Bothalia - African Biodiversity & Conservation
ISSN: (Online) 2311-9284, (Print) 0006-8241
Page 1 of 8
Original Research
Aspects of the population biology, life history and
threats to Aloe ortholopha Christian and Milne-Redh.:
A serpentine endemic from the northern
Great Dyke of Zimbabwe
Authors:
Ngoni I. Kunonga1
Tamuka Nhiwatiwa1
Mduduzi Tembani2
Shakkie Kativu1
Affiliations:
1
Department of Biological
Sciences, University of
Zimbabwe, Zimbabwe
Forest Research Centre,
Harare, Zimbabwe
2
Corresponding author:
Ngoni Kunonga,
nkunonga@gmail.com
Dates:
Received: 17 July 2018
Accepted: 11 Dec. 2018
Published: 26 Feb. 2019
How to cite this article:
Kunonga, N.I., Nhiwatiwa, T.,
Tembani, M. & Kativu, S.,
2019, ‘Aspects of the
population biology, life
history and threats to Aloe
ortholopha Christian and
Milne-Redh.: A serpentine
endemic from the northern
Great Dyke of Zimbabwe’,
Bothalia 49(1), a2396.
https://doi.org/10.4102/abc.
v49i1.2396
Copyright:
© 2019. The Authors.
Licensee: AOSIS. This work
is licensed under the
Creative Commons
Attribution License.
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Background: Aloe ortholopha is a rare endemic confined to serpentine soils of the Great Dyke
of Zimbabwe. Its International Union for Conservation of Nature (IUCN) status is listed as
Vulnerable; however, its population biology and life history are poorly documented.
Objectives: The aim of this article is to provide information on the population biology and life
history of A. ortholopha through assessment of its size-class distribution, population size and
density, reproductive output and fitness, and threats related to fire and mining.
Method: Circumference of A. ortholopha leaf rosette was used to ascertain size-class distribution.
Population size and density were determined by enumerating flowering individuals. Percapita reproductive output was determined as mean number of flowers per plant, fruit set and
mean number of seeds per fruit. Fitness was determined from seed germination capacity.
Impact of fire and mining were recorded photographically.
Results: Determination of size-class distribution of A. ortholopha from three study sites
(southern region [SR], central region [CR] and northern region [NR]) revealed a bell-shaped
curve dominated by intermediate size classes. Population size (number of flowering
individuals) ranged from 36 to 66 per site. This translated to a density of 4.0–7.3 flowering
plants per hectare. Per-capita reproductive output, measured as mean number of flowers per
plant, was significantly different in SR and CR compared to that in the NR region. Mean
number of fruits per plant did not significantly differ across the three regions. Mean seed set
per plant in CR and NR was significantly different to that in the SR region. Species fitness, as
determined from in vitro germination assays, showed that seeds harvested from fire-damaged
capsules have the lowest cumulative germination percentage. It was also observed that leaf
rosettes curled up to form a ball that protects the apical centre of plants from fire damage.
Conclusion: A. ortholopha occurs in small population clusters of low density. The species has a
low per-capita reproductive output characterised by production of many flowers, but with
very low percentage fruit and seed set. The species has low fitness as evidenced by nominal
recruitment of saplings and juveniles. Conspecific mates are frequently lost owing to fire and
mining activities.
Introduction and site description
The Great Dyke of Zimbabwe spans some 530 kilometres (km) in length, with a width ranging
between 2 km and 12 km. It runs in a linear south-southwest (SSW) to north-northwest (NNW)
direction (Figure 1). The flora of the Dyke includes 30 serpentine endemics, 16 of which are
restricted to serpentine-bearing soils of the northern Dyke, while eight occur in both the northern
and southern Dyke (Mapaura 2002). According to Lande (1998) and Siebert, Van Wyk and
Bredenkamp (2001), sites of high endemism and rare species need special protection in view of
threats from anthropogenic activities. To date, habitats of the Dyke endemics have not received
any protected status. Studies on the Dyke flora have largely been limited to checklists. BarclaySmith (1963) provided the first comprehensive checklist for a small section of the northern Dyke
known as the Horseshoe Intensive Conservation Area. A follow-up study by Wild (1965) provided
the first comprehensive compilation of a flora checklist covering the entire Dyke. That study
identified 322 species, of which 20 were endemic to serpentine soils. Kunonga, Nhiwatiwa and
Kativu (2017) provided the first report on population size structure of a serpentine endemic
(Ozoroa longipetiolata) from the Great Dyke of Zimbabwe.
