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Growth of the mangrove Avicennia marina in relation to soil salinity: does salinity
stunt tree height?
Sarah Refalo. School of Biological Sciences, The University of Queensland, 4072, Australia.
sarah.refalo@uqconnect.edu.au
Abstract
Mangroves are halophytic angiosperms that have to deal with many harsh environmental
conditions such as high variation in soil salinity. The grey mangrove Avicennia marina
dominates the seaward edge of many tropical mangrove zones, and is equipped with
morphological and physiological adaptations to combat hypersaline soils. In the intertidal
zone of the west coast of Fraser Island, transects were used to investigate whether
increased soil salinity was correlated with stunted A. marina height. As soil salinity
increased further towards the mainland, mean A. marina height (m) tended to
significantly decrease. At the 70m mark along the transect, salinity measured as high as
55ppt and mean A. marina height began to plateau at heights of less than a meter. The
reduction in growth displayed by A. marina with increasing salinity could be caused by
the physical inhibition of growth by built-up sodium chloride concentrations in plant
tissues, or the fact that A. marina has been shown to grow in an “optimal” salinity of half
that of seawater. It was concluded that further studies may reveal a complex interplay of
growth-limiting factors that aim to explain the growth patterns of these economically
important plant species.
Key words: Grey mangrove, Fraser Island, intertidal zone, PCQ method
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Introduction
The economic importance of the natural products and ecological services provided by
mangrove ecosystems is largely underestimated around the world (Rönnbäck 1999).
Mangrove forests are nursery grounds for many juvenile fish species that are of
commercial and recreational importance (Laegdsgaard and Johnson 1995), and they
provide a physical barrier to the coastline from wave action and damaging winds. The
many adaptive features of mangroves that have allowed them to successfully colonise
these harsh environments have acted as a trade-off to expending energy into other plant
roles such as producing fruit and growing larger (Macnae 1968; Saenger 1982; Clough et
al. 1982). Particularly, the adaptive ability to grow in hypersaline soil conditions is
thought to be a driving factor resulting in the stunted growth of some mangrove species
along the intertidal zone.
Mangroves are a taxonomically heterogeneous assemblage of plants that live between the
land and sea (Downton 1982). They are halophytic angiosperms that are almost
exclusively tropical, extending to about 320 north and 380 south of the equator. High soil
salinity is a common feature of these hostile environments; where waterlogged soils (low
O2 levels), periodic tidal inundation and exposure, low nutrient availability and high
temperatures also prevail (Hogarth 2007). Along the north-eastern coast of Australia,
mangrove vegetation is found to exist within zones, or bands of dominant species that lie
more or less parallel with the shore (Lear and Turner 1977). The most dominant species
at the seaward edge of the intertidal zone is the grey mangrove (Avicennia marina),
where trees are subjected to regular inundation by seawater (35ppt). This species is not
only abundant near the seaward margin, but also some way upshore, hence why it is said
to have a bimodal distribution (Hogarth 2007).
The distribution patterns of A. marina suggest that it can cope with a significant variation
in soil salinity, ranging from almost freshwater conditions upstream to the hypersaline
saltpans on the landward side of the mangrove fringe (Bunt et al. 1982; Wells 1982).
According to Hogarth (2007), the principle mechanisms deployed by mangroves to cope
with varying soil salinity include the exclusion of salt by the roots, tolerance of high
tissue salt concentrations, and the elimination of excess salt by secretion. The leaves of A.
marina taste strongly salty when licked, and upon close inspection, small salt crystals can
be seen on the surface of the leaves. This is due to the presence of salt glands, an
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adaptation utilised by “salt secreters” (classified by Scholander et al. 1962) like A.
marina to control salt balance by directly secreting sodium chloride through their leaves
(Tomlinson 1986).
The diversity of morphological and physiological mechanisms that mangroves have
adapted to combat high salinity vary greatly between species, and the interplay between
them is complex (Connor 1969). Walsh (1974) stated that no mangrove species is an
obligate halophyte, even though some are encouraged to grow in the salinity of sea water.
However, the ability for some species to tolerate continuously fluctuating salinity levels
is perhaps more of an ecological phenomenon than the ability to survive high salinity
alone (Chapman 1976; Chapman 1977). The major aim of the present study was to
investigate the growth of A. marina along the intertidal zone of the west coast of Fraser
Island and to test whether stunted growth is correlated with increased levels of soil
salinity.
Methods
Study site
A total of ten transects were set out along different regions of seaside mangrove habitat
on the north-west coast of Fraser Island (-25026.9’S, 152059.3’E). The transects began at
the seaward edge of the forest (dominated by A. marina), and a nearly continuous belt of
mangroves was measured for about 200m (see Appendix I for map). The Point Centred
Quarter method (PCQ) was used along the transects to sample the mangrove vegetation
of the intertidal zone (Mitchell 2007).
PCQ method
Starting at 0m (sea point), a line was run perpendicular to the transect and the sampling
world was divided into four quarters. In each quarter, the nearest adult tree to the
sampling point (with at least 20 nodes) was identified by species. Its distance from the
sampling point was also measured (m), as well as height (m) and diameter at breast
height (DBH, cm). The measurements of four separate trees equated to one sampling
point of data along the transect. This process was repeated every 5m along the transect up
until 90m towards the mainland.
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Soil Salinity
Every 5m along each transect, a water sample was taken by digging a hole into the
waterlogged soil and using a tube to siphon up ~5mL of water into a labelled vial. These
vials were then taken to the work station and left to stand for the sediments to settle. A
hand-held refractomter was used to measure the water salinity in parts per thousand (ppt)
from a few drops of each water sample. These measurements were recorded in a table and
used in accordance with the PCQ data.
