Regeneration mechanisms in Swamp Paperbark (Melaleuca ericifolia Sm.) and their implications for wetland rehabilitation



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Figure 7.2 Dendrogram using Single Linkage derived from the hierarchical cluster 
analysis. Distance metric is Euclidian Distance. Years highlighted in yellow represent 
years with unusual rainfall patterns. The five years with bolded dates represent years 
with well above-average rainfall (> 705 mm) with well above-average rainfall in 
spring and/or autumn.  
 
 
The five years with a Euclidean distance of greater than 19: 1974, 1950, 1969, 1993 
and 1951 (Figure 7.2) were distinguished from the seven other outlier years based on 
pattern of rainfall within these years. Each of these five years shared the common 
feature of heavy rainfall in spring. Four of the years 1950, 1969, 1993 and 1951 had 
an additional period of heavy rainfall in autumn. Heavy spring and/or autumn rainfall 
in these five outlier years falls outside of the ideal temperature range for germination 

 
166
and early establishment of M. ericifolia, namely October to January, although this is 
offset slightly for the year 1974. Spring and autumn rainfall in the five outlier years 
was significantly higher than the mean for these given seasons and ranged from 157 – 
205mm (Figure 7.4). Mean monthly rainfall in spring and autumn for the 61 years 
varied from 42 – 63 mm. The timing and amount of average and above-average 
rainfall for Dowd Morass is shown in Figure 7.3.  
 
Two potential recruitment events in spring-summer 1950 and 1993 had near identical 
climatic patterns: flood events in September/early October, average rainfall for mid 
October through January, followed by flood events in February (Figure 7.3). These 
two periods correspond to large recruitment events identified on the 1964 and 2003 
aerial photographs (Figure 7.1). Spring and autumn of 1951 exhibited a similar 
rainfall pattern to 1950 and 1993, with heavy rainfall occurring in August and March 
with average rainfall in the intervening months. It was not possible to distinguish 
recruitment events that occurred in 1950 and 1951 on the aerial photograph, as plants 
established in these years would have been of comparable size on the 1964 aerial 
photograph. The potential for two consecutive recruitment years in 1950 and 1951 
may account for the magnitude of recruitment evident on the 1964 aerial photograph.   
 
The potential recruitment event identified from climatic data for 1969 corresponds 
with minor recruitment on the 1978 aerial photograph. The pattern of rainfall for 
1969, while similar to 1950, 1951 and 1993 shows peak rainfall in November and 
March, slightly later than these three other years.  
 

 
167
The final recruitment year, 1974, deviates from the pattern of the four years in that 
heavy spring rainfall is not followed by heavy autumn rainfall. There is however, a 
minor recruitment event evident on the 1991 aerial photograph that would correspond 
to potential recruitment in 1974.  

 
168
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 7.3 Monthly rainfall data from the 5 outlier years from the hierarchical cluster 
analysis and mean monthly rainfall for the 61 years from 1943 - 2004.     A - Mean 
monthly rainfall July 1943 – June 2004, B - July 1951 – June 1952, C - July 1993 – 
June 1994, D - July 1974 – June 1975, E - July 1950 – June 1951, F - July 1969 – 
June 1970. 
 
 
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Temperature 
 
Ideal day:night temperature range for germination  (as identified in Chapter 5) occurs 
in two periods: October (18.5:8
o
 C) through November (21.5:9.5
o
 C) and again in 
April (20.5:8.5
o
 C) (BOM climate database, Commonwealth of Australia). Variation 
in mean maximum and mean minimum monthly temperatures was not significant over 
the 63 years for which data exists; standard deviation for mean maximum 
temperatures 1.6
0
 C, mean minimum temperature standard deviation 1.4

C.  
 
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5
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25
30
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Temperature (
o
C)
 
 
Figure 7.4 Mean monthly temperature levels from East Sale weather station, Sale, 
Victoria from July 1943 to June 2005 (BOM climate database, Commonwealth of 
Australia). Blue – mean monthly minimum temperature. Red – mean monthly 
maximum temperature. Error bars = standard error.  
 