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Original Research
Zimbabwe
Northern Region (NR)
Kariba
Chinhoyi
Harare
Kadoma
Hwange
Gweru
Masvingo
Bulawayo
Beitbridge
Central Region (CR)
Chinhoyi
Glendale
Mazowe
Southern Region (SR)
N
Harare
Key
Sampling point
City/Town
Planum Deposits
1:750,000
Lake Manyame
0
12.5
25
50 Kilometers
Great Dyke
Source: Map created by Rutendo Chekeche
FIGURE 1: Map showing the Great Dyke of Zimbabwe. The northern portion of the Dyke is where the southern region (SR), central region (CR) and northern regions (NR)
are situated.
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Studies by Werger, Wild and Drummond (1978) noted lower
floristic diversity on the Dyke serpentine soils compared to
adjacent non-serpentine soils. Proctor and Craig (1978)
observed that non-serpentine Brachystegia spiciformis thrived
on serpentine soils near riverine areas. According to Robertson
and Meakin (1980), the high nickel (Ni) levels in Dyke
serpentine soils prevent establishment of B. spiciformis by
arresting mitosis and disrupting the cell membrane. Other
studies by Wild (1974) and Brooks and Yang (1984) on mineral
levels of Pearsonia metallifera, Blepharis acuminata, Merremia
xanthophylla and Searsia wildii (=Rhus wildii) concluded that
these Dyke endemics were hyperaccumulators of Ni. These
findings were supported by studies of Hunter (1954) and
Robertson (1985) that demonstrated how high Ni levels,
associated with the Dyke serpentine soils, had deleterious
effects on tobacco and Zea mays plants. Furthermore,
transplant experiments revealed that non-serpentine species
failed to establish or had retarded growth on serpentine soils
of the Dyke (Wiltshire 1974).
Aloe ortholopha is an endemic succulent of the northern
Great Dyke that is listed on the Southern African Plant Red
Data List as Vulnerable (VU A1d A2b). The species is
threatened by farming and mining activities (Mapaura &
Timberlake 2002). According to Barclay-Smith (1963), miningand farming-related disturbances along the Dyke date back
to 1918. Maponga and Ruzive (2002) provided evidence for
impact of tribute mining on the northern Dyke landscape.
Original Research
to colonise steep slopes and form clusters that aggregate on
serpentine soils with varying magnitudes of current and
historical mining-related disturbances.
Sampling focused on selected accessible subpopulations
that constituted the southern (SR), central (CR) and NR
regions of the northern Great Dyke (Figure 1). Within each
region, three subpopulations at least 1 km apart were
randomly selected. A line transect spanning the length of
each subpopulation was established. Three 1 ha plots were
laid along each transect. Thus, for each region, nine plots
were established, totalling 9 hectare (ha) of sampling area
per region. The established plots accounted for at least 90%
of all aloe plants that constituted the subpopulation. Each
study site was visited several times from June through to
November 2016, as part of an assessment of population sizeclass distribution, reproductive output and disturbance
factors (fire and mining activities).
Size-class distribution
Size-class distribution was estimated by measuring the
circumference of the leaf rosette of both reproductive and
sterile individuals, including saplings and juveniles.
Population size and density
The primary objective of the present study was to report on
aspects of the population biology of A. ortholopha, which
includes its population size structure, reproductive output
and fitness, and fire and mining disturbance factors.
Population size was determined by enumerating all
flowering individuals across the sampled subpopulations
in each of the three regions of the northern Great Dyke.
Density of flowering individuals per region was calculated
by dividing the number of reproductive individuals by
nine (number of hectares).
