Diurnal dynamics of water transport, storage and hydraulic conductivity in pine trees under seasonal drought

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DOI: 10.3832/ifor2046-009
Abstract
The temporal dynamics of water transport and storage in plants have major implications for plant functioning and survival. In trees, stress on the conductive tissue can be moderated by water storage. Yet, trees can survive high percent loss of conductivity (PLC, up to 80%), suggesting efficient recovery. We assess the role of tree water storage and PLC recovery based on simultaneous measurements of leaf transpiration, branch hydraulic conductivity, and stem sap-flow from different seasons in three study years in mature Pinus halepensis (Miller) trees in a semi-arid forest. During the wet season the rates of transpiration (T) and sap flow (SF) peaked at high morning and through the midday. During the dry season T peaked at ~9:00 and then decreased, whereas SF lagged T and fully compensated for it only in the evening, resulting in a midday water deficit of ~5 kg tree⁻¹, and with up to 33% of daily T derived from storage. PLC of 30-40% developed during mid-day and subsequently recovered to near zero within 2-3 hr in the dry season (May, June, and September), but not in the wet season (January). The observed temporal decoupling between leaf water loss and soil water recharge is consistent with optimization of the trees’ water and gas exchange economy, while apparently facilitating their survival in the semi-arid conditions.
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iF o r e s t
F o r e s t
Biogeosciences and Forestry
Biogeosciences and Forestry
Diurnal dynamics of water transport, storage and hydraulic conductivity
in pine trees under seasonal drought
Tamir Klein (1-2),
Shabtai Cohen (2),
Indira Paudel (2-3),
Yakir Preisler (1-3),
Eyal Rotenberg (1),
Dan Yakir (1)
The temporal dynamics of water transport and storage in plants have major
implications for plant functioning and survival. In trees, stress on the conduc-
tive tissue can be moderated by water storage. Yet, trees can survive high per-
cent loss of conductivity (PLC, up to 80%), suggesting efficient recovery. We
assess the role of tree water storage and PLC recovery based on simultaneous
measurements of leaf transpiration, branch hydraulic conductivity, and stem
sap-flow from different seasons in three study years in mature Pinus halepen-
sis (Miller) trees in a semi-arid forest. During the wet season the rates of tran-
spiration (T) and sap flow (SF) peaked at high morning and through the mid-
day. During the dry season T peaked at ~9:00 and then decreased, whereas SF
lagged T and fully compensated for it only in the evening, resulting in a mid-
day water deficit of ~5 kg tree-1, and with up to 33% of daily T derived from
storage. PLC of 30-40% developed during mid-day and subsequently recovered
to near zero within 2-3 hr in the dry season (May, June, and September), but
not in the wet season (January). The observed temporal decoupling between
leaf water loss and soil water recharge is consistent with optimization of the
trees’ water and gas exchange economy, while apparently facilitating their
survival in the semi-arid conditions.
Keywords: Cavitation Reversal, Sap Flow, Semi-arid, Water Deficit, Xylem
Embolism.
Introduction
Pinus halepensis is a major tree species in
the Mediterranean region, where it is rou-
tinely exposed to a long summer drought.
This has led to multiple hydraulic adjust -
ments underlying its drought resistance
(Oliveras et al. 2003, Klein et al. 2011). Vari-
able cavitation levels (percent loss of hy-
draulic conductivity PLC, 0-80%) were
measured in drought-stressed saplings in
the greenhouse and in 20-year old trees in
the field (Klein et al. 2012), depending on
drought intensity and tree provenance. Re-
markably, most individual trees that suffer-
ed from the higher cavitation (PLC = 80%)
still survived, in line with other reports of
conifer survival at water potentials ap-
proaching 88% PLC, and in contrast with
angiosperms for which 50% PLC can be
lethal (Choat et al. 2012, Meinzer & McCul-
loh 2013). Field measurements in P. hale-
pensis under semi-arid conditions (Maseyk
et al. 2008) showed leaf water potential, Ψl
< -2.5 MPa throughout the day during the
entire dry season, approaching the points
of stomatal closure (Ψl = -2.8 MPa) and
PLC50 (Ψl = -3.9 MPa). Carbon uptake con-
tinues throughout the dry season, in spite
of the harsh conditions, mainly to support
fresh needle growth (Maseyk et al. 2008).
This “risky” behavior of P. halepensis was
contrasted by relatively high survival,
growth and productivity in a semi-arid for-
est site (Grünzweig et al. 2007, Maseyk et
al. 2008, Klein et al. 2014b). Together,
these observations suggest that the xylem
of P. halepensis regularly experiences high
cavitation levels during the dry season and
that this species must have some immunity
to the effects of cavitation, allowing its
survival and continuous activity under
drought.
Isohydric-like, low wood density species
such as P. halepensis are expected to have
a relatively high capacity to store water
(Meinzer et al. 2009) and possibly also to
refill cavitated xylem (as shown for angio-
sperm species – Taneda & Sperry 2008).
Some level of water storage is common in
tree stems and is active in the daily water
balance, which results in temporal decou-
pling of sap flow (SF) from transpiration
(T). A short (~0.5 hr) T vs. SF lag is usually
expected due to changes in Ψl, e.g., the
release of water from leaves in early morn-
ing, when Ψl decreases from its pre-dawn
value to an operational value. A T vs. SF lag
was discussed (Cruiziat et al. 2002) but not
frequently tested, presumably due to the
differentiation between studies using gas
(T) and liquid phase (SF) measurement
techniques. Two notable exceptions are
Zweifel et al. (2001) and Fisher et al. (2007),
who showed time lags between T and SF
of 0.5-3 hr. Comparing SF rates at the bot-
tom and high crown of large tropical trees,
Meinzer et al. (2003) found up to a 2 hr lag
time. The role of tree water storage was
shown in many studies (Zweifel et al. 2001,
Meinzer et al. 2003, McLaughlin et al. 2003,
Fisher et al. 2007, Meinzer et al. 2009),
some showing a major storage contribu-
© SISEF http://www.sisef.it/iforest/ e1 iForest (early view): e1-e10
(1) Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot
76100 (Israel); (2) Institute of Soil, Water and Environmental Sciences, ARO Volcani Center,
Beit Dagan 50250 (Israel); (3) Robert H. Smith Institute of Plant Sciences and Genetics in
Agriculture, Faculty of Agricultural, Food and Environmental Quality Sciences, the Hebrew
University of Jerusalem, Rehovot (Israel)
@
@ Tamir Klein (tamirkl@volcani.agri.gov.il)
Received: Mar 08, 2016 - Accepted: Jul 19, 2016
Citation: Klein T, Cohen S, Paudel I, Preisler Y, Rotenberg E, Yakir D (2016). Diurnal dynamics
of water transport, storage and hydraulic conductivity in pine trees under seasonal drought.
iForest (early view). – doi: 10.3832/ifor2046-009 [online 2016-08-21]
Communicated by: Silvano Fares
Research Article
Research Article
doi:
doi: 10.3832/ifor2046-009
10.3832/ifor2046-009
(Early View)
(Early View)
Klein T et al. - iForest (early view)
tion, e.g., 10-75% of daily T in potted, young
Norway spruce (Picea abies Zweifel et al.
