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Author(s): Hui Qian, Ai-Mei Dong, Marja Roitto, Di-Ying Xiang, Gang Zhang, Tapani Repo and 
Ai-Fang Wang. 
Title: Timing of Drought Affected the Growth, Physiology, and Mortality of Mongolian 
Pine Saplings 
Year: 2021 
Version: Published version 
Copyright:   The Author(s) 2021 
Rights: CC BY 4.0 
Rights url: http://creativecommons.org/licenses/by/4.0/ 
 
Please cite the original version: 
Qian, H.; Dong, A.-M.; Roitto, M.; Xiang, D.-Y.; Zhang, G.; Repo, T.; Wang, A.-F. Timing of Drought 
Affected the Growth, Physiology, and Mortality of Mongolian Pine Saplings. Forests 2021, 12, 
1491. https://doi.org/10.3390/f12111491 
Article
Timing of Drought Affected the Growth, Physiology,
and Mortality of Mongolian Pine Saplings
Hui Qian 1, Ai-Mei Dong 1, Marja Roitto 2 , Di-Ying Xiang 1, Gang Zhang 1, Tapani Repo 3 and Ai-Fang Wang 1,*






Citation: Qian, H.; Dong, A.-M.;
Roitto, M.; Xiang, D.-Y.; Zhang, G.;
Repo, T.; Wang, A.-F. Timing of
Drought Affected the Growth,
Physiology, and Mortality of
Mongolian Pine Saplings. Forests
2021, 12, 1491. https://doi.org/
10.3390/f12111491
Academic Editor: Maria C. Caldeira
Received: 15 September 2021
Accepted: 26 October 2021
Published: 29 October 2021
Publisher’s Note: MDPI stays neutral
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iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1 College of Horticulture, Hebei Agricultural University, Baoding 071001, China; qianhui14287@126.com (H.Q.);
dongaimei617@126.com (A.-M.D.); yyxdy@hebau.edu.cn (D.-Y.X.); zhanggang1210@126.com (G.Z.)
2 Ruralia Institute and Helsinki Institute of Sustainability Science, University of Helsinki, Lönnrotinkatu 7,
FI-50100 Mikkeli, Finland; marja.roitto@helsinki.fi
3 Natural Resources, Natural Resources Institute Finland (Luke), Yliopistokatu 6B, P.O. Box 68,
FI-80100 Joensuu, Finland; tapani.repo@luke.fi
* Correspondence: awang@hebau.edu.cn; Tel.: +86-151-3120-9091
Abstract: Background and Objectives: More frequent and severe droughts are occurring due to
climate change in northern China. In addition to intensity and duration, the timing of droughts may
be decisive for its impacts on tree growth, mortality, and the whole forest ecosystem. The aim of this
study was to compare the effect of drought occurring in the early- and mid-growing season on the
growth and physiology of Mongolian pine (Pinus sylvestris var. mongolica Litv.) saplings. Materials
and Methods: Four-year-old container saplings that were about to sprout were exposed to three
treatments: (i) regular irrigation throughout the growing season (CTRL), (ii) no irrigation in the
early growing season (weeks 1–5) followed by regular irrigation (EGD), (iii) no irrigation in the mid
growing season (weeks 5–10), and regular irrigation in the early and late growing season (MGD). We
measured the root and shoot growth, sapling mortality, and the physiological changes in the roots
and needles periodically. Results: Drought in the mid growing season was more harmful than in
the early growing season in terms of chlorophyll fluorescence, electrolyte leakage of needles, needle
length, stem diameter increment, and sapling mortality. The high mortality in the mid growing
season might be attributed to the joint effect of drought and high temperature. Drought in the early
growing season decreased root growth, and the starch and soluble sugars in roots as much as the
drought in the mid growing season. Abscisic acid concentration increased in fine roots, but decreased
in old needles after drought. Conclusions: Special attention should be paid on forest sites susceptible
to drought during afforestation in the face of ongoing climate change.
Keywords: abscisic acid; chlorophyll fluorescence; drought timing; electrolyte leakage; root length;
soluble sugar; starch
1. Introduction
The global land areas exposed to drought have remarkably increased over the past
decades [1]. Frequent and severe droughts have occurred in different parts of the world [2].
Severe droughts are expected to strongly impact forest ecosystems through affecting the
growth, mortality, and regeneration of many tree species. In addition to intensity and
duration, the timing of droughts has impacted plant productivity [3]. Huang et al. (2018)
found that legacies from extreme drought events during the dry season lasted longer and
impacted stronger on tree growth than those from extreme drought events during the wet
season, suggesting that the timing of drought is a crucial factor in determining the impact
on tree recovery [4]. Timing of drought may affect the mode, direction, and magnitude of
the impact on terrestrial ecosystems [5,6].
Previous studies have mainly focused on the effects of summer drought on tree func-
tioning. An irrigation experiment showed that summer drought limited Scot pine (Pinus
sylvestris L.) forest establishment by reducing growth and increasing seedling mortality [7].
Forests 2021, 12, 1491. https://doi.org/10.3390/f12111491 https://www.mdpi.com/journal/forests
Forests 2021, 12, 1491 2 of 16
Zang et al. (2012) reported that radial growth of Scots pine trees of two different diameter
groups were both limited by summer drought [8]. The severe late summer drought caused
visual drought-damage symptoms of trees in Finnish boreal forests [9], and the ecosystem
level water use efficiency showed a decrease during the severe soil drought [10].
In recent years, the effects of spring drought on the tree growth have been studied
increasingly. Spring drought strongly reduced the radial growth of European larch (Larix
decidua Mill.) and Norway spruce (Picea abies (L.) Karst) in Switzerland [11]. In temperate
mesic forests of Eastern North America, drought-induced stem radial growth reductions in
tree growth were the greatest when the droughts occurred at the beginning of the growing
season [12]. In the northern part of China, the drought conditions in recent decades have
been more severe than in other regions of China due to decreasing precipitation and
increasing temperature [3]. Moreover, spring drought occurs frequently, causing stress to
all plants in the early growth phase [3]. A study of Mongolian pine (Pinus sylvestris var.
mongolica Litv.) forests in northeastern China also showed that the effect of temperature
increases, caused by climate change, on tree growth depends on water availability during
the early growing season [13]. However, comparative studies between the effects of drought
occurred in the early- and mid- growing seasons on boreal and temperate tree species are
lacking. It is probable that air temperature interacts with the drought stress, thus affecting
the physiology and growth of tree saplings depending on the phase of the growing season.
Mongolian pine is a widely used tree species in afforestation in desertified areas of
North China, Northeast China, and Northwest China. It has been cultivated for water and
soil conservation and ecosystem recovery due to its quality of wood, fast growth, strong
resistance to adverse environments, and adaptability. Four-year-old container saplings
