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


µg per seed while M. parvistaminea produced  larger seeds, ranging between 31-33



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µg per seed while M. parvistaminea produced 
larger seeds, ranging between 31-33
 
µg per seed.  
 
Viability within M. ericifolia was related also to population size, distance to nearest 
population and disturbance. Populations bigger than about than 50 ha generally had 
the greatest seed viability (23 – 38 %), whereas those below 5 ha generally showed 
very low viability (< 10 %). Exceptionally, the largest population at Dowd Morass (> 
1,000 ha) had very low viability (6 %). Cades Road (Victoria) and The Apsley 
Marshes (Tasmania), both about 50 ha, exhibited viabilities of 13 and 14%, 
respectively. On further investigation of these three sites it was found that major 
alterations to water regime and accompanying secondary salinisation had occurred in 
all, and these environmental factors/disturbances were probably responsible for this 
lowered viability. Prior to European settlement all three of the above sites were 
freshwater swamps, but due to human interference have salinities that range up to 
approximately 20 g L
-1
 or more.   

 
201
 
9.3 Environmental requirements for seedling establishment 
 
9.3.1 Germination 
 
The germination requirements of M. ericifolia are poorly understood, despite the fact 
that this species is widespread and abundant in southern Australia and its priority 
listing for rehabilitation. Melaleuca ericifolia occur in a range of freshwater swamps 
and estuarine situations, many of which have been altered by human settlement 
(Bowkett and Kirkpatrick 2003). Several key environmental parameters, which may 
act individually or synergistically, were investigated to better understand germination.  
 
Seed of M. ericifolia is held in woody capsules on the plant and released after death of 
the branch from a range of factors, particularly fire, wind throw and damage by 
roosting colonies of birds, particularly ibis. Small quantities of seed are released 
throughout the year. Once seed is released from the seed capsules viability rapidly 
decreases with little or no germination after one year (Salter pers. comm.).  
 
The ideal conditions for germination in M. ericifolia are on the surface of the 
substratum but in darkness with temperatures of ~20
o
C and salinity of < 2 g L
-1

Salinity was found to be the strongest influence on germination, its effect becoming 
more pronounced under temperatures of 30
o
C. The adverse effect of salinity was 
greatly reduced if seed was subsequently flushed by fresh water.  
 
 

 
202
9.3.2 Hypocotyl hairs 
 
A second aspect of the successful recruitment of M. ericifolia plants is seedling 
survival. Many studies have examined the germination of wetland plant seed, but very 
few the factors that influence the subsequent establishment of the young seedlings. 
Hypocotyl hairs were shown to be a critical element in successful seedling 
establishment. Hypocotyl hairs (single celled outgrowth from the base of the 
hypocotyl not associated with the true root system) occur in a range of plant species, 
especially aquatic monocotyledons. The identification of hypocotyl hairs in M. 
ericifolia in this study is the first time such structures have been identified in this 
genus. Hypocotyl hairs develop in very young seedlings, usually before the true root 
system, and help to anchor seedlings to the substratum. Hypocotyl hairs are essential 
in the recruitment process of the few species in which they have been identified; those 
seedlings not producing them failing to recruit (Aronne and De Micco, 20004), and it 
is likely that a similar stricture holds for M. ericifolia.  
 
The sensitivity of hypocotyl hairs to a range of environmental conditions both before 
and after germination dictates their formation and subsequently the success of 
recruitment. Soaking of seeds inhibited the formation of hypocotyl hairs whereas 
more moderate levels of moisture availability in the substrate allowed for full 
development of hypocotyl hairs. Rapid imbibition of seed was found to rip the testa in 
the seed of other woody wetland species particularly Populus (Polya 1961).  
 
Salinity, light and temperature all had significant individual effects on the formation 
of hypocotyl hairs. Salinity exhibited the strongest effect, with salinities greater than  

 
203
1 g L
-1
 strongly inhibiting hypocotyl hair formation. Exposure to light inhibited 
hypocotyl hair development, as did higher temperatures. These last two factors 
increased moisture stress, quickly desiccating the single-celled hypocotyl hairs. The 
synergistic effect of combining salinity temperature and light was pronounced and 
highly detrimental to the formation of hypocotyl hairs.  
 
Successful recruitment of M. ericifolia from seed is strongly dependent on the 
formation of hypocotyl hairs, which allow the seedlings to anchor to the substratum 
and establish positive geotropism. Seedlings of M. ericifolia that did not form 
hypocotyl hairs, or those which the connection of the hypocotyl hairs to the 
substratum was broken, failed to establish. The environmental sieves created by 
salinity, light and temperature exhibit strong influence on germination and hypocotyl 
hair formation in M. ericifolia. For M. ericifolia to successfully recruit, a specific 
combination of environmental factors is needed, namely surface germination in dark 
situations, approximately 20
o
C, salinity levels below 1 – 2 g L
-1
 and moderate 
moisture levels in the substratum. These conditions are not normally found in the 
wetlands in which M. ericifolia grows, leading to the conclusion that recruitment 
events must be rare and episodic.  

