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Plant Ecol (2007) 191:67–75
DOI 10.1007/s11258-006-9214-4
ORIGINAL PAPER
Reproductive allocation, seed dispersal and germination
of Myricaria laxiflora, an endangered species
in the Three Gorges Reservoir area
Fang-Qing Chen Æ Zong-Qiang Xie
Received: 7 February 2006 / Accepted: 11 September 2006 / Published online: 10 November 2006
Springer Science+Business Media B.V. 2006
Abstract Myricaria laxiflora is an endangered
plant that grows in the flood zone along the
Yangtze River in the Three Gorges area from
70 m to 155 m above sea level. To understand the
spatial distribution patterns of the species and to
provide information for developing conservation
strategies, we used field surveys to study its seed
reproduction and dispersion, and used growth
chambers to study seed germination. Results
showed that M. laxiflora produced many flowering branches, inflorescences and seeds. Seeds
were very small and output was high although
biomass allocation to reproduction was low
(~4%). Reproductive allocation was strongly
correlated with the biomass of stems and leaves.
Seeds were dispersed either by the wind or the
river current. Wind-dispersed seeds usually settled within 25 m from parent plants leading to a
clumped distribution of individuals in populations. Water-dispersed seeds often landed and
established on strands of firth where the fine
F.-Q. Chen Z.-Q. Xie (&)
The Laboratory of Quantitative Vegetation Ecology,
Institute of Botany, Chinese Academy of Science,
Beijing 100093, P.R. China
e-mail: [email protected]
F.-Q. Chen
Graduate School of Chinese Academy of Science,
Beijing 100049, P.R. China
e-mail: [email protected]
sediment and gentle sloping were available.
Seedlings that emerged from water-dispersed
seeds were distributed along the water flood
line.The life-span of M. laxiflora seeds was about
7 days. Seeds could germinate within 24 h when
they absorbed adequate amounts of water. Soil
water content was a key factor limiting the
establishment ability of M. laxiflora. Experiments
showed that the minimum soil water content for
germination to occur was 10% on sand or 17% on
sandy soil substrates, and the optimal conditions
were on saturated soils. The water content of
sandy soils on the riverbank was lower than 10%
in autumn, the dry season, and seeds were able to
germinate only on sandy beaches that were
intermittently inundated by the fluctuating river
current. These characteristics of seed dispersal
and germination limit the ability for M. laxiflora
to expand its distribution. These results provide
information essential for the conservation and
reintroduction of this endangered species.
Keywords Soil water content Biomass Riverbank Distribution Three Gorges area
Introduction
A large of proportion of plant species are rare
and have limited distributions (Manfred et al.
2004; Mills and Schwartz 2005). For conservation
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purposes, it is important to determine what traits
limit a species from expanding its geographic
range, prevent it from increasing in abundance, or
allow it to persist even at low abundances. The
geographical distribution of plant species is
related to the development, dispersal ability,
and germination requirements of seeds (Ford
et al. 1983). Many plant species are rare and
endangered just because the processes of development, dispersal, and germination of diaspore
throughout are hampered or limited (Manfred
et al. 2004). Understanding these processes not
only will contribute to understanding the underlying mechanisms that promote a species rarity
but also provide important information for species conservation.
Resource acquisition and resource allocation
to reproduction determine seed output. In perennial plants, the reproductive strategy typically
shifts towards an increase in the proportion of
resources allocated with plant age (Zhang and
Jiang 2002; Pickering and Arthur 2003; Zhao
et al. 2004). Seed shape and volume, an important
factor for adaptation of species to their environment, determines the dispersal distance and
spatial area of seed coverage (Kelly and Purvis
1993; Jakobsson and Eriksson 2000).
