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Research Journal of Biology, 2: 53 - 59 (2014)
www.researchjournalofbiology.weebly.com
RESEARCH ARTICLE
Open Access
Allocation to Sexual Reproduction by the Common Reed
(Phragmites australis) is Highly Variable in Different Phases of
Estuarine Succession
Kai Aulio*
Department of Biology, University of Turku, FI-20014 Turun yliopisto, Finland; Present address: Lankakatu 3 D
16, FI-20660 Littoinen, Finland.
Abstract
The common reed, Phragmites australis (Cav.) Trin ex Steudel (= P. communis Trin.) showed appropriate strategies
in resource allocation to inflorescence formation in the varying phases of wetland succession in the rapidly changing
estuarine environment of the Kokemäenjoki River delta, western Finland. In the monocultural stands of the
common reed, three distinctive successional phases were separated. The biometric characteristics of P. australis
vary significantly according to the successional phases and also within each of the stand types classified. In general,
the height and weight of individual aboveground shoots (ramets) decreased in the order: Pioneer stage > Mature
stage > Regressing stage. The resource allocation to generative (sexual) reproduction showed different patterns.
The frequency of fertile (flowering) shoots was markedly higher in the pioneer and the regressing stages than in the
widest and longest-lasting communities of the mature stage. In determining the flowering frequency, the vegetative
growth dimensions must, however, be accurately considered. The frequency of fertile shoots is more than twenty
percent points higher in the dominant height class (“canopy layer”) than the average values calculated for the
whole community. Even more important is the detailed characterization of the sampling conditions in the
measurements of the actual investments that the plant individual or clone allocates to the sexual reproduction. The
average allocation of the current years’ production to flower and seed formation was high (up to 8.4 %) in the
pioneer stage, much less in the mature stage (4.3%), and the most importantly, the share of biomass allocated to
sexual reproduction peaked in the regressing phase of succession (8.8%). In the Kokemäenjoki River delta,
Phragmites australis showed biologically appropriate reproduction strategies by investing heavily to asexual
distribution in the young successional phases, whereas the proportion of investments to sexual, long-distance
reproduction and distribution were enhanced in the regressive, die-off phases of the plant’s successional history.
Key Words: Phragmites australis, common reed, resource allocation, sexual reproduction, reproduction strategies.
(Received: 12/05/2014; Accepted: 01/06/2014; Published: 22/06/2014)
efficient distribution through belowground rhizomes. On
the other hand, the common reed has a well-developed
sexual distribution ability by producing numerous and
easily dispersive seeds (Grime, 2001; Haslam, 2010).
According to the widely accepted assumption, plants
can choose the most efficient strategy in utilizing the
available environmental resources. Hence, the sexual
(generative) propagation is a means to escape insufficient
nutrient resources or space in situations, where intra- or
interspecific competition is severe. On the other hand,
asexual (vegetative) reproduction and distribution is the
most appropriate strategy for a plant individual or a clone
to maintain present position or to conquer new habitat
(Gardner and Mangel, 1999).
Complex dispersion patters can prevail in
heterogeneous or changing habitats, and thus various
strategies are needed in any respective growth conditions
(Grime, 2001). This applies exactly in the river deltas,
where water depth, water currents and wind strengths, as
well as availability of nutrients can change remarkably
between years or even within a growing season (Dobson
and Frid, 2009; van der Valk, 2012).
Introduction
The detailed characteristics in the forms and functions in
reproduction of Phragmites australis are surprisingly little
studied, although the common reed is a cosmopolitan
species – and in fact the most widely distributed of all
flowering plant species on the earth (Stott, 1981).
Successional trends – adaptations to changing
environments – are a characteristic feature in Phragmites
australis. Typically the common reed can invade and
colonize non-vegetated areas near the average water level
(in shallow water or a few centimetres above the mean
water level), and spread effectively through vegetative
underground rhizomes (Amsberry et al., 2000).
In the functional CSR theory of the plant strategies
presented by Grime (2001), Phragmites australis is Cstrategist, i.e. a species occupying habitats with high
productivity and long habitat duration. These
characterizations fit precisely to the present study area,
Pihlavanlahti Bay, in the Kokemäenjoki River delta,
western Finland, where Phragmites australis is the
dominant macrophytic plant species (Aulio, 1979, 2014).