Materials and methods
Per-capita reproductive output
Study area and sampling sites
Observations were made on the biogeographical range of
A. ortholopha, a species restricted to serpentine soils of
the northern Great Dyke of Zimbabwe that spans some
130 km from Lake Manyame (formerly Darwendale Dam,
at 17°43’10”S, 30°32’14”E) to Mavuradonha (16°29’00”S,
30°55’11”E). The width of the northern Dyke ranges from
2 km to 12 km. The entire Great Dyke land cover is estimated
to be 3000 km2 (Brooks & Yang 1984). The Dyke terrain
consists of undulating mountain chains, with steep slopes
whose facades are littered with loose rocks, which greatly
limit access to prospective sampling sites. Access to sampling
sites is also restricted by mining companies that have exclusive
mining rights to large swaths of the Dyke. In addition, sites
within the northern (NR) region are near the Mavuradonha
wilderness where wild animals roam. As such, accurate
determination of area of occupancy and population size of
A. ortholopha was unattainable.
In light of these challenges, we identified over 42
subpopulations across the species distribution range. Most of
these subpopulations were observed with the aid of binoculars
as they were not easily accessible. These subpopulations tend
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In October 2016, there was a brief phenophase captured prior
to floral abscission where the inflorescence of a reproductive
individual displayed both fruits and wilting flowers. An
enumeration of both developing fruits and wilting flowers
per plant was made over several days at each site, to estimate
the number of flowers and proportion of flowers that
developed into fruit (Wilson et al. 2009). Number of flowers
per plant and number of fruits per plant were determined
from all individuals in a sampling site (Cousins 2013). Thirty
fruits were harvested from five randomly selected individuals
per sampled subpopulation and seed set was determined by
dissecting the fruit and counting the number of seeds. This
measure gave an estimate of the mean number of seeds per
fruit (Wilson et al. 2009).
Reproductive fitness
Preliminary germination assays performed at room
temperature (25 degrees Celsius [°C]) revealed a percentage
cumulative germination of less than 5% (Figure 3). To
improve on percentage cumulative germination, assays
were performed using seeds placed under varying chemical
and temperature treatments. Assays were performed using
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incubators and protocols set to the International Seed Testing
Association standards (ISTA 2010). Seeds were treated with
different concentrations (10−3, 10−4 and 10−5 M) of gibberellic
acid (GA) or potassium nitrate (KNO3), and were prepared
with sterile distilled water, incubated at 20°C, 25°C, 30°C
and 20/30°C, respectively. The control was seed treated with
sterile, distilled water. Seed germination was defined as the
appearance of a radicle ≥ 2 millimetres (mm). Germinated
seeds were scored daily for 21 days, and periodically
moistened under the respective treatments. For each
treatment, four replicates containing 25 seeds were used, and
randomly placed in the respective incubators.
Fire and mining disturbances
Photo documentation of the species was conducted in
situ during the rainy season, dry season and then prior and
post-veld fire (VF). Individuals of different size classes
growing under shade and in open areas were documented.
Seeds of fruits damaged by VF were harvested for viability
tests by germination assays. Evidence of habitat destruction
and fragmentation because of mining and mining-related
activities was documented photographically.
Original Research
Statistical analysis
A one-way analysis of variance (ANOVA) was performed to
compare the per-capita reproductive output of the three
regions with regard to the mean number of flowers per
plant, mean number of fruits per plant and mean number of
seeds per plant. Tukey’s pairwise multiple comparison test of
means was performed (p < 0.05).
A two-way ANOVA was performed to assess the effects of
chemical treatments (seven levels) and temperature (four
levels) and their interaction on germination percentage of
A. ortholopha seeds. To determine the differences among the
treatment means, Tukey’s pairwise multiple comparison test
was performed (p < 0.05).
Results
Size-class distribution
A. ortholopha, a stemless succulent, has a leaf rosette that
exhibits seasonal plasticity. During the wet season or when
the plant grows under shade of a nurse plant or rock, the
leaf rosette displays a horizontal shape that increases its
a
b
c
d
e
f
Source: Photos taken by Ngoni Ishe Kunonga
FIGURE 2: (a) Nurse rock providing shade and possibly minimising fire damage to the plant. (b)-(d) Leaf rosette forming a ball protecting the core from direct sunlight and
veld fire damage. (e) Fruits exposed to veld fire in situ. (f) Rocky site occupation with little herbage.