Seed-dispersal patterns not only determine the
potential area for plant recruitment to occur but
also serve as a template for subsequent processes,
such as competition and genetic exchange (Tilman
and Kareiva 1997; Lookingbill and Zavala 2000;
Joseph et al. 2005). Effective seed dispersal is
influenced by spread time, spread range, recruitment ability of seeds, spatial distribution of
establishment sites, and landscape structure (Horn
et al. 2001; Oliver et al. 2003; Soons et al. 2004).
The dispersal of seeds and other propagulum is
often a complex, multi-step process. Seed-dispersal patterns can be studied by following the
fate of marked seeds from their sources, using
genetic markers to establish the source of dispersed seeds (Ouborg et al. 1999; He et al. 2004),
or by documenting variation in seed deposition or
density with distance from their source (Willson
1993). Seed-dispersal patterns also can be inferred
from the distributions of seedlings (Schupp and
Fuentes 1995). It is difficult to study seed dispersal
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Plant Ecol (2007) 191:67–75
in wetlands using seed traps as flooding washes
seeds out of the traps. Seedling and establishment
will reflect the pattern of seed dispersal given that
germination, establishment and survival are not
limiting. The distribution of seedlings around their
parent plants can be used to estimate seed
dispersal patterns. This approach has been applied
effectively in studies on seed dispersal of tree
species (Houle 1982; Clark 1998).
Favorable conditions for seed germination and
seedling establishment differ among species,
which can result in the zonation of species along
environmental gradients (Boeken et al. 2004;
Kunstler et al. 2004). Parolin et al. (2004) found
that the combination of different adaptive characteristics of seed germination and seedling
development resulted in a variety of growth
strategies among trees, which could explain species distribution patterns and zonation along the
flooding gradient within Amazonian floodplain
systems. Studies on seed traits and seed germination can provide important information for the
conservation of endemic species (Manfred et al.
2004).
Myricaria laxiflora is restricted to the riverbanks of the Yangtze River in the Three Gorges
area. Scattered populations have a narrow
distribution (north latitude 2941¢43¢¢–3103¢57¢¢,
east longitude 10658¢38¢¢–11055¢55¢¢; altitude
70–155 m) (Wang et al. 2003; Chen et al. 2005a,
b). The highest water level of the Three Gorges
Reservoir will reach 175 m with a new flood zone
between 145 m and 175 m when the dam is
completed in 2009. All the habitats of M. laxiflora
will be plunged under water owing to the
construction of the dam (Changjiang Water
Resource Commission 1997; Wu et al. 1998; Chen
et al. 2005b). Species reintroductions at the higher water mark are most likely required in order to
preserve this species. The present study aims to
uncover the factors that restrict the distribution of
M. laxiflora and provide important information
for the establishment and management in this
species if it is to be reintroduced. The specific
objectives are to (1) quantify plant reproductive
allocation, (2) determine spatial patterns of seeddispersal, and (3) determine the factors that
influence seed germination.
Plant Ecol (2007) 191:67–75
Materials and methods
Study species
Myricaria laxiflora, a shrub species of Tamaricaceae, is 1–1.5 m in height. Its old branches are
red-brown or purple-brown in color. Branches of
current year are green or red-brown. Every year,
after 3–6 months dormancy induced by flooding,
it begins to sprout leaves and grows in September.
The species usually sets flowers and fruits from
October to April in the following year, depending
on the time of its appearance above water.
Racemes are usually terminal and 6–12 cm in
length. Capsule are narrowly conical and 6–8 mm
in length. Seeds are 1–1.5 mm in length. Seeds are
mainly dispersed by wind, but they also can be
dispersed by river water flow. Following germination will occur on sand beach in the following
weeks after seeds’ disperse under favorable environmental conditions.
Reproductive allocation
We sampled 20 individuals of each size group in a
field population distributed in Zigui County in
April 2003. Plants were dug up and separated into
roots, stems, leaves, and inflorescence. Plant age
was determined based on its tree-ring from the
stem basis. All plant material was dried to a
constant mass in an oven at 60C and weighed.
The proportion of the total biomass accounted for
by inflorescences was used as an estimate of
reproductive allocation.
