Reconstructing Mallee fire history
using Callitris verrucosa tree rings
Heidi Zimmer, Peter Green,
David Cheal and Michael F. Clarke
2010
Arthur Rylah Institute for Environmental Research
Technical Report Series No. 215
Arthur Rylah Institute for Environmental Research Technical Series No. 215
Reconstructing Mallee fire history using
Callitris verrucosa tree rings
H. Zimmer1, P. Green2, D. Cheal1 and M.F. Clarke3
1
Arthur Rylah Institute for Environmental Research
123 Brown Street, Heidelberg, Victoria 3084
2
Department of Botany, La Trobe University,
Bundoora, Victoria 3086
3
Department of Zoology, La Trobe University,
Bundoora, Victoria 3086
December 2010
In partnership with
Arthur Rylah Institute for Environmental Research
Department of Sustainability and Environment
Heidelberg, Victoria
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Citation: Zimmer, H, Green, P., Cheal, D and Clarke, M.F. (2010) Reconstructing Mallee fire history using Callitris
verrucosa tree rings. Arthur Rylah Institute for Environmental Research Technical Report Series No. 215. Department of
Sustainability and Environment, Heidelberg, Victoria
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iii
Contents
List of tables and figures .................................................................................................................. v
Acknowledgements .......................................................................................................................... vi
Summary ........................................................................................................................................... 1
Introduction ...................................................................................................................................... 2
2 Methods .......................................................................................................................................... 4
2.1 Study area: Murray Sunset and Hattah Kulkyne National Parks ............................................. 4
2.2
Study species: Callitris verrucosa ........................................................................................... 4
2.3
Study area climate .................................................................................................................... 5
2.4
Proof of method ....................................................................................................................... 6
2.5
Sampling design ....................................................................................................................... 6
2.6
Dendrochonological analysis ................................................................................................... 6
3
3.1
Results ..................................................................................................................................... 8
Recruitment reconstruction ...................................................................................................... 8
3.2
Age–diameter relationship ..................................................................................................... 10
4
4.1
Discussion.............................................................................................................................. 11
Stand age structures and post-fire recruitment ....................................................................... 11
4.2
Fire severity and vegetation resistance to burning ................................................................. 11
4.3
Dune systems and variation in fire severity ........................................................................... 12
4.4
Grazing and rainfall limit post-fire recruitment ..................................................................... 12
4.5
Maintenance of long-unburnt vegetation in a fire prone landscape ....................................... 12
4.6
Utility of Callitris verrucosa for fire history reconstruction.................................................. 13
5
Conclusions ........................................................................................................................... 14
6
Fire policy and management considerations ..................................................................... 15
7
References ............................................................................................................................. 16
Appendix: Site photos .................................................................................................................... 19
iv
List of tables and figures
List of tables
Table 1. Site information and dendrochronological results summary. ................................................9
List of figures
Figure 1. Map of the Murray–Sunset – Hattah–Kulyne National Park complex showing MFB
project study mosaics and areas burnt between 1972 and 2007. ..............................................4
Figure 2. A large Callitris verrucosa c. 150 years old and C. verrucosa at mosaic 9 c. 30 years old
in Murray–Sunset National Park, Victoria. ...............................................................................5
Figure 3. Average monthly rainfall at weather stations close to Murray–Sunset National Park. .......5
Figure 4. Callitris verrucosa age structures. ......................................................................................8
Figure 5. Stem diameter vs age estimated from tree rings. ...............................................................10
Figure 6. A problematic increment core............................................................................................13
v
Acknowledgements
The authors acknowledge the work of the Mallee Fire and Biodiversity project team and
volunteers in establishing the study mosaics and in providing background data, especially Kate
Callister and Sally Kenny. A special thank you Angie Haslem for creating the maps for this report.
We thank Parks Victoria, particularly Kathryn Schneider for support in the field. We thank Patrick
Baker for providing technical advice and Katie Howard for assistance with fieldwork. Vivienne
Turner and David Dunkerley provided excellent advice on an earlier version of this manuscript.
Thank you to Steve Platt for assistance in drafting the ‘Fire policy and management
considerations’ section of this report.
Funding for this project was provided by the Department of Sustainability and Environment,
through the Fire Ecology Program.