have been commonly used in afforestation. According to forest practices, the best plantation
time is in the early spring. However, with the warming and drying of climate, water stress
seems to be one of the main factors causing a low sapling survival rate, and low growth
rate and dieback of this species in aging forests [14,15]. It is not well-known in which
season the saplings may tolerate drought stress the best. That knowledge would be helpful
for the forest practitioners to carry out the necessary irrigations.
In this study, we aimed to explore the impact of the timing of drought, particularly
when occurring in the beginning (after bud burst) and middle of the growing season, on the
above- and below-ground physiology and growth of Mongolian pine saplings. Different
physiological and growth parameters were measured to gain a comprehensive view of
sapling responses. Chlorophyll content and chlorophyll fluorescence were measured to
assess the photochemistry of photosynthesis [16]. Non-structural carbohydrates’ (mainly
soluble sugar and starch) allocation in different organs that are vital for the metabolism and
growth were monitored to evaluate the carbon balance under drought conditions [17,18].
Abscisic acid (ABA) is considered as a possible mediator of root-to-shoot communication,
and it is translocated from the drought-sensing roots to the shoots via the transpiration
stream, leading to the subsequent stomatal closure [19–21]. Therefore, ABA concentrations
in roots and needles were measured. Electrolyte leakage of the needles were measured
to evaluate the injury of the cellular membranes [22]. Root and needle length, stem
diameter increment, sapling mortality were measured to assess the growth. Therefore,
we assumed that these parameters would indicate sapling responses to drought during
different phases of growing season. We hypothesized that the Mongolian pine seedlings
are more susceptible to drought in the middle than in the early growing season, due to
their different physiological status, phase of growth and the higher air temperature in the
middle growing season.
2. Materials and Methods
2.1. Plant Cultivation and Drought Treatments
Four-year-old Mongolian pine saplings (height 46 ± 6 cm, basal stem diameter
8.56 ± 3 mm) (mean ± SE) were cultured in the containers with the mineral soil from
the local nursery in Longtoushan Seedling Farm, Mulan State Forest Management Bureau
Forests 2021, 12, 1491 3 of 16
(41◦99′ N, 117◦70′ E, 884 m above sea level), Hebei Province, from seeds originating from
local orchards. A total of 600 dormant container saplings were transported to Baoding,
Hebei Agricultural University (38◦50′ N, 115◦26′ E) in March 2018. Immediately after
arrival, the saplings with the soil clod (height 15 cm, diameter 10 cm) were replanted
into plastic pots (diameter 18.5 cm at the top and 12.5 cm at the bottom, height 22 cm)
with commercial organotrophic soil (N, P, K content >6%, organic matter content >45%,
pH = 5.5–6.5). The saplings were raised for one month with irrigation at 1-week intervals
before starting the drought treatments on April 26, when the saplings were about to start
sprouting.
The saplings were distributed into four replicate blocks in a plastic shed. The roof
was covered by plastic cloth to avoid the rain, and the four sides were open in order to
enhance the air circulation, except for rainy days. Each block comprised three plots; the
saplings of each drought treatment were put into one plot (50 saplings in one plot) and
the plots were randomly distributed in each block. The drought treatments were applied
within the first 10 weeks, when the buds burst and new needles started elongating (weeks
1–5), and shoot elongation was ending and needles was elongating quickly (weeks 6–10),
followed by the post-treatment growth phase (weeks 11–18). The three treatments were:
(i) regular irrigation throughout the growing season (CTRL); (ii) no irrigation in the early
growing season (weeks 1–5) followed by regular irrigation (EGD); and (iii) no irrigation in
the middle of the growing season (weeks 6–10) and regular irrigation in the beginning and
end of the growing season (MGD). Control seedlings were irrigated at intervals to maintain
adequate soil water content, and there was no irrigation in the EGD and MGD periods.
After the end of the drought treatment at week 10, all the saplings were watered weekly.
The daily maximum and minimum air temperatures in the plastic shed were recorded
during the experiment using a digital thermometer (HTC-1, Bing You, Shanghai, China).
The maximum and minimum air temperatures in the shelter were 32 ◦C and 8 ◦C during the
early growing season (weeks 1–5) and 38 ◦C and 15 ◦C during the middle of the growing
season (weeks 6–10), respectively. The average air temperatures were 20 ◦C during the
early growing season and 26 ◦C during the middle of the growing season, respectively
(Figure 1a).
The volumetric soil water content in the randomly selected 20 pots per treatment was
measured (TDR100, Spectrum Technologies, Inc., Plainfield, IL, USA) at 3 p.m. daily. The
corresponding soil relative water content (SRWC) was calculated based on the volumetric
soil water content, the field moisture capacity of soil (32.50% ± 1.2%) and soil bulk density
(1.53 ± 0.1 g/cm3) [23] as: SRWC = [(soil volumetric water content/soil bulk density)/field
moisture capacity] × 100. The soil relative water content in CTRL pots remained higher
than 60%, while it was from 36% to 21% between weeks 3 and 5 in EGD pots and it was
from 36% to 20% between the weeks 8 and 10 in MGD pots (Figure 1b).
Twenty-four saplings (2 saplings × 4 replicate blocks × 3 treatment) were taken to the
lab for the physiological measurements at week 3 (3 weeks EGD), week 5 (5 weeks EGD),
week 8 (3 weeks MGD), week 10 (5 weeks MGD), week 13 (3 weeks after ending MGD)
and week 16 (6 weeks after ending MGD).
2.2. Physiological and Growth Measurements
2.2.1. Chlorophyll Content and Chlorophyll Fluorescence of Needles
Previous-year and current-year needles (sample size 0.2 g fresh weight) was randomly
sampled from the middle of the previous-year shoots and middle of the current-year
shoots, respectively, in each of eight saplings per treatment (two saplings of each replicate
block) for total chlorophyll content measurement by the method described in Zhang et al.
(1986) [24]. The needles were cleaned and cut into pieces, followed by soaking in the mixed
solution of ethanol and acetone with ratio of 1:1 for 24 h in a dark place. When the needle
pieces turned white, extracting solution was measured spectrophotometrically (U-5100
Spectrophotometer, Hitachi High-tech Science, Tokyo, Japan) at 645 nm and 663 nm.
Forests 2021, 12, 1491 4 of 16
For the measurement of dark-acclimated chlorophyll fluorescence (Fv/Fm), five previous-
year needles were picked randomly from the middle of each sapling. The needles were
attached to a tape side by side and after dark-acclimation for 25 min at room tempera-
ture Fv/Fm was measured with a portable chlorophyll fluorescence meter (Handy-PEA,
Hansatech Instruments, King’s Lynn, UK).
Forests 2021, 12, x FOR PEER REVIEW 4 of 17 
 
