Wellington Mint-bush — the impact of fire
on population dynamics at Holey Plains
State Park
M. Kohout
2011
Arthur Rylah Institute for Environmental Research
Technical Report Series No. 219
Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
Arthur Rylah Institute for Environmental Research Technical Series No. 219
Wellington Mint-bush — the impact of fire on
population dynamics at Holey Plains State Park
Michele Kohout
Arthur Rylah Institute for Environmental Research
123 Brown Street, Heidelberg, Victoria 3084
June 2011
Arthur Rylah Institute for Environmental Research
Department of Sustainability and Environment
Heidelberg, Victoria
Report produced by:
Arthur Rylah Institute for Environmental Research
Department of Sustainability and Environment
PO Box 137
Heidelberg, Victoria 3084
Phone (03) 9450 8600
Website: www.dse.vic.gov.au/ari
© State of Victoria, Department of Sustainability and Environment 2011
This publication is copyright. Apart from fair dealing for the purposes of private study, research, criticism or review as
permitted under the Copyright Act 1968, no part may be reproduced, copied, transmitted in any form or by any means
(electronic, mechanical or graphic) without the prior written permission of the State of Victoria, Department of
Sustainability and Environment. All requests and enquiries should be directed to the Customer Service Centre, 136 186
or email customer.service@dse.vic.gov.au
Citation: Kohout, M. (2011). Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State
Park. Arthur Rylah Institute for Environmental Research Technical Report Series No. 219. Department of Sustainability
and Environment, Heidelberg, Victoria
ISSN 1835-3827 (print)
ISSN 1835-3835 (online)
Disclaimer: This publication may be of assistance to you but the State of Victoria and its employees do not guarantee
that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore
disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in
this publication.
Front cover photo: Heavily browsed Prostanthera galbraithiae plant at Holey Plains State Park. (M. Kohout).
Authorised by: Victorian Government, Melbourne
2
Contents
List of tables and figures ................................................................................................................ iv
List of tables ............................................................................................................................iv
List of figures ..........................................................................................................................iv
Acknowledgements ............................................................................................................................v
Summary ............................................................................................................................................1
Recommendations .....................................................................................................................1
1
Introduction .............................................................................................................................2
2
2.1
Methods....................................................................................................................................4
Site description..........................................................................................................................4
2.2
Survey methods .........................................................................................................................4
2.3
Data analysis .............................................................................................................................8
3
3.1
Results ......................................................................................................................................9
Population dynamics and time since last fire ............................................................................9
3.2
Browsing ................................................................................................................................12
4
Discussion .............................................................................................................................15
5
Recommendations ................................................................................................................17
References .......................................................................................................................................18
Appendix 1. Time since last fire of Prostanthera galbraithiae populations at Holey Plains
State Park in 2011. .........................................................................................................................19
Appendix 2. Populations of Prostanthera galbraithiae surveyed at Holey Plains State Park in
March 2011. ....................................................................................................................................20
3
List of tables and figures
List of tables
Table 1. Population structure and time since fire for P. galbraithiae at Holey Plains State Park....... 9
Table 2. The proportion of browsed and unbrowsed P. galbraithiae plants at each site .................. 12
List of figures
Figure 1. Schematic diagram of how management activities affect population dynamics of
P. galbraithiae at Holey Plains State Park (modified from Kohout et al. 2009). The dotted
lines show predicted transitions which require further research. .............................................. 3
Figure 2. Locations of Prostanthera galbraithiae populations studied at Holey Plains State Park in
March 2011 .............................................................................................................................. 5
Figure 3. The time since last fire of the Prostanthera galbraithiae populations examined at Holey
Plains State Park in March 2011. ............................................................................................. 6
Figure 4. Seed capsules of P. galbraithiae in March 2011, retained on the plant approximately....... 7
Figure 5. Measuring the length of new shoots of P. galbraithiae in March 2011. ............................. 7
Figure 6. The relationship between time since last fire and the number of primary stems of
P. galbraithiae plants. The Pearson correlation coefficient is shown..................................... 10
Figure 7. The relationship between time since last fire and the mean stem length of P. galbraithiae.
