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


Effects of environmental variables



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6.4.1 Effects of environmental variables 
 
Water availability had strong effects on the production of hypocotyl hairs on M. 
ericifolia seedlings.  Not only did it influence the development of geotropism, but 
water availability controlled the proportion of seedlings that developed fully 
functional hypocotyl hairs (e.g., see Fig. 3).  Furthermore, soaking seeds for periods 
of more than about one hour seemed to be highly inhibitory to the formation of 
hypocotyl hairs, even though the seeds of M. ericifolia are highly likely to be 
deposited into wet environments.  These results are consistent with those obtained by 
Polya (1961), who reported that soaking seed, even for very short periods of time, 
inhibited hypocotyl hair formation in Populus seedlings.  As the process of soaking 
simulates flood conditions, these results suggest that the longer the wetting period, the 
less likely M. ericifolia seedlings will produce functional hypocotyl hairs.  Even short 
periods of wetting, such as those experienced after heavy rainfall where seed may fall 
into puddles, may have a similar effect to long-term flooding in terms of hypocotyl 
hair production and presumably the successful establishment of young plants 
The results we obtained on the effects of water availability on hypocotyl hair 
formation are similar also to those of Aronne and de Micco (2004), who observed the 
development and average length of hypocotyl hairs was greater in Myrtus seedlings 
that germinated under dry conditions than those that germinated under moist or 
saturated conditions.  In combination, these findings suggest that hypocotyl hairs are 
either inhibited by exposure to flood-like conditions, as suggested by Polya (1961), or 
are primed by desiccation, as postulated by Aronne and de Micco (2004).  Polya 
(1961) suggested a causal mechanism for this response was that rapid swelling of the 

 
151
embryo in saturated conditions caused sudden distention of the cell membranes, 
resulting in tears in the protoplasm. 
 
When M. ericifolia seedlings devoid of hypocotyl hairs were transferred from 
an environment with high water availability (0.2 % w/v agar) to one with low water 
availability (10 % w/v agar), they produced a mass of short root hairs but none formed 
hypocotyl hairs and most died within a few days.  These results indicate that the 
ability of M. ericifolia seedlings to produce hypocotyl hairs is limited to the first few 
days of development, and perhaps is even stimulated prior to splitting of the seed coat. 
The implication is that timing of the wet and dry events may have significant impacts 
on the production of hypocotyl hairs and potentially on seedling survival under field 
conditions.  
 
As well as duration of immersion, temperature, salinity and light showed very 
strong effects on the development of hypocotyl hairs in M. ericifolia.  Hypocotyl hair 
development was inhibited strongly at salinities of more than about 1-2 g L
-1
 and, by 
4-8 g L
-1
 depending on light regime, hypocotyl hairs did not develop at all (e.g., see 
Figs. 5 and 6).  Such an extreme sensitivity to salt is somewhat surprising, given the 
salinity regime that exists in Dowd Morass (< 1-2 g L
-1
 to > 16 g L
-1
; commonly > 8 g 
L
-1
) and the well-reported salt tolerance of juvenile and adult M. ericifolia to salinity 
(Bird 1962; Ladiges et al. 1981; Robinson et al. 2006; Salter et al. 2007).  This 
sensitivity to salt could be a function of osmotic stress on unexpanded hypocotyl hair 
cells or on the developing hypocotyl hairs, or a direct consequence of toxic ion effects 
(e.g., see Kozlowski 1997; Barett-Lennard 2003).  The microscopy observations 
showed that hypocotyl hairs in M. ericifolia are single-celled outgrowths, about 20 

 
152
mm long x 30 
μ
m wide, with a consequently high surface-area-to-volume ratio.  The 
large contact area with the external environment suggests a high degree of sensitivity 
to osmotic stress and/or exposure to toxic ions, a factor that was identified by Young 
and Martens (1991) as important for the response to low moisture levels of hypocotyl 
hair development in the dryland plant Artemisia.   
 