 
 
Figure 1. Air temperatures (a) and soil relative water content (b) during the experiment that started 
on April 26. CTRL indicates the control; EGD and MGD indicate the drought stress treatment in the 
early- (EG, days 0–35) and mid- (MG, days 36–70) growing season, respectively. LG refers to the late 
growing season (days 71–126). Time indicates days from the beginning of the experiment. 
Twenty-four saplings (2 saplings × 4 replicate blocks × 3 treatment) were taken to the 
lab for the physiological measurements at week 3 (3 weeks EGD), week 5 (5 weeks EGD), 
week 8 (3 weeks MGD), week 10 (5 weeks MGD), week 13 (3 weeks after ending MGD) 
and week 16 (6 weeks after ending MGD). 
2.2. Physiological and Growth Measurements 
2.2.1. Chlorophyll Content and Chlorophyll Fluorescence of Needles 
Previous-year and current-year needles (sample size 0.2 g fresh weight) was ran-
domly sampled from the middle of the previous-year shoots and middle of the current-
year shoots, respectively, in each of eight saplings per treatment (two saplings of each 
replicate block) for total chlorophyll content measurement by the method described in 
Zhang et al. (1986) [24]. The needles were cleaned and cut into pieces, followed by soaking 
in the mixed solution of ethanol and acetone with ratio of 1:1 for 24 h in a dark place. 
When the needle pieces turned white, extracting solution was measured spectrophoto-
metrically (U-5100 Spectrophotometer, Hitachi High-tech Science, Tokyo, Japan) at 645 
nm and 663 nm. 
Figure 1. Air temperatures (a) and soil relative water content (b) during the experiment that started
on April 26. CTRL indicates the control; EGD and MGD indicate the drought stre s treatment in the
early- (EG, days 0–35) and mid- (MG, days 36–70) growing season, respectively. LG refers to the late
growing season (days 71–126). Time indicates days from the beginning of the experiment.
2.2.2. R lative Electrolyte Leakage (REL) of Needles
Four previous-year and current-year needles were randomly selected from the middle
of the previous-year shoots and middle of the current-year shoots, respectively, in each of
eight saplings per treatment (two seedlings of each replicate block) for relative electrolyte
leakage (REL) measurement. In total, 32 previous-year needles and 32 current-year needles
in each treatment at each sampling time were used. The samples, 10 mm in length, were
cut from the middle of the needles and rinsed with deionized water, then put into four
test tubes randomly with eight segments in each. Then, 12 mL of deionized water was
added into each tube. The tubes were shaken for 22 h, then the first conductivity value (C1)
(DDS-307 conductivity meter with DJS-1C electrode, INESA Scientific Instrument Co. Ltd.,
Shanghai, China) was measured. The samples were then heat-killed in a water bath (92 ◦C
Forests 2021, 12, 1491 5 of 16
for 30 min) and shaken again for 22 h before the second conductivity (C2) was recorded.
The REL was calculated as [25]:
REL =
C1
C2
× 100 (1)
2.2.3. Soluble Sugar and Starch Content
Eight current-year needles, one 5 cm long piece of stem and about 2 g of fine roots
(diameter < 2 mm) were randomly sampled from each of the saplings, that were used for
the root morphological and physiological measurements, for determining soluble sugar
and starch contents. The samples were dried at 40 ◦C to a constant weight and then
ground into a powder. The soluble sugars were extracted using 80% aqueous ethanol. The
concentration of total soluble sugars was determined spectrophotometrically at 630 nm
after the reaction with anthrone; D-glucose was used as the standard. The starch was
extracted from the residue using 35% perchloric acid and its content was determined
spectrophotometrically at 625 nm with anthrone and using starch in 30% perchloric acid as
a standard. The contents are given as percentage of dry mass of the needles [26].
2.2.4. Abscisic Acid (ABA) Determination
Five previous-year needles from middle of the previous-year shoots and about 0.2 g
of fine roots were collected from each of the seedlings that were used for the physiological
measurements at each sampling time and stored in liquid nitrogen for determining the
concentration of abscisic acid (ABA) by enzyme-linked immunosorbent assay (ELISA) [27].
2.2.5. Sapling Growth
Before the onset of the drought treatment, two saplings in each treatment of each block
(24 saplings in total) were randomly marked to measure the basal stem diameter and the
length of the current-year needles later on. The stem diameter was measured with a 6”
electronic caliper (PD-151, Prokit’s industries Co., Ltd, Taipei, Taiwan) at a marked position
above the root collar. In addition, a current-year shoot was labelled and five needles were
randomly selected for length measurement. The measurements of the same saplings were
repeated weekly.
At the sampling times, in the beginning of the experiment (Week 0), after five weeks
of EGD (Week 5) and MGD (Week 10) and resuming irrigation (Week 16), the roots of
two saplings from each of the four replicate blocks, i.e., eight saplings per treatment for
a total of 24 saplings used for physiological measurements were harvested. The whole
root system was cut out at the root collar and separated from the soil, cleaned with tap
water and deionized water, and then scanned to assess root morphology (Epson Expression
1640Xl scanner, Epson, Quebec, QC, Canada). The root length was analyzed using the
WinRHIZO program (WinRhizo, Régent Instruments Inc., Sainte-Foy, QC, Canada).
2.2.6. Sapling Mortality
The number of dead saplings was counted in all treatments one week after each
sampling. Mortality was calculated as the cumulative number of dead saplings divided by
the total number of saplings in each treatment in the beginning of the experiment.
2.3. Statistical Analysis
The effects of the treatments on the physiological and growth parameters were ana-
lyzed by means of a mixed linear model (procedure MIXED in SPSS 20.0, SPSS Inc., Chicago,
IL, USA). The model used was y = µ + treatment + time + treatment × time + ε, where
µ is a constant. The ‘treatment’ (i.e., CTRL, EGD and MGD) and ‘time’ (i.e., sampling
time) were regarded as fixed factors and ε as a random term. The significance of the
difference between the treatments at different sampling times was tested by contrasts using
Bonferroni-corrected significance levels. Normality and homogeneity of the variance of the
residuals were checked. Root length data were log-transformed to fulfill the assumptions
Forests 2021, 12, 1491 6 of 16
of the analyses. The mean value of the saplings in the replicate block was used in the
statistical analyses of the variables. The number of replicate blocks was four.
3. Results
3.1. Chlorophyll Content and Chlorophyll Fluorescence of Needles
Chlorophyll content of previous-year needles in CTRL increased and reached the
maximum at week 13 (Figure 2a). After three and five weeks from the start, it was
significantly lower in EGD than CTRL (p < 0.05). After resuming the irrigation, it increased
approximately to the same level as in CTRL, but then decreased afterwards except for the
last sampling time. After three weeks of MGD (week 8), it was similar to that of CTRL, but
after five weeks it was significantly lower in MGD than CTRL (week 10, p < 0.01), as well
as after three weeks of resuming irrigation.
Forests 2021, 12, x FOR PEER REVIEW 7 of 17 
 
 
 
Figure 2. Chlorophyll content in the previous-year (a) and current-year (b) needles of Mongolian 
pine saplings by treatment. CTRL indicates the control. EGD and MGD indicate drought 
stress treatment in the early- (EG) and mid- (MG) growing seasons. EG, MG and LG refer to the 
early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks 11−16) growing season, respectively. Time 
indicates weeks from the beginning of the experiment. Bars indicate standard error (n = 4). 
In CTRL, Fv/Fm of the previous-year needles was fairly stable between 0.79–0.83 dur-
ing the whole study period (Figure 3). Fv/Fm was lower in EGD than in CTRL after five 
weeks of drought and difference lasted until week 8, even though irrigation was resumed. 
Fv/Fm was significantly lower in MGD than in CTRL and EGD after five weeks of drought 
(week 10, p < 0.05). At the last two sampling times with similar irrigations in all treatments, 
there were no significant differences in Fv/Fm between the treatments. 
ll
li s y treat ent. CTRL i ates the control. EGD and MGD indicate drought stress
treatment in the early- (EG) and mid- (MG) growing seasons. EG, MG and LG refer to the early-
(weeks 1–5), mid- (weeks 6–10) and late- (weeks 11−16) growing season, respectively. Time indicates
weeks from the beginning of the experiment. Bars indicate standard error (n = 4).
Similar to the previous-year needles, chlorophyll content of current-year needles in
CTRL increased during the experiment and reached the maximum at week 13 (Figure 2b).
It was significantly lower in EGD than in CTRL after five weeks of EGD (week 5, p < 0.05).
However, chlorophyll content of current-year needles was not reduced after three weeks of
EGD like in the previous-year needles. After resuming the ir igation, chlorophyll content
Forests 2021, 12, 1491 7 of 16
in EGD increased quickly and were even higher than in CTRL at two sampling times.
MGD had similar effects on the chlorophyll content of the previous-year as current-year
needles, except in the last sampling at the end of experiment, the chlorophyll content of the
current-year needles was lower in MGD than in CTRL (Figure 2a,b).
In CTRL, Fv/Fm of the previous-year needles was fairly stable between 0.79–0.83
during the whole study period (Figure 3). Fv/Fm was lower in EGD than in CTRL after five
weeks of drought and difference lasted until week 8, even though irrigation was resumed.
Fv/Fm was significantly lower in MGD than in CTRL and EGD after five weeks of drought
(week 10, p < 0.05). At the last two sampling times with similar irrigations in all treatments,
there were no significant differences in Fv/Fm between the treatments.
Forests 2021, 12, x FOR PEER REVIEW 8 of 17 
 
 
 