2001), or a more minor contribution of 2-
10% in adult yellow poplar (Lirodendron
tulipifera McLaughlin et al. 2003) and
down to zero stem storage in the woody
monocot Bamboo suffering from cavita-
tion (Yang et al. 2012).
Sap flow is usually under large negative
pressures and subsequently trees live
under the threat of cavitation (Tyree &
Sperry 1988). Cavitation breaks the conti-
nuity of water columns and hence the
water supply to transpiring leaves. Xylem
embolism and cavitation reduce hydraulic
conductance and therefore bear major
implications for plant function and survival.
Cavitation can be fatal to trees (Tyree &
Sperry 1988, Brodribb & Cochard 2009,
Klein et al. 2011) and irreversible (Tyree &
Sperry 1988). Among P. halepensis prove-
nances, those with higher sensitivity to
cavitation generally had higher mortality
rates in the field (Klein et al. 2012). Since
water is transported in the xylem under
constant tension (i.e., water potential – Ψ <
0) the possibility of refilling was initially
ruled out. However, empirical evidence for
the reversibility of cavitation became avail-
able from experimental data on induced
and native embolism (Tyree et al. 1999). A
proposed refilling mechanism overcame
the tension enigma by segregation, i.e., the
refilling process is preceded by isolation of
the cavitated section from the water con-
ducting system (Holbrook & Zwieniecki
1999). Cavitation repair is usually allowed
once trees are re-watered (Ogasa et al.
2013), e.g., between hot day and cooler
night (Trifilò et al. 2014).
Cavitation removal was shown in gra-
pevine (Vitis viniferaZufferey et al. 2011)
where synchrotron x-ray microtomography
(microCT) facilitated visualization of the re-
filling process, confirming the segregation
hypothesis and showing water influx from
surrounding living cells (Brodersen et al.
2010). The new microCT technology per-
mits direct, nearly non-invasive embolism
measurement. Yet in mature trees in the
field, hydraulic conductivity measurements
still offer the best way to detect changing
levels of embolism.
The refilling process could result from
osmotic gradients generated by sucrose in
walls of embolized vessels (Secchi & Zwie-
niecki 2011). In the desert woody shrub En-
celia farinosa, embolism repair occurs at
night while stomata are open and transpir-
ing, without the need to isolate the cavi-
tated section (Espino & Schenk 2011). In
Bamboo (Sinarundinaria nitida) nocturnal
root pressure was sufficient to cause re-
covery from embolism in the leaves (Yang
et al. 2012). In conifers, mechanisms of
embolism repair are currently unclear and
might differ from those proposed for an-
giosperms due to differences in xylem
structure and chemical composition, but
embolism repair has been demonstrated in
at least eight conifer species (McCulloh et
al. 2011, Brodersen et al. 2010 and refer-
ences therein), although microCT showed
no refilling in Sequoia sempervirens sap-
lings (Choat et al. 2014). Additionally, some
of the measurements involving the cutting
of xylem under tension might have overes-
timated the levels of embolism and recov-
ery (Wheeler et al. 2013) and hence must
be taken with caution. In P. halepensis
under drought, embolism repair was de-
tected using acoustic emissions, a non-
quantitative method (Borghetti et al. 1991,
1998).
We hypothesize that water storage and
maintenance of hydraulic conductivity
must be integral components of the water
management in P. halepensis to sustain
activity and survival under continuous dry
conditions of the summer. To test this hy-
pothesis we applied a high temporal reso-
lution sampling strategy to study in vivo
the dynamics of hydraulic conductivity in
mature P. halepensis trees in a semi-arid
forest, where extreme seasonal drought is
the norm.
Materials and methods
Site description and environmental
conditions
Our study was conducted in the Yatir for-
est, a 45-yr-old P. halepensis plantation lo-
cated at the northern edge of the Negev
desert, Israel (31° 20 N, 35° 20E). The for-
est covers an area of 2800 ha and lies on a
predominantly light brown Rendzina soil
(79 ± 45.7 cm deep), overlying chalk and
limestone bedrock. The climate is hot (40-
yr average mean annual temperature is 18
°C) and dry (40-yr average mean annual
precipitation is 285 ± 88 mm). Rainfall is
restricted to the wet season between Nov-
ember/December and April/May, increasing
soil water content from 12-15 to 32-33 % v/v
(Fig. 1). Mean daily vapor pressure deficit is
mostly below 1.0 kPa during the wet sea-
son, and well over 2.0 and up to 5.0 kPa
during the long dry season. Stand density is
ca. 300 trees ha-1, leading to an average
leaf area index of about 1.50.
Study trees
In 2000, an instrumented flux tower was
installed in the geographic center of the
forest. Across the forest, trees were plant-
ed around the same time and at homoge-
neous density, and hence the tower stand
can be considered as a representative
stand. In 2009, sap flow probes were in-
stalled on sixteen trees ca. 70 m from the
tower, defined as the observation plot
within the tower stand. A subset of 1-4
trees of these sixteen trees was selected
for all other measurements and samples
taken between 2009 and 2014 (hydraulic
conductivity, needle gas exchange, water
potential, and relative water content).
These four trees had the longest and most
stable sap flow record in the observation
plot, and were arbitrarily selected as typi-
cal representative healthy trees; other
trees in the same stand died following the
drought years of 2008, 2009, and 2011
(Klein et al. 2014b), or showed some level
e2 iForest (early view): e1-e10
Fig. 1 - Site environmental conditions in
2011-2014. Daily precipitation (P); mean
soil water content at 30 cm below sur-
face (SWC); mean vapor pressure deficit
(VPD); and mean air temperature over
the forest canopy (T). Data are missing
due to technical impediments for T and
VPD during June-September 2011 and in
2014; for SWC during June 2011-July
2012; but not for P. Black arrows in the
bottom indicate major field day cam-
paigns.
iForest – Biogeosciences and Forest ry
Hydraulic dynamics in mature pine under drought
of drought stress (Klein et al. 2014a). The
forest inventory performed in 2010 in the
tower stand showed that tree height and
diameter at breast height (DBH) were 10.2
± 0.2 m (mean ± SE) and 19.8 ± 0.4 cm, re-
spectively (n = 177). In the observation plot,
tree height and DBH were 9.5 ± 0.3 m and
18.2 ± 0.9 cm, respectively (n = 16). Among
the subset of trees selected for intensive
measurements, tree height and DBH were
9.0 ± 0.6 m and 21.5 ± 1.3 cm, respectively
(n = 4). Therefore, the four selected trees
had, on average, slightly higher DBH than
their plot neighbors, but not significantly
higher than the stand-level mean. Consid-
ering the relatively low stand density and
LAI in Yatir forest, lower tree crowns are
sunlit and active, facilitating measurements
at 1.3-2.0 m aboveground.
Xylem hydraulic conductivity
Branch sections (~5 mm diameter, ~30 cm
length) were sampled from lower crowns
(1.3-2.0 m) of 1-4 P. halepensis trees every 1-
2 hr during five field days between January
2011 and September 2014. In P. halepensis in
Yatir forest, the outer 4 cm of the sapwood
is conductive (Cohen et al. 2008), which,
according to tree-ring counting, was for-
med within 27 ± 1 years. The 5 mm diameter
branches whose conductivity was mea-
sured were 13 ± 1 years old and hence fully
conductive. All measurements were done
according to the best practices at the time
of measurement, which changed during
the time-frame of this study, and we
adapted accordingly. Therefore, the proto-
col used in 2011-2012 was replaced by an
improved protocol in 2014, following the
validations made by Delzon et al. (2010)
and by Wheeler et al. (2013), which are
detailed below. However, the results from
the different protocols remained consis-
tent.