6.4.2 Implications for seedling establishment and plant recruitment 
 
This investigation indicated that M. ericifolia seeds that become soaked for more than 
about one hour or so by falling onto open water, especially if the salinity exceeds a 
few grams per litre, would be unlikely to form hypocotyl hairs.  Hair formation is 
decreased also by low (10
o
C) or high (30
o
C) temperatures and by exposure to light, 
although these two environmental variables exerted weaker effects that either soaking 
or salinity.  All species that have been reported to produce hypocotyl hairs (with the 
exception of Zostera marina, a submerged angiosperm growing in coastal waters) 
germinate on the sediment or soil surface.  Buried seed or disturbed seed died in 
studies of some species in the families Alismataceae and Hydrocharitaceae (Kaul 
1978), and substantial growth abnormalities were recorded when the connection of 
hypocotyl hairs and the substratum was disrupted in Artemisia (Young and Martens 
1991).  While seed of M. ericifolia and other species can germinate when flooded or 
after exposure to saline conditions (Ladiges et al. 1981; Robinson et al. 2006; Salter 
et al. 2007), the failure to produce hypocotyl hairs under these conditions would 
reduce markedly the subsequent ability of young plants to establish effectively into a 
wetland.   
 

 
153
This study thus considerably extends what is known about the environmental 
conditions that are required for successful establishment of M. ericifolia seedlings: it 
is not a simple matter of the conditions required for successful germination of the 
seeds alone, but also the rather limited conditions that are required for hypocotyl hairs 
to develop fully and allow the young seedling to establish successfully.  For 
germination to be successful, seed must find itself on moist, but not saturated, 
substrata of low salinity (Ladiges et al. 1981; Robinson et al. 2006).  Seed of M. 
ericifolia can germinate on the sediment surface, but requires reduced light levels, 
possibly in the shade that is provided at the base of other plants or in the pores of 
coarse- to medium-textured organic or mineral soils.  These conclusions are consistent 
with those of Polya (1961), who showed that the specific conditions for successful 
germination and establishment of the wetland tree species Salix and Populus were 
very narrow and broadly similar to those outlined above for M. ericifolia.  
 
For successful establishment of the young seedling once seeds have 
germinated, environmental conditions must be suitable for hypocotyl hairs to develop
in this regard the sensitivity of hypocotyl-hair formation to even brief soaking and to 
salinities of more than about 1-2 g L
-1
 would further limit the capacity of M. ericifolia 
seedlings to establish in coastal, brackish-water wetlands.  Such a strict combination 
of requirements, over diverse environmental variables including temperature, salinity 
and light, is even narrower than that required for seed germination and seedling 
establishment considered separately and would indicate that recruitment success 
would be limited to all but ideal conditions.  This conclusion is consistent with 
preliminary studies carried out by one of the authors (RWR) on the establishment and 

 
154
growth of M. ericifolia, which indicates that recruitment events are rare (less than one 
year in ten) under field conditions at Dowd Morass.   
 
Conditions for successful seed germination and hypocotyl hair formation are 
likely to have been even further limited by the human-induced modifications that 
affect many coastal, brackish-water wetlands that are vegetated with M. ericifolia.  
Increasing salinisation, for example following changes to water regimes after the 
construction of levees and other structures or to more frequent saline intrusions 
following sea-level rise and the artificial opening of previously intermittently open 
coastal lagoons, would further reduce opportunities for germination and seedling 
establishment.  Hydrological alterations that involve the replacement of alternating 
wet-and-dry cycles with permanently ponded water would similarly restrict the 
likelihood of young seedlings forming functional hypocotyl hairs, and thus further 
limit the chances of successful sexual recruitment. 
 
 
 
 

 
155
Chapter 7 
Historical recruitment events of Melaleuca ericifolia at 
Dowd Morass 
 
Abstract 
 
Determining specific safe site conditions in short-term studies can be difficult as 
recruitment in clonal species may take place only every few decades. This component 
of the project investigated how the combination of aerial photography, germination 
studies and historical rainfall and temperature data could be used to determine likely 
recruitment events for M. ericifolia over a period of ~ 60 years. Recruitment was 
found to be limited to specific events directly related to rainfall (flood events) and 
temperature. Recruitment occurred in just four or possibly five years over 60 years 
(1950, 1951, 1969, 1974 and 1993). Suitable climatic conditions in these years were 
related to average rainfall (> 800 mm, median 606 mm) and to magnitude of flooding 
(> 100 mm) at specific times (spring and autumn) with appropriate average rainfall 
during the intervening summer months. It is predicted that flooding or heavy 
precipitation would temporarily reduce salinity within the wetland bringing conditions 
within the range required for germination and establishment. Average rainfall during 
the putative period of establishment would reduce inundation and potential death of 
early-stage recruited seedlings.   
 