 
204
 
9.4 Safe sites for germination in M. ericifolia 
 
Three main characteristics have been identified in M ericifolia; a strongly clonal 
growth form, low fecundity, and specific recruitment requirements of seed and 
seedling. Low fecundity, coupled with specific recruitment requirements and limited 
availability of safe sites, suggests strongly that these factors are the main evolutionary 
driver of clonal growth in this species. Limited recruitment opportunities, sometimes 
only occurring only every few decades, is well known in woody wetland species 
throughout the world (Eriksson and Froborg 1996; Conner 2002; Gengerelly and Lee 
2005; Stokes and Cunningham 2006).  
 
Despite the fragility of conditions required for sexual recruitment, the adults of M. 
ericifolia are tolerant of a wide range of environmental conditions, a trait that is 
strengthened by the extensive clonal growth form. From the studies carried out in 
Chapters 5 and 6, it is evident that germination and particularly the formation of 
hypocotyl hairs occurs within a relatively narrow set of temporal and spatial 
conditions. 
 
9.4.1 Temporal requirements: climatic conditions  
 
Historical aerial photographs are commonly used to map changes to vegetation cover 
over time (Williams and Lyon 1997; Kadmon and Harari-Kremer 1999; Herwitz et al. 
2000; Fensham et al. 2003; Fensham and Fairfax 2002), and it proved especially 
useful for detecting cohorts of recruits at Dowd Morass. Using aerial photographs, 

 
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four successful recruitment events were tentatively detected in the 1950’s, 1970’s and 
1990’s. No other years in this 63 year-long period of analysis seemed suitable for M. 
ericifolia recruitment.  
 
Analysis of climatic data (rainfall and temperature) over the period of 63 years 
indicated that four years experienced markedly different weather from the norm 
(1950, 1951, 1974 and 1993). While monthly temperatures varied little over the 
period studied yearly and monthly rainfall data varied considerably. Yearly rainfall 
totals varied between 285 and 970 mm, monthly totals between 5 and 200 mm. For 
the years in which recruitment putatively occurred, monthly rainfall totals in the 
critical months of August to October and March to April were significantly higher 
than average. In general they were four times the long-term monthly average. As well 
as this period of high rainfall, average or below average rainfall for November to 
February were the norms for these four years. In effect these two rainfall patterns 
suggest a flood/flushing event followed by a period of stability; followed by another 
flood/flushing event is required for successful recruitment of M. ericifolia from seed. 
This convergence of rainfall events is critical to understanding recruitment in the 
salinised wetlands in southeast Australia, such as those of the Gippsland Lakes.  
 
9.4.2 Spatial requirements: the importance of hummocks  
 
Recording on-ground recruitment via field inspections, coupled with growth data of 
juvenile  M. ericifolia, suggested strongly that the juveniles had recruited during one 
of the few events predicted from the analysis of the climate data. Juveniles of M. 
ericifolia were recorded only on the tops of hummocks composed of herbaceous 

 
206
plants such as Phragmites,  Juncus,  Paspalum and Baumea. The tops of these 
hummocks are raised above the normal water level of the swamp and are composed 
almost entirely of organic matter. They provide the suitably moist, dark, low-salinity 
conditions need for germination and recruitment. These hummocks, however, are not 
always favourable to germination and recruitment as salinity in the upper portions of 
the hummock can become very high (e.g. > 20 g L
-1
). The flooding that occurs in 
critical years, especially during months of particularly heavy rain, presumably flush 
the salts out of the upper levels and dilute salinity in the surrounding waters, bringing 
the conditions within the range for successful recruitment. Work carried out on the 
hummocks at Dowd Morass by Coppolino (2007) strongly indicated that elevated 
precipitation and/or flooding modified salinity and moisture conditions within the 
upper portions of the hummocks to within a range suitable for germination and 
establishment of M. ericifolia.  

 
207
 
9.5 Implications of plant and germination characteristics for 
management of brackish wetlands  
 
 
The material summarised in earlier chapters shows clearly that natural recruitment of 
M. ericifolia is closely linked to specific climatic conditions: flood in periods outside 
ideal recruitment periods, average rainfall for 12-16 weeks during ideal temperature 
periods (day:night, 20:12
o
 C) and low salinity (< 2 g L
-1
) within the preferred 
recruitment sites (dark, shaded by standing organic matter on tops of hummocks).   
 
Manipulating the environment to provide ideal on-ground recruitment conditions is 
likely to be beyond the abilities of land managers. Alterations of stream flows, 
particularly to the LaTrobe River and water regimes in the wetlands of the Gippsland 
lakes (Dowd Morass, Heart Morass) has seen an overall decrease of potential flushing 
events and an inexorable increase in overall salinity due to the artificial opening to the 
sea at Lakes Entrance (Bird 1962; Grayson 2003). Mean and peak flows in these same 
wetlands and the rivers that feed them have been modified by the various 
impoundments (Boon et al. 2007). To achieve periodic flushing of the wetlands 
surrounding the Gippsland lakes, at least during peak flow periods, alteration to 
present environmental flow would need to be made. Such flows are largely over-bank 
flows, and thus beyond the aegis of day-to-day catchment management.   
 