The Pearson correlation coefficient is shown. ........................................................................ 10
Figure 8. The relationship between time since last fire and mean new shoot length of
P. galbraithiae plants. The Pearson correlation coefficient is shown..................................... 11
Figure 9. The relationship between time since last fire and the number of seed capsules present on
P. galbraithiae plants. The Pearson correlation coefficient is shown..................................... 11
Figure 10. Number of primary stems of P. galbraithiae plants at site 8a (fenced and unbrowsed)
and site 8b (unfenced and browsed). (mean ± SD) ................................................................. 13
Figure 11. Mean stem length of P. galbraithiae plants at site 8a (fenced and unbrowsed) and site
8b (unfenced and browsed). (mean ± SD) .............................................................................. 13
Figure 12. Mean new shoot length of P. galbraithiae plants at site 8a (fenced and unbrowsed) and
site 8b (unfenced and browsed). (mean ± SD)........................................................................ 14
4
Acknowledgements
Thanks to Kylie Singleton, Emily Willocks (DSE) and Paula Dower (PV) for assistance in the
field. Thanks to Dan Brown (PV) for providing information about the fire history of Holey Plains,
Josephine MacHunter for GIS mapping and Fiona Coates for help with design of methods.
Michael Duncan, Claire Moxham, Vivienne Turner and David Meagher provided comments on
drafts. This project was funded by Statewide Services, Department of Sustainability and
Environment, Gippsland.
5
Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
Summary
The perennial shrub Prostanthera galbraithiae (Wellington Mint-bush) is restricted to Holey
Plains State Park (SP) and adjacent private property (Dutson Downs) in Victoria. It is classified as
nationally vulnerable. Thirty-four populations are currently known from Holey Plains State Park.
Browsing by Swamp Wallabies and inappropriate fire regimes are the main threats to the ongoing
survival and condition of populations.
Prostanthera galbraithiae is an obligate seeder, depending on accumulated soil seed banks for
regeneration after adults have been killed by fire. The optimal fire frequency is 10 to 15 years,
with a minimum interval of five years. However, browsed populations are likely to have fewer
plants reaching reproductive maturity, leading to a much smaller soil seed bank. This will affect
both the minimum fire interval and optimal fire frequency for the regeneration of this species.
Population dynamics of P. galbraithiae were examined at 14 sites (all browsed except one) with
11 different time intervals since last fire. The population structure of P. galbraithiae varied with
time since fire: the proportion of seedlings was highest in the first year after fire, and mature adult
plants dominated the population structure by four years after fire. Seedlings also occurred 29 years
after the last fire. There was no significant correlation between time since last fire and the number
of primary stems, mean stem length, mean new shoot length or the number of seed capsules.
Browsing was evident at all unfenced sites. Browsed P. galbraithiae plants had a significantly
higher number of primary stems, shorter mean stem length and shorter mean new shoot length
compared to unbrowsed (fenced) plants. In addition, browsed plants showed no evidence of seed
production.
This study has provided two new insights into the ecology of P. galbraithiae. Firstly, long-unburnt
areas still support viable populations of the species. Secondly, fire might cue mass germination but
some germination from the soil seed bank can occur in the absence of fire. However, with high
browsing pressure, the rate of this recruitment is unlikely to be sufficient to maintain
P. galbraithiae populations.
Recommendations
The most important management recommendation for P. galbraithiae is to protect populations
from browsing. Without protection from browsing, fire management strategies are unlikely to be
effective. In particular:
•
Plants at site 8b need to be fenced and protected from fire until they have recovered from
browsing sufficiently to flower for at least two years. This will enable a soil seed bank to
become established. The recovery of this population should be monitored annually for the
next three years.
•
The recently burnt population at site 31a should be fenced to protect germinating seedlings
from browsing.
•
Sites not surveyed since 2005 need to be resurveyed to determine if plants are present.
•
Unbrowsed Prostanthera galbraithiae populations older than 10 years should be burnt to
encourage recruitment from the soil seed bank.
A study examining the identity, density, distribution, movement patterns and dietary preferences of
the herbivore(s) responsible for browsing P. galbraithiae at Holey Plains SP, particularly in
response to fire, would provide specific and targeted information about one of the main threatening
processes to the survival of P. galbraithiae.
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
1 Introduction
Prostanthera galbraithiae (Wellington Mint-bush) is a perennial shrub that is vulnerable
nationally (Carter and Walsh 2003). It is restricted to Holey Plains State Park (SP) and adjacent
private property (Dutson Downs) in south-east Victoria. It grows in heathy open forest, heathland
and heathy woodland in the South East Coastal Plain bioregion (Carter and Walsh 2003).
