the Sudan site with two Prosopis juliflora
trees (PS1 and PS2) outfitted with GS3 capacitance probes and underlain by ECH2O
5TE soil moisture probes at three depths, and (b) the US site with two Prosopis pubescens
trees (PU1 and PU2) and two Tamarix ramosissima trees (TU1 and TU2) each outfitted
with a GS3 probe and underlain by two 5TE and one 10HS ECH2O soil moisture probes.
The average groundwater level (GWL) at this site was about 1.23 m.
VZJ | Advancing Critical Zone Science p. 4 of 9
and the weight of the samples, meaning that the samples had been
almost saturated, the stem water content was changed by evapora-
tion in seven or eight steps between near saturation and air dry
for each stem. At each water content step, the values of the actual
stem water content were calculated by the weight of the stem with
the probe using an electronic balance. These water content values
are referred to here as actual q (qa, m3 m−3). All experiments were
conducted under a constant temperature condition (25°C). The
relationships between the actual water content (qa) and apparent
water content (q
p
) are shown in Fig. 2. Root mean square errors
(RMSEs) between qa and qp are also shown in Table 1. For stems
at the US site, qp was in relatively good agreement with qa (RMSE
= 0.0307–0.0724 m
3
m
−3
), indicating that the default calibration
equations for soils provided by the manufacturer can be used to
estimate the stem water content with relatively high accuracy for
some tree species. The relationship between q
p
and q
a
was fit using
a second-degree polynomial function for every stem and probe
combination:
( )
2
01 2
g aa aq = + q+ q [1]
where a0 to a2 are the fitting coefficients (Table 1).
Monitoring by the GS3 probes was severely affected by changes
in temperature in the stems, even though the stems were covered
with heat-insulating materials. Therefore, a calibration method
proposed by Saito et al. (2009) was applied to reduce the tempera-
ture effect. In this method, the probe output (q
p
) is expressed by
combining the calibration equations for water content (first term)
and temperature (second term):
( ) ( )
( )
pr
g f TTq = q+ q - [2]
where T is the temperature of the medium and Tr is the reference
temperature (25°C), g(q) is the relationship between qp and qa
at T
r
(Fig. 2 and Eq. [1]), f(q), which is the relationship between
dqp/dT (the slope values of the linear responses of qp to T) and q,
indicates the temperature dependence of qp on water content and
was obtained by applying temperature changes to samples under
constant water content steps in the calibration experiment. The
detailed derivation was shown by Saito et al. (2009). Furthermore,
Saito et al. (2013) proposed an approach to derive f(q) through
analysis of time series of field observation data without conduct-
ing laboratory experiments. Although this approach was originally
developed for monitoring soil water content, it was applied to the
monitoring results of the stem water content in the present study.
In this approach, f(q) can be derived using maximum and mini-
mum soil temperatures and the daily variation in q
p
for each day
in a monitoring period, on the assumption that daily variations in
Fig. 2. Relationships between actual stem water content (qa) and apparent stem water content (qp, manufacturer’s calibration) using GS3 probes at 25°C
for (a) two Prosopis juliflora trees (PS1 and PS2) at the Sudan site and b) two Prosopis pubescens trees (PU1 and PU2) and two Tamarix ramosissima
trees (TU1 and TU2) at the US site.
Table 1. Root mean square errors (R MSE) between actual a nd apparent
water content and fitting parameters for calibration equations.
Tree †
RMSE g(q)f(q)
m3 m−3 a0a1a2b0b1b2
PS1 0.0 617 0.1416 0.3742 0. 2861 −0.0075 0.0452 −0.050
PS2 0.0952 0.1836 0.040 0.5439 0.0003 −0.0 01 0. 0112
PU1 0.0469 0. 0744 0.8852 0.3719 0.0938 0 .4615 −0.5609
PU2 0.0724 0.1029 0.9786 0.6379 −0.0002 0.0126 0.0227
TU1 0.0371 0.0986 0.2947 0.9476 −0.0056 0.0289 −0.0234
TU2 0.0307 0.0388 1.0087 −0.1866 −0.0062 0.0424 −0.0529
† PS1 and PS2, Prosopis juliflora at the Sudan site; PU1 and PU2, Prosopis pube-
scens at the US site; TU1 and TU2, Tamarix ramosissima at the US site.