Figure 3. Dark-acclimated chlorophyll fluorescence (Fv/Fm) in the previous-year needles of Mongo-
lian pine saplings by treatment. CTRL indicates the control. EGD and MGD indicate 
drought stress treatment in the early- (EG) and mid- (MG) growing seasons, respectively. EG, MG 
and LG refer to the early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks 11–16) growing season, 
respectively. Time indicates weeks from the beginning of the experiment. Bars indicate standard 
error (n = 4). 
3.2. Relative Electrolyte Leakage (REL) of Needles 
In EGD, there was no immediate change in REL during the treatment in either previ-
ous- or current-year needles (Figure 4). However, in the previous-year needles the differ-
ences between EGD and CTRL were found at weeks 8 and 13 after resuming the irrigation 
(p = 0.073 and p < 0.05, respectively) (Figure 4a). In the current-year needles, the differences 
between EGD and CTRL were found after irrigation at weeks 8 and 10 (p < 0.01 and p < 
0.01, respectively) (Figure 4b). In contrast, REL increased significantly right after three and 
five weeks of drought in MGD in both previous- and current-year needles (p < 0.01 for 
each), being 1.6 and 2.5 times higher in MGD than in CTRL at week 10 for these two kinds 
of needles, respectively (Figure 4a,b). In the latter part of the growing season with similar 
irrigations, there were no significant differences in REL between the treatments for the 
previous-year needles at week 16 and for the current-year needles at weeks 13 and 16. 
Figure . r - li t l r yll fluorescence (Fv/Fm) in the previous-year needles of Mongolian
pi e saplings by treatm nt. CTRL indicates the control. EGD and M indicate drought stress
treatment in the e rly- (EG) and mid- (MG) growing seasons, respectively. EG, MG and LG refer to
the early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks 11–16) growing season, respectively. Time
indicates weeks from the beginning of the experiment. Bars indicate standard error (n = 4).
3.2. Relative Electrolyte Leakage (REL) of Needles
In EGD, there was no immediate change in REL during the treatment in either
previous- or cur nt-year needles (Figure 4). However, the p vious-year ne dles the
differences between EGD and CTRL were found at weeks 8 and 13 aft r r uming the
irrigation (p = 0.073 and p < 0.05, respectively) (Figure 4a). In the c rrent-year needles,
the differences between EGD and CTRL were found after irrigation at we ks 8 and 10
(p < 0.01 and p < 0.01, resp ctively) (Figu e 4b). In contrast, REL increased significantly
right after hr e and fiv weeks f drought in MGD in both previous- and current-year
ne dles (p < 0.01 for each), being 1.6 and 2.5 time higher in MGD tha in CTRL at week 10
for these two kinds of needles, respectively (Figure 4a,b). In the latter part of the growing
season with similar irrigations, there were no significant differences in REL between the
treatments for the previous-year nee les at week 16 and for the current-year needles at
weeks 13 and 16.
Forests 2021, 12, 1491 8 of 16Forests 2021, 12, x FOR PEER REVIEW 9 of 17 
 
 
 
Figure 4. Electrolyte leakage in the previous-year (a) and current-year (b) needles of Mongolian pine 
saplings by treatment. CTRL indicates the control. EGD and MGD indicate drought stress 
treatment in the early- (EG) and mid- (MG) growing seasons, respectively. EG, MG and LG refer to 
the early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks 11–16) growing season, respectively. 
Time indicates weeks from the beginning of the experiment. Bars indicate standard error (n = 4). 
3.3. Non-Structural Carbohydrates (NSC) in Roots, Stems and Needles 
The starch and soluble sugar content of fine roots did not differ between EGD and 
CTRL after three and five weeks of drought (Figure 5a,b). However, after resuming the 
irrigation, the starch and sugar contents were significantly lower in EGD than in CTRL at 
week 8 (p < 0.01 for both variables), week 10 (p < 0.01, p = 0.086, respectively) and week 13 
(p < 0.05, p < 0.001, respectively). The starch content was significantly lower in MGD than 
in CTRL after three weeks of drought, and the difference lasted for the rest of sampling 
times (p < 0.05 for each) even though the irrigation had resumed (Figure 5a). The soluble 
sugar content was significantly lower in MGD than in CTRL after three weeks of drought 
(p < 0.05) and during the post-drought period at weeks 13 and 16 (p < 0.001 and p < 0.01, 
respectively) (Figure 5b). There were no significant differences in starch and soluble sugar 
contents between EGD and MGD, except for the soluble sugar content at week 16. 
In stems, starch content was significantly lower in EGD than in CTRL at week 3 (p < 
0.01) and week 8 (p < 0.05) (Figure 5c). It was significantly lower in MGD than in CTRL 
after five weeks of drought (week 10, p < 0.05) and after resuming the irrigation (week 13, 
p < 0.05). The soluble sugar content increased significantly in EGD after three and five 
weeks of drought (p < 0.01) (Figure 5d). However, it was significantly lower in EGD than 
in CTRL during the post-treatment period at weeks 10 (p < 0.01) and 16 (p < 0.05). The 
Figure 4. Electrolyte leakage in the previous-year (a) and cu rent-year (b) n edles of Mongolian pine
saplings by treatment. CT indicates the control. EGD and MGD indicate drought st ess treatment
in the early- (EG) and mid- (MG) growing seasons, respectively. EG, MG and LG refer to the early-
(weeks 1–5), mid- (weeks 6–10) and late- (weeks 11–16) growing season, respectively. Time indicates
weeks from the beginning of the experiment. Bars indicate standard error (n = 4).
3.3. Non-Structural Carbohydrates (NSC) in R ots, Stems and N edles
The starch and soluble sugar content of fine roots did not di fer between EGD and
CTRL after thr e and five weeks of drought (Figure 5a,b). However, after resu ing the
i rigation, the starch and sugar contents were significantly lower in EGD than in CTRL at
w ek 8 (p < 0.01 for both variables), week 10 (p < 0.01, p = 0.086, respectively) and w ek 13
(p < 0.05, p < 0. 01, respectively). The starch content was significantly lower in MGD than
in CTRL after three weeks of drought, and the difference lasted for the rest of sa pling
times (p < 0.05 for each) even though the irrigation had resumed (Figure 5a). The soluble
sugar content was significantly lower in MGD than in CTRL after three weeks of drought
(p < 0.05) and during the post-drought period at weeks 13 and 16 (p < 0.001 and p < 0.01,
respectively) (Figure 5b). There were no significant differences in starch and soluble sugar
contents between EGD and MGD, except for the soluble sugar content at week 16.
In stems, starch content was significantly lower in EGD than in CTRL at week 3
(p < 0.01) and week 8 (p < 0.05) (Figure 5c). It was significantly lower in MGD than in CTRL
after five weeks of drought (week 10, p < 0.05) and after resuming the irrigation (week 13,
p < 0.05). The soluble sugar content increased significantly in EGD after three and five
weeks of drought (p < 0.01) (Figure 5d). However, it was significantly lower in EGD than in
CTRL during the post-treatment period at weeks 10 (p < 0.01) and 16 (p < 0.05). The soluble
Forests 2021, 12, 1491 9 of 16
sugar content was significantly lower in MGD than in CTRL at weeks 10 (p < 0.001) and 16
(p < 0.01).
In the current-year needles, starch content was lower in EGD than in CTRL and MGD
after three (p < 0.01) and five weeks of drought and even after three weeks of resuming
irrigation, but the statistical differences disappeared gradually (Figure 5e). Similarly, starch
content in MGD decreased after three weeks of drought (p < 0.001) and the significant
differences disappeared later. There were no significant differences in soluble sugar content
between EGD and CTRL, despite the slight reduction in EGD (Figure 5f). The soluble sugar
content was significantly lower in MGD than in CTRL after three and five weeks of drought
(week 8, p < 0.05; week 10, p < 0.01), while it was slightly higher in MGD than in CTRL
during the post-treatment period.
Forests 2021, 12, x FOR PEER REVIEW 10 of 17 
 
 
soluble sugar content was significantly lower in MGD than in CTRL at weeks 10 (p < 0.001) 
and 16 (p < 0.01). 
In the current-year needles, starch content was lower in EGD than in CTRL and MGD 
after three (p < 0.01) and five weeks of drought and even after three weeks of resuming 
irrigation, but the statistical differences disappeared gradually (Figure 5e). Similarly, 
starch content in MGD decreased after three weeks of drought (p < 0.001) and the signifi-
cant differences disappeared later. There were no significant differences in soluble sugar 
content between EGD and CTRL, despite the slight reduction in EGD (Figure 5f). The sol-
uble sugar content was significantly lower in MGD than in CTRL after three and five 
weeks of drought (week 8, p < 0.05; week 10, p < 0.01), while it was slightly higher in MGD 
than in CTRL during the post-treatment period. 
 