Prostanthera galbraithiae is an erect to spreading small shrub that grows 0.3 to 2 m high. It is
distinguished from other species of Prostanthera in having stalkless linear leaves and the lower
middle petal broader and longer than each of the upper two petals (Conn 1998). It flowers in
September to October (Walsh and Entwistle 1999).
Surveys conducted in 2004 and 2005 found 34 populations of P. galbraithiae from Holey Plains
SP (Trikojus and Reside 2004). These populations ranged in size from one individual to over 1000
plants, and were all close to vehicular tracks because of the method of survey (Trikojus and Reside
2004).
Browsing by Swamp Wallabies, inappropriate fire regimes, firebreak slashing and roadworks are
the main threats to the survival and condition of populations (Conn 1998, Carter and Walsh 2003,
Trikojus and Reside 2004). Of these threats, browsing and inappropriate fire regimes have a
significant impact on population dynamics (Kohout et al. 2009).
Prostanthera galbraithiae is an obligate seeder, depending on accumulated soil seed banks for
regeneration after adults have been killed by fire (Coates and Downe 2007). In the absence of fire,
some seed germination occurs from the soil seed bank. Mature individuals decline in vigour after
about 10 years (Carter and Walsh 2003), with reduced flowering, and it has been suggested that
the optimal fire frequency may be 10 to 15 years (Kohout et al. 2009). Seedlings regenerating after
fire can reach reproductive maturity in two years, and the minimum fire interval appears to be
about five years in the absence of browsing (Kohout et al. 2009). It is likely that browsing will
heavily influence both the minimum fire interval and optimal fire frequency for the regeneration of
this species. Browsed populations are likely to have fewer plants reaching reproductive maturity
over a longer time (i.e. greater than two years), which will result in a longer time to replenish the
soil seed bank compared to unbrowsed populations.
The impact of fire and browsing has been investigated in two populations which germinated from
soil-stored seed after a prescribed burn in April 2006 (Coates and Downe 2007, Kohout et al.
2009). One of these populations was fenced to protect young plants from wallaby browsing after
fire, and the other population was unfenced. In addition, an unburnt and slashed site, and an
unburnt site (both browsed) were also investigated. Three years of monitoring showed that there
was significant variation in growth and recruitment, attributed to different management regimes.
Burning promotes seed germination, while post-fire fencing protects seedlings from browsing and
substantially decreases mortality post-fire. Hence, browsing has a significant impact on population
size.
Figure 1 summarises the impact of fire and browsing on the population dynamics of
P. galbraithiae at Holey Plains SP, based on previous studies that examined the interaction
between browsing and fire at a small number of sites over five years (Coates and Downe 2007,
Kohout et al. 2009).
The focus of the study reported here was the more general impact of fire on population dynamics,
examining many sites with different time intervals since fire. The aims were to determine:
•
the time since last fire for each P. galbraithiae population
•
the relationship between population fecundity and structure with time since fire
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
•
the optimal range of age classes of P. galbraithiae populations for inclusion in fire
management planning
•
whether any P. galbraithiae populations require protection from fire
•
whether any populations require burning.
browsing,
no fire
fire
Germination
fencing
Holey Plains
Prostanthera galbraithaie
populations
Population decline
persistant browsing,
long-term
absence of fire
Population increase
no plants present
fencing, fire
no browsing, fire
soil seedbank
ceases to be
present and viable
removal of browsing
and application
of fire no longer
effective
EXTINCTION
Figure 1. Schematic diagram of how management activities affect population dynamics of
P. galbraithiae at Holey Plains State Park (modified from Kohout et al. 2009). The dotted lines
show predicted transitions which require further research.
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
2 Methods
2.1 Site description
Holey Plains State Park is an area of low sandy hills and undulating plains approximately 10 km
south-east of Rosedale in West Gippsland, Victoria. The vegetation is heathy woodland dominated
by stringybark and peppermints, including Eucalyptus globoidea, E. consideniana and
E. aff. willisii (Gippsland Lakes) on podsol soils. Banksia serrata is a frequent co-dominant.
The climate is mild and subhumid (Nicholson 1978). January is the hottest month with a mean
maximum temperature of 25.4 °C, and July is the coldest month with a mean maximum
temperature of 13.8 °C. Mean annual rainfall is 597 mm, with the most rain falling in November
and the least in July (Bureau of Meteorology 2011).