This project would not have been possible without support from La Trobe University and the
Mallee Fire and Biodiversity project. The Mallee Fire and Biodiversity Project was funded and
supported by Parks Victoria, Department of Sustainability and Environment (Vic), Mallee
Catchment Management Authority, NSW National Parks and Wildlife Service, Department of
Environment and Climate Change (NSW), Lower Murray Darling Catchment Management
Authority, Department for Environment and Heritage (SA), Land and Water Australia, Natural
Heritage Trust, Birds Australia (Gluepot Reserve), Australian Wildlife Conservancy (Scotia
Sanctuary), and the Murray Mallee Partnership.
The Mallee Fire and Biodiversity Project is grateful to the Doyle family for granting access to
Petro Station, and to the Barnes family for granting access to Lethero Station.
vi
Reconstructing Mallee fire history using Callitris verrucosa tree rings
Summary
The multi-year, tri-state Mallee Fire and Biodiversity (MFB) project aims to identify the properties
of mallee eucalypt habitat mosaics, produced by fire, that enhance the persistence and status of a
range of taxonomic groups. Large areas of mallee are described as ‘long-unburnt’ — that is, not
burnt for over 38 years. The purpose of this project, Filling Fundamental Gaps in the Fire History
of the Mallee, was to determine the ages of the various cohorts described as long-unburnt within
the MFB study mosaics.
Dendrochronology, the study of tree rings, can be used to determine tree age, annual growth rates,
and the relationships of age and growth with environmental conditions. The native Australian
conifer genus Callitris has known dendrochronological potential. Callitris verrucosa is fire
sensitive and occurs within long-unburnt patches of mallee and MFB study mosaics. The ‘proof of
method’ phase of this project (Zimmer et al. 2009) investigated the utility of C. verrucosa for
dendrochronological analysis. Sampling stands of known age, we discovered that growth rings in
C. verrucosa were anatomically distinct, and that ring counts from the majority of sampled trees
were approximately equal to years since fire. Tree rings were successfully visually cross-dated
(within tree), and at the mosaic-scale there was reasonable correlation among tree ring width
series. We concluded that ring counts of cross-dated C. verrucosa could be used to determine the
ages of long-unburnt cohorts within the MFB study mosaics.
This final report is based on work that aimed to reconstruct the fire history of long-unburnt areas in
the Murray-Sunset and Hattah–Kulkyne National Parks. Using C. verrucosa tree rings, we
reconstructed the age structures of 12 C. verrucosa stands, including eight stands in long-unburnt
areas. Despite our earlier findings, this study revealed that C. verrucosa was not an ideal candidate
for fire history reconstruction. Mature C. verrucosa can survive fire, and recruitment can occur
between fires. Nevertheless, stand age structures derived from tree rings may indicate ‘the period
for which fires have been sufficiently patchy to allow survival of C. verrucosa trees’; up to 180
years in some areas. Sources of error in tree-ring-derived estimates include intra-annual ‘false’ tree
rings and missing tree rings.
At several sites no recruitment pulse was detected, despite previous research suggesting serotiny in
C. verrucosa (e.g. Bradstock and Cohn 2002a). This may suggest that inter-fire recruitment is
more important than first thought, or that the influence of low-severity fire (scorching but not
killing trees, and releasing seed) is significant. However, our sample sizes were small and it is
possible that the true age structure of the stand was not captured by our sampling. Other potential
influences on C. verrucosa age structures are grazing pressure and drought.
That C. verrucosa is not killed by all fires and recruits between fires, places a qualifier on our
assumption that C. verrucosa would indicate the age of surrounding vegetation: this is only true in
the case of severe fire, with successful recruitment in the immediate post-fire years. Currently,
C. verrucosa tree-ring-derived age estimates may be best used in conjunction with other proxies,
e.g. bomb pulse dating (Hua 2009) and studies based on increases in stem diameter (Clarke et al.
2010).
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Introduction
1.1 Mallee vegetation
Mallee vegetation is characterised by small, multi-stemmed eucalypts that are adapted to fire and
regenerate from lignotubers or from seed released after fire (Noble et al. 1980). Mallee ecosystems
are widespread in the semi-arid shrublands of south-eastern Australia. The extensive distribution
of mallee eucalypts is both a response and contribution to the flammability of this landscape
(Cheal 2006). Gymnosperms of the genus Callitris are common in mallee vegetation as
monospecific stands or dispersed among mallee eucalypts. They regenerate solely from seed and
are considered to be ‘fire sensitive’ (Groves 1994). Callitris is the only woody genus displaying
this life history in south-eastern Australian mallee vegetation.
