Plant Science 171 (2006) 572–584
www.elsevier.com/locate/plantsci
Sap flow and daily electric potential variations in a tree trunk
Dominique Gibert a,*, Jean-Louis Le Mouël c, Luc Lambs b,
Florence Nicollin a, Frédéric Perrier c,d
a
Géosciences Rennes (CNRS UMR 6118), Université Rennes 1, Bât. 15 Campus de Beaulieu, 35042 Rennes cedex, France
b
Laboratoire Dynamique de la Biodiversité, CNRS, 29 rue Jeanne Marvig, 31055 Toulouse, France
c
Équipe de Géomagnétisme, Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris cedex 05, France
d
Commissariat à l’énergie atomique, 91680 Bruyères-le-Châtel, France
Received 11 January 2006; received in revised form 9 May 2006; accepted 2 June 2006
Available online 10 July 2006
Abstract
Electric potential has been monitored since December 2003 in the roots and at two circumferences and one vertical profile in a standing poplar
(Populus nigra). Electric potential is sampled using 6 mm diameter stainless steel rods, inserted 5 mm deep in the sapwood and is referenced to an
unpolarisable lead/lead chloride electrode installed 80 cm deep in the soil. Diurnal variations are observed with seasonal differences. During
winter, diurnal variations depend on the measurement point, with variable amplitudes and sometimes anti-correlations between electrodes. By
contrast, a stable and coherent organisation is established in the spring, with larger amplitudes, and lasts during summer. Dedicated experiments
have been performed to rule out a direct effect of temperature on the electrodes, and thus, demonstrate a genuine electrical source in the tree. Daily
electrical variations have been reported previously, and have been interpreted as electrokinetic effects associated with sap flow. However, a
comparison of the electrical signals with a direct measurement of the sap flow by a continuous heat flow method, shows that the electrical variation,
although clearly correlated to sap flow, is not simply proportional to it. In a living system, electrokinetic effects, in addition to thermoelectrical
effects, are probably modified significantly by additional electrochemical effects, such as membrane diffusion potentials, ion active transport by
proteins or action potentials. Electric potential variations in trees may thus, reveal physical mechanisms in living systems not accessible by other
methods. A better understanding of the electrical response of trees associated with sap flow may improve the knowledge of transfer processes
between the soil and the atmosphere. This is important for the understanding of adaptive response of trees, the modelling of water and carbon
balance in relation to climate change, and the quantification of the contribution of trees to the migration, retention and dispersion of contaminants.
# 2006 Elsevier Ireland Ltd. All rights reserved.
PACS: 02.70.Uu; 02.30.Zz; 91.25.Qi; 91.35.Pn; 91.40. k; 93.30.Vs
Keywords: Electric potential; Electrokinetic effects; Membrane potential; Sap flow; Temperature effects
1. Introduction
Estimating the effect of global climatic changes on
ecosystems and the associated feedback mechanisms requires
a better understanding of tree transpiration and carbon
assimilation [1]. These processes depend on sap flow, which
drives the whole tree physiology, the water balance and the
hydrology of the subsurface as well (e.g., [2–5]) and they
* Corresponding author. Tel.: +33 223236091; fax: +33 223236090.
E-mail addresses: dominique.gibert@univ-rennes1.fr (D. Gibert),
lemouel@ipgp.jussieu.fr (J.-L. Le Mouël), lambs@cict.fr (L. Lambs),
florence.nicollin@univ-rennes1.fr (F. Nicollin),
perrier@ipgp.jussieu.fr (F. Perrier).
0168-9452/$ – see front matter # 2006 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.plantsci.2006.06.012
have been studied in dedicated large-scale field experiments
(e.g., [6,7]). The direct measurement of sap flow up to now is
based on thermal methods [8], which can be separated into
heat pulse velocity techniques and heat balance techniques
[9]. However, the calculation of the total water flux remains
difficult. At the forest level, the water is shared between trees
of different species and sizes [10]. At the tree level, several
points need clarification, such as the contribution of night
respiration [11,1], the radial variation of the sap velocity in
the sapwood [12,13] or the occasional reverse sap flow in
roots [14]. Sap flow is different for the three major xylem sap
conducting systems (non-, diffuse- and ring-porous) and does
not bear a simple relationship with tree diameter or canopy
surface. Estimating the water flux from sap velocity
D. Gibert et al. / Plant Science 171 (2006) 572–584
measurement, thus, remains affected by large uncertainties
[10].