2.2 Survey methods
The fire history was estimated by overlaying the current fire mapping of the park with the location
of the 34 known P. galbraithiae populations using ArcviewGIS 3.3 (Appendix 1). The populations
were prioritised for field surveys by choosing sites with five or more individuals and with a range
of time intervals since last fire. Sites with an unknown fire history were classified as ‘50 years
since fire’ (i.e. a long time) for the purposes of analysis.
Population structure and fecundity were examined at 16 sites with 11 different time intervals since
the last fire (Figures 2 and 3) in March 2011. The site numbering follows that of Trikojus and
Reside (2004) (Appendix 2). Site 8 consists of a fenced site (8a), unfenced site (8b) and slashed
site (8c). A previously unrecorded site was also surveyed (site ‘x’). The location of each site was
recorded with a GPS, using GDA 94 coordinates. The location data differed slightly from that
given by Trikojus and Reside (2004) because of differences in GPS accuracy. As a consequence,
some sites had different actual time intervals since last fire than estimated by mapping (e.g. sites 3,
11, 25, 30, 31a). P. galbraithiae could not be relocated at sites 13 and 23.
At each site, up to 20 plants (where this number could be located) were randomly selected and the
following variables recorded (following the methods of Kohout et al. 2009):
•
Presence/absence of browsing.
•
Developmental stage: 1 = seedling, monopodial (unbranched), 2 = seedling, branched,
3 = mature shrub (reproductive), branched.
•
Number of primary stems.
•
Stem length (= plant height, in cm): the mean length of three stems chosen at random,
measured from the plant base to living tissue at the stem tip, using a retractable metal tape
measure; stems were gently straightened to align with the tape measure.
•
Number of seed capsules. Seed capsules are retained for about four months after flowering
(Figure 4). Counting seed capsules some time after flowering rather than flowers provides
a more accurate measure of fecundity since flowers may not always be pollinated
successfully.
•
New shoot length. This extra variable is a measure of current growth. The length (cm) of
three randomly selected shoots was measured from the tip of the new shoot to the
beginning of woody tissue (Figure 5).
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
N
0
Figure 2. Locations of Prostanthera galbraithiae populations studied at Holey Plains State Park in March 2011.
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
5
1 km
Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
Ú Prostanthera locations 2011
Ê
Years since last burnt
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
32
33
36
37
39
40
42
43
44
45
47
71
> 50
Rosedale-Longford Rd
N
Ê2
Ú
Ú3
Ê
Ú10
Ê
7ÊÚ
Êx
Ú
8cÊÚ 8a
Ú
Ê
8b
Ú
Ê
9
25
Ú
Ê
17
Ê
Ú
31a
11
Ú
Ê
Ú
Ê
30ÊÚ
Ú14
Ê
Ú
Ê
15
0
5 Kilometers
Figure 3. The time since last fire of the Prostanthera galbraithiae populations examined at Holey Plains State Park in March 2011.
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
Figure 4. Seed capsules of P. galbraithiae in March 2011, retained on the plant approximately
four months after flowering.
Figure 5. Measuring the length of new shoots of P. galbraithiae in March 2011.
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
2.3 Data analysis
Pearson correlation was used to test the relationship between time since fire and stem length,
number of stems, new shoot length and number of seed capsules. Tukey post hoc tests were used
to examine the influence of browsing on plant height, number of stems and new shoot length.
Results were considered significant if P < 0.05. Analyses were conducted using Mystat 12.
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
3 Results
3.1 Population dynamics and time since last fire
The structure of P. galbraithiae populations varied with time since fire (Table 1). The proportion
of seedlings was highest in the first year after fire, while mature adult plants dominated the
population structure four years after fire (Table 1). Mature adult plants started declining around 15
years after fire, and seedlings dominated the population structure 29 years after fire (but note that
some sites had a very low sample size).
There was no significant correlation between time since last fire and the number of primary stems,
mean stem length, mean new shoot length, and the number of seed capsules (Figures 6–9). Seed
capsules were found on plants at three sites only (sites 7, 8a and 15).
