Running head: Mortality sequence of mature trees under drought
Abstract
Drought-related tree mortality is increasing globally, but the sequence
of events leading to it remains poorly understood. To identify such
sequence, we used a 2016 tree mortality event in the semi-arid pine
forest of Yatir were dendrometry and sap flow measurements were carried
out in 31 trees, of which seven died. A comparative analysis revealed
three stages leading to mortality. First, a decrease in tree diameter in
all dying trees, but not in the living ones, eight months ‘prior to the
visual signs of mortality’ (PVSM; e.g., brown needles). Second, a decay
to near zero in the diurnal stem swelling/shrinkage dynamics, reflecting
the loss of stem radial water flow in the dying trees, six months PVSM.
Third, cessation of stem sap flow three months PVSM. Eventual mortality
could therefore be detected long before visual signs are observed, and
the three stages identified here demonstrated the differential effects
of drought on stem growth, water storage capabilities, and soil water
uptake. The results indicated that breakdown of radial stem water flow
and phloem functionality is a critical element in defining the ’point of
no return’ in the sequence of events leading to mortality of mature
trees.
Keywords: Dendrometer, Drought, Dying, Early-signs, Phloem
functioning, Radial water flow, Re-watering, Stem, Xylem.
*Corresponding author: ypreisler@fas.harvard.edu
Introduction
Global climatic trends in previous decades have shown a rise in
temperatures and drought intensity, leading to the increased
vulnerability of forests and tree mortality events in various biomes
(Breshears et al. , 2009; Allen et al. , 2010, 2015;
Preisler et al. , 2019). Tree mortality and canopy dieback can be
traced directly to water shortages, as well as to secondary stress
factors, such as insect damage (McDowell, 2011). Widespread forest
mortality has consequences for global carbon, water, and energy fluxes
(McDowell et al. , 2008; Adams et al. , 2012; Bastinet al. , 2017; Pugh et al. , 2019). Despite the significant
impacts of trees on the global biogeochemical cycles, the series of
physiological responses to drought before, during, and after a tree
mortality event are unknown—inflating uncertainty in the future of our
forests. Thus, understanding, quantifying, and detecting the tree
mortality sequences, from an ecophysiological perspective, is urgently
required.
Two physiological mechanisms leading to tree mortality are hydraulic
failure (embolism) and carbon starvation, which have been investigated
in many previous studies over the past decade (McDowell et al. ,
2008; Sala et al. , 2010; Choat et al. , 2012; Hartmannet al. , 2013; Sevanto et al. , 2014; Hartmann, 2015; Meiret al. , 2015; Adams et al. , 2017; Cailleret et al. ,
2017; Kono et al. , 2019). Identifying early warning indicators to
determine when trees cross the ’point of no return’, beyond which
mortality is inevitable, has been a major focus of recent studies, which
highlight plant water content, leaf wilting, and loss of hydraulic
conductivity as key parameters (Anderegg et al. , 2012; Hartmannet al. , 2018; Hammond et al. , 2019; Sapes et al. ,
2019). Experimental manipulations have largely been conducted on young
trees (e.g., seedlings, saplings) in controlled conditions measuring
physiological responses such as percent loss of conductivity (PLC),
water potential (Ψ), and nonstructural carbohydrates (NSC), while
large-scale observations (e.g., remote sensing) have focused on foliar
color, GPP, etc. (Ogaya et al. , 2015; Schwantes et al. ,
2016; Paz-Kagan et al. , 2018). There are only few studies on
mature trees – mainly due to logistic constraints; yet such studies are
likely to be the most important for understanding/predicting forest
mortality events in relation to future climate. Physiological indicators
and thresholds of vulnerability to drought may be similar or differ
quite considerably among species. Yet, the progression of the risk of
mortality is likely related to the coordination of many plant traits,
rather than a single trait (Hammond & Adams, 2019; Blackman et al.,
2019). Furthermore, the resistance in different species to drought
stress is also related to their anatomy and physiology. While some
species are expected to have minimal recovery after crossing their Ψ
associated with 50 % loss of xylem conductivity (P50) (Brodribb &
Cochard, 2009), many conifers have been observed to survive 90 % loss
of conductivity (Tyree & Sperry, 1988; McCulloh & Woodruff, 2012;
Klein et al. , 2016a; Hammond et al. , 2019). The
progression of tree mortality risk can be related to different traits,
such as stomatal control (Delzon & Cochard, 2014; Martin‐StPaulet al. , 2017), the use of internal water storage (Steppe et
al. , 2015; Colangelo et al. , 2017), cuticular water loss
(Cochard, 2019), NSC availability and mobility, and phloem transport
(Sala et al. , 2012; Klein et al. , 2014a; Sevanto et
al. , 2014; Hoch, 2015; Savage et al. , 2016; Wiley et al. ,
2017).
Recovery from embolism occurs during the radial growth of new sapwood
(conductive xylem) that replaces the old embolized xylem (Hammondet al. , 2019). Growing new conductive areas requires both
available carbon and a well-functioning phloem (De Swaef et al. ,
2015; Steppe et al. , 2015). This link between the xylem hydraulic
state and the functioning of the phloem is also expressed in the daily
stem diameter changes caused by radial water transport from the living
bark and phloem tissues to the transpiration stream, following the water
potential gradient between these neighboring tissues (Zweifel et
al. , 2001; Daudet et al. , 2005; Hölttä et al. , 2006,
2009; De Schepper & Steppe, 2010; Sevanto et al. , 2011, 2014;
Steppe et al. , 2015). Diameter variations are driven by
differences in xylem water potentials and phloem osmotic concentrations
(Hölttä et al. , 2009; Sevanto et al. , 2014). Diurnal
variations in stem diameters resulting from radial water flow between
xylem and phloem have been used for physiological assessments and
modeling efforts (Sevanto et al. , 2011; De Swaef et al. ,
2015). Diurnal fluctuations in diameter originate mainly from the
shrinkage and swelling of the living phloem and bark tissues (70 – 90
% of the total diurnal fluctuations), with the remaining contribution
(30 – 10 %) from the xylem. Some studies show a higher proportion of
the daily fluctuations are from the xylem (> 30 % of the
total) (Sevanto et al. , 2002; Zweifel et al. , 2014), but
in most cases, the phloem and bark are the major contributors
(> 50 %) (see e.g. Cermak and Nadezhdina, 1998; Irvine and
Grace, 1997; Scholz et al., 2008).