Sap flow may also be accessed through measurements of the
spontaneous electric potential of trees. Early measurements in an
elm trunk [15] showed that the electric potential of the trunk with
respect to a ground electrode shows daily variations with
amplitude of the order of 20–40 mV, and a minimum of the
potential in the afternoon. These daily variations have been
associated with the sap flow. Indeed, the motion of liquids in
porous media produces electric potential variations referred to as
streaming potentials or electrokinetic effects (e.g., [16–20]). This
effect is characterised by one coupling parameter, the streaming
potential coefficient, defined as the ratio of the potential
difference to the pressure difference. Electrokinetic effects have
been observed in laboratory experiments with rocks (e.g., [21]);
they lead to remarkable spatial and temporal variations of the
spontaneous electric potential (SP) in natural conditions
[22,17,23], especially near active volcanoes (e.g., [24]).
The early observation on the tree trunk was confirmed by an
experiment on a spruce tree [25], which in addition reported a
non-linear relationship between the daily electric potential
variation and the intensity of the solar radiation. Measurements
at the surface of a chestnut tree [26] indicated that the time
variations of the electric potential depend strongly on the
season. In summer, clear daily variations of the trunk potential
are observed, with a minimum in the afternoon when upper
electrodes are referred to lower electrodes, in agreement with
the previous experiments. In the late fall, after leaf shedding,
these clear daily variations disappear, as expected if sap flow is
the dominating mechanism for electric potential generation.
However, electrical signals, with more erratic time variations,
are still observed, indicating that other mechanisms may also
contribute, possibly unrelated to sap flow. The electric potential
was also monitored at several points of a Turkey oak (Quercus
cerris) during 2 years [27,28]. This experiment indicated that
the summer daily variations are in phase at the various points,
with again a minimum of the potential with respect to the
ground in the afternoon. The amplitude can vary by more than a
factor of two from point to point. Two periods for large daily
variations were observed: end of March and summer (end of
June–July).
Streaming potentials have also been studied in Salix alba L.
sapwood samples in the laboratory and the results have been
compared with the observed electrical daily variation in a
standing tree to obtain estimates of the sap flow velocity [29].
The sap flow velocity, thus, estimated from the electrical
variations, is 15–17 m h1, which is much higher than the
accepted values for S. alba L., which are of the order of 2–
3 m h1[29]. The positive sign of the streaming potential
coefficient obtained in the laboratory is in agreement with the
sign reported for most rocks in usual natural conditions (e.g.,
[30]), but does not agree with the sign of the observed electric
daily variation in the tree trunk. Indeed, for a positive streaming
potential coefficient, the potential of the trunk should be
positive with respect to the ground. While the observed daily
variation could be reasonably interpreted at first in terms of
streaming potentials, the relationship between the sap flow and
573
the electric potential variation therefore remains puzzling.
Other effects, such as thermo-electrical effects, membrane
potential and experimental artifacts, need to be considered
before a coherent physical interpretation can be proposed. It
appears, therefore, interesting to undertake new experiments to
investigate in more details the relationship between the sap flow
and the electric potential variations in a tree.
Monitoring of the electric potential in trees and more
generally in living plants, may be interesting for other reasons.
Electric activity has been evidenced in hibiscus and maize (e.g.,
[31,32]) and electrical signals have been observed in response
to changes in transpiration and photosynthesis in willow plants
[33] or Mimosa pudica [34]. The electric potential in trees may,
therefore, reflect a combination of physical, chemical and
physiological responses in relation with water transport,
photosynthesis and adaptive feedback mechanisms. The
seasonal variation of the electrical daily variation, for example,
may be closely associated with the poorly understood seasonal
variations of the enzymes of sucrose metabolism [35].
In this paper, we present a new experiment lasting more than
2 years in a poplar tree. In this experiment, we have focused on
the spatial and temporal variations of the electric potential
distribution in a single tree, using an increased number of
electrodes compared with previous work. The comparison of
several trees will be considered in a later stage. The
experimental set-up is first presented. After a brief overview
of the annual cycles of the tree, we present specific experiments
performed to constrain the physical mechanisms. In particular,
the electrical daily variations are compared with a direct and
independent measurement of the sap flow with a thermal
method. In addition, temperature effects, which can potentially
affect the measured electric potential, are studied in details. In
the conclusion, we discuss the generation mechanisms of
electric potential in the tree trunk.