Table 1. Population structure and time since fire for P. galbraithiae at Holey Plains State Park.
Site
31a
15
9
8a
8b
8c
14
25
2
10
7
17
x
3
11
30a
Time since
fire (year)
1
4
5
5
5
5
6
8
15
16
18
18
27
29
50
50
Population structure (%)
Number of
plants
surveyed
Monopodial
seedlings
Branched
20
20
20
20
20
20
20
4
11
20
20
5
20
4
4
15
60
5
10
0
0
0
0
0
0
0
0
0
5
50
100
20
40
0
15
0
0
0
0
0
9
10
0
40
10
0
0
47
seedlings
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
Mature shrubs
0
95
75
100
100
100
100
100
91
90
100
60
85
50
0
33
9
Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
8
Number of primary stems
7
6
R2 = -0.230
5
4
3
2
1
0
0
10
20
30
40
50
60
Time since fire (years)
Figure 6. The relationship between time since last fire and the number of primary stems of
P. galbraithiae plants. The Pearson correlation coefficient is shown.
180
160
Mean stem length (cm)
140
120
100
R2 = -0.156
80
60
40
20
0
0
10
20
30
40
50
60
Time since fire (years)
Figure 7. The relationship between time since last fire and the mean stem length of
P. galbraithiae. The Pearson correlation coefficient is shown.
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
35
Mean new shoot length (cm)
30
25
20
R2 = -0.193
15
10
5
0
0
10
20
30
40
50
60
Time since fire (years)
Figure 8. The relationship between time since last fire and mean new shoot length of
P. galbraithiae plants. The Pearson correlation coefficient is shown.
120
Number of seed capsules
100
80
60
R2 = -0.202
40
20
0
0
10
20
30
40
50
60
Time since fire (years)
Figure 9. The relationship between time since last fire and the number of seed capsules present
on P. galbraithiae plants. The Pearson correlation coefficient is shown.
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
3.2 Browsing
Browsing was evident at all sites except the fenced site (8a) (Table 2). Unbrowsed plants were
rare, only being recorded at three unfenced sites. These plants were often growing under the
protection of other shrubs. The highest proportion of unbrowsed and unfenced plants was at site
31a (all first year seedlings).
Table 2. The proportion of browsed and unbrowsed P. galbraithiae plants at each site.
Site
2
3
7
8a
8b
8c
9
10
11
14
15
17
25
30
31a
x
Browsed
(%)
100
75
100
0
100
100
100
100
100
100
100
100
100
80
60
100
Unbrowsed
(%)
0
25
0
100
0
0
0
0
0
0
0
0
0
25
40
0
At site 8, browsed (unfenced) P. galbraithiae plants (site 8b) had a significantly higher number of
primary stems, shorter mean stem length and shorter mean new shoot length than unbrowsed
(fenced) plants (site 8a) (Figures 10–12). Fenced plants had 104 ± 64 seed capsules, while there
was no evidence of flowering and seed production in the unfenced population.
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
Number of primary stems
8
t = -5.017, P < 0.001
6
4
2
0
8a
8b
Site
Figure 10. Number of primary stems of P. galbraithiae plants at site 8a (fenced and unbrowsed)
and site 8b (unfenced and browsed). (mean ± SD).
Mean stem length (cm)
250
200
150
t = 5.686, P < 0.001
100
50
0
8a
8b
Site
Figure 11. Mean stem length of P. galbraithiae plants at site 8a (fenced and unbrowsed) and site
8b (unfenced and browsed). (mean ± SD).
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
Mean new shoot length (cm)
45
40
35
30
t = 6.127, P < 0.001
25
20
15
10
5
0
8a
8b
Site
Figure 12. Mean new shoot length of P. galbraithiae plants at site 8a (fenced and unbrowsed)
and site 8b (unfenced and browsed). (mean ± SD).
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
4 Discussion
Prostanthera galbraithiae is an obligate seeder and responds positively to fire, with large numbers
of seedlings germinating after fire. Seedling numbers decrease over time with competition and
plants mature into flowering adults within two years after fire (Kohout et al. 2009). Given this
response, we may expect an initial decrease in plant numbers after fire, followed by a rapid
increase as seedlings germinate. In the presence of browsing, it might be expected that (a) large
populations of P. galbraithiae are unlikely to occur and (b) populations are unlikely to be
sustainable in the long-term, since browsing prevents plants maturing into flowering adults. In the
absence of fire, it would be expected that plants will gradually decline in vigour eventually leading
to a decrease in plant numbers. It is unclear whether total loss of above-ground populations will
always occur in the long absence of fire.