The fire history of the Mallee has recently changed from a majority of long-unburnt areas to
extensive recently burnt areas (Sandell et al. 2006). This has raised concerns based on the
assumption that large-scale fires have a negative impact upon flora and fauna (Bradstock et al.
2005) and because inappropriate fire management is recognised as a threatening process by the
Victorian Flora and Fauna Guarantee Act 1988. The paradigm of modern fire management for
ecological objectives is to create mosaics of vegetation with a diversity of fire histories and time
since fire (Parr and Andersen 2006). However, the characteristics of the ideal fire regime for many
species remain unknown (Cheal 2004, Clarke 2008). Time since fire determines the characteristics
and boundaries of vegetation communities (Bradstock and Cohn 2002b). Patchiness or micro-scale
variability in the landscape allows species with different fire responses to coexist (Williams et al.
1994), such as the mallee eucalypts and Callitris. Patterns in semi-arid vegetation are also
influenced by fine-scale nutrient and water distributions, driven by differences in soil texture,
which vary with topography (Noy-Meir 1974). Many mallee species have distributions confined to
narrow bands, e.g. on dune crests, slopes or in swales (Wevill and Read 2010). Communities may
occur on a site-specific basis, owing to interactions among fire history and local landscape factors
(Cheal 2006).
1.2 The Mallee Fire and Biodiversity project
The Mallee Fire and Biodiversity (MFB) project was conceived to identify the properties of mallee
eucalypt habitat mosaics produced by fire that enhance the persistence and status of a range of
taxonomic groups. The study area includes mallee vegetation spanning north-western Victoria,
south-western New South Wales and eastern South Australia.
Detailed and accurate fire histories are critical for the good management of pyrophytic landscapes.
Over the past 80 years fire maps have become increasingly accurate. For the Murray–Sunset
National Park, hand drawn maps provide records of fires in the 1930s, 1950s and 1960s.
Commencing in 1972, primarily using Landsat imagery, the timing and extent of fires in the
Mallee have been mapped at very fine spatial resolution. More recent fires have been mapped
using GIS. Nevertheless, large areas in the Murray–Sunset – Hattah–Kulkyne National Parks
complex are classified as long-unburnt, ignoring variation in vegetation age within these areas.
The MFB project used a chronosequence approach, in which sites that vary in time since last fire
were surveyed for the abundance and diversity of plants, birds, reptiles, small mammals and
invertebrates. The core sampling unit of the MFB project was a ‘mosaic’ — an area 4 km in
diameter that contained between one and five age classes (defined by time since last fire) of mallee
habitat (Figure 1). Detailed fire histories of these areas are fundamental, so that sites within
mosaics can be accurately assigned to age classes.
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Reconstructing Mallee fire history using Callitris verrucosa tree rings
1.3 Previous study of Callitris tree rings
Because C. verrucosa occurs within long-unburnt patches of mallee and in MFB study mosaics,
and previous research suggested that it is killed by fire and is bradysporous/serotinous (Bowman
and Harris 1995, Cheal 1999, Bradstock and Cohn 2002a), C. verrucosa was thought a good
candidate for reconstruction of Mallee fire histories. Several species of the genus Callitris are
known to produce reliable annual growth rings (Pearson and Searson 2002, O’Donnell et al. 2010).
That is, the age and annual growth rates of Callitris species can be determined from an analysis of
their wood anatomy or tree rings.
Using samples from areas that had experienced fire less than 38 years ago, we had discovered that
growth rings in C. verrucosa were anatomically distinct, and ring counts from the majority of
sampled trees were approximately equal to years since fire (Zimmer et al. 2009). Tree rings were
also successfully visually cross-dated (within tree), and at the mosaic-scale there was reasonable
correlation among tree ring width series. However, there is some uncertainty about the lag between
a fire event and tree establishment (Oliver and Larson 1996), and about tree-specific growth
anomalies such as intra-annual false rings. Despite these uncertainties, we believed that ring counts
of cross-dated C. verrucosa could be used to determine the ages of long-unburnt cohorts within the
MFB study mosaics.
1.4 Aim of this study
Building on Zimmer et al. (2009), this project aims to ‘Define the ages of long-unburnt vegetation
cohorts inside Mallee Fire and Biodiversity project study mosaics, using dendrochronology to
reconstruct fire histories’.