Phragmites australis is one of the most successful
competitors in established plant communities, due to
53
*Corresponding author: kai.aulio@gmail.com
Copyright © 2014 RJB
Aulio, 2014
Variations in the inflorescence frequency and the
timing of flowering are typical features in Phragmites
australis. Important environmental characteristics to
determine the inflorescence frequency and the start of the
flowering season are air temperature, water depth of the
site, exposition to sun, and the temperature of the
soil/sediment (Björk, 1967; Haslam, 2010). Factors
enhancing the panicle formation and the frequency of
inflorescences in the community are high temperature and
low water level. With respect to the panicle frequency, the
prevailing environmental conditions affect immediately,
during each growing season (Björk, 1967).
In the course of the life cycle, plants are capable of
changing the reproduction strategies to maximize the
utilization of available environmental resources. Hence, a
plant can transport essential nutrients from vegetative to
generative reproductive organs, or vice versa (Harper,
1977). The choice between the reproduction strategies is
important both for a plant individual and the plant species.
The strategy adopted often depends on competition, i.e.
whether the plant is growing free of struggle for resources,
or in a situation, where other plants are competing for the
limited resources (van Kleunen et al., 2002).
Phragmites australis is morphologically well-equipped
for sexual reproduction. The open habitats, dense
monospecific communities, and the structure of flowers
and inflorescences are all typical features for successful
anemochory (Glover, 2007). In addition, the seeds of the
common reed survive in aquatic environments, and thus
the species can distribute also through water currents
(Haslam, 2010).
northern reach of the Baltic Sea is brackish water with a
salt concentration of only 0.1–0.5 %.
Figure 1. Location of the study area
On the basis of water quality, the Pihlavanlahti Bay is
eutrophic. Biologically the estuary is very rich, and from
2004 the site has been part of the Natura 2000
conservation network of the European Union. The estuary
is also part of the international Ramsar Convention on
Wetlands network of valuable bird sanctuaries.
The biota of the study area is exceptionally rich as
compared to typical natural habitats in Northern Europe.
The number of plant taxa growing at the Pihlavanlahti Bay
is about 440. The flora includes several endangered
species included in the specially protected taxa by the
European Union (Suominen, 2013). The delta is
characterized as a birdlife paradise, and for reason. There
are about 110 bird species permanently living at the
Pihlavanlahti Bay area, and the estuary is one of the most
important resting and molting areas during migrations.
The number of bird species met annually at the
Pihlavanlahti Bay area reaches about 220 species. In
addition, the estuary is one of the most popular venues for
leisure fisheries in Finland, and the waters also support
some professional fisheries.
The development of communities dominated by
Phragmites australis usually involves phases of directed
and predictable succession. Typically, after the
establishment of the pioneer phase, reed communities
typically spread through the growth of belowground
rhizomes. In most coastal habitats, the succession of dense
and biologically competitive monoculture is usually
proceeding from a shallow water microhabitats towards
deeper water microhabitats (Amsberry et al. 2000; Dobson
and Frid, 2009; van der Valk, 2012). But in estuaries,
especially when the sedimentation processes of the delta
are strong and change the environmental conditions
rapidly, the succession often proceeds in the opposite
direction, i.e. from the deep water towards dry
microhabitats. This is the case in the Phragmites australis
reedswamps in the Kokemäenjoki river delta (Aulio, 1979,
2014).
Materials and methods
Study area
The succession of the macrophytic vegetation was studied
in the Kokemäenjoki River estuary, in western Finland
o
o
(Northern Europe; 61 34’N, 21 40’E) in 1990’s and again in
2013 (Figure 1). The estuary, discharging into the Baltic
Sea, is a shallow sedimentation basin, which is nearly
thoroughly covered with rich and exceptionally productive
macrophytic vegetation (Aulio, 1979, 2014). The delta in
the estuary is proceeding very quickly due to the
deposition of sediments carried by the River
Kokemäenjoki, the accumulation of autochthonous organic
(plant) matter, and due to land uplift, typical to the shores
of the Baltic Sea. At present, the extent of the land uplift in
the area is 5.5 millimeters a year. The deltaic deposits
(formation of new sandbanks and islands), as well as the
distribution of the vegetation zones are today moving
towards the sea at an average speed of 30 meters a year.