 
170
7.3.3 Salinity data 
 
Surface-water salinity data for the adjacent Lake Wellington (Grayson 2003), within 
0.85 km of the centre point of the area covered by the aerial photographs, exists for at 
least two of the periods identified in the climate data: July 1969 – June 1970 and July 
1974 – June 1975. There is an annual rise and fall in the salinity of the surface water 
of Lake Wellington, with peak salinity during winter and a minimum in spring-
summer. The magnitude and duration of maximum and minimum salinity varies 
widely not only from year to year but also within years, ranging from less than 1 g L
-1
 
to just under 20 g L
-1
 in the period studies by Grayson (2003), namely January 1965 – 
December 1991.   
 
The period of September 1969 through February 1970 represents a six month period 
when surface water salinity levels in Lake Wellington were 0.66 - 1.59 g L
-1
, well 
within the tolerance range for both germination and hypocotyl hair production in M. 
ericifolia (Chapter 6 & 7). After a short period when salinity ranged from 3 - 4 g L
-1
 
(March 1970 to May 1970), surface-water salinity of Lake Wellington fell to below 
1.0 g L
-1
, until April 1971. This represents a period of nearly 20 months of suitable 
conditions for recruitment and establishment of M. ericifolia seedlings.  
 
There was a 6-month period between August 1974 and January 1975 where salinity 
was below 2 g L
-1
. This period would be suitable for recruitment and establishment of 
M. ericifolia seedlings.  
 

 
171
Both of these periods (September 1969 - February 1970 and August 1974 - January 
1975) correspond with rainfall patterns identified in the hierarchical cluster analysis 
and which fall within the suitable temperature range for recruitment and establishment 
of M. ericifolia (Chapters 6 and 7). Surface waters in Lake Wellington during winter 
and outside of those listed times listed above were well above 4 g L
-1
 or dropped 
below 2 g L
-1
 for shorter periods of time, conditions that are unsuitable for 
recruitment and establishment of M. ericifolia.  
 
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linity
 (g/L
-1
)
 
 
Figure 7.5 Mean monthly salinity of surface waters in Lake Wellington, Sale, 
Victoria from July 1968 to June 1975. Data derived from Grayson (2003) 
 
 
 

 
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7.4 Discussion 
 
7.4.1 Aerial photographs 
 
The analysis of the aerial photographs indicated that there were only five recruitment 
events over the period of 46 years covered by the available aerial photography. 
Episodic recruitment is widely reported for many plants but is especially prominent in 
clonal wetland species where specific combinations of spatially and temporally 
suitable environmental conditions are rare (Eriksson and Froborg 1996; Nicol and 
Ganf 2000; Barsoum 2002; Griffith et al. 2004; Stokes and Cunningham 2006). 
Hydrology is generally assumed to be the major driver in the germination and 
establishment of wetland species, and a major influence on the persistence of adult 
plants and structuring of the vegetation (Keddy 1984; Keddy and Ellis 1985; Keddy 
and Constable 1986; Coops and Van der Velde 1995; Coops et al. 1996; Leck and 
Brock 2000). For example, a comparative study of four helophyte species 
(Phragmites australis, Phalaris arundinacea, Scripus maritimus and S. lacustris) in 
the Netherlands reported a strong differential growth response to a water-depth 
gradient (Coops et al. 1996). In a later study investigating these same species, 
germination response and seedling growth were determined by the same moisture 
gradients favoured by the adult plants (Coops and van der Veld 1995).  
 
Recruitment opportunities for species in the genus Melaleuca are very specific and 
related primarily to site conditions, particularly moisture and salinity (Browder and 
Schroeder 1981; Ladiges et al. 1981; de Jong 2000; Griffith et al. 2004). Germination 
of  M. ericifolia in laboratory situations has been shown to be limited to a range of 

 
173
conditions; 
∼20
oC
, < 2 g L
-1 
salt and darkness (Chapter 6). A further potential 
impediment to seedling establishment in M. ericifolia is the sensitivity of hypocotyl 
hairs that form in the initial days after germination (Chapter 7). These structures are 
thought to be critical in successful establishment as they play a major role in 
geotropism and root formation. Hypocotyl hair formation however is favoured only 
by the conditions required for germination (Chapter 7).  
 