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Reconstructing Mallee fire history using Callitris verrucosa tree-rings
2 Methods
2.1
Study area: Murray–Sunset and Hattah–Kulkyne National Parks
The study area is the Murray-Sunset Hattah-Kulkyne National Park Complex in north-western
Victoria (Figure 1), where C. verrucosa co-occurs with mallee eucalypts.
Figure 1. Map of the Murray–Sunset – Hattah–Kulkyne National Park complex showing MFB
project study mosaics (numbered circles) and areas burnt between 1972 and 2007. Areas
without colour are assumed to have burnt before 1972.
2.2
Study species: Callitris verrucosa
Callitris verrucosa (Scrub Cypress-pine) is a widespread species in eastern Australian mallee
(Figure 2). C. verrucosa is classified as an obligate-seeder, that is, it does not reproduce
vegetatively and only regenerates from seed (Cheal 1999, Bradstock and Cohn 2002a, Bradstock et
al. 2006). C. verrucosa is also killed by fire, although older trees may be more resistant to fire than
younger trees (Bowman and Harris 1995, Cheal 1999, Bradstock and Cohn 2002a). C. verrucosa
reaches maturity at 10–15 years, and the number of cones increases with age (Bradstock and Cohn
2002a). Small saplings do, however, produce a small number of cones, ostensibly as an ‘insurance
policy’ against frequent fire. Bowman and Harris (1995) suggested a potential life span of 250
years for Callitris species in general; demographic work for C. verrucosa is in agreement with this
estimate (Bradstock and Cohn 2002a). Seeds are relatively heavy, and dispersal distance is thought
to be short (Bradstock et al. 2006). Bradstock and Cohn (2002a) suggested that recruitment of
C. verrucosa may be confined to the post-fire period; this is true for other Callitris species
(O’Donnell et al. 2010). Consequently we assumed that, where C. verrucosa occurred in
vegetation communities that had experienced fire, all individuals were recruited immediately after
the same fire event.
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Reconstructing Mallee fire history using Callitris verrucosa tree rings
Figure 2. A large C. verrucosa c. 150 years old (left) and C. verrucosa at mosaic 9 c. 30 years old
(right) in Murray–Sunset National Park, Victoria.
2.3
Study area climate
Climatic data were obtained from the Bureau of Meteorology (2008) from weather stations close
to Murray–Sunset National Park, located at Mildura, Murrayville, Walpeup, Ouyen, Paringa and
Werrimull. The region has long-term mean annual rainfalls ranging from 271 mm (Werrimull) to
334 mm (Walpeup), and experiences an annual dry period from December to March, with a mean
monthly rainfall of under 21 mm (Figure 3). The highest rainfall is received in the May to
November period (over 28 mm per month). Approximately 66% of annual rainfall occurs within
this period. Mean daily maximum temperatures range from 32°C in January to 14°C in June.
Figure 3. Average monthly rainfall at weather stations close to Murray–Sunset National Park.
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Reconstructing Mallee fire history using Callitris verrucosa tree-rings
2.4
Proof of method
Callitris verrucosa trees were first sampled from areas where the time since the last fire was
known (see Zimmer et al. 2009). Sample sites were established within four areas that were last
burnt in 1980 (Mosaics 8 and 9) or 1975 (Mosaic 11 and Underbool Track)1. Whole stem sections
were taken from 10 trees within each sample site. Based on a comparison of known fire histories
with stand age structures (determined from tree rings), we concluded that the tree rings of
C. verrucosa were largely annual. Detailed information on this proof of method study can be found
in Zimmer et al. (2009).
2.5
Sampling design
After the proof of method study, samples were taken from long-unburnt areas, i.e. areas of
unknown time since fire. Sample sites were chosen based on the presence of C. verrucosa and the
absence of fire records. Sampling was undertaken in C. verrucosa stands within Mosaics 1, 3, 4
and 7 and sites on Pheenys Track and Bambill Road. At each stand up to 20 trees were sampled.
Two tree cores were taken from the base of each tree with an increment borer. The height from
which the tree core was extracted was recorded. Diameter was measured below branching at a
mean height of 30 cm.