This hydraulic link between xylem and phloem supports phloem transport
under well-watered conditions but limits it under dry conditions when
xylem tensions increase (Hölttä et al. , 2009). Phloem tissues
also act as a water reservoir for transpiration, buffering high
transpiration demands during the dry season, thus promoting tree
survival (Zweifel & Häsler, 2000; Pfautsch et al. , 2015a) and
growth (Hölttä et al. , 2010; Coussement et al. , 2018).
Direct damage to the phloem via insects or pathogenic attacks will
reduce the phloem’s ability to transport and allocate sugars (Schultzet al. , 2013), potentially resulting in growth declines or
increased risk of tree death probably as a result of alterations of
phloem transport as a response to drought (Salmon et al. , 2019).
Nonetheless, the direct effects of phloem damage to the xylem (in
particular to the sap flow), are still unclear (Savage et al. ,
2016; Salmon et al. , 2019).
While knowledge of the physiological mechanisms of tree mortality and
their sequences have been investigated in controlled experimental
conditions, it is unknown how mature trees in forests may differ in
their responses. The objectives of this study were: 1) to
identify the ’point of no return’ in mature trees exposed to a severe
drought (or water limitation); and 2) to improve our
understanding of the traits involved in the tree mortality sequence.
These objectives were addressed by using high-resolution physiological
measurements of both xylem and phloem hydraulic status. For these
purposes, we monitored stem diameter changes using band dendrometers
(Sevanto et al. , 2014; Zweifel, 2016) and water uptake with sap
flow sensors.
The radial growth and sap flow of the 50-year-old Aleppo pine
(Pinus halepensis ) trees growing in the world’s driest pine
forest (P/PET=0.17) in Yatir (a FLUXNET site), have been measured
continuously since 2009 (Klein et al., 2016; Tatarinov et al., 2016). In
the spring of 2016, mortality was observed among the sampled trees, with
visual indicators of tree death (brown and dry needles) beginning in
April 2016. Here, we describe the physiological events leading to tree
mortality in the mature Aleppo pine trees grown in the semi-arid forest
and demonstrate the link between phloem and xylem water transport under
drought conditions. The natural conditions of this study site, which
constitutes long and harsh dry period and short and concentrated rainy
period, together with the ongoing measurements enabled us to investigate
the potential processes that define the ’point of no return’ in the
mortality sequence of mature trees.
Methods
Experimental site
The study was conducted in the Yatir forest, located near the northern
edge of the Negev desert (31˚20’N, 35O03’E, 550–700 m
above sea level). This forest, planted mainly in the mid-1960s, is
dominated by Aleppo pine (Pinus halepensis Mill.) and covers an
area of about 3000 ha. Current stand density is ~300
trees ha-1, with a deep and unreachable ground-water
table (>-300 m). The soil type is classified as light
Rendzina (Haploxeroll) above chalk and limestone. The climate is
Mediterranean with prolonged summer drought periods from May to October
(average daily temperature in July is 25 ˚C) and a winter period with
low levels of precipitation (annual mean is 280 ± 90 mm) and moderate
temperatures (~10 ˚C in January), (Grünzweig et
al. , 2007; Rotenberg & Yakir, 2010).
Tree physiology
Tree sap flow (SF) was continuously measured since 2012 in 31 trees, by
home-made heat dissipation sensors (Granier, 1985) with site and species
specific adjustments (Appendix 1, Cohen et al. , 2008; Kleinet al. , 2014b). Variations in stem diameter at breast height
(DBH; 1.3 m height)) were measured by automatic band dendrometers (EMS,
Brno, Czech Republic) in five trees since 2012, and in 20 trees starting
in August 2015.
Intensive 16 to 24-h leaf-scale diurnal measurements were conducted in
13 trees equipped with dendrometers and sap flow sensors in six
campaigns from May 2014 to September 2015, 5-7 trees measured in each
campaign (Table S1). Measurements were taken every 1.5-2 hours,
including twig water potential (Ψleaf) using a pressure
chamber (PMS Instrument Company, Albany, OR, USA), needle water content,
and leaf gas exchange (LI-6400, Licor Inc., Lincoln, NE, USA). In total,
12-18 shoots per tree were sampled within each campaign (for detailed
methods, see Supplementary information methods).
Mortality event
In the spring of 2016, a mortality event occurred in the study plot
where the sap flow and dendrometry measurements were carried out. Visual
browning (Figure S1) of the needles was observed in seven trees,
starting in April 2016. Bark beetle (Orthotomicus erosus )
presence was observed in five out of the seven dead trees in June-July
2016, with typical galleries and exit holes (Figure S1). Some beetle
attacks (mature feeding of top canopy needles) were also observed in the
living trees. All seven trees which died during the mortality event had
SF sensors, and five had dendrometers. The last diurnal physiological
measurements, in August 2015 and September 2015, included eight trees,
of which three eventually died in Apr-May 2016 (18 branches were pruned
during each campaign, per tree).
Statistical analysis
Measured values from the physiological campaigns were averaged across
the sampled trees and sampling hours. For each season, differences
between the living and dying trees at each sampling hour (06:00 to
17:00) were analyzed using a paired t -test. Piecewise regression
analysis was applied to the half-hourly dendrometer data to determine
the branching point at which the DBH dynamics of the dying and living
trees started to diverge. A paired t -test was then used on the
two data sets (dying and living trees) for the periods before and after
the identified tipping point. Statistical analyses were performed in
SigmaPlot12 (Systat Software, Erkrath, Germany) and R (R Core Team
(2017) Version 1.1.447 Vienna,
Austria.)