7.4.2 Climate 
 
Climate, particularly rainfall, and its effect on the hydrological regime of wetlands 
critically alter site conditions influencing the spatial and temporal wetting and drying 
cycle (Bedford 1996). The five years identified in the climate analysis of this study 
(1950, 1951, 1977, 1969 and 1993), exhibited rainfall events of greater than 150 mm 
each year in spring, events that periodically flood the wetland (Figure 7.4). These 
rainfall events were, on average, three times greater than the mean rainfall for the 
months in which they occur. Floods of this magnitude flush brackish-water wetlands, 
remove or dilute salt and provide a short period of altered site conditions that may be 
suitable for germination (Pezeshki 2001; Middleton 2002; Riis and Hawes 2002; 
Warwick and Brock 2003). This essentially converts brackish-water wetlands to 
freshwater wetlands for short but variable lengths of time. In the case of Dowd 
Morass in the key recruitment years, near-freshwater conditions existed for about 6 
months, sufficient time for seedlings of M. ericifolia to establish. A period of release 
from hostile conditions (e.g. salinity, dryness) of 12-16 weeks has been found to be 
the most ideal for a range of herbaceous wetland species in swamps in New South 
Wales (Warwick and Brock 2003). 

 
174
 
It is events such as flooding, flushing or drawdown that provide the window of 
opportunity for recruitment of many clonal wetland plants for which the normal 
prevailing conditions are unsuitable for the highly sensitive seedling stages but 
suitable for growth of adult plants (Eriksson and Froborg 1996; Rand 2000; Noe and 
Zedler 2001; Peterson and Baldwin 2004). It is well established that some wetland 
plants are reliant on drawdown or periods of drying for establishment (Middleton 
1999; Nicol and Ganf 2000). However, a variety of otherwise freshwater species that 
occur in saline conditions are reliant on freshwater pulses to remove the osmotic 
stresses created by salt and allow germination and establishment (Churchill 1983; Gul 
and Weber 1999; Khan and Ungar 2001; Robinson et al. 2006). Melaleuca ericifolia 
and its responses to salinity and moisture suggest strongly that it is reliant on 
freshwater conditions for successful germination and early-stage establishment 
(Ladiges et al. 1981; Robinson et al. 2006).  
 
This study confirmed that recruitment of M. ericifolia is episodic and tied closely to 
climatic conditions. However, further investigation to actual on-ground conditions 
needs to be carried out to determine exactly what effect rainfall is having on 
recruitment sites. The following chapter investigates safe sites for germination at 
Dowd Morass.  

 
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Chapter 8  
Safe sites for recruitment of Melaleuca ericifolia in Dowd 
Morass
 
 
Abstract 
 
Safe sites are critical for the successful recruitment of many plants. For long-lived 
clonal plants, safe sites, or windows of opportunity for recruitment, may be rare and 
limited in both time and space. At Dowd Morass recruitment safe sites were limited to 
the tops of hummocks, where hydrologic and edaphic conditions were suitable. 
Periodic flooding/flushing of the wetland is posited to dilute potentially toxic salinity, 
and reduced the influence of low pH levels within hummocks, while at the same time 
maintain suitable moisture for seedling recruitment. The physical structure and 
composition of the hummocks coupled with flood pulsing, triggered by abnormally 
high rainfall and flooding, are proposed as the major influences on successful 
recruitment in M. ericifolia in brackish-water wetland. 
 
 
 
 

 
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8.1
 
Introduction 
 
 
Seedling establishment is dependent on the convergence of a series of precise events 
and conditions including temperature, moisture, light, substratum configuration, 
substratum chemistry and seed burial. These factors trigger germination and, if 
followed by favourable establishment conditions, recruitment of young plants into the 
established population is possible. Safe sites are the specific on-ground conditions, 
both spatial and temporal, that are conducive to the successful recruitment of plant 
species (Harper 1977).   
 
The co-occurrence of the abovementioned events and conditions in both space and 
time, particularly in heterogenous environments such as wetlands, is likely to be rare. 
The lack of convergence of these abiotic factors may explain the episodic nature of 
recruitment events in a wide range of plant species (Grubb 1977; Eriksson and 
Froborg 1996; Kellogg et al. 2003; Bell and Clarke 2004, Chapter 8). In coastal 
wetlands, these factors are further complicated by other influences, such as salinity, 
acid sulphate soils and human-altered water regimes (Noe and Zedler 2001). These 
additional influences may further restrict the already limited safe sites or windows of 
opportunity for germination and establishment (Harper 1977).   
 