In the first protocol, specific hydraulic
conductivity (Ks) was measured in the lab,
under low pressure (0.02 MPa) before and
after perfusing the xylem tissue at a high
pressure of 0.5 MPa. The branch section
was cut in the field and resin secretion
from the cuts was eliminated by placing
both ends (5 cm from cut tips) in a water
bath at 95 °C for 10 min (adapted from War-
ing & Silvester 1994). The resin treatment
was essential (Melcher et al. 2012) and also
helped remove trapped gas from branch
ends, which could otherwise create arti-
facts (Wheeler et al. 2013). Experimenting
with branches of well-watered Olea eu-
ropaea showed that the effect of this treat -
ment on Ks was <5%. Next, stem sections
were submerged in distilled water and
transferred to the lab. In the lab, the
upstream end of each branch was then fit-
ted with a rubber gasket, while still sub-
merged, to a latex pipe fed by a degassed
10 mM KCl solution reservoir. A hydrostatic
pressure of 0.02 MPa was applied by plac-
ing the reservoir exactly 2 m above the
stem section. For two hr, water dripping
from the distal end of the branch section
was collected and weighed every 20 min.
The flow rate was steady, producing a lin-
ear increase in the efflux water mass (r2 of
linear fits > 0.9) and indicating no rehydra-
tion of the branch. To test the effect of
hydrostatic pressure on Ks, nine branches
were exposed to hydrostatic pressures of
0.0015-0.0070 MPa. Ks increased from
practically zero to 0.4 kg m-1 MPa-1 s-1 at
pressure larger than 0.003 MPa (Fig. S1 in
Supplementary material). The water flow
rate (kg s-1) was divided by the pressure
gradient (0.02 MPa) along the ~30 cm stem
length to provide the hydraulic conductiv-
ity Kh (kg m MPa-1 s-1), as described by Tyree
& Alexander (1993). Kh was further divided
by the xylem cross-sectional area to get
specific hydraulic conductivity, Ks (kg m-1
MPa-1 s-1). Branch sections were then fitted
with high-pressure valve to a high-pressure
pipe fed by 10 mM KCl solution reservoir
placed inside a Scholander pressure cham-
ber. The segment was perfused at 0.5 MPa
for 5 min and then a second measurement
of Ks was made to get the maximum spe-
cific conductivity, Ks max. Measurements of
Ks and Ks max were further used to identify
loss of conductivity, which can be attrib-
uted to xylem cavitation according to (eqn.
1):
where PLC is the percent loss of conduc-
tivity (%) due to cavitation.
In the second protocol, resin secretion
was stopped by immediate immersion in
ice water. Branch sections were re-cut un-
der degassed water in the lab to a length
of 10 cm, to avoid potential artifacts re-
lated to induced cavitation due to cutting
in the field (Wheeler et al. 2013). Ks was
measured in a similar fashion but using a
lower hydrostatic pressure of 0.007 MPa
by placing a degassed 10 mM KCl solution
reservoir 0.7 m above the stem section.
Branches were not exposed to higher pres-
sure, to avoid sealing of the torus-margo
pit membrane (Delzon et al. 2010). For the
perfusion treatment, degassed water was
drawn through the stem overnight by
putting the stem in a beaker of degassed
water and connecting the other side to a
vacuum line whose negative pressure was
~0.07 MPa. Observation of the water level
in the beaker showed that a substantial
amount of water had perfused the stem
overnight. A second measurement at 0.007
MPa provided the reference conductivity,
Ks max.
Sap flow
Sap flow (SF) was measured continuously
since September 2009 on sixteen trees in
Yatir forest using lab manufactured ther-
mal dissipation sensors (Granier & Loustau
1994) calibrated by comparison with com-
mercial heat balance sensors (EMS, Brno,
Czech Republic) in the same trees. All
probes were installed at 1.3 m above the
ground and 2 cm into the sapwood. Mea-
surements were taken every 30 s and the
30 min average value was saved on a
CR1000® data-logger (Campbell Scientific
Inc., Utah, USA) and transmitted via inter-
net to the lab at the Weizmann Institute of
Science. Sap flow rates (kg hr-1) were calcu-
lated in relation to the minimum sap flux
during the 24 hr period (typically at night),
as shown in the empirical equation (eqn. 2)
of Granier & Loustau (1994), modified by
Kanety et al. (2014):
where SF is the half hourly sap flow rate;
LCF is the correction factor for the 2 cm
probes considering the radial distribution
of sap velocity (for P. halepensis in Yatir -
Cohen et al. 2008) and calculated specifi-
cally for individual trees (Paudel et al. 2013,
Kanety et al. 2014); CF is a calibration fac-
tor of 2.5 (Steppe et al. 2010, Paudel et al.
2013, Kanety et al. 2014), validated here by
comparison with the heat balance sensors;
ΔTr is the average half hourly temperature
difference between heated and non-
heated probes, and ΔTmax the maximum
temperature difference measured during
the 24 hr period (assumed to be at a negli-
gible sap flow rate, typically at night).
Stem diameter variations
Delicate changes in stem diameter (down
to ± 1 µm) were recorded continuously
since September 2009 with high-resolution
band dendrometer (EMS, Brno, Czech
Republic) fitted at 1.9 m around one of the
four P. halepensis trees used for intensive
measurements. To detect diurnal varia-
tions, output data were set to zero at mid-
night of each 24 hr period. Stem diameter
variations make a good proxy for tree wa-
ter relations (Zweifel et al. 2001) because
they reflect changes in the elastic tissues
of the stem (i.e., phloem and bark), which
have hydraulic connections to the xylem.
The negative pressure associated with wa-
ter transport in the xylem withdraws water
from these elastic tissues, thereby decreas-
ing stem diameter. This shrinkage stops as
soon as the tension is relaxed, and stem
diameter increases again.
Leaf gas exchange, water potential and
relative water content
Leaf photosynthesis (A), transpiration (T)
and stomatal conductance (gs) were mea-
sured using a LiCor 6400® photosynthesis
system (Licor Inc., Lincoln, NE, USA) on the
four trees every 30-60 min during five field
days between April 2010 and June 2011.
These were the trees used for hydraulic
conductivity and sap flow measurements.
Lower crown (1.3-2.0 m) cohorts of sunlit,
1-year old needles were marked for re-
peated measurement. Leaf chamber condi-
tions were adjusted to ambient and all gas
exchange parameters were expressed on a
projected needle area basis of 3 cm2 based
on the uniform area of twelve adjacent
iForest (early view): e1-e10 e3
iForest – Biogeosciences and Forest ry
PLC=100
(Ks maxKs)
Ks max
SF =LCFCF0.04284
[
ΔTmax−ΔTr
ΔTr
]
1.231
Klein T et al. - iForest (early view)
needles (Maseyk et al. 2008). Measure-
ments were upscaled to tree-scale transpi-
ration by multiplying by leaf area (Grünz-
weig et al. 2007), since for the low LAI (1.5)
it is fair to assume that all leaves can be
considered as sunlit and equally active.
Leaf water potential (Ψl) was measured
using the pressure chamber technique.
