3.4.2 Implications for revegetation
While the benefits of extensive physiologically integrated ramets and expansive clonality
are many, there are several distinct disadvantages in the longer-term. Reduction or loss of
genetic diversity or sexual reproduction in populations of clonal plants is well
documented (Widen et al. 1994; Jelinski and Cheliak 1992; Kreher et al. 2000; Eckert
78
2001). The evolution of the clonal growth form is believed to be a consequence of limited
opportunity for sexual recruitment (Mogori et al. 2003). Habitat change, including
alteration to water regimes and salinity levels can move the environment beyond the
ecological tolerances of the species forcing clonal plants to rely on asexual reproduction
for continued survival (Honnay and Bossuyt 2005; Wesche et al. 2005) and on sexual
reproduction for colonisation of new sites (Rea and Ganf 1994). The existing conditions
of the wetlands of the Gippsland Lakes have, through permanent opening of the
intermittent opening to the sea and human induced alteration to the water regime through
the installation of levee banks, severely impacted the ecological conditions of the
wetlands, increasing salinity levels and varying from historical wetting and drying
patterns (Bird 1962; Greyson 2003).
In the short-term, altered water regimes and salinity level changes seems to have
benefited M. ericifolia in the wetlands of the Gippsland Lakes, allowing expansion of
existing genets and recruitment of new genets (Raulings et al. 2006). However, longer-
term effects of these altered regimes are becoming apparent with decline of existing
genets and reduced sexual and asexual reproductive capacity (Raulings et al. 2006). The
requirements for sexual recruitment need to be understood to allow rehabilitation of these
now highly managed and degraded wetlands. Some studies investigating the hydrology,
microtopography and planting technique of M. ericifolia in the Gippsland Lakes wetlands
have been carried out (Raulings et al. 2006) although the planting techniques trialled
have been largely unsuccessful. Specific germination and recruitment condition
information is needed to allow planned intervention and site manipulation to achieve
maximum success. Knowledge of expansion rates and interactions of individual genets
has major implications for spatial and temporal appropriate planting techniques. Present
79
planting techniques that focus on dense plantings of individual plants (genets), usually on
2 x 2 to 3 x 3m centres, primarily to achieve rapid canopy cover, may be particularly
inappropriate for the long-term survival of M. ericifolia in brackish-water wetlands. If
hand-planting of seedlings is to be used, it may be more appropriate to plant far fewer
plants under ideal establishment conditions and allow the rapid expansion rates of the
species to come into play. Counts of numbers of genets per hectare, based on the work
carried out in this study, would indicate that there are as few as three genets per hectare
and that on average there are 7 per hectare. Present planting densities utilising 3 m
centres between planted seedlings utilises approximately 1,110 plants to revegetate one
hectare. Fewer, well-placed plants coupled with time represents both a significant cost
saving and probably a superior ecological outcome.
Chapter 4
Comparison of two contrasting life forms of Melaleuca in
south-eastern Australia.
Abstract
The viability of seed and the trade-off between sexual and asexual reproduction can
vary widely between plants of the same genus occupying different aspects of an
environmental gradient in wetlands. Viability can vary among plants of the same
species across the range of that species in response to different environmental
parameters. A comparative study was carried out between two co-occurring members
of the genus Melaleuca ( M. ericifolia and M. parvistaminea) with contrasting growth
type (rootstock regenerator vs. seed-only regenerator). Additional viability testing was
carried out on 23 populations across the southern part of the range of M. ericifolia in
Victoria and Tasmania in order to test the hypothesis that a range of factors including
population size, distance to nearest population and alterations to habitat may play a
role in viability over and above inherent viability. Seed weight and viability of the
rootstock regenerating M. ericifolia was consistently lower (<30 µg, < 38 %) than the
seed-only regenerating M. parvistaminea (>30 µg, > 70 %). Across the range of M.
ericifolia, viability varied from 0 – 38%, with large, relatively undisturbed
populations having higher percentage germination. Populations of M. ericifolia
affected by isolation, limited gene flow, disturbance and secondary salination had
markedly reduced viability regardless of area covered by the population.