There are future plans by the DSE to increase environmental flows in the Thompson, 
Latrobe and Avon Rivers that feed into the Gippsland Lakes: the first two supplying 

 
208
fresh water to Dowd Morass (WGCMA 2006).   One of the concerns is to ensure the 
flushing and subsequent dilution of salinity in the various wetlands of this region 
(West Gippsland CMA 2006) to restore more natural hydrological regimes and restore 
biodiversity values. Re-instating environmental flows, especially during peak flow 
periods, is likely to approximate the natural flushing regimes and overall water 
regimes of the wetlands of the Gippsland Lakes prior to water impoundments and 
manipulation measures installed in the past century.  
 
Landscape-scale recruitment of M. ericifolia from seed is reliant on natural flood 
pulsing to create the specific on-ground conditions for germination, hypocotyl hair 
formation and early-stage seedling establishment. Increased environmental flows and 
their effect on the hydrologic and edaphic conditions in the wetlands would have 
widespread benefits to both M. ericifolia and a range of other species dependent on 
these processes for recruitment and regeneration.  
 
Large-scale restoration of wetlands is generally based on the “self-design” method. 
Self-design assumes that restoration of key abiotic factors, such as hydrological 
regime, will lead to restoration of the plant and animal communities that formerly 
occurred in the restored wetland (Mitsch et al. 1998). Other restoration ecologists 
propose that many wetlands have become so damaged that there is the need to 
actually ‘design’ restoration, including plantings and reintroduction of animals 
(Montalvo  et al. 2002). There are many examples of highly damaged wetlands that 
lack vital elements, including keystone plant species and have suffered from a range 
of non-hydrological changes such as exotic flora and fauna (Pettit and Froend 2001; 
De Steven et al 2006). These damaged wetlands need some intervention other than 

 
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just re-instatement of water regime to achieve successful outcomes. Dowd Morass is a 
wetland that has changed significantly from its original configuration and there is 
little hope of fully restoring a hydrological regime that is similar to the pre-European 
pattern. Intervention in the form of planting or direct seeding of the keystone species 
may need to be carried out to achieve or maintain wetland function at Dowd Morass.  
 
Small to medium size rehabilitation of Dowd Morass and other similar wetlands 
throughout south-eastern Australia can be informed by the safe sites for recruitment 
identified in this study. Other species of Melaleuca, e.g. M. quinquinervia,  M. 
halmaturorum, have broadly similar requirements to M. ericifolia extending the 
possible relevance of this study to most of Australia and the introduced range of these 
species overseas (Florida, Hawaii).   Replanting trials carried out by the author and 
others (Raulings et al. 2006) clearly indicate that planting on sediments was not 
successful, with most planted stock not surviving for more than 4 months. A small 
preliminary trial carried out by Raulings et al. (2006) where seedlings were planted on 
hummocks and the adjacent surrounding sediments showed that nearly all seedlings 
planted on hummocks lived while the seedlings planted in the surrounding sediments 
died. Ninety percent of seedlings planted on hummocks were still alive as of January 
2007, over two years after initial planting, with many putting on over 1 m of growth 
in that time.  
 
The planting of brackish wetland species on natural and artificially created hummocks 
is widely used in the coastal wetlands of the USA for Taxodium distichum (Bald 
Cypress) and a range of other species (e.g., Clewell and Lea 1990; Titus 1990; Barry 
et al. 1996; Anon 2004; Bruland 2005) The specific hydrologic and edaphic 

 
210
conditions found on hummocks are conducive to the establishment of a wide range of 
species and contribute to overall diversity of wetlands by providing microtopographic 
diversity (Vivian-Smith 1997; Bruland and Richardson 2005).   
 
The unique combination of factors investigated in this study provides a framework for 
future work on clonal plants in a wide range of ecosystems, especially wetlands. Two 
such ecosystems in Australia where this thesis is of particular relevance are Melaleuca 
forests and woodlands (90,513 km
2
) and Hummock Grasslands (1,756,104 km
2

(National Land and Water Resources Audit 2001). Of particular note was the ability 
in this thesis to identify the specific edaphic conditions for germination and linking 
these with microtopography and climatic influences. The identification of extensive 
clonality, a feature common to many plants in a wide range of ecosystems in 
Australia, contributes significantly to our understanding of the vegetation dynamics of 
these systems. The lack of reliance of clonal plants on sexual reproduction reduces the 
imperative to ensure short-term sexual recruitment particularly in modified 
ecosystems. Clonality allows a land manager to adjust or modify on-ground 
conditions (e.g. water regime, salinity, topographical relief) relatively infrequently to 
ensure sexual recruitment and therefore the maintenance of genetic diversity within a 
population and sustainability of the species into the future.  
 
Of particular note is the need to adopt a holistic approach to the study of the life cycle 
of species to achieve effective management. Wide-ranging and primary research into 
the recruitment dynamics of a species, as outlined in this project, can uncover 
previously unobserved factors that may be critical to management of a species (e.g. 
hypocotyl hairs). Partial or assumed knowledge of a species life attributes or 

 
211
behaviours in relation to various edaphic conditions can lead to flawed management 
and lack of successful management outcome.  
 
 

 
212
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