Figure 5. Starch content (left) and soluble sugar content (right) in fine roots (a,d), stems (b,e) and 
current-year needles (c,f) of Mongolian pine saplings by treatment. CTRL indicates the control.  
EGD and MGD indicate drought stress treatment in the early- (EG) and mid- (MG) growing 
seasons, respectively. EG, MG and LG refer to the early- (weeks 1–5), mid- (weeks 6–10) and late- 
(weeks 11–16) growing season, respectively. Time indicates weeks from the beginning of the exper-
iment. Bars indicate standard error (n = 4). 
3.4. ABA Concentration in Roots and Needles 
In fine roots, the ABA concentration was significantly higher in EGD and MGD than 
in CTRL after three and five weeks of drought, and the differences lasted for five weeks 
Figure 5. Starch content (left) and soluble sugar content (right) in fine r ots (a,d), stems (b,e) and
current-year n edles (c,f) of Mongolian pine saplings by treatment. CT dicates the control. EGD
and MGD indicate rought stress treatment in the early- (EG) and mid- (MG) growing seasons,
respectively. EG, MG and LG refer to the early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks
11–16) growing season, respectively. Time indicates weeks from the beginning of the experiment.
Bars indicate standard error (n = 4).
3.4. ABA Concentration in R ots and Needles
In fine r ots, the ABA concentration was significantly higher in EGD and MGD than
in CTRL after thr e and five w eks of drought, and the di ferences lasted for five w eks
after resuming irrigation (p < 0.001 for each) (Figure 6a). The ABA concentration was
Forests 2021, 12, 1491 10 of 16
significantly higher in MGD than EGD after three weeks of MGD until the end of the
experiment (p < 0.001 for each sampling time).
In contrast to the fine roots, the ABA concentration in the previous-year needles was
significantly lower in EGD and MGD than in CTRL after three and five weeks of drought
(Figure 6b). The differences lasted for few weeks after resuming irrigation (p < 0.01 for
each), but disappeared at the end of the experiment.
Forests 2021, 12, x FOR PEER REVIEW 11 of 17 
 
 
after resuming irrigation (p < 0.001 for each) (Figure 6a). The ABA concentration was sig-
nificantly higher in MGD than EGD after three weeks of MGD until the end of the exper-
iment (p < 0.001 for each sampling time). 
In contrast to the fine roots, the ABA concentration in the previous-year needles was 
significantly lower in EGD and MGD than in CTRL after three and five weeks of drought 
(Figure 6b). The differences lasted for few weeks after resuming irrigation (p < 0.01 for 
each), but disappeared at the end of the experiment. 
 
Figure 6. Abscisic acid (ABA) concentration in fine roots (a) and previous-year needles (b) of 
Mongolian pine saplings by treatment. CTRL indicates the control.  EGD and MGD indicate 
drought stress treatment in the early- (EG) and mid- (MG) growing seasons, respectively. EG, MG 
and LG refer to the early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks 11–16) growing season, 
respectively. Time indicates weeks from the beginning of the experiment. Bars indicate standard 
error (n = 4). 
3.5. Length of Current-Year Needles 
The length of the current-year needles increased in the CTRL treatment until week 
15 (Figure 7a). After four weeks of EGD, the length of the new needles was shorter com-
pared to CTRL and the significant differences lasted until week 10 (p < 0.05 for each). After 
three weeks of MGD, the growth rate of the needle length decreased, and afterwards they 
were always shorter than in CTRL (p = 0.069 at week 8, p < 0.05 for the later sampling 
times). At the end of the experiment, length of current-year needles in MGD were the 
shortest among the treatments. 
Figure 6. Abscisic acid (ABA) concentration in fine r ots (a) and previous-year needles (b) of
Mongolian pine saplings by treatment. CTRL indica the control. EGD and MGD indicate drought
stress treatment in the early- (EG) and mid- (MG) growing seasons, respectively. EG, MG and LG refer
to the early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks 11–16) growing season, respectively.
Time indicates weeks from the beginning of the experiment. Bars indicate standard error (n = 4).
3.5. Length of Current-Year Needles
The length of the current-year needles increased in the CTRL treatment until week 15
(Figure 7a). After four we ks of EGD, th le gth of the new needles w s shorter compared
to CTRL and the significant differences lasted until week 10 (p < 0.05 for eac ). After three
weeks of MGD, the growth r te of the e dle length decreased, and afterwards they were
always shorter than in CTRL (p = 0.069 at w k 8, p < 0.05 fo the later sampling times). At
th end of the experiment, length of current-year n dles in MGD were the shortest among
the treatments.
Forests 2021, 12, 1491 11 of 16Forests 2021, 12, x FOR PEER REVIEW 12 of 17 
 
 
 
Figure 7. Mean current-year needle length (a) and basal stem diameter increment (b) of Mongolian 
pine saplings by treatment. CTRL indicates the control. EGD and MGD indicate drought stress treat-
ment in the early- (EG) and mid- (MG) growing seasons, respectively. EG, MG and LG refer to the 
early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks 11–18) growing season, respectively. Time 
indicates weeks from the beginning of the experiment. Bars indicate standard error (n = 4). 
3.6. Basal Stem Diameter Increment 
The basal stem diameter of the saplings increased quickly between weeks 4 and 11 in 
the CTRL treatment (Figure 7b). The stem diameter in the EGD saplings did not increase 
significantly between weeks 5 and 8 but increased thereafter. There were no statistically 
significant differences in stem diameter increment between MGD and CTRL at week 5 
and 6. After two weeks of MGD (week 7) until the end of the experiment, stem diameter 
increment was significantly smaller in the MGD than CTRL saplings (p < 0.05 at week 7, p 
< 0.001 for the other times). Stem diameter increment was significantly smaller in MGD 
than in EGD (p < 0.05 for week 10, p < 0.01 for weeks 11–18). 
3.7. Root Length 
The total root length was just slightly lower in EGD than that in CTRL after five 
weeks of drought stress (week 5). However, it was significantly smaller in EGD than in 
CTRL five weeks after resuming irrigation (week 10) and afterwards (p < 0.01, p < 0.05, 
respectively). The total root length was significantly lower in MGD than in CTRL after 
five weeks of drought stress (week 10, p < 0.001) and after resuming irrigation. There were 
no significant differences in total root length between EGD and MGD in the end of the 
experiment (Figure 8). 
i re . ean curre t-year ee le le t ( ) s l st i t r i t
i e saplings by treatment. CTRL indicates the control. EGD and MGD indicate drought stress
treatment in the early- (EG) and mid- (MG) growing seasons, respectively. EG, MG and LG refer to
the early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks 11–18) growing season, respectively. Time
indicates weeks from the beginning of the experiment. Bars indicate standard error (n = 4).
. . Basal Ste ia eter Incre e t
s l st i t f t s li i r i
t e
si ificantly bet een eeks 5 a t i ft r. i i ll
si ificant iffere ces i t i ter i r t t ee t
. fter t o eeks f ( ) til t f t ri t, st i t r
i cre e t as significantly smaller in the MGD than CTRL saplings (p < 0.05 at week 7,
p < 0.001 for the other times). Stem diameter increment was significantly s aller in
than in E (p < 0.05 for eek 10, p < 0.01 for weeks 11–18).
3.7. Root Length
The total root length was just slightly lower in EGD than that in CTRL after five
weeks of drought stress (week 5). However, it was significantly smaller in EGD than in
CTRL five weeks after resuming irrigation (week 10) and afterwards (p < 0.01, p < 0.05,
respectively). The total root length was significantly lower in MGD than in CTRL after
five weeks of drought stress (week 10, p < 0.001) and after resuming irrigation. There were
no significant differences in total root length between EGD and MGD in the end of the
experiment (Figure 8).
Forests 2021, 12, 1491 12 of 16Forests , , x FOR PEER REVIEW 13 of 17 
 