 

 
156
7.1
 
Introduction 
 
 
Predicting prior recruitment events is difficult due to the inherent speculation 
involved, lack of independent evidence for recruitment and the time lag needed to 
determine if predictions are correct. Melaleuca recruitment events in the field have 
been recorded only rarely (Di Stefano and Fisher 1983; de Jong 2000; Griffith et al. 
2004) and the specific convergence of germination and establishment conditions even 
less so (Griffith et al. 2004). Studies of the way wetland species recruit have relied 
heavily on laboratory or field-based observational studies of microtopographical relief 
or responses to biotic or abiotic conditions (Ladiges et al. 1981; Budelsky and 
Galatowitsch 1999; Cornett et al. 2000; Nicol and Ganf 2000; Barsoum 2001; 
Robinson  et al. 2006). Although predictive studies have used previously conducted 
laboratory-based studies to model potential recruitment sites (Green and Johnson 
1999; Gourlet-Fleury et al. 2005; Ordonez et al. 2006), these studies have not, on the 
whole, confirmed if predictions were correct through follow-up investigations.  
 
Aerial photography has been employed widely to map changes and landscape scale 
responses of vegetation (Williams and Lyon 1997; Kadmon and Harari-Kremer 1999; 
Herwitz  et al. 2000; Fensham and Fairfax 2002; Fensham et al. 2003). The 
combination of spatial resolution and extent, and long-term temporal coverage allows 
for tracking of precise changes over periods of many years or decades. In large clonal 
species, such as M. ericifolia, identification and tracking of individual plants may be 
possible, allowing genet/ramet dynamics and potential recruitment events to be 
identified (Hudon et al. 2005; Kyncl et al. 2006).  Studies of genet or ramet-level 

 
157
population dynamics of clonal shrubs is particularly uncommon due to difficulties of 
aging genets and the difficulty of tracking ramet growth using common demographic 
techniques (Pysek 1991; Schenk 1999; Lantz and Antos 2002; Kyncl et al. 2006).  
While aerial photography is commonly used for tracking overall vegetation change it 
is rarely used at the genet level even though it may be very useful (Hudon 2005; 
Kyncl et al. 2006).  
 
Historical climatic data has similar benefits to historical aerial photography in that it 
allows interrogation of information for specific combinations of abiotic conditions 
that may be suitable for germination and establishment. When climate data are 
combined with historical series of aerial photographs likely recruitment events and 
their potential climatic triggers may be ascertained. Germination responses and 
tolerances determined in the laboratory can be correlated to climatic data to more 
clearly elucidate relationships; the necessary background information on seed 
viability, germination conditions and seedling establishment has been reported in 
chapters 4-7 of this thesis.    
 
 Based on findings of Chapters 6 and 7, there was a strong indication that climatic 
variables and their influence on site conditions played a major role in creating spatial 
safe sites and temporal windows of opportunity for the recruitment of M. ericifolia in 
Dowd Morass. It was proposed that analysis of existing data, coupled with on-ground 
observations, would identify the possible climatic conditions that would permit 
successful recruitment.  
 
 

 
158
This component of the thesis aimed to use a combination of historical aerial 
photographs spanning 46 years, historical climatic information spanning 63 years and 
previously established laboratory-based germination and recruitment data (Ladiges et 
al. 1981; Robinson et al. 2006; Chapters 4-7) to predict when M. ericifolia recruited 
in Dowd Morass over the ~ past half century.  
 