2. Experimental set-up
2.1. Electrode array
The investigated tree is a poplar (Populus nigra L.) located in
Remungol1 (Brittany, France). The first part of the experimental
set-up, with a set of 26 electrodes, was installed on August 6,
2002, 21 electrodes in the trunk and five in two emerging roots
(Figs. 1 and 2). Trunk electrodes are arranged in three groups:
two circular rings and one vertical line. The lower ring, with
eight electrodes numbered E1 to E8, is located 1.0 m above the
ground and has a circumference of 2.7 m. The upper ring, with
eight electrodes numbered E11 to E18, is located 3.4 m above
the ground and has a circumference of 2.4 m. The vertical line
comprises five electrodes numbered from E30 (0.5 m above the
ground) to E34 (2.9 m above the ground) and aligned with E6
and E16 on the northern face of the trunk. Root electrodes E01,
E02 and E03 (Figs. 1 and 2) are implanted in a root running
1
WGS 84 geographic coordinates: 47 560 0500 N and 2 530 4500 W, i.e. Universal Time used in figures is almost the same as the local Solar Time.
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D. Gibert et al. / Plant Science 171 (2006) 572–584
Fig. 2. Plan view showing the canopy area, the trunk and the two roots equipped
with electrodes. Also shown are the two Petiau electrodes, P1 and P2.
Fig. 1. General view (from west) of the array of electrodes shown as black dots
on a simplified sketch of the tree. The box labelled T–H indicates the position of
the outdoor temperature and humidity sensors.
eastward from the trunk, partially emerging from the ground.
The diameter of this root is about 0.18 m, and the electrodes
E01, E02 and E03 are at distances of 0.45, 0.90 and 1.35 m
from the eastern edge of the trunk, respectively. Electrodes E04
and E05 are located at 0.6 and 1.2 m from the north-eastern
basis of the trunk on a shallow root with a diameter of 0.15 m
(Fig. 2). Seven electrodes (E21 installed on May 31, 2003, and
E22 to E27 installed on August 18 and 27, 2004) were added
along a vertical segment whose lower (E21) and upper (E26 and
E27) ends are at 5.6 and 10.5 m above the ground, respectively
(Fig. 1).
The electrodes have a diameter of 6 mm and were cut in
stainless steel rods which were carefully degreased 1 day
before the installation. The apparent metallic part of the
electrodes is cut to 15 mm, and each electrode is connected
by wrapping a cable which is secured with a epoxied cap of
thermo-sheath (Fig. 3). The connectivity of an electrode with
its cable is tested with an ohm-meter. The electrodes are
inserted in the tree by drilling a hole with a diameter of 8 mm
through the bark and another one with a diameter of 6 mm
and a depth of 15 mm in the wood. Next, the electrodes are
gently hammered until being embedded in the wood. In this
way, the non-isolated metallic part of the electrodes is fully
stuck in the wood so that the meteorological influences, such
as rain or fast temperature changes are reduced. The contact
resistance of the electrodes has been measured with an AC
ohm-meter and varies from 4700 to 6400 V with an average
of 5400 V. The electrode array is complemented with a nonpolarisable lead–lead chloride electrode [36] which is
used as a reference for all potential measurements. This
electrode is located 5.0 m away from the tree in the eastward
direction (Figs. 1 and 2) and is buried at a depth of 0.7 m
with clay as contact material. On May 31, 2003, another nonpolarisable electrode labelled E102, was buried using the
same method at a depth of about 0.3 m near the root
electrode E01.
2.2. Data acquisition system
Data acquired at the beginning of the experiment are not
used in the present study, and only those obtained with the
digital acquisition system installed in November 2003 are
presented here. The measurement device is a Keithley 2701
digital multimeter with an input impedance larger than 100 M
V and equipped with a relay matrix having 40 measurement
D. Gibert et al. / Plant Science 171 (2006) 572–584
575
Fig. 3. Top: detailed view of an electrode. The apparent metallic part of the electrode is fully embedded in the wood. The epoxied cap secures the cable which is
wrapped around the stainless steel rod. Bottom left: view of an electrode placed on the tree. The uncovered part of the electrode is fully embedded in the wood under
the bark in order to reduce meteorological influences. Bottom right: in the Granier’s method, the sap flux is obtained by measuring the thermal flux advected by the sap
flow with one heater (H) and two thermo-couples (T).
channels controlled through an acquisition software. Measurements of the electric potential are made at all electrodes with
a sampling interval of 1 min. We use a UT time base,
synchronised in real time to the Frankfurt atomic clock. Both
the computer and the multimeter are powered with a backup
generator which prevents breaks caused by short failures of the
electrical power line.