 
 
Figure 8. Root length of Mongolian pine saplings by treatment. CTRL indicates the control. 
EGD and MGD indicate drought stress treatment in the early- (EG) and mid- (MG) growing 
seasons, respectively. EG, MG and LG refer to the early- (weeks 1–5), mid- (weeks 6–10) and late- 
(weeks 11–18) growing season, respectively. Time indicates weeks from the beginning of the exper-
iment. Bars indicate standard error (n = 4). 
3.8. Sapling Mortality 
The total sapling mortality was low in CTRL throughout the whole experiment, being 
6.5% at the end of the experiment. In EGD, mortality increased after four weeks of drought 
and even after resuming irrigation (at weeks 6 and 9), being 22.6% at the end of the exper-
iment. In MGD, the mortality increased after four weeks of drought (week 9) and contin-
ued to increase quickly after resuming irrigation (at weeks 11, 14 and 17), reaching a final 
mortality of 45.2% (Table 1). 
Table 1. Sapling cumulative mortality of Mongolian pine saplings by treatment. CTRL indicates 
the control. EGD and MGD indicate drought stress treatment in the early- and mid- growing 
seasons, respectively. 
Time (Weeks) 
Sapling Cumulative Mortality (%) 
CTRL EGD MGD 
4 4 12.9 4.8 
6 4 16.1 6.5 
9 4.8 21.4 8.9 
11 4.8 21.4 19.4 
14 4.8 22.6 34.7 
17 6.5 22.6 45.2 
4. Discussion 
We studied the physiological and growth response of Mongolian pine saplings to 
drought during the early (the buds burst and new needles started elongating) and middle 
of the growing season (shoot elongation ending and needles elongating quickly). The re-
sults showed differences in response to different drought timing. This is consistent with 
Forner et al. (2018) [28] that the timing of extreme drought events can affect trees more 
than drought intensity. Our results confirmed our hypothesis that the saplings are less 
resistant to drought in the middle than in the beginning of the growing season. 
The drought in the early- and mid- growing season decreased stem diameter incre-
ment and needle length. This is in accordance with the previous study for European larch 
Figure 8. Root length of ongolian pine saplings by treat ent. CTRL indicates the control. EGD
and MGD indicate drought stress treatment in th early- (EG) and mid- (MG) growing seaso s,
respectiv ly. EG, MG and L refer to the early- (weeks 1–5), mid- (weeks 6–10) and late- (weeks
11–18) growing season, respectively. Time indicates we ks from the beginning of the exp rim nt.
Bars indicate standard error (n = 4).
3.8. Sapling ortality
The total sapling ortality as lo in CTRL throughout the whole experi ent, being
6.5 at the end of the experiment. In EGD, mortality increased after four eeks of drought
and even after resuming irrigation (at weeks 6 and 9), being 22.6% at the end of the
experiment. In MGD, the mortality increased after four weeks of drought (week 9) and
continued to increase quickly after resuming irrigation (at weeks 11, 14 and 17), reaching a
final mortality of 45.2% (Table 1).
Table 1. Sapling cumulative mortality of Mongolian pine saplings by treatment. CTRL indicates the
contr l. EGD and MGD indicate drought stress treatment in the early- and mid- growing seaso s,
respectiv ly.
Time (Weeks)
Sapling Cumulative Mortality ( )
CTRL EGD MGD
4 4 12.9 4.8
6 4 16.1 6.5
9 4.8 21.4 8.9
11 4.8 21.4 19.4
14 4.8 22.6 34.7
17 6.5 22.6 45.2
4. Discussion
We studied the physiological and growth response of Mongolian pine saplings to
drought during the early (the buds burst and new needles started elongating) and middle
of the growing season (shoot elongation ending and needles elongating quickly). The
results showed differences in response to different drought timing. This is consistent with
Forner et al. (2018) [28] that the timing of extreme drought events can affect trees more
than drought intensity. Our results confirmed our hypothesis that the saplings are less
resistant to drought in the middle than in the beginning of the growing season.
The drought in the early- and mid- growing season decreased stem diameter increment
and needle length. This is in accordance with the previous study for European larch and
Norway spruce concerning the effects of spring drought [11]. The MGD reduced the stem
Forests 2021, 12, 1491 13 of 16
diameter even more than EGD, which was probably due to the timing of MGD in the
rapid phase of radial growth in the middle of the growing season [12]. Conversely, the
EGD saplings (drought until week 5) could compensate for the reduced growth when the
irrigation started.
The total chlorophyll content and chlorophyll fluorescence of saplings decreased when
the soil relative water content dropped down to 20% in both the early- and mid-growing
season, indicating that the photosynthetic apparatus and photochemistry of photosynthesis
were negatively affected. On the contrary, the chlorophyll fluorescence of European beech
seedlings (Fagus sylvatica L.) was not affected by drought stress during the period of
leaf development [29]. In previous-year needles, chlorophyll content was lower in EGD
compared to CTRL when the soil relative water content dropped down to 36% without
the reduction of Fv/Fm. This might be explained because the chlorophyll synthesized
quickly in control saplings during the early growing season, and the drought inhibited the
synthesis. The elevated REL of needles of the MGD saplings indicates severe cell membrane
injuries, which might have contributed to the reduction of chlorophyll fluorescence and
increased mortality, as compared to EGD and CTRL saplings.
The concentration of ABA in fine roots increased with drought during both growing
phases. This is in accordance with Ding et al. (2016) results that showed that root ABA
accumulation was induced by drought stress in rice seedlings. Elevated ABA concentration
in roots increased the aquaporin expression and activity with the drought [30], which is
associated with enhanced root hydraulic conductivity [31] and regulating the root hair
growth [32]. However, ABA concentration in previous-year needles was reduced by
drought in this study. This is inconsistent with the study for Scots pine and Sitka spruce
(Picea sitchensis (Bong.) Carr.), where the ABA concentration of xylem sap extruded from
current-year shoots increased 11 times as the drought progressed [33]. ABA synthesized in
roots is translocated to leaves, leading to stomatal closure and plant growth reduction to
adapt to drought stress [34]. The lower ABA concentration in the previous-year needles
than current-year shoot after drought might be related to the age class of the measured
needles. We assume that ABA was transported via transpiration stream more to the current-
year shoots and needles than to the old needles, therefore stomata of current-year needles
were probably closed, and CO2 could not enter into the mesophyll cell, which would lead
to a decrease in starch content, as it proved to be.
Starch content of needles, stems and roots decreased already during EGD and MGD,
and/or just after resuming irrigation (e.g., in EGD of roots). In addition, soluble sugar