Small (5-7 cm long) twigs were cut from
the same trees used for the gas exchange
measurements at similar height and side
and put in a pressure chamber (Arimad 2®,
A.R.I, Kfar Charuv, Israel) fed by a Nitrogen
gas cylinder and equipped with a lamp-car-
rying magnifying glass. Gas pressure within
the chamber was gradually increased (~1
MPa min-1) until water emerged from the
protruding cut branch surface, and the
pressure value was recorded as leaf water
potential (Ψl). Due to staff and equipment
limitations, Ψl was only measured during a
few field days. Needle relative water con-
tent (nRWC) was determined from three
subsequent measurements: fresh leaf mass
(in situ), water-saturated mass by immer-
sion, and oven-dry mass (in the lab). Con-
secutive weightings verified complete satu-
ration and drying until stable mass was
achieved.
Statistical analysis
Diurnal changes in PLC were subjected to
analysis of variance (ANOVA) using the
JMP® software (Cary, NC, USA). Measure-
ments of individual trees (or individual
branches from one tree in the January and
May 2011 campaigns) were used as obser-
vations and sampling time was defined as
factor. Differences in PLC between sam-
pling times were considered significant
when type 3 sum of squares met the F-test
criterion at probability < 0.05. Means were
compared using the Student’s t test, where
different letters indicated significant differ-
ences between sampling times.
Results
Daily and seasonal dynamics of sap flow
and leaf transpiration
Sap flow rates showed strong interac-
tions between daily and seasonal dynamics
(Fig. 2). Moving from the wet season into
the dry season, the diurnal peak in SF
decreased from 4-5 kg hr-1 in February-April
to less than 1 kg hr-1 in June-October. The
timing of the SF peak tmax(SF) – also
changed, from ~11:00 in April to ~19:00 in
June (Fig. 2, Tab. 1). As a result, while in
April 96% of SF occurred during the day-
time, during September-October as much
as 70% of SF occurred at night (Fig. 2 inset).
The dynamics of leaf scale transpiration did
not reflect the major changes in SF. T was
always restricted to daytime throughout
the year, and its peak, tmax(T), was in the
morning in all seasons (Tab. 1), and moved
from mid-day in the wet season to mid-
morning (~9:00) in the dry season. Conse-
quently, the lag time between tmax(T) and
the less distinct tmax(SF) increased from 30
min in April up to a maximum of 9.5 hr in
July (Tab. 1).
Daily and seasonal dynamics of
hydraulic conductivity
The daily course of PLC, often used as an
indicator of xylem cavitation, showed a
major shift from consistent zero PLC in the
wet season (January) to highly fluctuating,
increasing up to 50% in the dry season (Fig.
3). In the dry season PLC peaked two
times during the day: first in the morning
and later again in the afternoon. In be-
tween, PLC declined to 0-5% and xylem
conductivity was fully restored. Morning
and afternoon PLC peaked in 1-3 hr and
consequently was reversed over a similar
time period. The standard error among
branches of the same tree was 7.4% on
average, and 16.4% among branches of mul-
tiple trees in 2012. Such variations are ex-
pected from a highly dynamic process that
may not occur uniformly throughout the
entire xylem and among multiple trees.
Nevertheless, using the improved protocol
in 2014 the standard errors decreased to
2.2% on average while showing a similar
pattern (i.e., there was still consistency be-
tween the old and newer methodologies,
which add robustness to the results). In
May 2011 and September 2014 differences
in PLC in different times of the day were
significant, e.g., between 10:40 and 11:40 in
May 2011, and between 8:00 and 8:30 in
September 2014. The exact timing of the
PLC highs and lows changed between
observation days, between the beginning
of the dry season (May) and its end (Sep-
e4 iForest (early view): e1-e10
Tab. 1 - Diurnal maxima of leaf transpiration [tmax(T)] and sap flow [tmax(SF)] along the
year in the Yatir forest and the time difference between them [Lag time: tmax(SF)-
tmax(T) (hr)].
Date tmax(T)tmax(SF) Lag time
1-Oct 11:00 19:00 8.0
1-Nov 10:30 16:30 6.0
1-Dec 10:00 12:30 2.5
1-Jan 09:00 14:30 5.5
1-Feb 09:30 13:30 4.0
1-Mar 09:30 14:00 4.5
1-Apr 09:30 10:00 0.5
1-May 09:30 13:00 3.5
1-Jun 09:30 18:30 9.0
1-Jul 09:00 18:30 9.5
1-Aug 09:00 18:00 9.0
1-Sep 09:30 18:00 8.5
Fig. 2 - Diurnal changes in the monthly
average sap flow (SF) rate in Pinus
halepensis in the Yatir forest, with the
fraction of nocturnal sap flow shown in
the inset (horizontal line represents the
annual average, 0.16). Nighttime is indi-
cated by a light or dark background,
depending on the date. Error bars,
shown for April and June for ease of
reading, represent the standard error of
the mean (n = 16 trees × 28-31 days), and
are not observed when smaller than
symbols.
iForest – Biogeosciences and Forest ry
Hydraulic dynamics in mature pine under drought
tember), consistent with previously ob-
served seasonal shift in the peak of the
daily activity cycle at the site (Maseyk et al.
2008). Xylem conductivity partially recov-
ered in the afternoon of May 2011 and 2012,
but not in September 2014. The lower va-
por pressure deficit (VPD) in September
vs. May meant that stomatal gas exchange
was maintained throughout the afternoon
hours in September despite low soil water
availability, in turn inhibiting reversal of
cavitation during the day.
The dynamics of branch hydraulic conduc-
tivity (Ks) and water potential (Ψl) in neigh-
boring trees were further examined. At
predawn of the September 2014 field day
trees had various levels of Ks and a uniform
Ψl of -2.25 MPa (Fig. 4). During the next 5
hours Ks reached its minimum and subse-
quently, maximum values, while Ψl de-
creased monotonously by 1.0 MPa. During
the rest of the day and until sunset, Ks con-
tinued to decline gradually, while Ψl re-
mained around -3.15 MPa, a level associ-
ated with 33% PLC (Fig. S2 in Supplemen-
tary material).
Contemporary changes in tree
hydraulics and gas exchange
More detailed 24-hr gas exchange and hy-
draulic measurements in both the wet and
dry periods were used to help understand
the dynamics of the interactions reported
above. In the wet season, photosyntheti-
cally active radiation (PAR) was between
200 and 900 µmol m-2 s-1 during most of the
light hours (8:15-15:15), and VPD was low
(Fig. 5A-B). Under these favorable condi-
tions, net carbon assimilation (A) reached
17 µmol m-2 s-1 at noontime and stomatal
conductance (gs) was >0.1 mol m-2 s-1
throughout the entire day. Water transport
rates in the xylem (SF) and from the leaf
(transpiration, T) were relatively synchro-
nized (Fig. 5C) and the short lag could be
explained by the effect of changes in Ψl
(see Introduction). Stem diameter de-
creased by 40 µm between 8:15 and 12:15,
iForest (early view): e1-e10 e5
Fig. 3 - Percent loss of conductivity, PLC (a
measure of xylem cavitation) in Pinus halepen-
sis branches sampled in the Yatir forest in 2011,
2012, and 2014 (upper, middle, and bottom pan-
els, respectively). Changes in vapor pressure
deficit (VPD) are indicated by dashed lines.