Data Analysis
The PCQ method data from all ten transects was combined in tables and manipulated to
produce a table describing the approximate zonation of mangrove vegetation in the
region. The mean values of A. marina height along the ten transects were then calculated
and plotted against the salinity data of one transect.
Results
As soil salinity increased further towards the mainland, the mean heights (m) of A.marina
trees were found to significantly decrease (Fig. 1). From 55-90m towards the end of the
transect, A. marina tree height tended to plateau as soil salinity abruptly increased to a
maximum of 55 parts per thousand (ppt) at the 70m mark.
The logarithmic line of best fit for the tree height data had a more significant slope than
the linear line that was fitted to the salinity data. The exponential line of best fit for the
height data had an R2 value of 0.84. The R2 value calculated for a linear fit of the salinity
data was 0.17.
5
10
60
50
8
7
40
6
5
30
4
20
3
2
Soil salinity (ppt)
Mean A.marina height (m)
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10
1
0
0
0
10
20
30
40
50
60
70
80
90
100
Distance along transect (m)
Figure 1 Mean Avicennia marina height (dark-shaded diamonds) plotted with soil
salinity (light-shaded squares) against distance along the transect (where 0m = sea point).
Standard error bars represent tree height data that was averaged from 10 sampling
transects. Tree height is shown to decrease further along the transect as soil salinity peaks
to 55ppt at the 70m mark.
Discussion
The results of the current experiment tend to support the hypothesis that increased soil
salinity is correlated with stunted A. marina growth. Soil salinity was found to increase
further towards the mainland, and this is known to occur due to a number of contributing
factors. Oliver (1982) describes some of these factors as tidal inundation, soil type and
topography, run-off from adjacent terrestrial areas and evaporation. The mangrove forest
under study is heavily influenced by tidal inundation, and therefore evaporative loss and
flooding frequency are most likely the major determining factors resulting in increased
levels of salinity. At 55ppt, A. marina tree height formed a plateau on the graph, where
trees past 70m on the transect did not measure higher than a meter.
This reduction in growth displayed by A. marina in high salinities may be due to the
physical inhibition of growth by high sodium and/or chloride concentrations in plant
tissues (Clough 1984). It may also be that A. marina has been found to have an “optimal”
salinity of half that of seawater (Connor 1969). Or perhaps, a more biological explanation
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could be that soil salinity is simply not the only factor acting upon mangrove tree height.
This could explain why the exponential line of best fit for the tree height data was more
significant than the salinity data (R2 = 0.841 compared to 0.171)
In an experiment comparing the growth and salt balance of A. marina and Rhizophora
stylosa (red mangrove), Clough (1984) found that A. marina is physiologically able to
avoid severe water deficits in plant shoots more efficiently than R. stylosa. It does this by
accumulating high concentrations of sodium and chloride in the xylem sap of its leaves
and stems, whilst possessing salt-secreting glands to gradually expel this salt from the
leaves. This adaptation of maintaining salt balance in plant tissues means that the plant
has to expend more energy into its adaptive features rather than investing that same
energy into growing taller. This physiological trade-off could explain why A. marina
height was stunted in growth in areas of high salinity on Fraser Island.
The optimal salinity of A. marina is an important factor to consider when giving
explanations for trends in tree height. Downton (1982) raised A. marina seedlings in
several different seawater concentrations in the laboratory to test the effect of salinity on
plant growth. It was found that plants raised on sodium chloride-free media did not
perform well in terms of plant growth and biomass production. This showed that A.
marina actually requires sodium chloride for successful growth in the long term, a
finding consistent with Clarke and Hannon’s (1970) similar experiment. Optimal growth
was found to be 58-290mM chloride (50-250mM sodium) which is typical of several
other halophytes (Flowers et al. 1977; Greenway and Munns 1980). This value is
obviously not experienced along the entire intertidal zone, and therefore A. marina trees
growing outside this optimal range cannot grow to their full potential.
Furthermore, a few other factors that could be working alongside soil salinity in a
complex way to contribute to A. marina growth include nutrient availability, water stress,
temperature (atmosphere and water) and inter- and intra-specific competition (Hutchings
and Saenger 1987). Perhaps the trees were stunted in growth towards the mainland solely
because they faced a higher level of both inter- and intra-species competition, and this
will inevitably vary with the zonation patterns of the site under study. Further studies
should focus on achieving an accurate approximation of species abundance and biomass
in the region to gain a clearer understanding of plant-plant interactions such as
competition and facilitation.
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Also for future application, multiple salinity levels should be measured over spatial and
temporal scales to improve the statistical power of the experiment. It is also important to
acknowledge that soil analyses in the field are constantly fluctuating with the seasons,
thus snapshot surveys are not an adequate representation of the entire zone (Chapman
1976). Future experiments could also consider measuring response variables such as
osmotic relations, number of leaves per plant and dry weight rather than just assessing
plant height (Downton 1982; Clough 1984). The increasing need to conserve mangrove
ecosystems around the world due to their economic and ecological benefits should see
support for near-future advancements in mangrove community studies (Mudie 1974).
Acknowledgements
Thank you to all of my lecturers and course co-ordinator Myron Zalucki, who over the
past weeks have provided me with the necessary theory and skills required to work
successfully collecting data in the field. Thank you also to my peers who contributed to
the dataset of this report. Finally, I would like to personally thank Lucy Hurrey, the
mangrove activity tutor, whose insight and wealth of knowledge inspired me to learn
more about these fascinating ecosystems in the first place.
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Appendix I – Map of north-west coast of Fraser Island indicating transects set along
mangrove vegetation by the sea (s1-10.)
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