content of needles was not affected by EGD, but increased in stems and decreased with a
delay in roots. In MGD, soluble sugar content decreased in needles, stems and roots, but
increased in needles after resuming irrigation. Therefore, production of carbohydrates by
photosynthesis was not affected by drought only, but its timing during the growing season
was also important too. In addition, drought seemed to change the allocation of starch
among the organs to support respiration and structural and reproductive growth [35].
Transportation of energy-rich compounds to roots decreased, thus leading to carbon star-
vation and shortage of energy for root function [36,37]. This was corresponding to the
reduced root length during the post-treatment period. Root growth was also reduced in
two-year-old Scots pine saplings after drought stress [38].
Drought occurring both in the early- and mid- growing season decreased the survival
rate of the saplings. Tree mortality associated with drought has been observed frequently
both in the boreal forest and globally [39,40]. As shown in this study, the timing of drought
also affected the mortality. The mortality of the saplings remarkably increased when
drought occurred in the mid growing season, when air temperature was temporarily high
and reached 38 ◦C at maximum. With the climate change, the extreme high temperature
is expected to increase in the distribution area of Mongolian pine in northern China [41].
In this study, the mortality of the control saplings did not increase, even though they
experienced the same temperature as the saplings in the drought treatments. This indicates
that the high temperature alone did not increase the sapling mortality. The joint effect
Forests 2021, 12, 1491 14 of 16
of high temperature and drought probably led to the sapling mortality. Many studies
demonstrated that mortality can occur faster in hotter drought in a review by Allen et al.
(2015) [42]. This is because that hotter drought increased risks of both hydraulic failure
(irreversible xylem dysfunction) and carbon starvation [43]. Carbon starvation might
have occurred since the starch and sugar content in fine roots, current-year needles, and
stems were reduced by drought in the mid growing season. In this study, the high sapling
mortality in MGD treatment might be also related to hydraulic failure, which was enhanced
by rising atmospheric moisture demand [44], as atmospheric moisture demand increases
largely with hotter temperature when accompanying drought [45].
5. Conclusions
The impacts of drought in the middle of the growing season on the Mongolian pine
saplings was more harmful in terms of the needle length, stem diameter, electrolyte leak-
age of needles and sapling mortality. In spite of the less effect of drought in the early
growing season on the saplings, it reduced root growth and allocation of non-structural
carbohydrates to roots as much as the drought in the middle of the growing season. Special
attention should be paid on forest sites susceptible to drought during afforestation in face
of ongoing climate change.
Author Contributions: Designing the experiment, A.-F.W. and G.Z.; Implementation of the experi-
ment and data collection, A.-M.D.; Data analysis and interpretation, A.-M.D., H.Q. and A.-F.W.; and
Manuscript writing, H.Q., A.-F.W., M.R., D.-Y.X. and G.Z., T.R. All authors have read and agreed to
the published version of the manuscript.
Funding: This work was funded by the Start-up Fund for Introduced Talents of Hebei Agricultural
University (YJ201813), the Fund for Introduced Returned Overseas Scholars of Hebei Province
(C20190339) and National Natural Science Foundation of China (32101250).
Acknowledgments: We thank Jun-Feng Tong for the assistance during the experiment and referees
for their comments to improve the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Dai, A. Characteristics and trends in various forms of the Palmer Drought Severity Index during 1900–2008. JGR-Atmos. 2011,
116, D12115. [CrossRef]
2. Zhao, M.; Running, S.W. Drought-induced reduction in global terrestrial net primary production from 2000 through 2009. Science
2010, 329, 940–943. [CrossRef]
3. Zhang, L.; Xiao, J.; Zhou, Y.; Zheng, Y.; Li, J.; Xiao, H. Drought events and their effects on vegetation productivity in China.
Ecosphere 2016, 7, e01591. [CrossRef]
4. Huang, M.; Wang, X.; Keenan, T.F.; Piao, S. Drought timing influences the legacy of tree growth recovery. Glob. Chang. Biol. 2018,
24, 3546–3559. [CrossRef]
5. Misson, L.; Limousin, J.M.; Rodriguez, R.; Letts, M.G. Leaf physiological responses to extreme droughts in Mediterranean
Quercus ilex forest. Plant Cell Environ. 2010, 33, 1898–1910. [CrossRef] [PubMed]
6. Camarero, J.J.; Franquesa, M.; Sanguesa-Barreda, G. Timing of drought triggers distinct growth responses in Holm Oak:
Implications to predict warming-induced forest defoliation and growth decline. Forests 2015, 6, 1576–1597. [CrossRef]
7. Castro, J.; Zamora, R.; Hódar, J.A.; Gómez, J.M. Alleviation of summer drought boosts establishment success of Pinus sylvestris in
a Mediterranean mountain: An experimental approach. Plant. Ecol. 2005, 181, 191. [CrossRef]
8. Zang, C.; Pretzsch, H.; Rothe, A. size-dependent responses to summer drought in Scots pine, Norway spruce and common oak.
Trees 2012, 26, 557–569. [CrossRef]
9. Muukkonen, P.; Nevalainen, S.; Lindgren, M.; Peltoniemi, M. Spatial occurrence of drought-associated damages in Finnish boreal
forests: Results from forest condition monitoring and GIS analysis. Boreal Environ. Res. 2015, 20, 172–180.
10. Gao, Y.; Markkanen, T.; Aurela, M.; Mammarella, I.; Thum, T.; Tsuruta, A.; Yang, H.; Aalto, T. Response of water use efficiency to
summer drought in a boreal Scots pine forest in Finland. Biogeosciences 2017, 14, 4409–4422. [CrossRef]
11. Vitasse, Y.; Bottero, A.; Cailleret, M.; Bigler, C.; Fonti, P.; Gessler, A.; Lévesque, M.; Rohner, B.; Weber, P.; Rigling, A.; et al.
Contrasting resistance and resilience to extreme drought and late spring frost in five major European tree species. Glob. Chang.
Biol. 2019, 25, 3781–3792. [CrossRef]
Forests 2021, 12, 1491 15 of 16
12. D’Orangeville, L.; Maxwell, J.; Kneeshaw, D.; Pederson, N.; Duchesne, L.; Logan, T.; Houle, D.; Arseneault, D.; Beier, C.M.; Bishop,
D.A.; et al. Drought timing and local climate determine the sensitivity of eastern temperate forests to drought. Glob. Chang. Biol.
2018, 24, 2339–2351. [CrossRef]
13. Zhang, X.; Manzanedo, R.D.; D’Orangeville, L.; Rademacher, T.T.; Li, J.; Bai, X.; Hou, M.; Chen, Z.; Zou, F.; Song, F.; et al.
Snowmelt and early to mid-growing season water availability augment tree growth during rapid warming in southern Asian
boreal forests. Glob. Chang. Biol. 2019, 25, 3462–3471. [CrossRef]
14. Li, M.-Y.; Fang, L.-D.; Duan, C.-Y.; Cao, Y.; Yin, H.; Ning, Q.-R.; Hao, G.-Y. Greater risk of hydraulic failure due to increased
drought threatens pine plantations in Horqin Sandy Land of northern China. For. Ecol. Manag. 2020, 461, 117980. [CrossRef]