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circumference, exposing the rosette core (Figure 2a). During
the dry, hot season, the leaf rosette curls up towards the
centre, forming a ball that covers the apical centre
(Figure 2b–d). Population size-class distributions were
determined in the three regions to assess the recruitment
capacity of the species. A. ortholopha exhibited a bell-shaped
frequency distribution in all three Dyke regions (Figure 3).
a
25
(SR)
Frequency (%)
20
15
10
5
0
0
20
40
60
80
100 120 140 160 180 200 220 240
Circumference (cm)
b
50
(CR)
The least number of sapling and/or juvenile size classes
were recorded in the NR region. The largest size classes were
observed in the SR and CR region (Figure 3).
Population size, density and per-capita
reproductive output
Population size, as a count of number of flowering
individuals, revealed that the CR region had the highest
number of flowering individuals, 66 on 9 ha plots with a
corresponding density of 7.3 flowering individuals per ha
(Table 1). The CR region also exhibited the largest number of
non-flowering individuals, 118 per 9 ha plot, with a
corresponding density of 13.2 non-flowering individuals per
ha. Assessment of the per-capita reproductive output showed
that the CR regions had the highest total number of flowers
produced at 6692. A one-way ANOVA revealed no significant
(p < 0.05) differences with respect to the mean number of
flowers per plant between SR and CR regions; however, both
regions were significantly different to the NR region. There
was no significant difference across the three regions with
respect to mean fruit set per plant. However, the mean
number of seed set per fruit was statistically similar between
the CR and NR regions, while both regions were significantly
different from the SR region.
Reproductive fitness
A two-way ANOVA showed significant (p < 0.05) interaction
between temperature and treatment that affected the
cumulative percentage of germination of A. ortholopha seeds
(Table 2). Germination was poor in almost all the treatments
at a temperature of 30°C (Figure 3). When treatments were
assessed separately, potassium nitrate treatments (KNO3,
KNO4 and KNO5) set at concentrations 10−3, 10−4 and 10−5 M,
respectively, recorded higher germination percentage
at 25°C. Seed exposed to VF exhibited low germination
40
Frequency (%)
Original Research
30
20
10
TABLE 1: Summary statistics of A. ortholopha surveyed on 9 ha plots.
0
0
20
40
60
80
100 120 140 160 180 200 220 240
Circumference (cm)
c
50
(NR)
Frequencey (%)
40
Variables
SR
CR
Number of non-flowering plants
66
118
83
Population size (flowering plants)
36
66
45
Population density (number of
flowering plants/ha)
4
7.3
5
Fruit set percent per plant (%)
NR
33.5
42.7
65.1
Mean number of flowers per plant
122.4 ± 71.9a
101.4 ± 72.7a
71.7 ± 41.6b
Mean number of fruits per plant
40.9 ± 38.3.9a
42.5 ± 39.4a
46.7 ± 39.5a
Mean number of seed per fruit
72.8 ± 13.4a
91.2 ± 14.7b
86 ± 13.2b
Note: The acronyms SR, CR and NR represent the southern, central, northern regions of the
Great Dyke of Zimbabwe respectively (Figure 1). Means with different superscript letters
denote groups that are statistically different (p = 0.05). For example, the mean number of
flowers per plant of SR and CR regions shares a same superscript (a) denoting that they are
statistically similar to each other while both are statistically different to NR region with the
superscript (b).
30
20
TABLE 2: A two-way analysis of variance showing the effects of chemical
and temperature treatments and their interaction on the germination of
A. ortholopha seeds.
10
Source of variation
0
0
20
40
60
80
100 120 140 160 180 200 220 240
Circumference (cm)
FIGURE 3: Frequency of size-class distribution of A. ortholopha in the three
regions: northern region (NR), central region (CR) and southern region (SR).
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SS
Mean square
F
Pr > F
Treatment
91.0937500
13.0133929
3.51
0.0021
Temperature
122.5937500
40.8645833
11.02
< 0.0001
Treatment × temperature
190.5312500
9.0729167
2.45
0.0017
Note: SS is the sum of squares; F-value represents the difference in variation among
sample means to variation among groups, while Pr > F provides the probability for the
F-value.