The establishment of woody species of plants in wetlands has been shown to to occur 
only rarely, sometimes only once every few decades, due to the specific requirements 
of the seeds and seedlings in genera such as ChamaecyparisNyssaSalix, Taxodium 
and Vaccinium (Eriksson and Froborg 1996; Conner 2002; Gengerelly and Lee 2005; 

 
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Stokes and Cunnigham 2006). Climatic change may shift microsite characteristics 
beyond the range that could be considered safe, particularly species that are persisting 
at the edge of their distributional or tolerance limits, such as Juniperus sabina in 
Mongolia. In this case, complete failure of recruitment for many decades was reported 
by Wesche et al. (2005). The loss or reduction of sexual reproduction, through the 
lack of safe sites for recruitment, is reported as being a factor in the evolution of long-
lived clonal species such as Melaleuca ericifolia (Eckert 2001).  
 
Safe site provision in heterogenous wetlands, although highly restricted, nevertheless 
does occur in both space and time. Hummocks, a common feature of wetlands, greatly 
alter the substratum topography providing a range of conditions, and commonly 
function as safe sites for wetland plants (Vivian-Smith 1997; Roy et al. 1999; 
Nungesser 2003; Peach and Zedler 2006; Raulings et al. 2007). Several authors have 
found that the majority of plants in wetlands were reliant on hummocks for 
recruitment and to maintain structural and floristic diversity in the broader vegetation 
community (Rheinhardt and Hershner 1992; Jones et al 1994; Crain and Berness 
2005)  
 
Hummocks in Dowd Morass are composed primarily of living and dead plant 
material. Hummock height varies greatly depending on the species that form 
hummocks and the subsidiary species that colonise them. Over time, through 
deposition of organic matter and consolidation by plant roots, hummocks can attain 
heights of a metre or more, raising the upper parts of the hummock above the 
prevailing water level of a wetland (Bertness et al. 1992; Fogel et al. 2004)  
 

 
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At Dowd Morass a range of species form hummocks, primarily: Juncus spp., 
Melaleuca ericifolia,  Paspalum distichum and Phragmites australis. The hummocks 
formed by each of these species have different configurations and heights depending 
on the growth habits of the plants. Paspalum distichum forms low, broad hummock 
on the very edge of the wetland in periodically inundated sites. Juncus spp occupies a 
similar zone to P. distichum but forms more narrow upright hummocks. Both 
Phragmites and Melaleuca form tall hummocks (> 1 m high) that may be several 
metres across. Hummocks of both Phragmites and Melaleuca occur in areas that are 
permanently or near-permanently flooded and on the edges of wetlands that have less 
permanent standing water (Boon et al. 2007). Although Melaleuca and Phragmites 
hummocks occur in flooded conditions, the tops of the hummocks are elevated above 
the prevailing water level and rarely, if ever, become submerged. A wide range of 
herbaceous species is known to occupy the upper levels of the hummocks throughout 
Dowd Morass (Raulings et al. 2006).  
 
Woody plant species are known to colonise hummocks in wetlands due to their 
favourable germination conditions: moist, well aerated and with generally higher 
temperatures than the surrounding substrates (Titus 1990; Eriksson and Froborg 1996; 
Gengerelly and Lee 2005). While hummocks provide relief from flooding, anaerobic 
stress and generally provide favourable temperatures, high transpiration rates coupled 
with saline conditions may create toxic conditions for seedlings. The specific 
microsite conditions created by individual species may influence the availability of 
safe sites by altering soil chemistry, moisture availability and degree of competition 
(Hatton 2004). 
 

 
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This part of the study carries on from Chapter 8, which attempted to identify the 
temporal occurrence of safe sites for recruitment of M. ericifolia at Dowd Morass. 
The specific aims are to determine the spatial occurrence of safe site, in particular to:  
 
•  Characterise the environmental conditions where potential safe sites may 
occur, and 
•  Determine potential safe sites for germination and establishment on-the 
ground.   
 
It was predicted that recruitment would be limited in space, specifically in relation 
to suitable safe sites that provided salinity below 2 g/L
-1
, day:night temperatures 
~20
o
 C:12
o
 C, moderate moisture (neither dry or saturated/inundated) and 
darkness. 
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