Results
The hydrological years relevant to this study (2013/14, 2014/15, and
2015/16 (October to September in each year) were characterized by above
average precipitation, 323, 357, and 349 mm, respectively, and 2016/17
was relatively dry, with 246 mm of total annual precipitation, compared
with the long-term mean value of 285 mm (data not shown;
http://gaia.agraria.unitus.it/home/site-details?id=IL-Yat).
Lack of stem growth as an indication of
mortality
The mean annual stem diameter growth (2012-2016) of the Aleppo pine
trees in the study site was 2.0 (± 1.4) mm (n=5 and 20, from 2012-2015
and 2015-2016 respectively). Stem growth started after the first rain
event (around December) and ceased in April. From April to the next
rainy season, trees stem diameters were either relatively stable or
shrank up to 0.9 (± 0.5) mm (Figure 1), depending on the precipitation
regime of the preceding rainy season.
No significant differences were found between the seasonal stem diameter
trends of the living and dying trees prior to September 2015 (p
> 0.05). For trees which eventually died, the
characteristic phenological trends in stem diameter changed in 2015
(Figure 2). The stem diameter of the dying trees started to decline at
the end of the growing season (May 2015), similar to the living trees,
but then diverged from the living trees from September 2015 to the end
of the record in June 2016 (p < 0.001). Stem diameter declined
in dying trees by 2.2 (± 0.003) mm, but grew by 2.5 (± 1.4) mm in living
trees between September 2015 and June 2016. Notably, in contrast to the
living trees, the dying trees did not show any positive response
(increase) in their diameter to the onset of the rainy season in
December 2015 or to any subsequent rain events.
Cessation of xylem water transport as an early indication of mortality
Seasonal patterns of sap flow were characterized by maximum values (34.5
± 3.1 L tree-1 day-1) in the second
half of the rainy season between February and April and minimum values
at the end of the dry season around October - November (7.8 ± 0.6 L
tree-1 day-1) (Figure 2b; Figure
s2). The SF in the living and dying trees co-varied until December 2015
when it started to differ. After the first big rain event in January
2016, the SF increased dramatically in the living trees (midday maximum
of 2.0 L/hr), whereas in the dying trees, the SF showed only a minor
recovery of 0.2 L tree-1 hr-1(Figure 2b). From February 2016 SF, in the dying trees essentially
stopped, with the sensors showing unreliable values (maximum values at
midnight, indicating dry wood and thermal artifacts of the sensors).
Evidence of xylem desiccation in the dying trees is supported by
increasing temperature differences between the heated and reference
probes of the SF sensors, expressed in the voltage readings
(dT, mV; Figure 3) from February 2016 to June 2016. When the wood is
dry, the ability of transferring the heat generated by the heating probe
decreases, resulting in lower heat conductivity and higher dT in the
dying trees. Therefore, in addition to the changes in the SF trend
itself, dT depends on the xylem thermal conductivity, which, in turn,
depends on the wood water content. The increasing trend in dT of the
dying trees during the period when SF showed a sharp recovery
(January-April 2016) in the living trees, strongly indicated a drying
trend in the xylem of the dying trees.
Reduction of daily radial stem water flows as an early
indication of mortality
Diurnal dynamics of stem diameter provides insights into the radial
water fluxes between the phloem and bark to the xylem. The mean
(2012-2017) diurnal dynamics of the stem diameter in the rainy season
were characterized by a morning maximum at around 08:00 and a minimum at
the end of the day at around 17:00 hours (Figure S3). The mean diurnal
trend of the live and dying trees was similar until November 2015, when
it started to differ significantly both in amplitude and trend (Figure
4; p<0.05).
Another way to present the diurnal fluctuation in stem diameter is the
maximum daily shrinkage (MDS; calculated as the difference between
diurnal maximum and minimum of the DBH). The MDS shows one value per day
and it highlights long term changes along the year. The amplitude of the
MDS did not differ significantly between the living and dying trees
prior to November 2015 (p>0.05; Figure 5). However, after
accumulated precipitation of 37 mm in November 2015, the MDS began to
diverge significantly between the living and dying trees (p <
0.001). Namely MDS in the living trees started to increase, indicating
high diurnal fluctuations implying high radial water flow, whereas in
the dying trees it remained approximately constant (Figure 5). Notably,
the seasonal amplitude of the MDS signal did not clearly correlate with
annual precipitation, indicating that other controlling factors are
involved, such as rain distribution (see Fig. 5 bottom).
Gas exchange and hydraulic traits prior to
mortality
Overall, no consistent physiological differences were found between the
living and dying trees during the diurnal campaigns that took place
between May 2014 and August 2015 (Table 1), prior to the mortality
event. Leaf transpiration and Ψleaf were higher in the
live trees than in the dying trees in May 2014 (P < 0.005 and
< 0.001, respectively; two years prior to mortality), but
these differences were not consistent over time. In the diurnal campaign
of August 2015 (7 months prior to tree decline), daytime needle WC was
higher in living than in dying trees (p = 0.006), but note that the
sampling day in August 2015 occurred during an extremely hot and dry
period, with VPD >7 kPa, and that this difference between
tree types was not observed in the measurement campaign one month later.
Discussion
In this study, on-going comparative measurements over several seasons in
a mature pine forest stand provided the opportunity to reconstruct the
development of a drought-induced mortality event when its visible signs
appeared. The dying trees in this study had dry brown needles in
April-May 2016, but as shown recently (Hammond et al. , 2019;
Blackman et al. , 2019), needle color is a poor indicator of when
a tree dies. It probably occurs after a tree is already past the ‘point
of no return’ leading to its mortality. Identifying the evolution of the
mortality process, especially in mature trees, will improve our
understanding of key traits to forest resilience and, potentially,
management steps to minimize its impact. Obtaining information on the
mortality process under natural field conditions is invaluable in
complimenting studies of small trees or saplings in controlled
environments (e.g., Garcia-Forner et al., 2016; Grossiord et al., 2017;
Hammond et al., 2019).