PLC values are means from three branches of
one tree (2011) or from three-four different
trees (2012, 2014); PLC was determined using
the first protocol in 2011 and 2012 (see Meth-
ods) or the second protocol in 2014. Different
letters indicate significant differences among
sampling times, with P-values indicated at
upper right corners. Error bars represent the
standard error of the mean.
iForest – Biogeosciences and Forest ry
Klein T et al. - iForest (early view)
when SF was increasing, or at its peak of
4.5 kg hr-1 (Fig. 5C-D). No cavitation was
observed at any of the four sampling times
during the day.
Under the regular dry season drought,
net assimilation A was observed in the light
hours but was not synchronized with PAR
(Fig. 5E), probably due to the development
of high VPD around midday (>3 kPa - Fig.
5F; see also Maseyk et al. 2008) The in-
creased VPD resulted in reduced gs from
~0.05 mol m-2 s-1 at 7:45 to below 0.01 mol
m-2 s-1 at 12:15 (Fig. 5F). In the afternoon,
around 16:00, VPD decreased below 3 kPa
and gs increased two folds to ~0.02 mol m-2
s-1, still lower than the morning peak, pre-
sumably due to the low light levels ap-
proaching sunset (when PAR = 50 µmol m-2
s-1 by 17:45). Consistent with this pattern,
the diurnal curve of A also showed a morn-
ing peak (3 µmol m-2 s-1) followed by a mid-
day depression and a smaller afternoon
peak (1 µmol m-2 s-1, Fig. 5E). In contrast to
A, the observed diurnal changes in T
reflected the combined effects of gs and
VPD, where variations in T can be approxi-
mated by variations in gs × VPD. Therefore
T peaked at 0.73 mmol m-2 s-1 at 9:15 and
declined to 0.15 mmol m-2 s-1 at 13:45 (Fig.
5G). Between 14:00 and 18:30 T stabilized
at around 0.25 mmol m-2 s-1, reflecting the
inverse relationship between gs and VPD.
Comparing the dynamics in T and SF (Fig.
5G) indicated that: (1) T had large fluctua-
tions, while changes in SF were more grad-
ual; and (2) T ceased at night whereas SF
continued long after sunset. Stem shrink-
age of 30 µm matched the time-frame be-
tween the start of SF increase (8:15) and
decrease (18:45), reflecting the longer SF
time-window in dry vs. wet season. But
unlike the constant shrinkage rate of 10 µm
hr-1 in January, the dry season shrinkage
rate changed from 3 µm hr-1 (8:15-10:15) to a
moderate 1 µm hr-1 (10:15-15:15) and then to
6 µm hr-1 (15:15-18:45). Stem contraction
during the dry season day did not reach
negative values due to higher relaxation at
night. Fluctuations in xylem conductivity
dynamics were associated with the decou-
pling between T and SF, reflected by rapid
changes in the PLC (Fig. 5H), with two
transient peaks of 30% and 40% at 9:45 and
17:45, respectively. The PLC dynamics on 6
June 2011 were generally similar to those
observed in May 2011 and 2012 (Fig. 3),
albeit conductivity did not fully recover in
the evening. PLC and transient decreases
in needle relative water content (nRWC,
from 77 to 63% - data not shown) occurred
simultaneously with high gs, while the re-
covery in PLC and nRWC was at decreasing
gs, which may indicate that PLC reversal
was dependent on reduced rates of water
loss and increasing nRWC.
Seasonal changes were observed in the T
vs. SF dynamics, between wet and dry sea-
son (Fig. 5C and Fig. 5G, respectively). The
increase in T from 6:00 to 9:00 occurred in
both seasons, but the SF supply of water
for T was substantially delayed in the dry
season. Upscaling leaf-scale transpiration
measurements (mmol m-2 s-1) to tree-scale
transpiration (kg hr-1), using individual tree
leaf area estimates based on our own site-
specific allometric equations (Grünzweig et
al. 2007), showed that large (3-5 kg) water
deficits developed in the tree during the
day, but the trees also re-hydrated daily,
i.e., tree water content did not decline dur-
ing the season. Integrating the dry season
diurnal water use of an average tree in
Yatir forest yields a value of 15 kg day -1. This
implies that during the light hours of a typi-
cal dry season day 20-33% of the transpira-
tion came from water storage.
Discussion
Temporal decoupling between leaf
transpiration and stem sap flow
In the studied semi-arid forest ecosystem,
leaf transpiration and xylem sap flow are
never quantitatively synchronized (Fig. 5C),
and even at the time of peak activity, when
sap fluxes surpassed 5 kg hr-1 (April) there
was still a 30 min lag of SF peak behind
that for T (Tab. 1). Such short lags are ex-
pected due to changes in tree capacitance
or water content as Ψl decreases during
the morning (Fig. 4). The interaction of leaf
conductance with water potential can in-
volve long response times, sometimes pro-
ducing sustained stomatal cycling with an
average period of approximately 70 min
(Dzikiti et al. 2007). In our study, diurnal
peaks of T and SF (the latter peak was not
as distinctive) were 9.5 hr apart, compared
to 0.5-3 hr reported in other studies (Zwei-
fel et al. 2001, Meinzer et al. 2003, Fisher et
al. 2007). These are exceptionally long de-
lays, which might be expected in this case
of extreme drought. The lag time was in-
versely related to the water flux, peaking
at 9.5 hr in the height of the dry season
(July – Tab. 1), when peak sap fluxes never
exceeded 1 kg hr-1. This could be partly
explained by a higher contribution of a few
sinker roots to the tree water use in the
dry season, thereby reducing the overall
flow and increasing the mean water path
length. The inverse relationship noted
above is in agreement with measurements
in young spruce (Zweifel et al. 2001), where
the lag increased from 30 min on high-flux
days to 110 min on low-flux days. Brodersen
et al. (2010) showed in grapevine that de-
lays are related to water deficits, where
cavitated vessels could not refill until addi-
e6 iForest (early view): e1-e10
Fig. 4 - Diurnal changes in xylem specific hydraulic conductivity (Ks) and shoot water
potential (WP) in four neighboring Pinus halepensis trees in the Yatir forest during 9
Sep 2014.
iForest – Biogeosciences and Forest ry
Hydraulic dynamics in mature pine under drought
tional water was introduced. Here we
show that SF was often delayed into the
night (Fig. 2), and consequently up to 70%
of the total diurnal SF occurred after sun-
set. This high nocturnal SF proportion,
which occurs when nighttime leaf transpi-
ration is negligible, is much larger than the
values of up to 35% observed in a California
oak species, Quercus douglasii (Fisher et al.
2007), and up to 25% in other trees.
The calculated water storage capacity,
contributing 20-33% of daily T, is within the
range of 10-75% reported for potted, young
Norway spruce (Zweifel et al. 2001), but
much higher than the 2-10% reported for
yellow poplar (McLaughlin et al. 2003). This
also agrees with the hypothesis that isohy-
dric-like, low wood density species like P.
halepensis should have relatively high ca-
pacity to store water (Meinzer et al. 2009).
This reliance on daily water storage is
equivalent to other forms of water redistri-
bution, such as hydraulic lift and soil water
storage (Oishi et al. 2010).