Seed dispersal
Seedling dispersal of wind-dispersed seeds was
investigated on two common riverbank landform
types, convex and concave banks. Seedling abundance was determined around five parent plants
on each landform type. The number of seedlings
in the distribution area was counted in concentric
circles around each parent plant with radial
intervals of 1 m. A secondary dispersal pattern
was studied by investigating the distribution of
seedlings that were water dispersed and established on river bends where no parent plants were
present. We investigated six contrasting sites with
69
to survey the relationship between seedling
establishment and environment. The appropriate
habitat for water-dispersal seed germination and
seedling establishment was indicated in a sketch
map to give a concept model.
Seed germination
Inflorescences were collected from riverbank,
Zigui county, Hubei province in October. All
fruits were stripped off the inflorescences manually, and then wrapped in cheesecloth. The
wrapped fruits were put in a plastic bag and kept
in an icebox at 5C. Mature seeds were selected
for germination experiments. As seeds germinate
and seedling established on riverbanks during
several days following seed dispersal, the critical
environmental factors limiting seed germination
and seedling establishment were soil water content and soil type. A factorial experiment was
designed to test the effects of soil type and soil
water content on seed germination by using three
soil types, eight soil water content treatments, and
four replicates. The three soil types were sand
substrate, sandy soil, and a clay soil. All soil materials were taken from the study area. Soil water
content treatments (w/w) included 5, 9, 13, 17, 21,
25, 29, and 31%. The experimental unit consisted
of 50 mature seeds placed on a plate filled with
50 g of a soil substrate treated with one of the
eight soil water content treatments. Seed germination trials were performed in growth chamber
in October, under 12 : 12 h light to dark cycle.
Temperature was kept constant at 18C, the
average temperature in field in October. Water
content was adjusted every day, and germinated
seeds were counted and removed on a daily basis.
The experiment lasted for 7 days.
One hundred seeds were placed on a sandy
substrate with saturated soil water content and
germinated in a culture box at 18C to study
the germination dynamics of seeds over time.
Germinated seeds were removed every 2 h over a
48 h period.
Some collected inflorescences were kept under
natural conditions (Temperature 13–23C, Humidity 75%). One hundred seeds released from
fruits were germinated on a sandy substrate with
saturated soil water content at 18C every day for
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Plant Ecol (2007) 191:67–75
a week to observe changes in seed viability
over time.
25
20
The correlation between biomass and plant age
was examined by Pearson’s correlation coefficient.
Simple Linear Regression Analysis with the
dependent variable ‘inflorescence biomass’ and
independent variables ‘stem biomass’ and ‘leaf
biomass’, respectively, and Stepwise Linear
Regression analysis with dependent variable
‘inflorescence biomass’ and independent variables
‘stem biomass’ and ‘leaf biomass’ were conducted.
Multiple analysis of variance (MANOVA) with
the dependent variable ‘seed germination’ and
fixed factors ‘soil type’ and ‘soil water content’ was
used first to investigate the effects of soil type, soil
water content, and their interaction on seed
germination. Then the effects of variables were
analyzed using one-way analysis of variance
(ANOVA) after In-transformation of data and
means were compared with LSD test. All analyses
were conducted using SPSS software (11.0).
Seedling (%)
Data analysis
15
10
5
0
5
10
15
20
Dispersal distance (m)
Fig. 1 Dispersal distance of seedlings from parent plants.
The seedlings emerged from wind dispersed seeds on
convex riverbanks. All the 1-year-old seedlings in the
distribution areas were counted. About 90% seedlings
were distributed within a distance of 10 m from the parent
plant
Inflorescence biomass was significantly related
to the biomass of the stem and leaves but not the
root. The following forms are their correlation
equations.