81
4.1 Introduction
Melaleuca ericifolia (Swamp Paperbark) and Melaleuca parvistaminea are two
members of the large serotinous genus Melaleuca (Myrtaceae) that occurs primarily in
Australia (Spencer 1996). There is considerable overlap in the distribution of the two
species and until recently both were considered varieties of M. ericifolia (Albrecht
1987). Differences in regeneration and reproductive ability are two of the main
distinguishing features between these taxa: Melaleuca parvistaminea is a seed-only
regenerator while M. ericifolia forms extensive colonies of vegetatively reproduced
stems through root suckering (Albrecht 1987). Both species occur in wetlands, but
are usually spatially and ecologically segregated: M. parvistaminea occurs in areas in
south-east Australia with only intermittent, short-term inundation in primarily
freshwater swamps ( Figure 4.1), whereas M. ericifolia occurs in wetter habitats that
may be flooded for many months, ranging from fresh to brackish swamps (Figure
4.2). Accordingly, it is not uncommon to find both species growing in the same
wetland but spatially separated along an elevational gradient. The co-occurrence of
these two species in many wetlands throughout south-eastern Australia provides an
ideal opportunity to investigate the hypothesis that there is a trade-off between sexual
and asexual reproduction and that this may be expressed as reduced seed viability in
the clonal species.
82
Figure 4.1 Distribution of Melaleuca ericifolia in Australia (Australian Virtual
Herbarium)
Figure 4.2 Distribution of Melaleuca parvistaminea in Australia (Australian Virtual
Herbarium).
83
It is commonly assumed that there is a trade-off between sexual and asexual
reproduction in plants due to the high costs (energetic and nutrient) of sexual
reproduction (Lovett Doust 1989; Reekie 1999; Eckert 2001). There are, however,
conflicting findings regarding resource allocation to sexual reproduction in clonal
plants, with some authors finding reduced commitment to sexual reproduction (in
Mimulus: Sutherland and Vickery 1988) and others no reduction in sexual
reproductive outputs (Rea and Ganf 1994).
While there are some seed viability studies of several populations of M. ericifolia in
South Gippsland (Ladiges et al. 1981; Robinson et al. 2006) there are no
comprehensive studies of this species over the majority of its range from northern
New South Wales through to western Victoria and Tasmania. Such studies are
essential if generalisations about trade-offs between sexual and asexual reproduction
are to be made. No references to the seed viability of any populations of M.
parvistaminea were found.
The aims of this component of the thesis are to:
1. Compare the seed viability in the clonal M. ericifolia to the non-clonal M.
parvistaminea,
2. Determine the importance of site isolation and population size on the seed
viability of M. ericifolia and M. parvistaminea, and
3. Determine the importance of site degradation on the seed viability of M.
ericifolia and M. parvistaminea.
84
4.2 Methods
4.2.1 Seed collection
Seed capsules were collected between April 2004 and November 2005 from 12 – 25
adult trees (different clones) scattered throughout various populations of M. ericifolia
across its range in Victoria and Tasmania, including several Bass Strait Islands (Table
4.1). Seed was collected from seven populations of M. parvistaminea in South
Gippsland that were known to be natural occurrences ( Table 4.2) were the range of
this species overlaps with M. ericifolia. The characteristic phalanx (dense advancing
front) growth form and dome shape was used to determine individual plants of M.
ericifolia; seed was collected from widely separated plants that were clearly not
related clonally. Approximately 12 individual plants were sampled scattered
throughout each population of M. parvistaminea. Approximately 500 seed capsules
were collected from each putative individual of each species. Seed loses viability after
approximately 1 year, the short period of storage, mostly over the cooler winter
months was not viewed as materially altering overall viability.
Information was collected for each of the populations of M. ericifolia, including
locality (latitude and longitude), adjacent disturbance and an estimate of area covered
by the species. Area of population for M. ericifolia was visually assessed and classed
into categories shown in Table 4.3. Site salinisation and distance to nearest population
was calculated using site records from the Waterwatch and the National Herbarium
Melbourne
85
Table 4.1 Populations and location of seed collection sites for M. ericifolia across
southern Australia. Localities marked with an * are potentially introduced
populations.