Dynamics of xylem hydraulic
conductivity
We detected high loss of hydraulic con-
ductivity, which is usually associated with
cavitation (PLC of 40%) in the xylem of P.
halepensis under drought, which was re-
versed on a time scale in the order of sev-
eral hours (Fig. 3). These observations
were consistent across four field days
along three dry seasons, and across the
two measurement protocols, with minor
differences due to the day in the dry sea-
son. Measured PLC levels were also in
agreement with the P. halepensis branch
vulnerability curve, predicting 33% em-
bolism at Ψl = -3.15 MPa (Fig. S2 in the Sup-
plementary material). Measurements in
grapevine stems revealed a diurnal cycle of
cavitation and recovery, with a 3-5-hr refill-
ing time (Brodersen et al. 2010, Zufferey et
al. 2011, respectively). Shorter refilling
times (~2 hr) were reported for poplar (Po-
pulus trichocarpa) stems under induced
embolism in the lab (Secchi & Zwieniecki
2011). However, we noticed that some of
the measurements involving the cutting of
iForest (early view): e1-e10 e7
Fig. 5 - Diurnal changes in
net assimilation (A), pho-
tosynthetically active
radiation (PAR), stomatal
conductance (gs), vapor
pressure deficit (VPD),
leaf transpiration (T), sap
flow (SF), stem diameter
variation, and percent
loss of hydraulic conduc-
tivity (PLC) in Pinus
halepensis during wet
season (A-D) and dry sea-
son (E-H) days in the
Yatir forest. Note differ-
ent scales for the same
parameters in wet and
dry seasons. Dry season
curves were measured
on 6 Jun 2012; wet sea-
son curves on 9 Feb 2011,
except for A, gs, T (mean
curves for Jan-Feb) and
PLC (9 Jan 2011). Error
bars represent the stan-
dard error of the mean
(n = 1-4, depending on
the parameter), and are
not observed when
smaller than symbols.
The dashed vertical lines
in the right-hand panels
denote the times 6:30,
9:30, and 11:30 discussed
below.
iForest – Biogeosciences and Forest ry
Klein T et al. - iForest (early view)
xylem under tension might have overesti-
mated the levels of embolism and recovery
(Wheeler et al. 2013) and hence must be
taken with caution. However, our observa-
tions of two consecutive sub-diurnal cycles
in PLC may be, to the best of our knowl-
edge, the most dynamic cavitation/ recov-
ery cycle reported for trees in the field.
Although embolism repair has been re-
ported in conifers (Borghetti et al. 1998,
McCulloh et al. 2011) and in P. halepensis
specifically (Borghetti et al. 1991), its mech-
anism is yet to be resolved. If ray paren-
chyma cells and non-structural carbohy-
drates (NSCs) are involved, as suggested
by Brodersen & McElrone (2013), P. hale-
pensis has both components at notable lev-
els (Esteban et al. 2010, Klein et al. 2014a,
respectively). Live parenchyma cells are
also expected in the young branches stud-
ied here (13 ± 1 years, see Methods). The
kinetics of xylem refilling depends on the
volume of the embolized lumen, and hence
on species-specific traits like conduit diam-
eter (Brodersen et al. 2010). With its nar-
row tracheids (9.4 µm Oliveras et al.
2003), P. halepensis makes a good candi-
date for rapid refilling. Frequent cavitation
and recovery cycles can induce “cavitation
fatigue”, decreasing the ability of the xy-
lem to recover (Hacke et al. 2001). How-
ever, Taneda & Sperry (2008) reported
repeated overnight xylem refilling through-
out most of the summer in an oak species,
similar to our observations.
Interestingly the cavitation dynamics did
not always correlate with the level of wa-
ter potential (Ψl) in the studied branches
(Fig. 4), although in situ measurements
were in line with the vulnerability curve
(Fig. S2 in the Supplementary material).
Cavitation occurred at decreasing Ψl but its
reversal was not followed by increased Ψl
as might have been expected. Hölttä et al.
(2009) proposed that Ψl might increase
during cavitation due to a capacitive effect.
It is possible that such an effect, together
with a 0.5-1.5 hr delay, was masked by cavi-
tation reversal.
Stomata, VPD, leaf water potential and
cavitation
For a first approximation, we validate our
observations of cavitation and recovery
cycles semi-quantitatively based on the
basic flux equations for approximate tran-
spiration (T) and sap flow (SF eqn. 3,
eqn. 4):
where K is the overall plant hydraulic con-
ductivity. Under wet season conditions Ψl =
-1.5 MPa (Tab. 2), T greatly exceeds the
tree capacitance, and T and SF are continu-
ous and nearly temporally coupled (T ~ SF
at the same point in timeFig. 5C). In the
dry season, the soil water potential (Ψs)
decreases from the wet season value of
-0.1 MPa to -0.9 MPa (Tab. 2 – Klein et al.
2014b), thereby decreasing ΔΨsoil-leaf from
1.4 in the wet season to 0.6 MPa in the
early morning of a dry season day and,
using eqn. 4, decreasing the driving force
for SF. Together with the observed de-
crease in K for the same time-frame, from
3.0 to 0.5 kg hr-1 MPa-1, SF decreases from
4.5 kg hr-1 in wet season to 0.1 kg hr-1 in the
early morning of a dry season day and up
to 1.3 kg hr-1 at 11:30 (Tab. 2).
Conditions change rapidly during the dry
season mornings, as in our measurement
campaign. VPD increases from 0.4 kPa at
6:30 to 1.9 kPa at 9:30 (Tab. 2). This seems
to trigger two parallel processes with con-
trasting consequences: (1) Ψl decreases
from -2.3 to -3.3 MPa (Fig. 4), meaning the
development of xylem embolism; and (2) T
increases (eqn. 3) from 0.13 mmol m-2 s-1 at
6:30 to 0.58 mmol m-2 s-1 at 9:30. This in-
crease reflects the small change in gs (0.03
mol m-2 s-1 – Tab. 2) while VPD increased.
Evidently, while process (1) decreases the
tree hydraulic conductance, process (2) in-
creases the tree hydraulic demand. This, in
turn, seems to trigger the next sequence
of events: the cavitation of part of the xy-
lem (30% – Tab. 2) decreases overall K from
0.50 to 0.35 kg hr-1 MPa-1 (Tab. 2). This fur-
ther decouples SF from T, increases the
water deficit (~3 kg tree-1 by 9:30) and
decreases nRWC (from 77% to 63% in our
case). Only then, stomatal closure follows,
reducing gs from 0.030 mol m-2 s-1 at 9:30 to
0.007 mol m-2 s-1 at 11:30 (Tab. 2). This over-
compensates for the VPD increase (from
1.9 to 3.4 kPa), and hence T decreases from
0.58 mmol m-2 s-1 at 9:30 to 0.25 mmol m-2 s-1
at 11:30 (eqn. 3) and nRWC increases. The
parallel decrease in T, increase in SF, and
recovery in hydraulic conductance by 11:30
permit the closure of the hydraulic deficit
that developed earlier.
Oscillations in gs in water-stressed woody
plants have been long related to the feed-
back loop between T and gs spanning over
2-4 hr in orange trees (Cohen & Cohen
1983). High VPD may be one of the signals
that lead to stomatal closure (Oren et al.