The cascade of tree mortality
process
Based on our results we identified three discrete early signs of tree
mortality, which could be detectable several months before the visual
signs of mortality were observed (Figure 6). The first “early warning”
indicator was a decrease in stem diameter, which started in September
2015, 8-9 months prior to any visual signs of mortality. Decreases in
stem net diameter can result from a combination of cessation of growth,
tissue destruction, and tissue shrinkage due to water loss, all of which
can occur during prolonged drought (Fernandez & Cuevas, 2010; Devine &
Harrington, 2011; Urrutia-Jalabert et al. , 2015). In the Yatir
forest site, net stem shrinkage and apparent negative growth are not
unusual, usually occurring towards the end of the dry season
(September-November). The start of the enhanced stem shrinkage of the
dying trees, compared with the living ones, occurred in that period,
showing similar initial dry-season trend (in May) and before the clear
partitioning between the groups of trees was apparent (in September).
The net stem shrinkage was associated with a significant decrease in
diurnal stem diameter fluctuations in the dying trees, which appeared to
be the second step identified in the mortality sequence. Here too,
because the diurnal amplitude is rather small during the dry season
(<0.1 mm) it did not clearly differ from the live trees before
November 2015, when the first rain events triggered the more pronounced
daily diameter amplitudes in the living trees. Fluctuations in stem
diameter can be a combination of several processes including part of the
permanent irreversible stem growth, swelling and shrinking of live
tissues, water potential changes in xylem conduits (tension changes),
and wood daily thermal expansion (Zweifel, 2016; Zweifel et al. ,
2016). The diurnal fluctuations are mainly a result of live tissue
radial water flow that support the transpiration stream in the morning
hours and rehydrate during the night (Sevanto et al. , 2002;
Daudet et al. , 2005; Sevanto et al. , 2011). Therefore, the
striking complete stop of the diurnal fluctuations (Fig. 4,5) strongly
indicates impediment to the radial flux capacity in the dying trees (see
also below).
The crucial role of radial water flow in the ’point of no
return’
process
Radial water flow, from living tissues (mainly bark and phloem) to the
xylem and then to the transpiration stream is a well-known process
(Zweifel et al. , 2001; Sevanto et al. , 2011; Pfautschet al. , 2015b) that helps to buffer the high evaporative demand
in dry conditions, and reduces the associated drop in xylem Ψ values,
which may lead to hydraulic failure (Hölttä et al. , 2009; Sevantoet al. , 2014). Therefore, the ability to draw on stored water
within the stem to support the transpiration stream contributes to the
likelihood that a tree will survive prolonged drought. In contrast,
plants that are unable to exploit this trait might cross their ’point of
no return’ toward mortality faster (Lamacque et al. , 2020).
Based on our findings we suggest that interruption of the radial flow,
as reflected in the MDS decline (Figure 4 & 5), is irreversible. The
trees could not recover from this effect even with re-establishing
sufficient water supply (above average annual precipitation), and the
large MDS response in the living trees. Furthermore, we suggest also
that the results indicate that the capacity for radial water flow is a
critical trait for tree recovery from drought, by mainly supporting the
recovering water transport in the xylem and, sustaining its hydraulic
status, and possibly supporting the production of new xylem (Nardiniet al. , 2017; Hammond et al. , 2019). It is important to
stress that at the end of the dry season, under severe drought
conditions, diurnal fluctuations in stem diameter mainly reflect radial
water flow as the capacity for growth is strongly limited.
Note that the differences in the MDS between the dying and living trees
became visible in the data only in November 2015, two months after the
net diameter started to diverge, but this could be because the natural
increase in the live trees MDS was delayed until the first rain events.
But it seems clear that the ability to recover from drought stress is
also linked on the ability to transfer water radially, as also shown
recently in Lavender plants (Lamacque et al. , 2020).
Why did sap flow continue after diurnal fluctuations
stopped?
The results demonstrated differential development of the disruption of
radial and the sap fluxes. The ability to have active sap flow, two
months after the MDS of the dying trees started to diverge from the live
trees, is probably due to the renewed water supply from the soil, when
the volumetric SWC in the root zone was >25 % between
November 2015 and February 2016 (data not shown). The final or total
hydraulic failure (i.e, SF cessation) occurred 2.5 months after the
radial water flow cessation was identified. Although embolism was not
quantified directly, the gradually drying of the wood (Figure 3) may
indicate that air continuously accrued in the stems during these two
months when xylem transitioned from at least partially fluid-filled,
non-flowing state, to an air-filled, embolized state. The wood drying
expressed in the dying trees’ dT values may be a good indication for
embolism and is supported by the direct link between the increase in dT
and the decrease in xylem water content shown in previous studies
(Tatarinov et al. , 2005; Vergeynst et al. , 2014; Hölttäet al. , 2015). Our results are consistent with the findings of
hydraulic failure accumulation during lethal drought events (Tyree &
Sperry, 1989; Borghetti et al. , 1998; Choat et al. , 2012;
Anderegg et al. , 2012; Sperry & Love, 2015), and indications of
the redundancy of the xylem (and the production of new xylem) reducing
the risk of hydraulic failure due to embolism (Hammond et al. ,
2019; Körner, 2019). During the dry season, in a monthly means
resolution shown in figure 2, the low SF rates makes the differences
between the living and the dying trees hardly detectable. Yet, no clear
proof for separation between the living to the dying trees was found
until the end of January 2015 when SF values in dying trees became
unreliable data (figures 2b & 3).