In order to check the entire electrical system, and in
particular, the high input impedance of the recorder, we
performed several injections of electrical current in the soil
near the basis of the tree. The applied voltage at the grounded
electrodes was 12 V for an electrical current of 2.5 mA.
Fig. 4 shows the electrical potential measured at several
electrodes during two current injections of opposite signs.
The response due to the injections is clearly seen on all
channels. In addition, the electric potential response happens
within one sampling time. This indicates that our measurements are not affected by artificial delay times or capacitive
effects, that could, for example, have resulted from an
insufficient input impedance of the acquisition system. Such
tests are performed regularly.
2.3. Meteorological measurements
In November 2003, a meteorological station was installed to
record the main atmospheric parameters in the neighbourhood
of the tree. The measured parameters include the speed and
direction of the wind, the atmospheric pressure, the outside
temperature and humidity, the rain events. Temperature and
humidity are also measured indoor in order to identify a
possible sensitivity of the electronic devices to the environmental conditions. All meteorological parameters are recorded
every 15 min and synchronised with the time base used for the
potential measurements.
2.4. Influence of temperature on the electrode array
Fig. 4. Electrical potential measured at several electrodes during an injection of
electrical current in the soil near the tree. Hours are given in Universal Time.
Stainless-steel electrodes might be affected by significant
temperature effects, and it is important to discuss purely
thermal effects as the cause of the observed diurnal variations of
the electric potential. In principle, this is possible, as the
temperature sensitivity of the electrode–wood contact may be a
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D. Gibert et al. / Plant Science 171 (2006) 572–584
present in the curve of the outdoor temperature measured in the
vicinity of the tree (Fig. 1). This condition is, however, not due
to a temperature effect at the electrodes, but to a global effect
affecting the whole tree. This was further checked by studying
the correlation between the potential and temperature curves. In
particular, we observed that the correlation between the E6 and
E31 potential curves is maximal for zero-time delay both before
and, significantly, after the installation of the insulating pot on
E31. Conversely, when the pot is installed, the correlation
between the outdoor and E31 temperature curves is maximum
for a time-delay of 45 min. This is another manner to
demonstrate that the delayed correlation between electric
potential and outdoor temperature results from the activity of
the tree.
2.5. Sap flow measurements
Fig. 5. Thermal tests made on electrode E31 to verify that no important
potential variations are induced by temperature. Potential at nearby E6 electrode and outdoor temperature are given for reference. The transient variations
of E31’s potential are observed when introducing the warm (or cold) bottle in
the box covering the electrode. Hours are given in Universal Time.
rapidly varying function of the water content in the wood and
the air humidity. Temperature influence on the tree electrodes
would introduce temporal effects depending on position. For
instance, the amplitude of the potential variations should be
larger on the south side exposed to solar heating. Note that this
is not what we observe actually; however, dedicated thermal
experiments were performed to check for a possible
temperature effect on our electrode array.
As shown by Petiau [36], the temperature sensitivity of the
grounded electrochemical electrode used as as reference is
negligible (smaller than 30 mV= C), and our thermal experiment focused on the steel electrodes. In August 2004, we
applied large artificial thermal perturbations to E31 which was
previously covered with an insulating pot. Fig. 5 shows the
results obtained during both a heating and a cooling test done by
introducing a bottle of hot water or ice in the pot. Excepted for
sharp transient variations of the potential occurring when
introducing the bottle, no perturbation of the E31 potential is
observed when compared with the potential measured on the
nearby E6 electrode. This clearly rules out a direct effect of
temperature on the measured electrode potential.
Looking at Fig. 5 more closely, we observe that potential
variations at both E31 and E6 have features similar to those
During a limited period of time (from June 21 to July 15 of
2004), direct measurements were performed in addition to the
electrode measurements. The sap flow is measured with the
heat-balance technique initially proposed by Granier [9]. This
method uses a pair of probes, each equipped with a small
resistor to produce heat and a miniature thermocouple to
measure the temperature inside the wood. The diameter of a
probe equals 2 mm for a length of 20 mm. Only the upper
heater is turned on and both probes are used to make
differential temperature measurements in order to measure the
thermal imbalance due to sap flow (Fig. 3). One pair of probes
was placed in the root near E01 and two pairs were placed
10 cm apart between E31 and E32 (Fig. 1). The part of the
trunk holding the probes was covered with an insulating
blanket in order to attenuate the thermal perturbations
produced by the atmospheric variable conditions. The
temperature was measured every 5 min and these data were
later converted into sap flow current through an experimentally
derived calibration [9].