The delta of the Kokemäenjoki River at Pihlavanlahti
Bay shows the most rapid change in any landscape in the
Northern Europe. The water of the estuary is essentially
fresh water carried by the River Kokemäenjoki. The water
of the river and of the estuary were highly polluted and
eutrophicated during past decades, but due to effective
water purification and conservation efforts, the aquatic
environment of the area is now considered clean and
healthy (Aulio, 2010). The penetration of sea water into
the estuary is restricted by road embankments, and also
naturally by many small islands. The water of the adjacent,
Collection and analyses of the plant materials
The plant samples of Phragmites australis were collected
by the time of the maximum biomass of reeds (late
August, during several successive years in 1990’s and again
54
Copyright © 2014 RJB
Res. J. Biol., 2014 [2:53-59]
E-ISSN: 2322-0066
Table 1. The average shoot height (cm) in Phragmites australis at
the three successional stages in the Kokemäenjoki River delta,
western Finland.
in 2013). The sampling was made randomly in the middle
of the uniform, monocultural stands at three successional
phases (Aulio, 1979, 2014). In this study, the height of the
individual shoots (or ramets, as an individual member of
plant clone is defined) was determined from the
water/sediment interface up to the tip of the uppermost
leaf axil (Haslam, 1973, 2010). The biometric
measurements as well as the frequency of the flowering
ramets were taken for 200–250 randomly sampled
individuals at each of the successional stages. The water
depth of the sites was determined by 10 measurements in
each site. Such sampling methods and the number of
replicates are considered reliable in describing the growth
and production characteristics of tall helophytic plants like
Phragmites australis (Gouraud et al. 2008). The sampling
and measurement policy of the present study followed the
international standards used in hydrobiological studies
(Vollenweider, 1969).
Total, all
shoots
Fertile
(flowering)
shoots
Sterile (nonflowering)
shoots
Mature
218.48±
49.87a
262.94
±27.17a
Regressing
165.38 ±
25.80b
188.34 ±
10.42b
177.21 ±
24.14a
174.03
±16.83a
142.43 ±
16.67b
Mean ± standard deviation (S.D.). N = 200–250 ramets in each successional
stage. Statistical significance of the differences: In ANOVA, the different
superscript letters in the horizontal rows indicate highly significant
difference (P < 0.01).
Table 2. The average weight of shoots (grams, dry weight/ramet)
of Phragmites australis at the three successional stages in the
Kokemäenjoki River delta, western Finland.
Statistical analyses and terminology
The statistical analyses used follow Sokal and Rohlf (2012).
The parametric (mean ± standard deviation of the mean,
one-way analysis of variance; ANOVA), and nonparametric (Kruskal–Wallis one-way analysis of variance)
statistical analyses of the numerical data were performed
by using the Analyse-it for Microsoft Exel (version 2.12)
program package (2008).
The terminology of biological concepts and principles
follows the latest edition of the Oxford Dictionary of Plant
Sciences (Allaby, 2012).
Fertile
(flowering)
shoots
Sterile (nonflowering) shoots
Pioneer
15.49±
1.22a
Mature
10.35±
1.11b
Regressing
5.71 ± 0.60c
9.62± 2.01a
6.51±
1.86b
4.32 ± 1.67c
Mean ± standard deviation (S.D.). N = 50–75 ramets in each successional
stage. Statistical significance of the differences: In ANOVA, the different
superscript letters in the horizontal rows indicate highly significant
difference (P < 0.01).
Besides the variations in the habitat characteristics,
wide variation was seen in the shoot parameters between
the fertile (flowering) and sterile (non-flowering) ramets in
each of the successional stages. In all the habitats studied,
the fertile ramets were significantly taller and more robust
than the sterile shoots. The differences in the individual
ramets’ weight between the flowering and non-flowering
shoots were statistically highly significant (P < 0.001) in all
the three successional stages (Table 2).
The shoot dimensions of Phragmites were further
analyzed by comparing the differences within each of the
three successional phases. Remarkable intra-clonal
variations – difference between the flowering and nonflowering ramets (the vertical columns in the Tables 1 and
2) – in the shoot heights and weights were seen in all the
three successional stages. The differences were analyzed
by the one-way analysis of variance (ANOVA), and the
results were as follows: Pioneer stage, F = 34.86**, Mature
stage, F = 47.71**, Regressing stage, F = 82.57** (** =
statistical significance, P < 0.01).
The main biometric parameters of Phragmites
australis, i.e. the height and the weight of individual
ramets, are highly significantly correlated in the
reedswamps in each of the successional phases at the
Kokemäenjoki River estuary (Table 3).