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percentage regardless of temperature. Seeds treated with
10−5 M GA exhibited the highest cumulative germination
rate at 25°C. A. ortholopha recorded low seed cumulative
germination rates under different temperature and chemical
treatment, suggestive of low overall fitness (Figure 4).
Fire and mining disturbances
Outer leaves of the rosette formed a parasol protecting the
core of both juvenile and adult plants from direct sunlight
by providing shade to the delicate core while also protecting
the core from annual VF (Figure 2c–d). The fruit formation
phenophase of A. ortholopha coincided with annual VFs,
with fruits displaying fire damage (Figure 2e). Figure 2f
shows that individuals growing on rocky soils were less
35
20 °C
25 °C
30 °C
KN03
KN04
KN05
20/30 °C
Germinaon (%)
30
25
20
15
10
5
0
Control
GA3
GA4
GA5
Original Research
prone to fire damage. Saplings establishing on rock crevasses
were also less prone to fire damage.
Figure 5a–c illustrates the anthropogenic-driven habitat
destruction and fragmentation because of open cast mining.
Open cast mining formed large mining dumps on selected,
disused dumps. There was evidence of re-establishment of
the species on these disused dumps (Figure 5d).
Discussion
A. ortholopha exhibited a bell-shaped size-class distribution
dominated by middle size-class individuals, with fewer
small and large class sizes. The bell-shaped distribution
pattern is generally indicative of an unstable population with
low recruitment and loss of adult reproductive class sizes.
For threatened rare endemics, recurring fire disturbance is an
important factor in their recruitment effort. To this end, postVF visits showed loss of A. ortholopha saplings because of
incineration of individuals on sites with herbage. A. ortholopha
saplings that established on rock crevices appeared to be
protected from fire. Kumara plicatilis (=Aloe plicatilis), from the
Western Cape province of South Africa, exhibits a similar
fire avoidance strategy. Cousins (2013) reported that for
K. plicatilis, there appeared to be a trade-off between growing
on unfavourable rocky sites and protection from VFs, which
resulted in stunted growth. With no historical records, it is
not clear whether A. ortholopha on rocky microsites exhibits
size plasticity similar to that recorded for K. plicatilis.
VF
Treatments
FIGURE 4: Germination assay results performed for 21 days using seeds pooled
from three regions (southern, central and northern regions). Under the
respective temperature, the control was seed treated with sterile distilled water.
The remaining seed were treated with gerberellic acid (GA3, GA4 and GA5) or
potassium nitrate (KNO3, KNO4 and KNO5) set at concentrations 10−3, 10−4 and
10−5 M, respectively. Seeds harvested from fruits exposed to veld fire (VF) were
incubated at the respective temperatures.
Several studies have attributed the bell-shaped population
size distribution to anthropogenic disturbances, episodic
recruitment and rainfall pattern. Anthropogenic disturbance
was a key factor in determining a bell-shaped population
size distribution pattern of Sclerocarya birrea (Helm &
Witkowski 2012). In the present study, episodic recruitment
c
a
d
b
Source: Photos taken by Ngoni Ishe Kunonga
FIGURE 5: (a)-(c) Sites showing habitat loss and destruction by open cast mining and shallow shaft mines that litter the Dyke. (d) Reclamation of an abandoned mining site
by A. ortholopha.
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of A. ortholopha at the subpopulation level was observed at
Mapinge Pass and Vanad Mine, as no flowering individuals
were observed during the 2012 and 2014 flowering
seasons, yet in all other years, the subpopulations flowered.
Studies by Brum (1973), Van Blerk (2013), and Cousins,
Witkowski and Pfab (2014) indicated correlation between
total precipitation and episodic recruitment resulting in
a bell-shaped curve of in Aloidendron dichotomum (=Aloe
dichotoma), Carnegiea gigantean (a desert succulent) and
K. plicatilis (=A. plicatilis), respectively.
Fire appeared to retard or lower the germination capacity of
seeds harvested from capsules exposed to VF in situ. Seeds
exposed to fire generally recorded the lowest germination
rates, which partly explains the low recruitment levels
represented by the bell-shaped distribution pattern.