3. Presentation of data
3.1. Overview of electric potential variations
Figs. 6–9
present the data for seven electrodes from
December 1, 2003 to May 21, 2005. This choice of seven
electrodes is representative of the whole electrode array: one
root electrode (E01), E6 on the lower ring, E11 and E18 on the
upper ring, E34 located on the vertical line just beneath the
upper ring, E21 and, when available, E25 located on the vertical
line 5.50 and 9.50 m above the floor, respectively. Two
breakdowns of long duration interrupt the curves, the first from
January 12, 2004 to February 17, 2004, the second from
September 4, 2004 to September 15, 2004.
These recordings of long duration show that the electrical
activity is present on all electrodes over the whole time span
although with varying amplitudes and a high variability from
one electrode to another. Large daily variations are present on
most electrodes from spring to summer. A first important
observation is the persistence of the electrical activity during
D. Gibert et al. / Plant Science 171 (2006) 572–584
577
Fig. 6. Top: potential signals measured on six electrodes representative of the entire electrode array for the December 2003–April 2004 period. Relative potential
values. Bottom: outdoor temperature measured near the tree (see Fig. 1 for location). Tick marks fall at midday.
Fig. 7. Same as Fig. 6 for the April–August 2004 period.
Fig. 8. Same as Fig. 6 for the September 2004–January 2005 period.
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D. Gibert et al. / Plant Science 171 (2006) 572–584
Fig. 9. Same as Fig. 6 for the January–May 2005 period.
winter (Figs. 6 and 8) when the tree activity is expected to be
greatly reduced. During this period, a daily variation is
observed on all electrodes, sometimes with a high amplitude as
large as the one seen in spring and summer. This is, for
example, the case for electrode E18 whose diurnal amplitude
remains of the order of 30–50 mV all year long. Note that in
December, the E6 curve appears to be the upper envelope of the
E18 signal. In winter, the signals appear one-sided with daily
positive jumps with respect to a smoothly varying base level. In
spring and summer, this one-sided structure is less visible and
replaced by a more symmetrical pattern. This is particularly
clear when comparing the E18 curve in September and
December 2004 in Fig. 8.
At many points in the tree, long-term and daily electric
potential variations appear anti-correlated with the variations of
the outdoor temperature. This is, for example, the case for the
long-term trends of E01 and E6 in December 2003 (Fig. 6), or
for the long-term trend and the daily variations of E6 March–
April 2004. Note, however, that in March–April 2004, there is
no clear relationship between E01 and the temperature,
indicating that this anti-correlation between electric potential
and temperature is complex and time-dependent. When
considering the diurnal signal of a given electrode on a
long-term basis (Figs. 6–9), larger temperature daily variations
are not systematically associated with larger electrical diurnal
amplitudes as would be expected for a dominant temperature
effect. These observations, together with the thermal tests
discussed above, rule out the hypothesis that the observed
electrical variations are due to a purely thermal artifact at the
electrical contact between steel and wood. Instead, a physical
mechanism indirectly connected to the temperature and
involving the activity of the tree must be invoked, as proposed
by previous authors.
Additional peculiar observations are made when looking at
the period from February to March 2004, a period which is also
shown with an enlarged time scale in Fig. 10. During this
period, all electrodes, except electrode E18, display large
negative peaks, for example, around February 23, March 7 or
March 13 (Fig. 10), while E18 shows a more regular daily
variation. Interestingly, these large negative peaks on the
Fig. 10. Enlargement of a part of Fig. 9 showing the occurrence of negative daily jumps of the electric potential (E6, E34, E11 and E21) corresponding to negative
outdoor temperature. Notice the persistence of the daily positive jumps for electrode E18. Tick marks of horizontal axis fall at midday.
D. Gibert et al. / Plant Science 171 (2006) 572–584
579
Fig. 11. (a) Sap flow measured with two probes located near E32. (b) Outdoor temperature. (c) Electrical potential measured at E32. (d) First time-derivative of the
E32 potential shown in (c). Tick marks of horizontal axis fall at midday.
potential occur precisely when the temperature goes below
zero. The same phenomenon is observed during the winter of
2004–2005 (Fig. 9), especially in the second half of February
2005.