The relationship between inflorescence biomass and leaf biomass:
Results
Y ¼ 0:131 þ 1:622 Xleaf
Reproductive allocation
The relationship between inflorescence biomass
and stem biomass:
Myricaria laxiflora began to flower and to fruit
1–2 months after the floodwaters receded, and
many plants in the field were in flower and
fruiting from October of the current year to April
in the next year after the flood season (June–
October). Inflorescence biomass usually accounted for about 4% of the total plant biomass
(Fig. 1), however, seed production was very high
and seeds were small (0.15 g per 1,000 seeds). A
mature plant typically developed 5–60 flowering
branches, and the length of a flowering branch
ranged from 10 cm to 100 cm. At times, a
flowering branch developed a raceme, which
would have 20–170 fruits per raceme. Each fruit
produced about 100 seeds. A flowering branch
sometimes developed only a few branches but it
also developed racemes so that the seed output of
individual plants was consistently high (from
5 · 103 to 10 · 106).
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Y ¼ 0:569 þ 0:0642 Xstem
r ¼ 0:919; P\0:001
r ¼ 0:655; P\0:05
The relationship between inflorescence biomass
and leaf biomass/stem biomass:
Y ¼0:0612 þ 0:0059 Xstem þ 1:549 Xleaf
r ¼0:920; P\0:001
The biomass of inflorescence, stems, leaves, and
roots were positively related to plant age
(Table 1). We could infer indirectly that seed
output was also positively related to plant age.
Plants usually began to fruit in 2-year-old plants.
However, reproductive allocation did not show
any positive relationships with age. Reproductive
allocation reached the highest levels in 3- to 4year-old plants and then declined in 5- to 6- yearold plants.
Plant Ecol (2007) 191:67–75
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Table 1 Biomass and biomass allocation of different aged Myricaria laxiflora plants
Age (year)
Total
mass (g)
Mean ± SD
Root
mass (g)
Mean ± SD
Stem
mass (g)
Mean ± SD
Leaf
mass (g)
Mean ± SD
Inflorescence
mass (g)
Mean ± SD
Reproductive
allocation (%)
Mean ± SD
1–2
3–4
5–6
n
r
23.99 ± 10.19
56.29 ± 33.12
82.65 ± 34.67
20
0.80ns
7.23 ± 3.75
21.75 ± 21.69
38.40 ± 14.27
20
0.82*
14.59 ± 6.21
30.29 ± 11.94
39.38 ± 18.60
20
0.76*
0.85 ± 0.07
1.66 ± 1.24
1.68 ± 1.01
20
0.34ns
1.31 ± 0.55
2.59 ± 2.16
3.19 ± 1.59
20
0.45ns
5.47 ± 0.90
5.63 ± 5.10
3.77 ± 0.87
20
0.44ns
Mass means the dry mass; n is the individuals; r is the Pearson’s correlation coefficients between plant age with biomass
(total, root, stem, leaf inflorescence) and reproductive allocation. *The correlation is significant at P < 0.05 level; nsThe
correlation is not significant at P < 0.05 level
Seed dispersal
Seeds were dispersed primarily by wind. Field
investigations showed that wind-dispersal resulted in the distribution of seedlings in fanshaped zone. Landform played an important role
in determining the seed dispersal pattern. In the
riverside convex riverbanks seedling distribution
zone, about 90% of the seedlings were distributed
within a distance of 10 m of the parent plant
(Fig. 1). To the contrary, in concave or sunken
riverbanks seedling distribution zone, about 90%
of the seedlings were distributed within a distance
of 5 m of the parent plant. Both of the seedling
distribution patterns only indicated the seed
dispersal patterns around parent plant. Some
seeds probably traveled too far to be followed
their fate.
The seed hairs on the surface of M. laxiflora
seeds enhanced the ability for seeds to adhere to
the soil surface and germinate. The seed hairs also
helped seeds to float on the water surface and
spread to new habitats over long distances when
washed off the riverbank. The landscape of the
riverbank influenced the establishment and
resulting spatial patterns of water-dispersed
seeds. Water-dispersed seeds were deposited on
banks of river bends where the fine sediment and
gentle sloping were available and germinated
along the water flood line (Fig. 2).