1999), thereby facilitating recovery from
cavitation. The decrease in gs in mid-morn-
ing and its subsequent increase in the
afternoon seem to correspond to similar
trends in VPD and inverse trends in water
deficit. The time course of change in gs, in
turn, may influence that of the cavitation/
recovery cycles. For example, in May maxi-
mum daily VPD in the Yatir forest is usually
below 3.0 kPa (Maseyk et al. 2008) and
hence the midday depression in gs is re-
stricted to 10:30-13:30, compared to 10:00-
16:00 in June (Fig. 5F). This can explain the
shorter cavitation/recovery cycles observ-
ed in May (Fig. 3) than in June. The lower
vapor pressure deficit (VPD) in September
vs. May meant that stomatal gas exchange
was maintained throughout the afternoon
hours, which can explain the lack of hy-
draulic recovery during the day in our Sep-
tember field day (Fig. 3 and Fig. 4). Causal-
ity is of course difficult to infer from these
patterns. While some studies claim that
stomatal activity protects the xylem (Crui-
ziat et al. 2002, Yang et al. 2012), others
propose that the xylem imposes a limit on
gs and water loss (Brodribb & Cochard
2009, Zufferey et al. 2011). Nardini & Salleo
(2000) concluded that in very dry condi-
tions some cavitation could not be
avoided, because it acts as a signal regulat -
ing gs. This is consistent with the observa-
tion that stomatal responses both at 9:15
and 17:15 (Fig. 5F) coincided with a PLC
(cavitation level) of ~30% (Fig. 3) and indi-
cates a possible threshold for stomatal re-
sponse, with the value for initiation of clo-
sure corresponding to a water deficit of
~2.5 kg tree-1. Based on these observations,
we speculate that while there are complex
feedbacks between gs and PLC, actual
changes in leaf conductance do not corre-
late with threshold or cumulative changes
in PLC, such that moderate levels of cavita-
tion and recovery of xylem in our
extremely dry conditions can be routine
during daytime, thus permitting or maxi-
mizing leaf gas exchange.
e8 iForest (early view): e1-e10
iForest – Biogeosciences and Forest ry
T=gsVPD
SF =KΔ Ψsoilleaf
Tab. 2 - Summary of changes in vapor pressure deficit (VPD), leaf water potential (Ψl),
soil water potential (Ψs), stomatal conductance (gs), hydraulic conductivity (K), per-
cent loss of conductivity (PLC), leaf transpiration (T), and sap flow (SF) in Pinus
halepensis in the Yatir forest. Observed T and SF rates are compared with values cal-
culated using eqn. 3 and eqn. 4, respectively.
Values Parameters Wet season Dry season
11:30 6:30 9:30 11:30
Observed VPD (kPa) 1.5 0.4 1.9 3.4
Ψl (MPa) -1.5 -1.5 -3.0 -3.0
Ψs (MPa) -0.1 -0.9 -0.9 -0.9
gs (mol m-2 s-1) 0.18 0.030 0.030 0.007
K (kg hr-1 MPa-1) 3.00 0.50 0.35 0.55
PLC (%) 0 7 30 0
T (mmol m-2 s-1) 3.15 0.13 0.58 0.25
SF (kg hr-1) 4.50 0.10 0.73 1.30
Calculated T = gs × VPD (mmol m-2 s-1) 2.70 0.12 0.57 0.24
SF = K × ΔΨs-l (kg hr-1) 4.20 0.30 0.73 1.16
Hydraulic dynamics in mature pine under drought
Interactions between hydraulic and
physiological adjustments
The pine trees in the semi-arid study site
have relatively high productivity and a
range of adjustments to the dry conditions.
Furthermore, these trees maintain photo-
synthetic activity throughout the long
annual dry season, sufficiently to support
the growth of new needles during this
period (Maseyk et al. 2008). At the same
time xylem resistance to embolism is not
high in P. halepensisPLC50 > -4.0 MPa) simi-
larly to other pine species (Brodribb et al.
2014).
Based on the results reported above we
speculate that the observed temporal de-
coupling between water loss from the
leaves and water recharge from the soil
(Fig. 5G) helps to optimize the trees’ water
and gas exchange economy, facilitating
their survival in the semi-arid conditions.
Such a system operates with a relatively
narrow safety margin, at the cost of signifi-
cant loss in hydraulic conductivity, relying,
in turn, on rapid recovery (Fig. 3). The re-
sults seem to indicate that high vulnerabil-
ity to cavitation does not necessarily re-
flect low levels of resistance to drought,
but may be part of the overall optimization
between reduced water availability and the
requirement for maintaining gas exchange
under dry conditions. Equally important,
the redistribution of water at both soil and
tree levels is an essential component.
Further research is needed to focus on
the different aspects of the cavitation/re-
covery cycle process. In addition, better
tools are needed to measure cavitation in
the field, facilitating more frequent sam-
pling, which could give insight into the dif-
ferent environmental and physiological fac-
tors involved in forest sustainability under
increasingly dry conditions in the Mediter-
ranean and other regions.
List of Abbreviations
A: net carbon assimilation;
gs: stomatal conductance;
K: hydraulic conductivity;
nRWC: needle relative water content;
PAR: photosynthetic active radiation;
PLC: percent loss of conductivity;
SF: sap flow;
T: transpiration;
VPD: vapor pressure deficit.
Acknowledgments
Measurements were performed jointly by
all authors. TK conceived the study and
wrote the first draft of the manuscript,
with all authors contributing to the writing.
TK acknowledges the Karshon foundation
grant provided through KKL-JNF and the
Rieger foundation grant. TK thanks Dr. Jiri
Kucera of EMS Ltd., Czech Republic, and
Prof. Katarina Strelcova of Zvolen Technical
University, Slovakia, for training with the
sap flow measurement system; and Tal
Kanety of ARO Volcani, Israel, for assis-
tance in sap flow sensor manufacturing.
We thank Prof. Harvey Scher of the Weiz-
mann Institute of Science for helpful dis-
cussions.
Funding
This work was supported by KKL-JNF and
the Israel ministry of Agriculture; the Suss-
man Center and the Cathy Wills and Robert
Lewis Program in Environmental Science of
the Weizmann Institute; the Israel ministry
of Science (France-Israel High Council for
Research Scientific and Technological Coo-
peration, project 3-6735); the Minerva
Foundation, The Israeli Science Foundation
(ISF), and the COST FORMAN (Forest Man-
agement and the Water Cycle, FP0601) pro-
gram.
The authors declare no conflict of inter-
est.
Data Accessibility
All data are included in the manuscript
and supporting information.
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Supplementary Material
Fig. S1 - The effect of hydrostatic pressure
on specific hydraulic conductivity (Ks) of
Pinus halepensis branches.
Fig. S2 - Xylem vulnerability curve for Pinus
halepensis branches: percent loss of hy-
draulic conductivity (PLC) as function of
branch water potential (WP).
Link: Klein_2046@suppl001.pdf
e10 iForest (early view): e1-e10
iForest – Biogeosciences and Forest ry
iForest_ifor2046-009_Klein.pdf
1.08 MB
  • ... Nocturnal sap flow has been traditionally regarded as insignificant (or zero) [17,18], which has led to the underestimation of total sap flow. Nocturnal sap flow has now been documented on various plant species and in different environments [17][18][19][20][21][22]. It is known that vapor pressure deficit, soil moisture, air temperature, and wind speed can be important drivers of nocturnal sap flow [18,20], but more regional and species-specific studies are needed to determine its cause and evolutionary significance [17]. ...