The zoomed signals shown in Fig. 10, reveal the occurrence
of dipolar transient signal with a duration of several hours on
March 3 and 6, 2004. Such signals frequently occur with a
duration varying from several minutes to several hours. The
amplitude of the dipolar signal increases with the altitude of the
considered electrode, a feature observed for all similar signals
we examined, which rules out the possibility that these signals
are caused by instabilities of the reference ground electrode.
The amplitude of these transient signals depends more strongly
on altitude than observed for the step-like signals recorded
during the test of current injection in the soil (Fig. 4). It is then
unlikely to explain these transient signals by invoking telluric
currents, either natural or anthropic, originating from the
ground. Rain or storms, which could have been a first
explanation, must also be excluded using our meteorological
data. The origin of such signals, which appear occasionally,
remains unclear and deserve further study.
A big change occurs in the recorded signal from winter to
spring in both years 2004 and 2005 with the onset of a stable
daily activity at all electrodes. This onset is particularly clear in
April and May of years 2004 (Fig. 7) and 2005 (Fig. 9) with
starting times differing of several weeks from one electrode to
another. For instance, activity begins around March 20, 2005
for E18 and around May 1, for E25 (Fig. 9). Once established,
the daily activity remains high during summer and autumn, and
the return to the low winter activity also occurs at noticeably
different dates depending on the electrode (Fig. 8).
3.2. Comparison with sap flow measurements
We now turn to the comparison between electrical signals
and sap flow measurements. The sap flow curves corresponding
to the probes located near E32 are shown in Fig. 11 a for the
period going from June 21 to July 15, 2004. The two curves
show clear diurnal variations; they have a similar appearance
but their amplitudes differ up to a factor of two on certain days.
The electrical potential (Fig. 11c) measured at E32 and its first
time-derivative (Fig. 11d) also show a diurnal variation with a
variable amplitude which does not appear to be directly related
to the amplitude of the sap flow. The same temporal variation of
the electrical signal was observed during the days preceding the
installation of both the thermal probes and the insulating
blanket. This ensures that the electrical variations are not due to
the heating device.
To look more closely at the correlation between the
electrical signal and the sap flow, a period of 4 days was
selected (Fig. 12). This detailed view allows to examine
precisely the temporal coincidence between the sap flow and
the electrical variations, either in the potential curve or in its
time-derivative. The variations goes from the early time in the
morning, when the sap flow begins to be reactivated, to the
mid-morning when the sap flow reaches its maximum value.
During this period of sap flow reactivation, the timederivative of the potential curve shows a conspicuous bipolar
variation with a high amplitude, beginning with a positive
lobe. The late negative lobe of the bipolar event reaches its
minimum value (black dashed line) precisely when the sap
flow stops increasing and reaches a plateau. At that time, the
potential begins to decrease and becomes minimal slightly
after the sap flow has left its plateau-level and begins to
diminish sharply. The electrical potential then returns to its
original value in about 8 h, while a zero sap flow is reached
faster, after about 4 h. The time when zero sap flow is reached
(red dashed line) or keeps decreasing slowly asymptotically
to zero, also coincides approximately with the maximum of
the time-derivative of the electrical potential. Notice that a
large electrical signal still remains while the sap flow has
completely vanished.
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D. Gibert et al. / Plant Science 171 (2006) 572–584
Fig. 12. Same as in Fig. 11 for a limited period showing the correlations between electrical and sap flow events. Hours are given in Universal Time.
electrodes. Our new observations make this interpretation quite
questionable. Indeed:
4. Discussion
In the rather small number of experiments made on standing
trees, listed in the introduction, the measured electric potentials
were attributed to an electrokinetic effect forced by the sap flow
[15,26,25]. The rough agreement of the value of the
electrokinetic coupling coefficient measured in the laboratory
with the value required to account for the observed magnitude
of the potentials in the standing tree was taken as a strong clue
in favour of this hypothesis, although the sign does not appear
to be the correct one. Furthermore, these previous experiments
were generally performed over short time periods, with few
(1) A negative potential is observed on all the trunk electrodes
with respect to the ground.
(2) In the framework of electrokinetic theory, the electric field
is proportional to the pressure gradient, hence, to the sap
flow per unit surface. This implies, at least in average, for
example, taken over the eight electrodes of a ring, a linear
increase of the electric potential amplitude with height in
the trunk (recall that the reference electrode is located in the
ground). This variation is not observed at all (Fig. 13).
Fig. 13. Amplitude of the electric potential variation from the 5:00 a.m. to 5:00 p.m. period of June 1, 2004.