Seed germination
MANOVA tests showed that both the effects of
soil type, soil water content, and their interaction
had a significant effect on seed germination
Fig. 2 The distribution of seedlings germinated from
water-dispersed seeds. Seedlings typically established on
river bends where the fine sediment and gentle sloping
were available
(Table 2). Seed germination increased with an
increase in soil water content and reached maximum germination rates when soils were saturated (Fig. 3). The water holding capacity of the
soil and soil pore space differed among soil types,
and, consequently, seed germination differed
significantly among soil types. Seed germination
was highest on the sand substrate and lowest on
the sandy soil substrate. The minimum and
optimal soil water content for seed germination
also differed among soil types. Seeds began to
germinate on the sand substrate at 10% soil water
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Plant Ecol (2007) 191:67–75
Table 2 The effect of soil type and soil water content on germination of Myricaria laxiflora seeds
Treatments
Type III of squares
df
Mean square
F
Mean ± SD
P
Soil water content
Soil type
Soil water content*soil type
Error
Total
20277.78
8055.25
5886.31
1238.67
76356.00
7
2
14
48
72
2896.83
4027.63
420.45
25.81
112.26
156.08
16.29
23.83 ± 23.35
23.83 ± 23.35
<0.001
<0.001
<0.001
The result of MANOVA with fixed factors soil water content and soil type
60
Sandy soil
Sand
Caly soil
60
50
40
30
20
10
0
5
10
15
20
25
30
35
Soil water content (%)
Fig. 4 Effect of soil type on seed germination (means ± SE). Seeds germinated in all three kinds of soils, and
sand was the most fitted one. However, the clay soil was
suitable for seed germination when soil water content
ranged from 25% to 30%
Discussion
The seed reproduction of M. laxiflora is very high
and its seeds are very small and light. High seed
a
a
40
b
30
20
100
c
10
0
b
c
c
5
10
15
20
25
30
35
Soil water content (%)
Fig. 3 The effect of soil water content on the seed
germination of Myricaria laxiflora seeds. Values are means
of all pots exposed to each moisture level (means ± SE).
One-way ANOVA with dependent variable germination
percentage and fixed factor revealed that the germination
percentage under treatment with different letters is
different significantly at P < 0.05 level. The germination
percentage increased with an increase in the soil water
content but declined when soil water content was over
30%
123
Seed germination (%)
Seed germination (%)
a
50
70
Seed germination (%)
content, and the highest germination rates
occurred at 23% soil water, which was the
saturated soil water content where a water film
appeared on the surface of the sand substrate.
The water film allowed the seed hair to cling to
the substrate surface. The minimum and optimal
soil water content for seed germination on the
sandy soil were 17% and 26%, respectively, and
13% and 29%, respectively, for the clay soil
(Fig. 4). Seed germination decreased when the
water layer over the soil surface was too deep.
Seeds of M. laxiflora on saturated soils began
to germinate within 8 h at 18C, and germination
increased sharply between 11 h and 15 h. All
seeds germinated within a 24-h period (Fig. 5).
Seed vitality and germination declined after 24 h
since the time of released from the fruit, and
seeds almost lost their ability to germinate after a
week (Fig. 6).
80
60
40
20
0
8
10
12
14
16
18
20
22
24
26
Time (hr)
Fig. 5 Germination of Myricaria laxiflora seeds over time
on soil with saturated water content. Seeds began
to germinate in 8 h and nearly all of them germinated
within 24 h
Plant Ecol (2007) 191:67–75
73
Seed Germination (%)
100
80
60
40
20
0
1
2
3
4
5
6
7
Time (Day)
Fig. 6 The change in Myricaria laxiflora seed vitality over
time following release from the parent plants
output is an ecological adaptation by species that
experience low seedling recruitment (Willson
1993; Parolin 2001). M. laxiflora experiences
many environmental stresses during seed dispersal and germination, such as drought and
flooding, and only a few of seeds survive and
germinate. High seed production results in a
greater chance of having a seedling successfully
establish at a new site.