Possible non-drought contributions to mortality
No clear evidence or early signs for the coming mortality was observed
in the leaf-and branch scale physiological measurements in the two years
prior to the visual mortality. This goes in line with the idea that the
dying process starts from the stem radial water transport and not from
the leaves. The significant pruning that took place during these
physiological measurements, for measurement purposes, may have led to
some damage to the trees defense system (e.g., resin production) and
exposed the trees to biotic damage. However, no link between the branch
sampling and mortality could be established, as both living and dying
trees were sampled to the same extent throughout the study. The higher
transpiration and Ψ values in May 2014, in the dying trees (Table 1)
were probably not an indication of the early signs of mortality, since
it was measured two years before mortality, and because such differences
were not observed subsequently.
The clear MDS and diameter decline discussed above, might also reflect
damage to the bark and phloem tissues by pathogens. Indeed, the drought
period was followed by a bark beetle attack at the site. However, we
note that the time of the onset of the bark beetle attack was not known
and it could be a consequence of the stress effects and weakening of the
tree (Dobbertin et al. , 2007; Preisler et al. , 2019),
rather than its cause. Furthermore, our qualitative assessment of the
bark beetle damage indicated that its effects were similar in the living
and dying trees and therefore cannot explain the specific mortality
sequence. Interestingly, the bark beetle attacks were also noted in two
additional trees in the stand in 2017. One of these trees died, while
the other one survived showing similar DBH responses in the living vs.
the dying trees (i.e.- decrease in DBH during the wet season of the
dying tree) , as shown above (data not shown). It seems likely,
therefore, that the bark beetle damage was not part of the
drought-induced mortality development discussed above but could have
been the last phase in the described mortality sequence.
Seeking the ’point of no return’ main
trait/s
There is aggregating evidence from recent studies that emphasizes the
fact that hydraulic stress leading to runaway cavitation and loss of
conductivity is not the single cause of tree death (Sevanto et
al. , 2014; Blackman et al. , 2019). Our results support the view
that the coordination of several traits leads trees to an irrecoverable
state, and that this suites of interacting traits must be taken into
consideration when considering the sequences leading up to tree
mortality, including stem drying and shrinkage, embolism, and cessation
of phloem radial flow.
Furthermore, our results indirectly suggest that the capability of trees
to transfer water radially, e.g., from the bark and phloem to the xylem,
may be critical for resilience to drought. The exact ’point of no
return’ in the steps leading to final mortality (Fig. 6) cannot be
categorically determined in this study since this natural sequence of
mortality was not linked to a recovery study, as would be initiated by
experimental watering earlier than the following rainy season. However,
we suggest the cessation of daily stem fluctuations as a likely key
trait in the ’point of no return’ definition. Radial water transport may
also be linked to damage to the phloem transport and can lead to carbon
limitations, and damage to the roots, and further contribute to critical
steps leading to mortality (Mencuccini et al. , 2015). The evident
effects of the direct damage to the phloem can also harm the phloem
transport coupled to the hydraulic failure. This is supporting the idea
that the mortality process is an integrative process linking hydraulic,
carbon limitations, and tree’s ability to develop new xylem (e.g.,
McDowell et al., 2008; Améglio et al. , 2002; Spicer & Groover,
2010; Hammond et al. , 2019).
The results discussed above seem to indicate that ultimate mortality
could be implied when water uptake by the tree ceased. This, in turn,
could be brought about in two ways: 1) sap flow stopped due to embolism
damage as a consequence of air infiltration via the de-hydrated bark and
phloem tissues; or/and 2) phloem transport failure led to damage to
roots reducing water uptake and inducing xylem embolism. These two
scenarios are likely to coexist, and it maybe inconsequential to rank
their contribution. Eventually, the cascade of events identified in this
study led to the lack of response to the renewed water supply in the
following rainy season, and complete failure of the xylem water
transport identified by February 2016. This was nearly four months prior
to visual mortality.
The sequence of steps leading to tree mortality (Fig. 6), and the
definition of tree mortality itself has been investigated over at least
the last decade (McDowell et al. , 2008; Sala et al. , 2010;
Sevanto et al. , 2014; Garcia-Forner et al. , 2016b; Adamset al. , 2017) and yet, our understanding is still limited. This
case study using a naturally occurring mortality events and in-situ
measurements may help advance our understanding of the the steps, and in
particular the role of radial water movement and phloem transport,
leading to mortality of mature trees. Such knowledge can help identify
the key traits to seek in our management attempts to reduce the impact
of forest mortality. It should also motivate research to identify the
factors that contribute to the extreme spatial heterogeneity in tree
mortality, even within the same uniform stand, such as due to
small-scale variations in the sub-surface conditions to which individual
trees are exposed (Preisler et al., 2019).
Acknowledgments
The work was financially supported by KKL (project number:
90‐10‐012‐11), and the United States-Israel binational agriculture
research and development fund (BARD) grant number- FI-584-2019. We want
to thank AS, BH, NMH and members of our lab for valuable discussions and
comments to the manuscript.
Reference
Adams HD, Luce CH, Breshears DD, Allen CD, Weiler M, Hale VC,
Smith AMS, Huxman TE . 2012 . Ecohydrological consequences of
drought‐and infestation‐triggered tree die‐off: insights and hypotheses.Ecohydrology 5 : 145–159.
Adams HD, Zeppel MJB, Anderegg WRL, Hartmann H, Landhäusser SM,
Tissue DT, Huxman TE, Hudson PJ, Franz TE, Allen CD, et al.2017 . A multi-species synthesis of physiological mechanisms in
drought-induced tree mortality. Nature Ecology and Evolution1 : 1285–1291.
Allen CD, Breshears DD, McDowell NG . 2015 . On
underestimation of global vulnerability to tree mortality and forest
die-off from hotter drought in the Anthropocene. Ecosphere6 : 1–55.
Allen CD, Macalady AK, Chenchouni H, Bachelet D, McDowell N,
Vennetier M, Kitzberger T, Rigling A, Breshears DD, Hogg EH (Ted),et al. 2010 . A global overview of drought and
heat-induced tree mortality reveals emerging climate change risks for
forests. Forest Ecology and Management 259 : 660–684.
Améglio T, Bodet C, Lacointe A, Cochard H . 2002 .