As a strong competitor, Phragmites australis is
capable of forming and maintaining permanent
monocultures, unless the environmental conditions do not
change markedly (Dobson and Frid, 2009). In river deltas,
where new grounds are created by flood-borne sediments,
the habitat variability is often unpredictable, and thus
flexibility in growth strategies is essential for the success
and existence of the vegetation. Phragmites australis can
adapt to such extreme conditions, and the species can
Results and discussion
Variation in shoot size in the three successional
phases
The common reed Phragmites australis (Figure 2) shows
remarkable variability in all the biometric measures
according to the successional phases in the Kokemäenjoki
River estuary (Aulio, 2014). The height and weight of the
ramets of P. australis in the present study are summarized
in Tables 1 and 2, respectively. The height of the
Phragmites shoot was measured from the sediment
interface up to the uppermost leaf axil (thus, the length of
the inflorescence was not included in this analysis to
ensure a straight comparison).
Figure 2. Dense monocultures of Phragmites australis at a pioneer
successional stage in the Kokemäenjoki River delta, western
Finland.
55
Copyright © 2014 RJB
Pioneer
229.54 ±
49.34a
266.93 ±
20.06a
Aulio, 2014
even modify the habitat to be even more competitive in
the continuously changing estuarine succession (Aulio,
2014; Dobson and Frid, 2009).
dominant (“canopy”) layer than in the whole community
combined. The within-class differences (the vertical
columns in Table 4) were statistically highly significant (P <
0.01).
Table 3. Height vs. weight -ratio in the shoots of Phragmites
australis at the three successional stages in the Kokemäenjoki
River delta, western Finland. Equations and statistical significance
in the linear regression analysis.
Successional
stage
Pioneer
Mature
N
Regressing
71
62
59
Regression
equation
154.5 + 4.34x
-20.81+
0.1297x
-12.12+
0.1189x
R2
Significance
0.73
0.71
P < 0.0001
P < 0.0001
0.84
P < 0.0001
The development in the height and weight of the
individual ramets in Phragmites australis is rather regular
and linear from the start of the growing season towards
the end of the summer. Within-site variations occur
naturally in all biological communities. In the growth
dynamics of the common reed, the variations – as
analyzed by the coefficient of variation – are significant
only during the start, i.e. the first two months or so. In the
mid-summer and early autumn, the time of the most
intensive growth and the prevalence of the maximum
shoot biomass – the variation is minor (Hara et al. 1993).
Figure 3. Typical form of panicle-inflorescence of Phragmites
australis in the Kokemäenjoki River delta, western Finland.
In the comparison between the successional phases,
the flowering frequencies were markedly higher in the
pioneer and the regressing stages of the succession than in
the established, mature stands (Table 4). The frequency of
fertile ramets was statistically significantly (P < 0.05) lower
in the established stands than in the other two
successional classes.
Frequency of fertile (flowering) shoots
The patterns in flowering frequency in Phragmites
australis are versatile, and both the sampling and
interpretation of the results must be described carefully
and in absolute detail. The number of the flowering shoots
(Figure 3) as related to the total number of shoots per unit
area differs markedly when different size classes of
individuals are analyzed. This is logical and natural per se,
but there are two distinctively different approaches: (i) In
the wind-pollinated (anemochorous) species like
Phragmites australis, only the inflorescences developed at
the tops of the dominant height are of any biological
significance. The modal height in wide monocultures is
usually uniform, and the frequency of panicles is highest in
the tall ramets. (ii) But the shorter shoots (ramets) bear
also panicles – although usually small and weakly
developed ones. Such flowers can hardly be windpollinated, and thus they are of marginal significance for
the propagation of the species.
In the present study, the frequencies of the fertile
shoots were determined separately for the dominant
height, consisting of the ramets reaching the uppermost
one meter of the rather uniform canopy layer. On the
other hand, the flowering frequency was determined also
for all the ramets in the sampling areas. The results
showed two distinctive patterns: (i) The flowering
frequency differed significantly between the dominant
height class and the whole set of shots, and (ii) the
frequency of the fertile shoots showed marked and
appropriate variations according to the three phases in the
succession of Phragmites australis (Table 4).
The investments for sexual propagation in Phragmites
australis showed two different trends. Hence, the
frequency of the fertile (flowering) ramets was, on the
average, more than twenty percent points higher in the
Table 4. The frequency (%) of the fertile (flowering) shoots in
Phragmites australis at varying successional phases in the
Kokemäenjoki River delta, western Finland.
All
aboveground
shoots in the
sampling areas
Shoots reaching
the dominant
height (“the
canopy layer”)
Mature
63.39 ± 5.87bA
91.58 ±
2.73aB
86.33 ± 5.65bB
Regressing
66.74 ±
3.88bA
93.57 ±
2.02aB
Mean ± standard deviation (S.D). N = 150–200 in each successional stage.