D. Gibert et al. / Plant Science 171 (2006) 572–584
(3) The heterogeneity of the diurnal amplitudes of the electric
potential from electrode to electrode is difficult to explain in
terms of an electrokinetic effect. Of course, we may expect
lateral heterogeneity in the xylem vessels through which the
sap is pumped up. However, the tree is a conducting
medium, and some homogenisation of the potential at a
given height is expected.
(4) The strongest argument comes from the comparison of the
electric potential curves with the sap flow curves (Figs. 11
and 12). Clearly, the electric potential is not at pace with
the sap flux, assuming that the thermal method measures
the sap flux with negligible retardation. Actually, our
measurements of the sap flow at two different points
provide the same time variation and support this
assumption. In particular, we note that a strong negative
potential is still present after the sap flow has receded,
around 10 p.m. (Fig. 12); it is at this very moment that the
curve of the time-derivative of the potential reaches its
maximum value. Similarly, in the morning, during the 2-h
time interval taken by the sap flux to reach its maximum,
the electric potential hardly changes. These observations
cannot be reconciled with the electrokinetic mechanism in
its simple form.
(5) The electric potentials in a root and in the nearby soil are not
equal (Fig. 14), suggesting the presence of an electrical
barrier between the tree root system and the soil.
Nevertheless, the curves of Figs. 11 and 12 evidence a stable
functional relationship between the sap flow and the electric
potential. To simplify the picture, it appears that the sap flow
polarises (electrifies) negatively the whole tree trunk with
respect to the ground; but this polarisation process has a long
time constant, on the order of hours; when the sap flow
vanishes, in the evening, it is with this long time constant that
the polarisation also goes to zero. Another remarkable fact is
that the rapid onset of the sap flux in the morning generates a
specific transient variation on the electric potential, particularly
outstanding in its time-derivative (Fig. 12).
581
A well known phenomenon in geophysical prospecting,
although still poorly understood, is the so-called induced
polarisation (e.g., [37,38]). Injecting electric current into the
ground through two electrodes generates an electric field in
their neighbourhood; when switching off the current source, the
potential goes back to zero with a time constant which can
sometimes reach several seconds, depending on the nature of
the ground and of the electrodes (e.g., [39,37]). Such induced
polarisation effects can be particularly strong in the presence of
ore deposits, which is the basis of its use as an empirical
prospecting method. This observation reveals the ubiquitous
presence of electromotive forces in the ground after turning off
the external current source. In their absence, the ground would
be back instantaneously equipotential.
However, the time constant involved in the present
experiment is so large that it seems necessary to call for a
mechanism specific to biological living systems. Note that all
the curves corresponding to the different electrodes behave in a
similar fashion with respect to the sap flow curve, while the
amplitudes of this daily variation differ greatly from point to
point. This time constant does not appear specific to a given
neighbourhood but characterises the whole tree.
The mechanism that could be invoked to generate such a
trunk polarisation and time constant remains unclear. As
mentioned in the introduction, the observed electric potential
may reflect a combination of physical, chemical and
physiological responses to the sap flow, photosynthesis and
adaptive feedback controls of the tree. While we are not able to
propose a comprehensive model at this stage, we can sketch the
following scenario.
We have to find a mechanism able to accommodate the
peculiar electrical system observed in the tree trunk. The trunk
is a conductor, but the electrical potential, which depends on the
position vector r and time t, must be able to sustain a temporal
relaxation of the form:
@Vðr; tÞ Vðr; tÞ
þ
¼ F½r; SðtÞ;
@t
t
(1)
Fig. 14. Example of diurnal potential variation measured on a chemical electrode (E102) placed in the soil near the root electrode E01. Note the different vertical
scales used for both potential curves (left scale for E01 and right scale for E102). Major tick marks of horizontal axis fall at midday.
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D. Gibert et al. / Plant Science 171 (2006) 572–584
Fig. 15. (a) Main electrical structure of a sectional cross-section of the tree
trunk. Small longitudinal capacitive elements are composed by insulating
xylem elements embedded in the conductive trunk and containing the circulating and conducting sap (b).
where t is some relaxation time and F is some positiondependent functional of the sap flow SðtÞ, vanishing when
the sap flow vanishes. As t is of the order of several hours,
some poorly conducting medium must be present within the
conductor, able to generate a significant capacitance. In addition, we know that F is a negative function of S, basically
constant with height z, but smoothly varying around the circumference. This imposes a configuration of the source with
translational symmetry along the tree axis, as depicted in
Fig. 15 a. The spatial configuration of the potential results
from leakage of charge accumulated within this supposed nonperfect isolator. Note that a grounded perfect conductor with an
inner distribution of charge is characterised by constant potential and spatially varying surface charge distribution. Here, we
have a non-perfect insulator embedded in a non-perfect conductor, which allows to maintain a small volume current.