VZJ | Advancing Critical Zone Science p. 5 of 9
q
p
were caused only by daily f luctuations in T except for rainy days.
The qp and T values for the Sudan site (July–October 2012) and
the US site (March–September 2013) were analyzed, after which
f(q) was derived using the responses of qp to daily fluctuations
in T for each stem and probe combination, and f(q) was fit using
a second-degree polynomial function for every stem and probe
combination:
( )
2
01 2
f bb bq = + q+ q [3]
where b0 to b2 are fitting coefficients (Table 1). Substituting Eq. [1]
and [3] into Eq. [2] gives the calibration equation describing the
probe output (q
p
) as a function of q and T. The calibrated water
content was obtained by solving Eq. [2] by substitut-
ing the values of qp and T from the GS3 probes and
T
r
(= 25) into the equation for every stem and probe
combination. The same approach was applied to the
soil water content monitoring results at the Sudan
site by the ECH2O 5TE probes because these were
also affected by soil temperature.
6Results and
Discussion
Temperature Calibration
and Daily Variation
in Stem Water Content
Examples of comparisons of the calibrated stem
water content (q
st
, m
3
m
−3
) with the apparent stem
water content (qp) are presented in Fig. 3. The qp
of both PS1 and PU2 showed clear daily variations
that correspond to the daily fluctuation of T. In
contrast, such variations in qst were remarkably
reduced, indicating that the approach of Saito et al.
(2013) successfully reduced the effects of the daily
fluctuation of T on qst. The qst of PS1 increased
clearly after the rainfall event. It appeared that q
p
of PS1 also increased after the rainfall event; how-
ever, it is difficult to judge whether this increase in
q
p
was caused by the rainfall event because T also
increased after the rainfall event. The qp of PU2
did not appear to respond to the rainfall event at
all, even though q
st
clearly responded. These results
suggest that temperature calibration is essential for
correct interpretation of monitoring data of stem
water content in arid environments.
Although the temperature calibration remarkably
reduced the effect of T, qst still showed slight varia-
tions that seemed to be caused by daily f luctuations
in T. Similar slight variations were also seen in all
other stems in this study, suggesting that the daily
variations in stem water content caused by water consumption
(transpiration) and/or storage (root water uptake) within 1 d could
not be observed because of error in the temperature calibration.
This probably occurred because (i) the daily variation in stem water
content of trees in arid environments was originally too small to
be detected by the GS3 probe considering the error of the tem-
perature calibration, and (ii) the daily fluctuation of T in the thin
stems (4–11 cm in diameter) was large in the arid environments.
Moreover, the differences between T measured at the sensor over-
molds and the actual temperature in the stems might have affected
the results of the temperature calibration. Therefore, (i) improve-
ment of the calibration method, (ii) further ingenuity to reduce
temperature variations of stems, and (iii) additional monitoring of
Fig. 3. Comparisons of the calibrated stem water content (qst) with the apparent stem
water content (qp) and stem temperature for the (a) Prosopis juliflora PS1 tree at the
Sudan site (26 July–9 Aug. 2012) and (b) Prosopis pubescens PU2 tree at the US site (21
Jan.–3 Feb. 2013).
VZJ | Advancing Critical Zone Science p. 6 of 9
the temperature inside the stems may be needed to evaluate daily
variations in stem water content in arid environments.
Monitoring Results for the Sudan Site
Variations in qst of the mesquite trees, soil water content, daily
mean stem temperature (PS1), and daily precipitation in the rainy
season in 2012 are shown in Fig. 4. The groundwater level data
(24.8 m deep on average) are not shown because the data had been
disturbed frequently by the effect of pumping from nearby wells.