Statistical significance of the differences: In ANOVA, the different
superscript letters in the horizontal rows indicate significant difference (P <
0.05), and the DIFFERENT SUPERSCRIPT CAPITAL LETTERS in the vertical
columns indicate highly significant difference (P < 0.01).
The present results were consistent with the patterns
recognized in other studies. In the wide reedswamp
monocultures – such as the communities in the
Kokemäenjoki River estuary – several shoots are shorter
than the modal height, and these ones often remain sterile
(non-flowering). The frequency of panicle formation is
typically highest in the tallest shoots (Haslam, 2010).
In poor habitat conditions the flowering frequency
remains low, and sometimes there are no panicles at all.
Low temperature and poor nutrient status of the habitat
are obvious reasons for Phragmites australis for not to
flower during some growing seasons (Björk, 1967). The
same habitat can, however, have very high flowering
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Copyright © 2014 RJB
Pioneer
72.04 ±
1.64aA
Res. J. Biol., 2014 [2:53-59]
E-ISSN: 2322-0066
Table 5. The average weight of flowers and seeds (grams dry
weight/ramet) and the proportion of inflorescences of the total
ramet weight (%) in fertile shoots of Phragmites australis in the
three successional stages in the Kokemäenjoki River delta,
western Finland.
frequency in the following summer. Flowering in
Phragmites is mainly determined at the beginning of the
annual life cycle, by the bud characteristics and size. The
wider the bud, the taller the shoot, and the more likely the
shoot is to bear a panicle at the end of the growing
season. But the flowering can be disturbed also in the late
phase of the year’s life cycle. Bad weather conditions just
before or during the flowering period can prevent or
decrease the panicle formation (Haslam, 2010).
The flowering frequency in Phragmites australis
varies widely due to differences in physical growth
conditions, e.g. temperature, and also due to genetic
variability. In theory, every fully developed shoot can
develop a panicle. Hence, if a ramet does not flower, it can
be considered to be hindered by some reason, e.g. habitat
conditions or developmental characteristics. The flowering
frequency in a reed stand can be anything between 0% to
100% (Haslam, 2010). In the present study this was shown
by the lower flowering frequency in the shorter ramets at
each of the successional stages compared.
The shoot height is a significant determinant in the
flowering process of Phragmites. But in natural stands,
panicles can be formed in short as well as in tall stands.
Flowering is usually most equal in stands, where the modal
height of the shoots is uniform. Thus, even in a stand,
where the modal height is less than half a meter, the
flowering frequency can be high (Haslam, 2010).
The remarkably high flowering frequency in the
regressing successional phase of the common reed in the
present study area indicates an appropriate reproduction
strategy to ensure the existence of the plant species, even
though the individuals or clone at the present habitat are
vulnerable or threatened. These observations are
consistent with the results from Italy, where the
production of seeds was enhanced in the unfavorable, dieback stands of Phragmites australis (Reale et al. 2011).
Pioneer
Mass of
inflorescences/
ramet (g dry
weight)
Share of
inflorescences of
the ramet weight
(%)
Regressing
0.45 ±
0.06b
0.50 ±
0.06b
8.39
4.35
8.76
Mean ± standard deviation (S.D). N = 50–75 in each successional stage.
Statistical significance of the differences: In ANOVA, the different
superscript letters in the horizontal rows indicate highly significant
difference (P < 0.01).
In the pioneer phase – when the stand is young and
limited in the area – the ramets of Phragmites australis
were taller, thicker, and heavier than in the other two
stages. And accordingly, in this stage the proportion of
flowers and fruits of the total aboveground biomass was
much higher than in the established, mature stands. But
the vitality of an individual or the clone is not the only
guide to investments in sexual reproduction. In the
present study, the common reed allocated the highest
proportion (up to 8.8 %) of the aboveground biomass into
flowers/seeds in the regressing stage – i.e. in the final
phase of the successional history of the species.
In the life cycle of a plant, reproduction requires
remarkable investments. Thus, the share of resources
allocated to produce flowers and seeds are largely
determined by the condition of the plant species or the
clone (Grime, 2001). This was shown also in the present
study. The weight of flowers and seeds was
overwhelmingly highest in the pioneer stage of succession,
where the shoots (ramets) of P. australis were tallest and
most robust (Tables 1, 2 and 5).
As in the case of flowering frequency, the regressing
stands showed marked enhancement in the flower/seed
production as compared to the mature stage. The present
results of the enhanced resource allocation to sexual
reproduction in the regressing stage of succession support
the results from dying reed stands, widely recognized in
Europe. In the die-back communities in Italy, the
frequency of viable seeds was remarkably high in the
declining stands of Phragmites australis (Reale et al. 2011).