A. ortholopha subpopulations tend to be small, isolated,
with sparsely distributed individuals. Populations with
similar structure were shown to be susceptible to stochastic
disturbance factors that could lead to local extirpation
(Munzbergova 2006). Small populations tend to have low fruit
set, seed set and seed germination capacity. A. ortholopha had
low numbers of reproductive individuals. Such populations
were reported to suffer from increased inbreeding (Morgan
1999; Reed 2005). They also tend to face increased mate finding
challenges when conspecific mates are lost because of habitat
destruction. This subsequently results in reduced population
size and density (Steven & Waller 2007). Continued loss of
conspecific mates, coupled with small population size and low
density, may lead to genetic bottlenecks (Gascoigne et al.
2009). Courchamp, Berec and Gascoigne (2008), on the other
hand, pointed out that threatened conspecific subpopulations
could be rescued by meta-populations (also referred to as
‘rescue effect’). In addition, small fragmented populations
were shown to be less attractive to pollinators (Agren 1996;
Dauber et al. 2010). With each subpopulation being sparsely
populated, this presents another reproductive hurdle as
pollinators are not attracted to sparsely populated areas,
which require higher energy expenditure for nectar foraging
(Mustajarvi et al. 2001). The adaptive mechanism of
A. ortholopha in attracting pollinators could include displaying
large numbers of flowers per plant, making each pollination
visit profitable for nectar seekers during periods when
competing plants are not in flower.
The use of leaves as insulators against fire damage is a
recurring phenomenon among aloes. Tall, single stemmed
aloes, such as Aloe ferox, have their stem protected from fire
damage and browsers by the remains of dead leaves (Bond
1983). High mortality rates recorded for A. ferox because
of fire damage were primarily in size classes of less than
2.0 metres (m), as they have a thinner insulator layer of
dead leaves. Fires of a height range of 0.2 m – 1.0 m produce
the hottest flames, and may prove detrimental to aloes
(Bond 1983). A. ortholopha is a stemless aloe whose height is
less than 1.0 m (Kimberley 1974). The adaptation strategy of
A. ortholopha against fire damage appears to be similar to that
of Aloe peglerae (a stemless aloe) where the leaf rosette forms
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Original Research
a protective ball that shields the delicate apical centre (Arena,
Witkowski & Symes 2015). The protective ball forms during
the late dry season prior to the onset of rains. Results from
the present study showed that individuals growing under
shade or nurse rock do not form a protective ball. In addition,
A. ortholopha appears to avoid fire damage by growing on
shallow, rocky soils and in rock crevasses. These microsites
are not favourable for the growth of herbage forming species,
thus minimising fuel load. Mwafongo (2000) observed a
positive linear correlation in the number of fire undamaged
K. plicatilis plants and rockiness of the microsite. This is an
adaptive strategy observed in several plants, including the
fan aloe (K. plicatilis) and A. peglerae (Arena et al. 2015;
Cousins, Witkowski & Pfab 2016). Leaves of A. ortholopha
saplings are too small to cover and protect the rosette core
from fire damage. It is, therefore, not clear to what extent fire
affects recruitment in A. ortholopha. This is a subject for future
studies.
Conclusions and recommendations
Survival of A. ortholopha on the northern Great Dyke
of Zimbabwe appears to be threatened by habitat loss
resulting from mining-related activities and fire-related
disturbances. Lack of historical records on population
biology of the species and its conservation status prior to the
present-day anthropogenic activities present challenges to
the understanding of recruitment pattern and long-term
survival of the species.
Acknowledgements
The authors are grateful to the invaluable assistance provided
by the technical team from the Department of Biological
Sciences at the University of Zimbabwe and that from the
Forest Research Centre, Harare. The teams assisted in field
sampling and technical assistance on the use of laboratory
equipment.
Competing interests
The authors declare that they have no financial or personal
relationships that may have inappropriately influenced them
in writing this article.
Authors’ contributions
S.K. supervised the project and assisted in experimental
design and drafting of the manuscript. T.N. assisted in
data analysis and drafting of the manuscript. N.I.K. was
responsible for the experimental design, fieldwork and
drafting of the manuscript. M.T. assisted in designing and
monitoring of seed germination assays.
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