First, we focus on the variation in qst of PS1. On 30 July, 10.4 mm
of precipitation was observed at the site and only the soil mois-
ture sensor at 5 cm responded to this event. On 1 and 2 August, a
total of 47.0 mm of precipitation was observed and the soil water
content at 15 and 30 cm increased drastically, at which point the
qst of PS1 increased clearly. These findings suggest that PS1 used
the soil water below the 15-cm depth after the heavy rainfall event.
Although the soil water content started decreasing the next day, q
st
of PS1 increased continuously with time, and the maximum value
reached 0.64 m3 m−3 on 29 August. The qst started decreasing
from next day, at which point the soil water content at the 15-cm
depth was 0.19 m3 m−3 and at 30 cm was 0.21 m3 m−3. These
soil water content values were approaching the water content at
the wilting point (0.18 m3 m−3) obtained from the water reten-
tion curve of the soil at this study site, indicating that q
st
started
decreasing because root water uptake became difficult owing to
drying of the soil. These results suggest that Prosopis juliflora uses
soil water from below 15 cm during rainy seasons, even though
this species is generally considered to use groundwater through
deep tap roots. The q
st
of PS2 also responded to the rainfall events;
however, the values and variations of q
st
of PS2 were smaller (0.20
0.30 m3 m−3) than those of PS1 (0.38–0.64 m3 m−3) throughout
the monitoring period. One of the reasons may have been because
the stem of PS2 was thinner (4-cm diameter) than that of PS1
(7-cm diameter).
A large variation in qst between the dry season (0.38 m3 m−3) and
rainy season (0.64 m3 m−3) was observed, meaning that a large
amount of water was stored in the stem in the rainy season. Waring
et al. (1979) estimated that 30 to 50% of the transpired water was
extracted from water stored in the stem sapwood of Scots pine.
Holbrook and Sinclair (1992) estimated that water stored in the
stem supplied 20 to 40% of the total water lost by transpiration of
palm trees. The water stored in the stem of mesquite in Sudan also
seemed to contribute considerably to transpiration, especially after
30 August when the soil was dry and only qst decreased. However,
it is difficult to perform a quantitative evaluation for the contribu-
tion only from q
st
monitoring because the daily consumption and
storage of the stem water could not be observed due to the remain-
ing error in the temperature calibration. Further studies to enable
accurate monitoring of q
st
and the combinatorial monitoring of q
st
and other factors (e.g., sap flow and transpiration measurements)
related to the water use characteristics of trees are warranted.
Fig. 4. Variations in stem water content (qst) of two mesquite (Prosopis juliflora) trees (PS1 and PS2), soil water content, daily mean stem temperature of
PS1, and daily precipitation from July (the beginning of the rainy season) to October (the end of the rainy season) in 2012 at Soba, Khartoum, Sudan.
VZJ | Advancing Critical Zone Science p. 7 of 9
Monitoring Results for the US Site
Variations in q
st
, soil water content, groundwater level, daily mean
stem temperature (TU1), and daily precipitation from April 2012
to November 2013 are shown in Fig. 5. The groundwater level
was shallow (1.23 m deep on average) throughout the monitoring
period and showed a reverse trend from the stem temperature, i.e.,
Fig. 5. Variations in stem water content (qst) of (a) two screwbean mesquite (Prosopis pubescens) trees (PU1 and PU2) and (b) two tamarisk (Tamarix
ramosissima) trees (TU1 and TU2), groundwater level, daily mean stem temperature of TU1, soil water content, and daily precipitation from April
2012 to November 2013 at Riverside, NV.
VZJ | Advancing Critical Zone Science p. 8 of 9
the groundwater level was low in summers and high in winters.