Winter embolism, mechanisms of xylem hydraulic conductivity recovery and
springtime growth patterns in walnut and peach trees. Tree
Physiology 22 : 1211–1220.
Anderegg WRL, Berry J a, Field CB . 2012 . Linking
definitions, mechanisms, and modeling of drought-induced tree death.Trends in plant science 17 : 693–700.
Bastin J-F, Berrahmouni N, Grainger A, Maniatis D, Mollicone D,
Moore R, Patriarca C, Picard N, Sparrow B, Abraham EM . 2017 .
The extent of forest in dryland biomes. Science 356 :
635–638.
Blackman CJ, Creek D, Maier C, Aspinwall MJ, Drake JE, Pfautsch
S, O’Grady A, Delzon S, Medlyn BE, Tissue DT, et al.2019 . Drought response strategies and hydraulic traits
contribute to mechanistic understanding of plant dry-down to hydraulic
failure. Tree Physiology 39 : 910–924.
Borghetti M, Cinnirella S, Magnani F, Basilicata Á, Vegetale P .1998 . Impact of long-term drought on xylem embolism and growth
in Pinus halepensis Mill . : 187–195.
Breshears DD, Myers OB, Meyer CW, Barnes FJ, Zou CB, Allen CD,
McDowell NG, Pockman WT . 2009 . Tree die-off in response to
global change-type drought: mortality insights from a decade of plant
water potential measurements. Frontiers in Ecology and the
Environment 7 : 185–189.
Brodribb TJ, Cochard H . 2009 . Hydraulic Failure
Defines the Recovery and Point of Death in Water-Stressed Conifers.Plant Physiology 149 : 575–584.
Cailleret M, Jansen S, Robert EMR, Desoto L, Aakala T, Antos JA,
Beikircher B, Bigler C, Bugmann H, Caccianiga M, et al.2017 . A synthesis of radial growth patterns preceding tree
mortality. Global Change Biology 23 : 1675–1690.
Cermak J, Nadezhdina N . 1998 . Sapwood as the scaling
parameter-defining according to xylem water content or radial pattern of
sap flow? In: Annales des Sciences forestieres. EDP Sciences, 509–521.
Choat B, Jansen S, Brodribb TJ, Cochard H, Delzon S, Bhaskar R,
Bucci SJ, Feild TS, Gleason SM, Hacke UG, et al. 2012 .
Global convergence in the vulnerability of forests to drought.Nature 491 : 752–755.
Cochard H . 2019 . A new mechanism for tree mortality
due to drought and heatwaves.
Cohen Y, Cohen S, Cantuarias-Aviles T, Schiller G .2008 . Variations in the radial gradient of sap velocity in
trunks of forest and fruit trees. Plant and Soil 305 :
49–59.
Colangelo M, Camarero JJ, Borghetti M, Gazol A, Gentilesca T,
Ripullone F . 2017 . Size Matters a Lot: Drought-Affected
Italian Oaks Are Smaller and Show Lower Growth Prior to Tree Death.Frontiers in Plant Science 8 : 1–14.
Coussement JR, De Swaef T, Lootens P, Roldán-Ruiz I, Steppe K .2018 . Introducing turgor-driven growth dynamics into
functional–structural plant models. Annals of botany121 : 849–861.
Daudet FA, Améglio T, Cochard H, Archilla O, Lacointe A .2005 . Experimental analysis of the role of water and carbon in
tree stem diameter variations. Journal of Experimental Botany56 : 135–144.
Delzon S, Cochard H . 2014 . Recent advances in tree
hydraulics highlight the ecological significance of the hydraulic safety
margin. New Phytologist 203 : 355–358.
Devine WD, Harrington CA . 2011 . Factors affecting
diurnal stem contraction in young Douglas-fir. Agricultural and
Forest Meteorology 151 : 414–419.
Dobbertin M, Wermelinger B, Bigler C, Bürgi M, Carron M, Forster
B, Gimmi U, Rigling A . 2007 . Linking increasing drought stress
to Scots pine mortality and bark beetle infestations.TheScientificWorldJournal 7 Suppl 1 : 231–9.
Fernandez JE, Cuevas M V . 2010 . Irrigation scheduling
from stem diameter variations : A review. Agricultural and Forest
Meteorology 150 : 135–151.
Garcia-Forner N, Adams HD, Sevanto S, Collins AD, Dickman LT,
Hudson PJ, Zeppel MJB, Jenkins MW, Powers H, Martínez-Vilalta J,et al. 2016a . Responses of two semiarid conifer tree
species to reduced precipitation and warming reveal new perspectives for
stomatal regulation. Plant Cell and Environment 39 :
38–49.
Garcia-Forner N, Sala A, Biel C, Save R, Martínez-Vilalta J .2016b . Individual traits as determinants of time to death under
extreme drought in Pinus sylvestris L. Tree Physiology36 : 1196–1209.
Grossiord C, Sevanto S, Adams HD, Collins AD, Dickman LT,
McBranch N, Michaletz ST, Stockton EA, Vigil M, McDowell NG .2017 . Precipitation, not air temperature, drives functional
responses of trees in semi-arid ecosystems. Journal of Ecology105 : 163–175.
Grünzweig JM, Gelfand I, Fried Y, Yakir D . 2007 .
Biogeochemical factors contributing to enhanced carbon storage following
afforestation of a semi-arid shrubland. Biogeosciences4 : 891–904.
Hammond WM, Adams HD . 2019 . Dying on time: traits
influencing the dynamics of tree mortality risk from drought. Tree
Physiology 39 : 906–909.
Hammond WM, Yu K, Wilson LA, Will RE, Anderegg WRL, Adams HD .2019 . Dead or dying? Quantifying the point of no return from
hydraulic failure in drought-induced tree mortality. New
Phytologist : 0–3.
Hartmann H . 2015 . Carbon starvation during
drought-induced tree mortality–are we chasing a myth?
Hartmann H, Moura CF, Anderegg WRL, Ruehr NK, Salmon Y, Allen
CD, Arndt SK, Breshears DD, Davi H, Galbraith D . 2018 .