This study has provided two new insights into the ecology of P. galbraithiae. Firstly, fire history
mapping across the range of the species at Holey Plains SP has shown that long-unburnt areas still
support viable populations of the species, so the need to use fire at regular intervals to maintain the
species might not necessarily be supported. It would be worth examining the age of P. galbraithiae
plants, as they may not be as ‘old’ as suggested by the fire history. Secondly, this study shows that,
although fire might cue mass germination, some germination from the soil seed bank can occur in
the absence of fire. However, under high browsing pressure the rate of this recruitment is unlikely
to be sufficient to maintain P. galbraithiae populations.
This study highlights the necessity for keeping detailed records about fire history. Fire is
characterised by many components, including intensity, season of burn and frequency, extent of
fires in the landscape (Bond and Keeley 2005). This study is based only on the time since last fire.
Of most value to the management of P. galbraithiae would be to examine the effects of fire
intensity at Holey Plains SP. Given the potential for some fires to be ‘hot’, because of their timing
(in summer) or fuel loads, some evidence is needed that P. galbraithiae seedling recruitment is
advantaged by fire per se, or whether it is related to the intensity (i.e. fires that do not heat the soil
too much). Germination studies of Prostanthera askania showed that smoke and no heat treatment
gave the highest germination response, suggesting that low-intensity fires would be optimal for
management of this species (Tierney 2006). Prostanthera galbraithiae may have similar
requirements.
The lack of a relationship between fire history and the population structure, growth or fecundity of
P. galbraithiae was not unexpected given the heavy browsing pressure in Holey Plain SP. Site 8a
and 8b clearly demonstrate the impact that browsing can have on a population with the same time
since last fire; the fenced plants were significantly taller, had longer new shoots and were
flowering compared to unfenced plants, which were shorter, had shorter new shoots and were not
flowering. Browsing is not only having a significant negative impact on P. galbraithiae plants, but
is also obscuring the (positive) effect that fire can have on a population.
Herbivory differs from fire as a disturbance because it applies a more continuous and less intense
pressure on a population (Staver et al. 2009). Fire, on the other hand, consumes both living and
dead plant material, has broad dietary preferences, but can also be selective (Bond and Keeley
2005). Herbivory has been shown to create a ‘browse trap’ or demographic bottleneck, where
plants can be suppressed from becoming reproductive adults (Staver et al. 2009). This is likely to
apply to P. galbraithiae at Holey Plains SP, since unfenced seedlings germinating post-fire at site
8 show no sign of flowering compared to those that are fenced. The fence at site 8 will be extended
to include previously unfenced seedlings at site 8b in May 2011 (P. Dower, pers. comm.) and it
will be interesting to see the response of the plants to release from browsing pressure, in particular
the time to flowering.
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
In the case of managing sustainable P. galbraithiae populations, it is vital to understand the impact
of fire regime on herbivores. Fire stimulates vegetative regrowth (Gill et al. 1995) and the pattern
of burning, size of the area burnt, density and distribution of herbivores and their mobility will
then determine the pattern of browsing (Di Stefano 2005). Burnt areas that are small in size tend to
act as herbivore ‘sinks’ and attract high numbers of browsers post-fire when vegetation is
regenerating. It may be possible to diffuse browsing pressure on P. galbraithiae seedlings in the
first few years after fire by burning larger areas in Holey Plains SP. An examination of the
movement and habitat selection of herbivores post-fire at Holey Plains would provide an insight
into the options for the management of browsing pressure through landscape burn planning.
Swamp Wallabies (Wallabia bicolor) are thought to be the main herbivore browsing
P. galbraithiae at Holey Plains SP (Carter and Walsh 2003). However, this is based on casual
observations and has not been confirmed. Other herbivores may include kangaroos, rabbits, deer,
possums and wombats. In order to better understand and manage browsing of P. galbraithiae it is
necessary to (a) confirm which species is (or are) responsible for browsing, (b) quantify the
browsing pressure and (c) define how much browsing is acceptable, in terms of sustaining
P. galbraithiae populations. Herbivore distribution and density might be examined by using a
faecal pellet count at a range of sites over a period of time (Di Stefano et al. 2007). This would
provide information about whether the pressure is constant throughout the year or seasonal, and if
all P. galbraithiae populations experience a similar intensity of browsing.