The soil water content at the site was basically high because of
(i) a continuous supply of capillary water from the groundwater
and (ii) overestimation of the water content caused by the salinity
dependence of the ECH
2
O probes (e.g., Saito et al., 2008) because
the groundwater at the site is shallow and saline (electrical conduc
-
tivity: 18 dS m−1). The qst of both tree species responded to some
rainfall events (A, B, and C in Fig. 5); however, variations having
no relationship with rainfall were also seen. Furthermore, the
responses to the rainfall events were weaker than those of Prosopis
juliflora at the Sudan site. These results indicate that Prosopis
pubescens and Tamarix ramosissima at this study site depend on
the shallow saline groundwater without depending heavily on
rainwater. It was found that both species at this site depend on
groundwater because the stable O isotope ratios of water in the
stems of both species were almost equal to that of the ground-
water (Saito et al., 2014). These results also indicate that Prosopis
pubescens has salt tolerance equal to that of tamarisk, which is well
known to be tolerant of high salt levels. The qst of PU1 and PU2
appeared to decrease sharply several times in winter (D in Fig.
5); however, these were not actual decreases in water content but
rather the result of decreased permittivity in response to partial
freezing of the stem water.
The long-term variations in qst of the screwbean mesquite and
tamarisk were significantly different throughout the monitoring
period. The values of qst for PU1 and PU2 at the beginning of
the monitoring period were 0.37 and 0.29 m
3
m
−3
, respectively,
while they were 0.40 and 0.36 m
3
m
−3
at the end of the monitor-
ing period. In contrast, qst of TU1 and TU2 showed a monotonic
decrease, with lower levels occurring during summers and almost
constant levels during the other seasons. This appeared to be in
response to damage caused by feeding on the trees by the tamarisk
leaf beetle, which had been released as a biological control mea-
sure. At the US site, large outbreaks of the beetles were observed
in 2012 (after their arrival at the site in 2011), and the leaves of
the tamarisk trees are fed on repeatedly throughout the growing
season. Therefore, the tamarisk trees did not maintain normal vital
activities including root water uptake and transpiration owing to a
lack of leaves; hence, they were not able to acquire enough water to
maintain the stem water content, especially during summer.
6Conclusions
Monitoring of qst of mesquite, tamarisk, and screwbean mesquite
trees was conducted using newly developed capacitance sensors
(GS3) under different arid environments. Monitoring of q
st
and
the soil water content clarified the various water use characteristics
of these trees. The mesquite trees in Sudan used soil water during
the rainy season, even though this species is generally considered to
use groundwater obtained through deep tap roots. The trees at the
US site depend on the shallow saline groundwater without heav-
ily depending on rainwater. Monitoring of qst indicated that the
condition of the tamarisk trees was declining because of feeding
damage caused by the tamarisk leaf beetle. These findings indicate
that dielectric sensors may be useful as tools for evaluating the
biological control of tamarisk.
The temperature calibration equations derived though analysis
of field observation data greatly reduced variations in qst caused
by daily fluctuations of stem temperature, suggesting that these
are essential for correct interpretation of qst monitoring data
collected from arid environments. However, daily variations of
qst caused by water consumption (transpiration) and/or storage
(root water uptake) within 1 d could not be observed because of
the remaining error in the temperature calibration. Accordingly,
further improvements in the calibration method and measure-
ment techniques are needed. Simultaneous analysis of qst and
the electrical conductivity and/or sap flow also has the potential
to clearly quantify the water use characteristics of trees. Further
studies to enable accurate monitoring of qst and the combinato-
rial monitoring of q
st
and other factors related to the water use
characteristics of trees are warranted.
Acknowledgments
This work was supported by JSPS KAKENHI Grants no. 24780233, 23404014, 25257006,
and Adaptable and Seamless Technology Transfer Program through target-driven R&D, JST,
International Platform for Dryland Research and Education, Tottori University, and Joint Re-
search Program of Arid Land Research Center, Tottori University. We thank Misses Misaki
Inagaki and Mayu Tsukumo for helping with data collection and analyses.