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  • ... High levels of embolism are assumed to impair water supply to the foliage and ultimately lead to tissue desiccation. While some studies suggest that xylem embolism may play an important role during exceptional and devastating drought events ( Anderegg et al., 2016), it is still discussed whether xylem embolism is a common phe- nomenon in mature trees under nonlethal drought events (Cochard & Delzon, 2013;Klein et al., 2016). The depletion of nonstructural carbohydrate pools has been suggested to result from the extended closure of stomata during drought leading to reduced photosynthesis and eventually a shortage of carbohydrate metabolites in different tree tissues (Hartmann, 2015). ...
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    1.Temperate forests are predicted to experience an increased frequency and intensity of climate change‐induced summer droughts and heat waves in the near future. Yet, while previous studies clearly showed a high drought sensitivity of different temperate tree species, the vulnerability of the physiological integrity of these trees remains unclear. 2.Here, we assessed the sensitivity of six temperate tree species to severe water limitation during three consecutive growing seasons including the exceptional 2015 central European summer drought and heat wave. Specifically, we assessed stem increment growth, sap flow, water potentials, hydraulic vulnerability, and non‐structural carbohydrate contents in leaves and branches to determine how mature temperate trees responded to this exceptional weather event and how the observed responses relate to variation in xylem embolism and carbohydrate economy. 3.We found that the trees’ pre‐dawn water potentials reached their minimum values during the 2015 summer drought and most species reduced their sap flow by up to 80%. Also, increment growth was strongly impaired with the onset of the drought in all species. Despite the strong responses in the trees’ growth and water relations, all species exhibited minimum midday shoot water potentials well away from values associated with severe embolism (P50). In addition, we detected no distinct decrease in non‐structural carbohydrate contents in leaves, bark and stems throughout the drought event. 4.Synthesis: This study shows that mature individuals of six common central European forest tree species strongly reacted to a severe summer drought by reducing their water consumption and stopping growth. We found, however, no indications for xylem embolism or carbohydrate depletion in these trees. This suggests, that xylem embolism formation and carbohydrate reserve depletion are not routine in temperate trees during seasonal strong drought and reveals a low vulnerability of the physiological integrity of temperate trees during drought events as we describe here. This article is protected by copyright. All rights reserved.
  • ... The circum-Mediterranean conifer Aleppo pine (Pinus halepensis Mill.) is the most widely distributed tree species in the Mediterranean basin. It shows an extensive ecological breadth and is seemingly adapted to a broad range of abiotic stressors and perturbations, espe- cially fire and drought (Ne'eman et al., 2004;Schiller and Atzmon, 2009;Klein et al., 2011Klein et al., , 2016. P. halepensis is also extensively used in afforestation programs in the region. ...
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    Thermal dissipation probes (TDPs) were calibrated in three diffuse porous fruit trees and one ornamental species in the field by comparison with heat pulse probes (nectarine and persimmon), in a greenhouse on lysimeters (apple and persimmon) and in the laboratory by pushing water through cut branches (apple, Peltophorum and nectarine). Two operational methods were used: continuous (constant thermal dissipation, CTD) and discontinuous, or transient, heating (transient thermal dissipation, TTD). Correction for the radial distribution of sap flux density was with an analytical function derived from a linear decrease in flux density with depth, as measured with a multi-depth 'Tmax' heat pulse system. When analyzed with previous calibration factors, the measured sap flow was <50% of actual value. The underestimations were consistent, and calibrations for each species in the field, greenhouse and laboratory gave approximately the same factors. Reasonable values of tree water use were obtained with the new calibration factors. Evidence is provided that even though the xylem was diffuse porous, the underestimations were caused by contact of the probes with inactive xylem along their length. The average portion of probe in contact with inactive xylem, measured in stained branches following laboratory calibrations, was 0.2-0.24. Using the measured fractions to correct temperature differentials between heated and unheated probes for CTD and TTD, based on Clearwater et al. (in Potential errors in measurement of nonuniform sap flow using heat dissipation probes. Tree Physiol 1999;19:681-687) almost completely compensated for the underestimations. Calibrations are given for each species both before and after corrections of temperature differentials, along with a multispecies calibration. These results should be an important step in reconciling many reports of different calibration factors for TDP probes.
  • Article
    1. Xylem hydraulic properties play an essential role in supporting growth and photosynthesis and influence sensitivity to environmental conditions such as drought and freezing. Consequently, stem hydraulic conductance can be used as a comparative measure of overall hydraulic adaptation across species and to assess the impact of environmental variation, especially drought, on water transport. 2. We summarize the main methods currently in use for measurements of stem xylem hydraulic properties. Measurements can be accomplished in a number of ways, including using a pipette, an analytic balance or a ‘pressure-drop’ flow meter. We provide new details on the design of a relatively inexpensive and easily field-deployable flow meter that is flexible for a variety of applications. The biological challenges associated with these measurements arise from the difficulties of working with diverse living tissues of variable geometry. 3. We provide a review of best practices and provide technical guidance, emphasizing measurements on detached samples using portable equipment.
  • Article
    A 3-year irrigation trial provided basic information on the response of persimmon (Diospyros kaki cv. Triumph) water use and development to irrigation levels. Constant experimental factors applied to recommended “baseline” crop factors resulted in ratios of irrigation (I) to FAO56 reference crop evapotranspiration (ET0) ranging from 0.35 to 1.14. Vegetative and reproductive growth, sap flow, stem water potential (SWP), and local climate were monitored. An overall increase in yield and vegetative growth in response to irrigation was found, which suggests a potential yield increase for higher irrigation levels (40 tons/ha for annual irrigation of 1,000 mm). At high irrigation, the yield response curve levelled off and the marginal contribution of additional water declined. The up to threefold increase in number of fruits with irrigation, with no influence on natural abscission, suggests that differences in fruit quantities stem from response to irrigation at the earlier growth stages. Mean fruit size and fruit quality, as indicated by the ratio of rejected fruit, increased with irrigation up to I/ET0 of ~0.8. Relative yield increased linearly with relative transpiration. However, post-harvest quality was not influenced. SWP, sap flow, and non-transpirable water fractions indicated that the seasonal irrigation tables were not well tuned. Initial adjustments were made during the final season of the experiment and a new table was developed based on our results. The new table should be a basis for further trials.
  • Article
    We investigated the common assumption that severing stems and petioles under water preserves the hydraulic continuity in the xylem conduits opened by the cut when the xylem is under tension. In red maple and white ash, higher PLC in the afternoon occurred when the measurement segment was excised under water at native xylem tensions, but not when xylem tensions were relaxed prior to sample excision. Bench drying vulnerability curves in which measurement samples were excised at native versus relaxed tensions showed a dramatic effect of cutting under tension in red maple, a moderate effect in sugar maple, and no effect in paper birch. We also found that air injection of cut branches (red and sugar maple) at pressures of 0.1 and 1.0 MPa resulted in PLC greater than predicted from vulnerability curves for samples cut 2 min after de-pressurization, with PLC returning to expected levels for samples cut after 75 min. These results suggest that sampling methods can generate PLC patterns indicative of repair under tension by inducing a degree of embolism that is itself a function of xylem tensions or supersaturation of dissolved gases (air injection) at the moment of sample excision. Implications for assessing vulnerability to cavitation and levels of embolism under field conditions are discussed.