Seed dispersal is the critical stage of a plant’s
life history that allows it to move. Dispersal
patterns not only dictate the potential range of
new seedlings but also provide a means to access
new suitable environments that ultimately determine the spatial pattern and distribution of new
recruits (Harper 1977; Guariguata and Pinardc
1998; Kunstler et al. 2004). Howe and Smallwood
(1982) suggested that seed dispersal has three
potential benefits: escape from competition, colonization of new suitable habitat, and high
probability of seedling establishment. The ideal
dispersal system maximizes all three benefits, but
few, if any, single means of dispersal seem
capable of doing so. Some forms of seed dispersal
comprise two phases, with distinctly different
mechanisms (Wall and Longland 2004). Because
seeds of most wetland species can float, secondary
dispersal of seeds by water currents is common in
wetland species (Cook 1987; Middleton 1999).
Seeds of M. laxiflora are dispersed by two
mechanisms, i.e., wind and water. The dispersal
distance of seeds by wind is related to landform
and wind strength. Most of the wind-dispersed
seeds scatter within 20 m of the parent plant in a
fan-shaped zone, so that natural populations of
M. laxiflora seedlings have clustered distribution
patterns. Secondary dispersal by the river current
allows for long distance dispersal of seeds from
upstream toward downstream. As a result, M. laxiflora tends to have scattered and isolated populations along the Yangtze River in the Three
Gorges Reservoir (Wang et al. 2003). Secondary
dispersal promotes the exchange of genes among
the populations at a certain extent (Li et al. 2003).
Fiedler and Ahouse (1992) classified rare
species into one of several groups based on
their geographic distribution and population size:
(1) narrow distribution but large population
sizes; (2) narrow distribution and small population sizes; and (3) wide geographic distribution
but small population sizes. Conservation efforts
have focused on rare species with narrow distributions, discrete occurrences, or strict habitat
preferences. All rare species have specific key
environmental factors that limit their distributions. For M. laxiflora, the soil water content of
the site where seeds dispersed was a key factor
limiting their germination and thus distribution.
Seeds of M. laxiflora mature in fall, which is the
dry season in the Three Gorges area, but the soil
water content of most soils is lower than the
minimum required for germination to occur
during this time. Because the life-span of seeds
is very short under natural conditions, seeds will
lose their ability to germinate if suitable soil
conditions are not met during this time. The
restricted distribution of this species in the flood
zone along the river is therefore most likely due
to limitations imposed by the low soil water
content of upland soils during seed dispersal and
germination.
All the natural habitats of M. laxiflora will be
submerged, when the Three Gorges dam is
completed and water levels rise to 175 m above
the original river levels. The seasonal hydrological environment also will change from summer
flooding with winter drought to summer drought
with winter flooding (Chen et al. 2005b). The life
cycle of M. laxiflora is not adapted to these
reversed seasonal hydrological conditions and will
not survive making in situ conservation impossible (Chen et al. 2005a). Ex-situ reconstruction of
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74
the populations is the only viable approach to
preserve the species. The flood zones above
175 m in branches of the Yangtze River, which
have similar physical and ecological features of
the original habitats of M. laxiflora and are not
affected directly by the dam, might be suitable
sites for population reconstruction. Liu et al.
(2006) reported that the gene diversity in natural
populations of M. laxiflora was moderate, while
genetic differentiation among natural populations
was significant. Natural populations should be
reintroduced as many as possible in order to
conserve genetic diversity of the species.
Acknowledgements We would like to thank Dr. Ke-Ping
Ma, Dr. Ruth E. Sherman and Dr. Zheng-Qing Li for their
helpful comments on this manuscript. The Three Gorge
Project Construction Committee of the State Council
provided fund for this research. Two anonymous referees
gave us very interesting comments to the final
improvement of this work.
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