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... Plant water storage and its dynamics are different based on soil water availability, tree species, tree size, wood properties, drought tolerance, and hydraulic strategy (Oliva Carrasco et al., 2014;Longuetaud et al., 2016;Matheny et al., 2016). Most other studies have focused on water storage quantities, measurement method, component connection, and species-specific water storage characteristics (Clevers, Kooistra & Schaepman, 2010;Longuetaud et al., 2017;Matheny et al., 2016;Paz-Kagan & Asner, 2017;Saito et al., 2016). Researchers have reported that trunk storage appears to provide a buffer to water demands during transpiration (Matheny et al., 2016). ...
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... [17]). Dielectric moisture/salinity sensors that use the capacitance method to observe fertiliser leaching are available [18][19][20][21], including the GS3 Greenhouse Sensor (METER Inc.), which is a low-cost dielectric moisture/salinity sensor that is easy to operate [17,22]. It can simultaneously monitor the soil water content, soil solute concentration (as electrical conductivity [EC]), and temperature. ...
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... Therefore, FDR is a better choice for measurement of θ stem in trees of small diameter. However, FDR measurements are moderately affected by diel fluctuation in stem temperature (Saito et al., 2016) and by waveguide configuration (Wang et al., 2009), and further study is required to provide suitable calibrations for the measurement of θ stem by FDR. ...
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Invasive alien plants such as mesquite (Prosopis juliflora) alter patterns of groundwater uptake. We measured the groundwater level beneath a mesquite stand in an arid area in Sudan. The changes in groundwater level closely followed plant water uptake. The groundwater level started to decline a few hours before sunrise, recovered around noon, and then continued to decline until a few hours after sunset, before recovering again during the night. Thus, groundwater level showed two peaks: just before sunrise and around midday. The midday recovery was due to the depression of photosynthesis by heat and light.
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Dielectric sensors have been widely used for nondestructive determination of volumetric soil water content (0). Since the output of such sensors is influenced by soil temperature (T), the calibration for the dependence is indispensable for accurate determination of e. The objectives of this study were (i) to apply an existing empirical temperature calibration method for capacitance probes to other types of commercial dielectric probes and (ii) to propose a new calibration method using time series of field data that requires no laboratory experiments for temperature calibration. Laboratory experiments were performed to obtain probe outputs at various T (5-35 degrees C) and 0 (air dry near saturation), using two soils and seven probes. From the results, a calibration equation describing the probe output as a function of 0 and T was derived for every soil probe combination and confirmed that this method is applicable not only to capacitance probes but also to impedance and TDR probes. Time series of field data (0 and T) had been previously monitored with ECH2O EC-5 and EC-20 capacitance probes in the Loess Plateau, China. Hourly outputs from the probes and Tat the 5-cm depth were analyzed, and calibration equations were derived using the responses of the probe outputs to daily fluctuation of T. The derived calibration equations from the field data were in good agreement with those from laboratory experiment for both probes. The equation was applied to field data and validly removed the effect of Ton the probe output.
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Phytoextraction is an established method of removal of heavy metals from contaminated soils worldwide. Phytoextraction is most efficient if local plants are used in the contaminated site. We propose that Prosopis pubescens (Screw bean mesquite) would be a successful phytoextractor of copper in our local soils. In order to determine the feasibility of using Screw bean mesquite, we utilized inductively-coupled plasma-optical emission spectroscopy (ICP-OES) and elemental analysis to observe the uptake of copper and the effects on macro and micro nutrients within laboratory-grown seedlings. We have previously shown that P. pubescens is a hyperaccumulator of copper in soil-grown seedlings.Light and transmission electron microscopy demonstrated death of root cells and ultrastructural changes due to the presence of copper from 50 mg/L – 600 mg/L. Ultrastructural changes included plasmolysis, starch accumulation, increased vacuolation and swollen chloroplasts with disarranged thylakoid membranes in cotyledons. Inductively coupled plasma-optical emission spectroscopy analyses of macro- and micro-nutrients revealed that the presence of copper sulfate in the growth medium of Petri-dish grown Prosopis pubescens seedlings resulted in dramatic decreases of magnesium, potassium and phosphorus. At 500-600 mg/L of copper sulfate, a substantial increase of sulfur was present in roots.