Population location
Latitude
Longitude
Population
size (ha)
Little River (Victoria)
37
o
56' 17" 144
o
28' 33" >1 ha
Cape Nelson (Victoria)
38
o
24' 39 '
141
o
23' 41
1-5 ha
*Mount Wellington (Tasmania)
42
o
53' 09" 147
o
15' 32" >1 ha
Heritage Golf Course (Victoria)
37
o
40' 55" 145
o
21' 05" >1 ha
Brushy Creek (Victoria)
37
o
46' 00" 145
o
17' 00" 1-5 ha
Yanakie Peninsula (Victoria)
38
o
48' 52" 146
o
14' 09" 1 ha
Dowd Morass (Victoria)
37
o
28' 58" 149
o
40' 38" > 1000 ha
*Kemps Marsh (Lake Sorell) (Tasmania)
42
o
47' 31" 147
o
33' 26" 1-5 ha
Spadoni Reserve (Victoria)
37
o
28' 58" 149
o
40' 38" 1-5 ha
Boggy Creek (Victoria)
37
o
42' 00" 145
o
33' 12" 1-5 ha
Cades Road (Victoria)
37
o
33' 38" 149
o
07' 13" 10-50 ha
Apsley Marshes (Tasmania)
41
o
59' 59" 148
o
15' 53" 10-50 ha
Lightwood Creek, Point Nepean Nation Park (Victoria)
38
o
25' 5"
144
o
56' 19" 5-10 ha
Garfield (Victoria)
38
o
05' 06" 145
o
36' 38" 5-10 ha
Cann River (Victoria)
37
o
32' 35" 149
o
07' 07" 10-50 ha
Gypsy Point (Victoria)
37
o
28' 58" 149
o
40' 38" 10-50 ha
Yarram (Victoria)
38
o
37' 55" 146
o
40' 05" 10-50 ha
Cape Barren Island Population 1 (Tasmania)
40
o
24' 43" 148
o
01' 42" 10-50 ha
Cape Barren Island Population 2 (Tasmania)
40
o
26' 28"
148
o
08' 14" 10-50 ha
Flinders Island Population 1 (Tasmania)
39
o
57' 10" 148
o
10' 49" 50-100 ha
Flinders Island Population 2 (Tasmania)
40
o
13' 17" 148
o
17' 40" 50-100 ha
Gladstone (Tasmania)
40
o
55' 01" 147
o
54' 30" 50-100 ha
Narawntapu National Park (Tasmania)
41
o
10' 72" 146
o
36' 70" 100-1000 ha
Table 4.2 Populations and locations of seed collection sites for M. parvistaminea in
South Gippsland, Victoria
Population location
Latitude
Longitude
Population size (ha)
Rosedale 38
o
10'4" 146
o
57'24"
1-5 ha
Maffra 37
o
56'2" 146
o
48'41"
1-5 ha
Fernbank 37
o
52'31"
147
o
16'16"
1-5 ha
Providence Ponds
37
o
57'46"
147
o
19
'
18
"
5-10
ha
Heyfield 37
o
56'33"
146
o
43'34"
5-10 ha
Briagalong 37
o
49'9" 146
o
58'39"
5-10 ha
Sale Common
38
o
8'2" 147
o
5'0" 5-10
ha
86
Table 4.3 Classification of population size of M. ericifolia across southern Australia.
Classification Population
size
Very small
Under 1 ha
Small
1-5 ha
Medium
5-10 ha
Medium/Large
10-50 ha
Large
50-100 ha
Very Large
100-1000 ha
Expansive
Greater than 1000 ha
Capsules were stored in paper bags at 20
0
C for one week. The bags were lightly
shaken to release seed from the capsules and the contents sieved to remove the empty
capsules and other detritus. Seed was placed in clean paper bags for a further three
days to remove excess moisture, then transferred to sealed glass containers and stored
at 20
0
C in darkness until used, usually within 2 months of harvest. Preliminary
germination trials began in late April 2004, with the main trials carried out in July
2004 and December 2005.
4.2.2 Seed viability
The number of seeds per gram was determined by taking five samples of known
weight (0.003-0.007 g), which were counted by eye. The average number of seeds
was calculated from the mean of the aggregated totals of the five samples. The weight
of individual seeds was determined by dividing the number of seeds per sample by the
weight of the sample (Association of Official Seed Analysts 1990).
87
Viability testing followed procedures outlined by the Association of Official Seed
Analysts (1990), except that the number of seeds per replicate run was increased from
25 to 100 to improve statistical rigour (Robinson et al. 2006). A small pilot study was
carried out before the main trials to determine statistical power and the number of
replicates needed to detect significant responses to environmental variables (Zar
1999). These trials indicated that four replicates, each using 100 seeds, yielded a
power of >0.99.