Some previously recorded populations of P. galbraithiae could not be located during this study.
This may be because of the limited time available for searching, or because previously recorded
plants had died. Small populations are valuable as they are likely to add to the genetic diversity of
P. galbraithiae. However, the protection of populations of P. galbraithiae may need to be
prioritised, especially where expensive management options are used (e.g. fencing). It may be
more cost-effective to protect larger populations first, since they will have a better chance of being
sustainable into the future.
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
5 Recommendations
The most important management recommendation for P. galbraithiae is to protect populations
from browsing. Without protection from browsing, fire management strategies are unlikely to be
effective. In particular:
•
Plants at site 8b need to be fenced and protected from fire until they have recovered from
browsing sufficiently to flower for at least two years. This will enable a soil seed bank to
become established. The recovery of this population should be monitored annually for the
next 3 years.
•
The size of the recently burnt population at site 31a needs to be determined and the
majority fenced to protect germinating seedlings from browsing.
•
Sites not surveyed since 2005 need to be re-surveyed to determine if plants are present.
In general, any P. galbraithiae populations older than 10 years since last fire could be burnt.
However, this will depend on whether plants have been able to flower and create a soil seed bank
in the presence of browsing. The maximum longevity or ‘age’ of P. galbraithiae should be
determined in order to decide whether populations that are long unburnt have been replacing
themselves in the absence of fire. In addition, it would be useful to examine cues for germination
in the absence of fire.
A study examining the identity, density, distribution, movement patterns and dietary preferences of
the herbivore(s) responsible for browsing P. galbraithiae at Holey Plains SP, particularly in
response to fire, would provide specific and targeted information about one of the main threatening
processes to the survival of P. galbraithiae.
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
References
Bond, W.J., and Keeley, J.E. (2005). Fire as a global 'herbivore': the ecology and evolution of
flammable ecosystems. Trends in Ecology & Evolution 20: 387-394.
Bureau of Meteorology. (2011). www.bom.gov.au/climate/averages.
Carter, O., and Walsh, N. (2003). National Recovery Plan for Prostanthera galbraithiae
(Wellington Mint-bush) 2004-2008., Department of Sustainability and Environment,
Heidelberg, Victoria.
Coates, F., and Downe, J. (2007). Population structure of Prostanthera galbraithiae (Wellington
Mint-bush) in response to different management regimes at Holey Plains State Park.,
Arthur Rylah Institute for Environmental Research., Department of Sustainability and
Environment, Heidelberg, Victoria.
Conn, B.J. (1998). Contributions to the systematics of Prostanthera (Labiatae) in south-eastern
Australia. Telopea 7: 319-331.
Di Stefano, J. (2005). Mammalian browsing damage in the Mt. Cole State forest, southeastern
Australia: analysis of browsing patterns, spatial relationships and browse selection. New
Forests 29: 43-61.
Di Stefano, J., Anson, J.A., York, A., Greenfield, A., Coulson, G., Berman, A., and Bladen, M.
(2007). Interactions between timber harvesting and swamp wallabies (Wallabia bicolor):
space use, density and browsing impact. Forest Ecology and Management 253: 128-137.
Gill, A., Bradstock, R., Bradstock, R., Auld, T., Keith, D., Kingsford, R., Lunney, D., and
Sivertsen, D. (1995). Extinction of biota by fires. Conserving biodiversity: threats and
solutions: 309-322.
Kohout, M., Coates, F., Hirst, M., and Downe, J. (2009). Wellington Mint Bush - population
responses to fire and browsing at Holey Plains State Park., Arthur Rylah Institute for
Environmental Research, Melbourne.
Nicholson, B.M. (1978). A study of the land in the Gippsland lakes area. Soil Conservation
Authority, Victoria.
Staver, A.C., Bond, W.J., Stock, W.D., van Rensburg, S.J., and Waldram, M.S. (2009). Browsing
and fire interact to suppress tree density in an African savanna. Ecological Applications
19: 1909-1919.