 
159
7.2
 
Methods 
 
7.2.1 Historical aerial photographs 
 
The historical series of aerial photographs used in Chapter 4 was re-examined to 
determine evidence of a limited number of successful recruitment events over this ~ 
50-year period. Recruitment events were determined by examining each photograph 
for cohorts of plants: these were then classified into either major or minor events 
depending on the number and distribution of young plants on each photograph. Major 
events represented a large number of plants scattered throughout the area represented 
in each air photograph. Minor events were limited to small numbers of plants 
restricted to small areas on the air photographs. The area represented in the aerial 
photographs is approximately 1.5 x 1.2 km.  
 
7.2.2 Climatic and salinity data  
 
Climatic data (rainfall, air temperatures) covering the years 1943 to 2005 were 
obtained from the Bureau of Meteorology (Government of Australia). These data 
contained daily rainfall and max/min air temperature readings for the East Sale 
weather station (38
o
 12’ S 147
o
 13’ E) approximately 1 km north of Dowd Morass.  
To determine whether there were potential patterns to climatic events, data were 
analysed using a single linkage method (nearest neighbour) hierarchical cluster 
analysis (hca) (SPSS vers. 15, SPSS Inc, Chicago, Illinois, USA), using Euclidean 
distance as the distance metric. Each data set (rainfall, mean maximum temperature
mean minimum temperature), was analysed separately in two runs; the first with daily 

 
160
and then with monthly data. Dendrograms were produced for mean monthly rainfall 
data.  
 
Salinity modelling data for Lake Wellington, immediately adjacent to the study site, 
was obtained from Grayson (2003).  These data were used, where possible, to further 
elucidate the patterns established using climate data and recruitment events observed 
on the aerial photographs.  
 

 
161
7.3 Results 
 
 
7.3.1 Recruitment events determined using aerial photographs 
 
Recruitment events were evident on four photographs in the historical series of aerial 
photographs; 1964, 1978 and 1991 and 2003 (Figure 7.1).  The largest and most 
widespread recruitment was evident in the 1964 aerial photograph, with obvious 
young plants scattered across the whole area of the 1.8 km
2
 image. As seen in Figure 
7.1, M. ericifolia plants appear as dark points amongst the lighter coloured stands of 
Phragmites australis. Minor recruitment appears on the 1978 and 1991 aerial 
photographs. A final major recruitment event was evident on the 2003 aerial 
photograph, with small plants appearing amongst the much older established clones.  
 
 
 
 
 
 
 
 
 
 
 
 

 
162
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 7.1 Photos of a single section of Dowd Morass at six time periods from 1957 
to 2003. Light coloured areas are Phragmites australis, dark areas Melaleuca 
ericifolia. Circular patches are individual clones of M. ericifolia. Scale 1:150. Red 
arrows indicate areas of recruitment.  
1957 
1991 2003
1978 1982
1964

 
163
7.3.2 Climate data  
 
Rainfall 
 
The use of a hierarchical cluster analysis was, in many respects, an exploratory 
process to see if there were any patterns in the data that might help to explain the four 
recruitment events identified on the historical aerial photographs. It was assumed that 
the normal pattern of climatic events was not conducive to recruitment and that 
unusual climatic events would account for the four periods that supported recruitment.  
 
Hierarchical cluster analysis of the 61 years of climate data revealed 4 main clusters 
(Figure 7.2). Forty-nine of the years clustered very tightly, occurring at a Euclidean 
Distance (ED) of 4-11. The remaining 12 years (outliers) formed three discreet 
clusters: seven years ranged from a ED of 13-16, three years at a ED of 19-21 and two 
years at a ED of 25.    
 
Average rainfall over the entire period from July 1950 to June 2003 was 613 mm per 
annum (mean 606 mm), although individual years ranged from a low of 285 mm in 
2002 to a high of 971 mm in 1951. Of the 12 outliers identified by the hierarchical 
cluster analysis the five years with a ED of greater than 19 had yearly rainfall 
averages well above the norm (> 705 mm) indicating a generally ‘wet’ year. The 
remaining 7 years, with an ED of 13-16 had below average to above average yearly 
rainfall (505-704 mm).    
 
 

 
164
 
              0         5        10        15        20        25 
        Year  +---------+---------+---------+---------+---------+ 
 
          1987   
òûòòòø
 
          2005   
ò÷
   
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òòòø
 
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          1971   
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          1959   
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          1953   
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165
          1944   
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          1990   
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          1991   
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