Seeds were surface sterilized by placing them in small sealed muslin bags and
plunging the bags in 10 % w/v sodium hypochlorite solution for 20 seconds and then
rinsing them three times in distilled water. For each viability-trial replicate, 100 seeds
were evenly spaced in a grid pattern on a disc of Whatmans #3 filter paper in a 9-cm
diameter petri dish. Each paper disc was wetted with 8 mL of distilled water and the
dish sealed with Labfilm to reduce moisture loss. A total of four replicate petri dishes
(i.e., 400 seeds) were used for each viability test. Seeds were germinated and then
incubated in growth cabinets with day-time temperatures of 20
o
C and night-time
temperatures of 10
o
C. A 12:12 hour light:dark cycle was used. A bank of fluorescent
tubes designed for plant growth provided light, emitting a PAR of 40
μ
E m
-2
s
-1
at the
level of the seeds. All replicates were randomly shuffled daily within the cabinet to
randomise spatial variability, and germination was measured after 7, 14 and 21 days.
The trial was terminated at 21 days because no additional germination was recorded
after Day 14. Germination was judged by the emergence of the base of the hypocotyl
from the testa.
88
4.3 Results
4.3.1 Melaleuca ericifolia
Viability of seed from various population of M. ericifolia varied widely, from 0 % for
two isolated populations in western Victoria and one from southern Tasmania through
32 % for populations from East Gippsland and 33–38 % for the majority of
Tasmanian and Bass Strait Island populations ( Table 4.4). Germination percentage
was closely correlated with seed weight (R
2
= 0.933, see Figure 4.3) with little
deviation from a linear relationship.
Populations with lower viability had a large percentage of unfilled seed. The three
populations with zero germination were excluded from data analysis (rows 1-3 Table
4.4). There was little relationship between viability and population size with three
larger populations, Apsley Marshes, Cades Road and Dowd Morass (10-50 ha and >
1000 ha, respectively) exhibiting a clear deviation from the mean (Figure 4.4).
Populations under 5 ha also exhibited wide divergence from the mean (Table 4.4 and
Figure 4.4). Features in common with all sites with very low viability were the degree
of adjacent human disturbance, secondary salinisation, and isolation from the nearest
population.
89
Table 4.4 Population size, seed weight and viability of various populations of
Melaleuca ericifolia in Victoria and Tasmania, Australia
Population location
Population
size (ha)
Number
of
seeds
per mg
Mean
Seed
weight
(µg)
Percent
germination
(%)
Little River (Victoria)
> 1 ha
0
0
0
Cape Nelson (Victoria)
1-5 ha
0
0
0
Mount Wellington (Tasmania)
> 1 ha
0
0
0
Heritage Golf Course (Victoria)
> 1 ha
61.5
16
0.3
Brushy Creek (Victoria)
1-5 ha
66.4
15
0.5
Yanakie Peninsula (Victoria)
1 ha
55.3
18
5.5
Dowd Morass (Victoria)
> 1000 ha
55.6
18
6.0
Kemps Marsh (Lake Sorell)
(Tasmania) 1-5
ha
53.6
19
6.0
Spadoni Reserve (Victoria)
1-5 ha
53.4
19
6.3
Boggy Creek (Victoria)
1-5 ha
49.9
20
9.3
Cades Road (Victoria)
10-50 ha
49.2
20
12.8
Apsley Marshes (Tasmania)
10-50 ha
47.3
21
14.5
Lightwood Creek Point Nepean
National Park (Victoria)
5-10 ha
42.5
23
21.5
Garfield (Victoria)
5-10 ha
45.2
22
22.8
Cann River (Victoria)
10-50 ha
43
23
23.3
Gypsy Point (Victoria)
10-50 ha
41.4
24
27.0
Yarram (Victoria)
10-50 ha
39.2
26
32.0
Cape Barren Island Population
1 (Tasmania)
10-50 ha
39
26
32.3
Cape Barren Island Population
2 (Tasmania)
10-50 ha
38.7
26
33.0
Flinders Island Population 1
(Tasmania)
50-100 ha
34.2
29
34.0
Flinders Island Population 2
(Tasmania) 50-100
ha
34.4
29
34.5
Gladstone (Tasmania)
50-100ha
37.9
26
35.8
Narawntapu National Park
(Tasmania)
100-1000
ha
37.7 26 38.0
90
y = 3.0579x - 48.442
R
2
= 0.933
0
5
10
15
20
25
30
35
40
45
0
5
10
15
20
25
30
35
Mean Seed weight (µg)
G
e
rm
inat
ion (%
)
Figure 4.3 Linear regression of seed weight versus germination rate for Melaleuca
ericifolia.
0
5
10
15
20
25
30
35
40
Population size (ha)
G
e
rminat
ion (
%
)
< 1
1
1 -
5
5 -1
0
10
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