15. Liu, Y.-Y.; Wang, A.-Y.; An, Y.-N.; Lian, P.-Y.; Wu, D.-D.; Zhu, J.-J.; Meinzer, F.C.; Hao, G.-Y. Hydraulics play an important role in
causing low growth rate and dieback of aging Pinus sylvestris var. mongolica trees in plantations of Northeast China. Plant Cell
Environ. 2018, 41, 1500–1511. [PubMed]
16. Baker, N.R. Chlorophyll fluorescence: A probe of photosynthesis in vivo. Ann. Rev. Plant. Biol. 2008, 59, 89–113. [CrossRef]
[PubMed]
17. Wiley, E.; Huepenbecker, S.; Casper, B.B.; Helliker, B.R. The effects of defoliation on carbon allocation: Can carbon limitation
reduce growth in favour of storage? Tree Physiol. 2013, 33, 1216–1228. [CrossRef]
18. Camarero, J.J.; Sangüesa-Barreda, G.; Vergarechea, M. Prior height, growth, and wood anatomy differently predispose to
drought-induced dieback in two Mediterranean oak species. Ann. For. Sci. 2016, 73, 341–351. [CrossRef]
19. Blake, J.; Ferrell, W.K. The association between soil and xylem water potential, leaf resistance, and abscisic acid content in
droughted seedlings of Douglas-fir (Pseudotsuga menziesii). Physiol. Plant. 1977, 39, 106–109. [CrossRef]
20. Zhang, J.; Davies, W.J. Increased synthesis of ABA in partially dehydrated root tips and ABA transport from roots to leaves. J.
Exp. Bot. 1987, 38, 2015–2023. [CrossRef]
21. Trejo, C.L.; Davies, W.J. Drought-induced closure of Phaseolus vulgaris L. stomata precedes leaf water deficit and any increase in
xylem ABA concentration. J. Exp. Bot. 1991, 42, 1507–1515. [CrossRef]
22. Wang, A.F.; Zhang, G.; Qie, Y.X.; Xiang, D.Y.; Di, B.; Chen, Z.P. Effects of drought on physiological characteristics of needles of
Pinus bungeana Zucc. seedlings. Plant. Physiol. J. 2012, 48, 189–196, (In Chinese with English abstract).
23. Wang, A.-F.; Di, B.; Repo, T.; Roitto, M.; Zhang, G. Responses of parameters for electrical impedance spectroscopy and pressure–
volume curves to drought stress in Pinus bungeana seedlings. Forests 2020, 11, 359. [CrossRef]
24. Zhang, X.Z. Determination of chlorophyll content in plants—Acetone and ethanol mixture method. Liaoning Agric. Sci. 1986, 3,
26–28.
25. Zhang, G.; Li, Y.-Q.; Dong, S.-H. Assessing frost hardiness of Pinus bungeana shoots and needles by electrical impedance
spectroscopy with and without freezing tests. J. Plant. Ecol. 2010, 3, 285–293. [CrossRef]
26. Hansen, J.; Møller, I. Percolation of starch and soluble carbohydrates from plant tissue for quantitative determination with
anthrone. Anal. Biochem. 1975, 68, 87–94. [CrossRef]
27. Yang, J.; Zhang, J.; Wang, Z.; Zhu, Q.; Wang, W. Hormonal changes in the grains of rice subjected to water stress during grain
filling. Plant. Physiol. 2001, 127, 315–323. [CrossRef]
28. Forner, A.; Valladares, F.; Bonal, D.; Granier, A.; Grossiord, C.; Aranda, I. Extreme droughts affecting Mediterranean tree species’
growth and water-use efficiency: The importance of timing. Tree Physiol. 2018, 38, 1127–1137. [CrossRef] [PubMed]
29. Gebauer, R.; Plichta, R.; Urban, J.; Volarˇík, D.; Hájícˇková, M. The resistance and resilience of European beech seedlings to drought
stress during the period of leaf development. Tree Physiol. 2020, 40, 1147–1164. [CrossRef] [PubMed]
30. Ding, L.; Li, Y.; Wang, Y.; Gao, L.; Wang, M.; Chaumont, F.; Shen, Q.; Guo, S. Root ABA accumulation enhances rice seedling
drought tolerance under ammonium supply: Interaction with aquaporins. Front. Plant. Sci. 2016, 7, 1206. [CrossRef]
31. Olaetxea, M.; Mora, V.; Bacaicoa, E.; Baigorri, R.; Garnica, M.; Fuentes, M.; Casanova, E.; Zamarreño, A.M.; Iriarte, J.C.; Etayo, D.;
et al. Abscisic acid regulation of root hydraulic conductivity and aquaporin gene expression is crucial to the plant shoot growth
enhancement caused by rhizosphere humic acids. Plant. Physiol. 2015, 169, 2587–2596. [PubMed]
32. Zhang, Y.; Xu, F.; Ding, Y.; Du, H.; Zhang, Q.; Dang, X.; Cao, Y.; Dodd, I.C.; Xu, W. Abscisic acid mediates barley rhizosheath
formation under mild soil drying by promoting root hair growth and auxin response. Plant Cell Environ. 2021, 44, 1935–1945.
[CrossRef]
33. Jackson, G.E.; Irvine, J.; Grace, J.; Khalil, A.A.M. Abscisic acid concentrations and fluxes in droughted conifer saplings. Plant Cell
Environ. 1995, 18, 13–22. [CrossRef]
34. Wilkinson, S.; Davies, W.J. Drought, ozone, ABA and ethylene: New insights from cell to plant to community. Plant Cell Environ.
2010, 33, 510–525. [CrossRef]
35. Hartmann, H.; Trumbore, S. Understanding the roles of nonstructural carbohydrates in forest trees—From what we can measure
to what we want to know. New Phytol. 2016, 211, 386–403. [CrossRef] [PubMed]
36. Sala, A.; Piper, F.; Hoch, G. Physiological mechanisms of drought induced tree mortality are far from being resolved. New Phytol.
2010, 186, 274–281. [CrossRef]
37. McDowell, N.G. Mechanisms linking drought, hydraulics, carbon metabolism, and vegetation mortality. Plant. Physiol. 2011, 155,
1051–1059. [CrossRef] [PubMed]
Forests 2021, 12, 1491 16 of 16
38. Pearson, M.; Saarinen, M.; Nummelin, L.; Heiskanen, J.; Roitto, M.; Sarjala, T.; Laine, J. Tolerance of peat-grown Scots pine
seedlings to waterlogging and drought: Morphological, physiological, and metabolic responses to stress. For. Ecol. Manag. 2013,
307, 43–53. [CrossRef]
39. Allen, C.D.; Macalady, K.A.; Chenchouni, H.; Bachelet, D.; McDowell, N.; Vennetier, M.; Kitzberger, T.; Rigling, A.; Breshears,
D.D.; Hogg, E.H.; et al. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for
forests. For. Ecol. Manag. 2010, 259, 660–684. [CrossRef]
40. Peng, C.; Ma, Z.; Lei, X.; Zhu, Q.; Chen, H.; Wang, W.; Liu, S.; Li, W.; Fang, X.; Zhou, X. A drought-induced pervasive increase in
tree mortality across Canada’s boreal forests. Nat. Clim. Chang. 2011, 1, 467–471. [CrossRef]
41. Zhang, G.W.; Zeng, G.; Yang, X.Y.; Jiang, Z.H. Future changes in extreme high temperature over China at 1.5 ◦C–5 ◦C global
warming based on CMIP6 simulations. Adv. Atmos. Sci. 2021, 38, 253–267. [CrossRef]
42. Allen, C.D.; Breshears, D.D.; McDowell, N.G. On underestimation of global vulnerability to tree mortality and forest die-off from
hotter drought in the Anthropocene. Ecosphere 2015, 6, 1–55. [CrossRef]
43. Adams, H.D.; Collins, A.D.; Briggs, S.P.; Vennetier, M.; Dickman, L.T.; Sevanto, S.A.; Garcia-Forner, N.; McDowell, N.G.
Experimental drought and heat can delay phenological development and reduce foliar and shoot growth in semiarid trees. Glob.
Chang. Biol. 2015, 21, 4210–4220. [CrossRef]
44. McDowell, N.; Pockman, W.T.; Allen, C.D.; Breshears, D.D.; Cobb, N.; Kolb, T.; Plaut, J.; Sperry, J.; West, A.; Williams, D.G.; et al.
Mechanisms of plant survival and mortality duringdrought: Why do some plants survive while others succumb to drought? New
Phytol. 2008, 178, 719–739. [CrossRef] [PubMed]
45. Breshears, D.D.; Adams, H.D.; Eamus, D.; McDowell, N.G.; Law, D.J.; Will, R.E.; Williams, A.P.; Zou, C.B. The critical amplifying
role of increasing atmospheric moisture demand on tree mortality and associated regional die-off. Front. Plant. Sci. 2013, 4, 266.
[CrossRef]