As the potential varies with azimuth, we have to conclude
that the charge distribution maintained by the insulator is not
cylindrical. This probably requires a non-cylindrical sap flow as
well. In our tree, given the asymmetric distribution of branches
and roots around the axis of the trunk (Fig. 2), it may not be
unexpected. Note that the two measurements of sap flow also
support a non-cylindrical sap velocity. At each electrode, the
potential is the result of the potential distribution due to sum of
all the individual electrical sources, namely the leakage of all
the supposed elementary capacitors (Fig. 15).
Once this electrical mechanism is proposed, the role of the
sap flow can be sketched as follows. Let us consider a xylem
element (Fig. 15b). The conducting sap pervades the trunk and
makes it a conductor; when photosynthesis and transpiration
are active, it is circulating upwards in xylem elements. The
walls of such elements are made of insulating ligneous cell
walls. In the case of a solid rock surface, elementary charges are
adsorbed, creating an excess charge in the electrolyte. It is the
motion of this excess charge that creates the electrokinetic
effect. In our case, however, a different situation happens,
which completely changes the resulting distribution of currents.
Indeed, imagine that the electric charges are not just adsorbed at
the solid surface, but are now allowed to freely diffuse within
the wall. What now dominates the electrical current is not the
circulation along the axis of the trunk but the radial charge
transfer. When the sap flow is suppressed, the charge carriers
slowly diffuse back to the sap electrolyte or alternatively may
get neutralised within the wall.
This situation is analogous to a heat source in an
underground cavity [40]. The temperature in the cavity during
heating is rising linearly after some time, corresponding to a
steady state heat transfer from the cavity to the surrounding
rock. When the heat source is removed, the temperature is
going back to the initial state, but with a dynamics controlled by
the diffusion of heat in the walls. If this analogy is relevant for
the tree potential, then the relaxation should actually not be
exponential but a power law of time. The data in the tree at this
time do not allow to distinguish between the two different types
of relaxation.
To summarise, we propose that the spatio-temporal
characteristics of electrical potential in the tree trunk might
be explained by a biological mechanism allowing the diffusion
of charge carriers (e.g., heavy protein fragments or charge
carrying hormones) across the insulating gel of the xylem walls.
Maybe this charge transport is controlled actively by dedicated
ionic channels in the wall, followed by molecular diffusion
once across the membrane. Such a mechanism then suggests the
presence of electrically active structures within the xylem
channels. Thus, the xylem may play a more active role in the
electrical response than would have been expected from nonliving cells with pure conduction.
5. Conclusion
This experiment confirms the existence and largely extends
the investigation of specific daily variations of the electric
potential distribution in a tree trunk as mentioned in previous
studies [15,26–28]. These daily variations bear a definite
relation to the simultaneously sap flow in summer. The
D. Gibert et al. / Plant Science 171 (2006) 572–584
observations of electrical variations in winter would then imply
a sporadically varying sap flow, both in space and time, during
this season. These results, deduced from measurements on a
single tree, should be confirmed by performing analogous
studies on several trees in order to get a statistical significance.
While the order of magnitude of the observed variation is the
same as observed by previous workers, an electrokinetic
mechanism seems to be unable to account for the independence
of the potential with respect to altitude. Instead, we propose a
different mechanism based on charge diffusion from the
conductive sap flow channels into the resistive xylem walls.
This mechanism could be clarified by a dedicated experimental
program, for instance, by performing measurements on small
trees in the laboratory or by using ion-selective microelectrodes
or energy dispersive X-ray microanalysis.
Thus, electrical monitoring of a living tree can reveal both
unexpected patterns of the sap flow and new mechanisms of
charge exchange in xylem elements. These results should raise
a renewed interest in electrical measurements in trees.
Experiments with long-term monitoring using a large number
of distributed electrodes are needed to make progress.
Acknowledgments
The authors thank Frédéric Conil for technical assistance and
Pierre Morat for guidance. This work is IPGP contribution no.
2149. We thank M.L. Gicquel for providing us with facilities
during the experiment. This work was financially supported by
the CNRS and ANDRA through the GdR FORPRO and corresponds to the GdR FORPRO contribution number 2006/03 A.
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