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Tamarisk (Tamarix spp.) is an invasive shrub or tree species in the United States. Glands within Tamarix leaves secrete salt that may increase surface soil salinity and consequently result in an unviable ecosystem for native riparian tree species. To increase our understanding of the effects of invasive Tamarix on the ecosystem and its native riparian vegetation, we investigated salt secretion of a stand of Tamarix ramosissima in the Las Vegas Wash, Nevada. Short-term and diurnal salt secretion rates were monitored in the stand from September to October 2009. Soil samples were collected beneath the canopy so that water and salinity conditions could be determined. We used weather data from a Las Vegas database in conjunction with soil temperature data collected for this study. The major cation secreted by salt glands was sodium. Diurnal variation in the salt secretion rate had a maximum during the day, suggesting that salt secretion is dependent on salt transportation to leaves. Variations in the salt secretion rates were mostly influenced by solar radiation, suggesting that salt secretion in T. ramosissima would be enhanced by light in natural conditions.
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Time Domain Reflectometry (TDR) has previously been used to determine the water content of soils. Here, TDR is assessed as a method of tracking the seasonal change in water content of the stems of mature trees (Pinus sylvestris L.). The longer probes used for soil were replaced by 50 mm probes, inserted radially into the stems at 1 m above the ground. Tests on blocks of wood in the laboratory suggested that the probe is influenced by the water content several centimetres around it, and the sensitivity of the system enables volumetric water content (W) in the physiological range to be measured to a resolution greater than 0.01 m3 (H20) m−3. The use of very short probes prevents the development of a universal calibration between measured dielectric constant and W. Calibrations for both 50 mm and 20 mm long probes are reported. The effect of temperature on the calibration was negligible. The system was used successfully on mature trees in 1995, and no evidence of wound reaction around the permanently installed probes was apparent in the measurements.
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Since 1990s, time domain reflectometry (TDR) has been applied to estimate the stem water content of living trees. Here, new calibration equations relating the apparent dielectric constant (Ka) to the volumetric water content (θ) were developed for two Mediterranean oak species. Our calibration equations differ from those previously calculated for other species, suggesting that stem water contents could be monitored more accurately using species-specific curves. The stem water content in the trees of these species and the surrounding soil were monitored with TDR to examine the feasibility of this technology for recording changes in trunk water storage. The average stem water contents of the oaks reflect the soil water contents, and the temporal differences observed (17%) point to the importance of trunk water for coping with soil water deficit. Although it would be very useful to obtain a single function to estimate the stem water content of trees, it remains necessary to obtain the results in more species.
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Close, direct, and accurate monitoring of the plant water status may serve as a practical (irrigation scheduling) and a research (climate- environmental induced physiologic changes) tool. Methods for high- frequency capacitance measurement (e.g., time domain reflectometry (TDR)) possess the potential for high resolution dielectric measure- ments with minimal dependence on properties of the measured matrix. The objective of this study is to test the accuracy, response time, and sensitivity of the TDR methodology in measuring changes in water status in a mango (Mangifera indica L., Cultivar 'Kent') tree stem exposed to several perturbations concerning water, salinity, fruit load, and radiation. Under several induced stress conditions, stem and root zone water content (u) and electrical conductivity (s) were simulta- neously measured. Our study is distinct in its detailed and frequent measurements of stem water content (ustem) using short (29-70 mm) TDR probes in trees growing in a detached medium. We have found that ustem response to root zone applied salinity and water stress were negative and positive, respectively. Stem electrical conductivity (sstem) was primarily dependent on ustem and only negligibly on stem cells salinity. The ustem response time to water application was ~4 h. Two practical outcomes of our study were: (1) Because the salt content of the tree cells only slightly affected sstem, stem resistivity measurements could be used to represent dielectric changes, and(2)quiteshort probes could be used to include young trees of slim tree branches.