Sexual propagation and dispersal of Phragmites
australis are – at least theoretically – very effective
because the seed production is high. One flowering ramet
of the common reed can produce hundreds (up to 1000)
viable seeds during every growing season (Maheu-Giroux
and de Blois, 2007; Haslam, 2010). And the dispersal ability
of the light seeds (0.12 grams per seed, on average) is
well-developed. On the basis of sexual propagation
abilities, Grime (2001) includes Phragmites australis
among the W-strategists, i.e. plants, in which the
reproduction involves numerous wind-dispersed seeds.
The habitat conditions prevailing in the present study
area are difficult for any helophytic macrophyte to occupy
new territories sexually through seed germination, in spite
of the huge seed production of the area’s own vegetation.
Allocation of biomass to reproductive organs
Whether a plant is flowering or not is only one option in
determining the reproductive capacity. The number of
flowers and seeds, and the real vitality of the generative
organs finally determine the success of the plant. In this
regard, the weight and the proportion of the biomass are
more important factors than the average flowering
frequencies. Such a varying – and biologically appropriate
– trend was seen in the flower and seed production of
Phragmites australis in the Kokemäenjoki River delta.
The proportion of the total aboveground production
allocated to the sexual reproductive organs, i.e. flowers
and fruits, varied markedly between the successional
phases of Phragmites australis.
The allocation of biomass into the reproductive
organs was highest in the pioneer stage, and accordingly,
lowest in the established, mature and most widely
distributed stands. But in the regressing stands the
common reed begins to invest in sexual reproduction,
again. The trend was apparently the same, whether the
share of investments was calculated per total
aboveground biomass of the whole community or as the
percentage of individual flowering shoots (Table 5).
57
Copyright © 2014 RJB
Mature
1.30 ±
0.27a
Aulio, 2014
In exceptionally dense monocultures in the Kokemäenjoki
River delta, there is only very limited space for new
individuals to establish from seeds.
The best germination conditions for the seeds of P.
australis are moist, fertile littoral soils above the average
water level (Szczepaoski, 1978). The availability of
nutrients in the substratum is often limited because the
small seeds can support growth only during the short
initial period after germination. Thus, the generative
distribution on sandy, nutrient-poor sediments – typical
for littoral shore habitats – is rather rare in P. australis
(Szczepaoski, 1978). The seeds of Phragmites australis are
capable of germinating in permanently inundated
(underwater) substrata. But in natural environments, such
a generative establishment of new reed stands is,
nevertheless, relatively uncommon (Weisner et al., 1993).
Suitable conditions supporting sexual propagation of
Phragmites australis prevail in the Kokemäenjoki River
delta only after exceptionally high spring flood periods,
after which the huge amounts of river-borne sediments
are deposited establishing new, barren sandbanks. So, the
sexual propagation in the common reed here serves
principally long-distance distribution of the species.
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Summary and Conclusions
The trends in the allocation to sexual reproduction in
Phragmites australis were appropriate as far as the plants’
distribution potential and the future survival in the
Kokemäenjoki River estuary are concerned.
(i) In the most vital pioneering phase of succession,
the plant can utilize the maximal resources of space and
nutrients, and in this stage the common reed colonizes
habitats mainly vegetatively through fragments of
rhizomes carried by the river. In this phase, the plants are
vital, and the flowering frequency is high – providing
propagules for long-distance distribution of the species.
(ii) In the mature phase of the succession, the
communities of Phragmites australis are very wide and
very dense, and thus the possibilities for establishing new
plants through seed germination are minimal. And in those
habitat conditions, the common reed “seems to know the
investment for seed production would be wasted,
anyway”, and the proportion of resources allocated to
sexual propagation are less than in other two stages.
(iii) Hence, it is appropriate that the plant starts
maximizing the investments into sexual propagation again
in the final, regressing phase of successional development
of the reedswamps. Through long-distance dispersal, the
common reed can at least hope to transfer the genes and
thus maintain the survival of the plant as a species.
(iv) The height of the ramets is a significant
determinant for the flowering. Thus, panicles are formed
mostly at the top of the dominant (“canopy”) layer of the
monocultures.
(v) Biologically, in determining the appropriate
investments to sexual reproduction, the proportion of
biomass allocated into panicle formation is more
important than the flowering frequency of the ramets
within a monoculture.
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