Research frontiers for improving our understanding of drought‐induced
tree and forest mortality. New Phytologist 218 : 15–28.
Hartmann H, Ziegler W, Trumbore S . 2013 . Lethal
drought leads to reduction in nonstructural carbohydrates in Norway
spruce tree roots but not in the canopy. Functional Ecology27 : 413–427.
Hoch G . 2015 . Carbon reserves as indicators for carbon
limitation in trees. In: Progress in botany. Springer, 321–346.
Hölttä T, Linkosalo T, Riikonen A, Sevanto S, Nikinmaa E .2015 . An analysis of Granier sap flow method, its sensitivity
to heat storage and a new approach to improve its time dynamics.Agricultural and Forest Meteorology 211 –212 :
2–12.
Hölttä T, Mäkinen H, Nöjd P, Mäkelä A, Nikinmaa E .2010 . A physiological model of softwood cambial growth.Tree Physiology 30 : 1235–1252.
Hölttä T, Mencuccini M, Nikinmaa E . 2009 . Linking
phloem function to structure: Analysis with a coupled xylem-phloem
transport model. Journal of Theoretical Biology .
Hölttä T, Vesala T, Sevanto S, Perämäki M, Nikinmaa E .2006 . Modeling xylem and phloem water flows in trees according
to cohesion theory and Münch hypothesis. Trees 20 :
67–78.
Irvine J, Grace J . 1997 . Continuous measurements of
water tensions in the xylem of trees based on the elastic properties of
wood. Planta 202 : 455–461.
Klein T, Cohen S, Paudel I, Preisler Y, Rotenberg E, Yakir D .2016a . Diurnal dynamics of water transport, storage and
hydraulic conductivity in pine trees under seasonal drought.IForest 9 .
Klein T, Cohen S, Paudel I, Preisler Y, Rotenberg E, Yakir D .2016b . Diurnal dynamics of water transport, storage and
hydraulic conductivity in pine trees under seasonal drought.IForest 9 : 710–719.
Klein T, Hoch G, Yakir D, Körner C . 2014a . Drought
stress, growth and nonstructural carbohydrate dynamics of pine trees in
a semi-arid forest. Tree physiology 34 : 981–992.
Klein T, Rotenberg E, Cohen-Hilaleh E, Raz-Yaseef N, Tatarinov
F, Preisler Y, Ogée J, Cohen S, Yakir D . 2014b . Quantifying
transpirable soil water and its relations to tree water use dynamics in
a water-limited pine forest. Ecohydrology 7 : 409–419.
Kono Y, Ishida A, Saiki S-T, Yoshimura K, Dannoura M, Yazaki K,
Kimura F, Yoshimura J, Aikawa S . 2019 . Initial hydraulic
failure followed by late-stage carbon starvation leads to
drought-induced death in the tree Trema orientalis. Communications
biology 2 : 8.
Körner C . 2019 . No need for pipes when the well is
dry—a comment on hydraulic failure in trees. Tree Physiology39 : 695–700.
Lamacque L, Charrier G, dos Santos Farnese F, Lemaire B, Ameglio
T, Herbette S . 2020 . Drought-induced mortality: branch
diameter variation reveals a point of no recovery in lavender species.Plant Physiology : pp.00165.2020.
Martin‐StPaul N, Delzon S, Cochard H . 2017 . Plant
resistance to drought depends on timely stomatal closure. Ecology
letters 20 : 1437–1447.
McCulloh KA, Woodruff DR . 2012 . Linking stomatal
sensitivity and whole-tree hydraulic architecture. Tree
physiology 32 : 369–372.
McDowell NG . 2011 . Mechanisms linking drought,
hydraulics, carbon metabolism, and vegetation mortality. Plant
physiology 155 : 1051–9.
McDowell N, Pockman WT, Allen CD, Breshears DD, Cobb N, Kolb T,
Plaut J, Sperry J, West A, Williams DG, et al. 2008 .
Mechanisms of plant survival and mortality during drought: why do some
plants survive while others succumb to drought? The New
phytologist 178 : 719–39.
Meir P, Mencuccini M, Dewar RC . 2015 . Drought‐related
tree mortality: addressing the gaps in understanding and prediction.New Phytologist 207 : 28–33.
Mencuccini M, Minunno F, Salmon Y, Martínez‐Vilalta J, Hölttä
T . 2015 . Coordination of physiological traits involved in
drought‐induced mortality of woody plants. New Phytologist208 : 396–409.
Nardini A, Savi T, Trifilò P, Gullo MA Lo . 2017 .
Drought stress and the recovery from xylem embolism in woody plants. In:
Progress in Botany Vol. 79. Springer, 197–231.
Ogaya R, Barbeta A, Başnou C, Peñuelas J . 2015 .
Satellite data as indicators of tree biomass growth and forest dieback
in a Mediterranean holm oak forest. Annals of forest science72 : 135–144.
Paz-Kagan T, Vaughn NR, Martin RE, Brodrick PG, Stephenson NL,
Das AJ, Nydick KR, Asner GP . 2018 . Landscape-scale variation
in canopy water content of giant sequoias during drought. Forest
Ecology and Management 419 : 291–304.
Pfautsch S, Hölttä T, Mencuccini M . 2015a . Invited
review Hydraulic functioning of tree stems — fusing ray anatomy ,
radial transfer and capacitance. : 706–722.
Pfautsch S, Renard J, Tjoelker MG, Salih A . 2015b .
Phloem as capacitor: Radial transfer of water into xylem of tree stems
occurs via symplastic transport in ray parenchyma. Plant
Physiology 167 : 963–971.
Preisler Y, Tatarinov F, Grünzweig JM, Bert D, Ogée J, Wingate
L, Rotenberg E, Rohatyn S, Her N, Moshe I, et al. 2019 .
Mortality versus survival in drought-affected Aleppo pine forest depends
on the extent of rock cover and soil stoniness. Functional
Ecology 33 : 901–912.