Tierney, D.A. (2006). The effect of fire-related germination cues on the germination of a declining
forest understorey species. Australian Journal of Botany 54: 297-303.
Trikojus, N., and Reside, J. (2004). Distribution and abundance of the Wellington Mint-bush
Prostanthera galbraithiae within the Holey Plains State Park. Wildlife Unlimited
Consultancy, Bairnsdale.
Walsh, N.G., and Entwistle T.J. (1999). Flora of Victoria. Volume 4: Dicotyldedons Cornaceae to
Asteraceae. In: Flora of Victoria. Inkata Press, Melbourne.
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
Appendix 1. Time since last fire of Prostanthera galbraithiae
populations at Holey Plains State Park in 2011.
The year burnt was estimated using GIS to overlay the locations of Prostanthera galbraithiae
populations mapped by Trikojus and Reside (2004) on current fire history mapping.
Site
Easting
Northing
Year burnt
1
2
3
4
5
6
7a
7b
8a
8b
8c
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29a
29b
30a
30b
31a
31b
32
33
34
490529
490466
492267
492354
492100
492482
492601
493039
495739
496339
496081
497259
489763
490694
489122
489041
488733
488541
495903
495267
494216
494129
494190
495013
495592
489808
488754
488534
488560
493579
492251
493003
492963
489184
489185
496812
496956
494582
496266
496439
5775257
5775105
5775134
5775499
5775026
5774835
5774527
5773826
5773371
5773292
5773351
5773159
5774659
5770896
5769523
5769037
5767214
5766843
5771807
5771549
5772267
5772407
5769783
5768541
5769705
5772350
5771960
5772677
5772851
5770830
5775263
5773785
5773754
5770087
5770136
5771440
5771418
5771449
5769029
5769568
1985
1996
1993
1984
1982
1993
1993
2010
2006
2006
2006
2006
1995
1991
1994
1994
2005
1995
2006
1993
2006
2006
2002
2009
2010
1995
1995
2005
1982
2010
1982
n/a
n/a
1994
1994
1993
1993
2010
1966
2010
Years since
last fire
26
15
18
27
29
18
18
1
5
5
5
5
16
20
17
17
6
16
5
18
5
5
9
2
1
16
16
6
29
1
29
n/a
n/a
17
17
18
18
1
45
1
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
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Wellington Mint-bush – the Impact of Fire on Population Dynamics at Holey Plains State Park
Appendix 2. Populations of Prostanthera galbraithiae surveyed at Holey Plains State Park in
March 2011.
Site numbering follows that of Trikojus and Reside (2004). In addition, a previously unrecorded site was surveyed and labelled ‘x’. Site description data
are from Trikojus and Reside (2004) and information collected during this study are given. The GPS datum is GDA 94.
Site
Easting
Northing
2
3
7
8a
8b
8c
9
490462
492257
492644
496166
496171
496081
497243
5775095
5775163
5774254
5773296
5773295
5773351
5773141
10
11
489753
490721
5774675
5770898
14
15
488731
488593
5767246
5766868
17
25
495255
488488
5771555
5772528
30
489186
5770082
31a
x
496825
492913
5771415
5773909
Site description
Crookes Tk, plants in a depression along the track, 150 m south
of North Boundary Tk
Whites Tk, 15 m west of intersection of Whites Tk and Spring Tk
Spring Tk
Berlin Wall, fenced
Berlin Wall, unfenced
Berlin Wall, slashed
Berlin Wall, east from site 8
Whites Tk, 200-250 m north of intersection with Hacketts Tk.
Population consists of approx 30-50 plants
Birmingham Tk
On both sides of Jacks Tk, 1.5 km south of Chessum Rd and
Jacks Tk intersection
South Boundary Tk
North side of Chessum Rd surveyed, approx. 10 m from the
road
Pipeline Tk
Jacks Tk, top of the rise, approx. 200 m N of the dam on Box
Track.
South Pipeline, on south side of the track, ~300 m from Emu
Track and Pine Plantation Tk.
Spring Tk
Arthur Rylah Institute for Environmental Research Technical Report Series No. 219
Year last
burnt
Years since
last fire
1996
1982
1993
2006
2006
2006
2006
15
29
18
5
5
5
5
1995
?
16
~50
2005
2007
6
4
1993
2003
18
8
?
~50
2010
1984
1
27
20
20