Pugh TAM, Arneth A, Kautz M, Poulter B, Smith B . 2019 .
Important role of forest disturbances in the global biomass turnover and
carbon sinks. Nature geoscience : 1–6.
Rotenberg E, Yakir D . 2010 . Contribution of semi-arid
forests to the climate system. Science (New York, N.Y.)327 : 451–4.
Sala A, Piper F, Hoch G . 2010 . Physiological
mechanisms of drought‐induced tree mortality are far from being
resolved. New Phytologist 186 : 274–281.
Sala A, Woodruff DR, Meinzer FC . 2012 . Carbon dynamics
in trees: feast or famine? Tree physiology 32 : 764–775.
Salmon Y, Dietrich L, Sevanto S, Hölttä T, Dannoura M, Epron D .2019 . Drought impacts on tree phloem: From cell-level responses
to ecological significance. Tree Physiology 39 :
173–191.
Sapes G, Roskilly B, Dobrowski S, Maneta M, Anderegg WRL,
Martinez-Vilalta J, Sala A . 2019 . Plant water content
integrates hydraulics and carbon depletion to predict drought-induced
seedling mortality. Tree physiology 39 : 1300–1312.
Savage JA, Clearwater MJ, Haines DF, Klein T, Mencuccini M,
Sevanto S, Turgeon R, Zhang C . 2016 . Allocation, stress
tolerance and carbon transport in plants: how does phloem physiology
affect plant ecology? Plant, Cell & Environment 39 :
709–725.
De Schepper V, Steppe K . 2010 . Development and
verification of a water and sugar transport model using measured stem
diameter variations. Journal of Experimental Botany 61 :
2083–2099.
Scholz FC, Bucci SJ, Goldstein G, Meinzer FC, Franco AC,
Miralles-Wilhelm F . 2008 . Temporal dynamics of stem expansion
and contraction in savanna trees: withdrawal and recharge of stored
water. Tree Physiology 28 : 469–480.
Schultz JC, Appel HM, Ferrieri A, Arnold TM . 2013 .
Flexible resource allocation during plant defense responses.Frontiers in plant science 4 : 324.
Schwantes AM, Swenson JJ, Jackson RB . 2016 .
Quantifying drought-induced tree mortality in the open canopy woodlands
of central Texas. Remote sensing of environment 181 :
54–64.
Sevanto S, Hölttä T, Holbrook NM . 2011 . Effects of the
hydraulic coupling between xylem and phloem on diurnal phloem diameter
variation. Plant, Cell and Environment 34 : 690–703.
Sevanto S, Mcdowell NG, Dickman LT, Pangle R, Pockman WT .2014 . How do trees die? A test of the hydraulic failure and
carbon starvation hypotheses. Plant, Cell and Environment37 : 153–161.
Sevanto S, Vesala T, Perämäki M, Nikinmaa E . 2002 .
Time lags for xylem and stem diameter variations in a Scots pine tree.Plant, Cell & Environment 25 : 1071–1077.
Sperry JS, Love DM . 2015 . What plant hydraulics can
tell us about responses to climate-change droughts. New
Phytologist 207 : 14–27.
Spicer R, Groover A . 2010 . Evolution of development of
vascular cambia and secondary growth. New Phytologist186 : 577–592.
Steppe K, Sterck F, Deslauriers A . 2015 . Diel growth
dynamics in tree stems: Linking anatomy and ecophysiology. Trends
in Plant Science .
De Swaef T, De Schepper V, Vandegehuchte MW, Steppe K .2015 . Stem diameter variations as a versatile research tool in
ecophysiology. Tree Physiology 35 : 1047–1061.
Tatarinov FA, Kučera J, Cienciala E . 2005 . The
analysis of physical background of tree sap flow measurement based on
thermal methods. Measurement science and Technology 16 :
1157.
Tatarinov F, Rotenberg E, Maseyk K, Ogée J, Klein T, Yakir D .2016 . Resilience to seasonal heat wave episodes in a
Mediterranean pine forest. New Phytologist 210 :
485–496.
Tyree MT, Sperry JS . 1988 . Do woody plants operate
near the point of catastrophic xylem dysfunction caused by dynamic water
stress?: answers from a model. Plant physiology 88 :
574–580.
Tyree MT, Sperry JS . 1989 . Vulnerability of xylem to
cavitation and embolism. Annual review of plant biology40 : 19–36.
Urrutia-Jalabert R, Rossi S, Deslauriers A, Malhi Y, Lara A .2015 . Environmental correlates of stem radius change in the
endangered Fitzroya cupressoides forests of southern Chile.Agricultural and Forest Meteorology 200 : 209–221.
Vergeynst LL, Vandegehuchte MW, McGuire MA, Teskey RO, Steppe
K . 2014 . Changes in stem water content influence sap flux
density measurements with thermal dissipation probes. Trees -
Structure and Function 28 : 949–955.
Wiley E, Hoch G, Landhäusser SM . 2017 . Dying piece by
piece: carbohydrate dynamics in aspen (Populus tremuloides) seedlings
under severe carbon stress. Journal of experimental botany68 : 5221–5232.
Zweifel R . 2016 . Radial stem variations - a source of
tree physiological information not fully exploited yet. Plant,
Cell and Environment 39 : 231–232.
Zweifel R, Drew DM, Schweingruber F, Downes GM . 2014 .
Xylem as the main origin of stem radius changes in Eucalyptus.Functional Plant Biology 41 : 520–534.
Zweifel R, Haeni M, Buchmann N, Eugster W . 2016 . Are
trees able to grow in periods of stem shrinkage? New Phytologist211 : 839–849.
Zweifel R, Häsler R . 2000 . Stem radius changes and
their relation to stored water in stems of young Norway spruce trees.Trees 15 : 50–57.
Zweifel R, Item H, Häsler R . 2001 . Link between
diurnal stem radius changes and tree water relations. Tree
Physiology 21 : 869–877.