2.6
Dendrochonological analysis
Increment cores were prepared using standard dendrochronological procedures (Stokes and Smiley
1968). Samples were dried, glued to mounts, and then sanded using increasingly fine grits of
sandpaper (from 180 to 800 grit). The samples were scanned at 2400 dpi resolution using a
ScanMaker 1000XL scanner (Microtek International Inc.). Scanned samples were cross-dated
using WinDendro (Regent Instruments, Canada) image-analysis software. Cross-dating is a
fundamental practice in dendrochronology; in this process, ring-width series of radii across
sections are compared to ensure that annual growth rings are allocated to the years in which they
were formed (Fritts 1976, Grissono-Mayer 2001).
Chronologies for individual trees were then compared to others within the stand using COFECHA
(Holmes 1983), a statistical program which calculates cross-correlations (or average inter-series
correlations) between each individual ring-width series and an average chronology generated from
all the ring-width series from the stand or site (Grissino-Mayer 2001). Some individual ring-width
series showed irregular growth patterns and/or unclear ring structure, and correlated poorly with
the average chronology. These series were re-checked and either corrected or removed from
further analyses. This allowed us to reduce the effects of local disturbance only recorded in
individual trees (Grissino-Mayer 2001). The remaining set of ring-width series was then
standardised to create a master chronology and calculate age structures for each stand and site.
Age structures provide a graphical representation of recruitment patterns; periods of intense
recruitment, called ‘recruitment pulses’, may be used to reconstruct the disturbance history, e.g.
when the study species is a post-fire recruiter. Mean inter-series correlation is reported for each
site; a mean inter-series correlation (MIC) of 0.4 or greater is acceptable for dendrochronological
studies (Grissino-Mayer 2009).
1
At the time of publication of the proof of method study (Zimmer et al. 2009), we were not aware of
annually resolved dates for fires in Mosaics 8, 9, 11 and Underbool. These dates are contained in the updated
results summary in this report. Furthermore, we later discovered records of several pre-1970 fires in
Murray–Sunset National Park. Sampling of long-unburnt stands, detailed in this report, was not undertaken
in these areas.
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If the core did not intersect the pith, the missing distance to the pith was estimated using Duncan’s
geometric method (Duncan 1987). The estimated distance to the pith was then divided by the
average ring-width of the five rings closest to the pith to estimate the number of missing years
between the last ring on the core and the missed pith.
To account for the number of years for the tree to grow to coring height, we used the relationship
described by O’Donnell et al. (2010), where the number of missing rings is equal to the coring
height  0.23, bounded by a 95% confidence interval of ±4.63 rings. Although this relationship
was developed for Callitris preissii, we believe it is broadly transferable to C. verrucosa because
the trees record comparable mean tree ring widths — 0.46 in C. preissii (O’Donnell et al. (2010),
and 0.45 – 1.01 in C. verrucosa (site-dependent).
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Reconstructing Mallee fire history using Callitris verrucosa tree-rings
3 Results
3.1
Recruitment reconstruction
Sampled trees from long-unburnt areas were up to 180 years old (Bambill Road, established 1830).
Age structures are presented for eight long-unburnt C. verrucosa sites (Figure 4 and Table 1). The
age structures for the long-unburnt areas indicate that recruitment has occurred over a long period.
In contrast, age structures for areas burnt since 1972 suggest recruitment was more episodic.
Figure 4. Callitris verrucosa age structures. Annual rainfall is presented at the top. Arrows
indicate introduction and then removal of livestock.
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Table 1. Site information and dendrochronological results summary.
Study site
Number
of trees
Mean Establishment
Year (Max, Min)
Mean Inter-series
Correlation (MIC)
Known fire history
Mosaic 1a
16
1872 (1834, 1904)
0.120
None
Mosaic 1b
9
1895 (1855, 1938)
0.154
None
Mosaic 3
9
1899 (1857, 1933)
0.262
None
Mosaic 4
11
1935 (1925, 1950)
0.200
Near 1932
Mosaic 7
9
1908 (1880, 1931)
0.223
None
13
1868 (1830, 1928)
0.165
Near 1957
Pheenys Tk 1
4
1889 (1852, 1938)
0.202
None
Pheenys Tk 2
20
1910 (1879, 1937)
0.135
None
Bambill Rd
The age structures of Mosaic 1a and Mosaic 1b demonstrate a wide range of ages and no
recruitment pulse. At Mosaic 1a, establishment occurred in all but one of the 5-year periods
between 1870 and 1940. The oldest trees in Mosaic 1a and Mosaic 1b were 138 and 115 years old
(established in 1872 and 1895 respectively), suggesting the absence of severe fire for 138 years
and 115 years, respectively. In Mosaic 1a both the oldest and the youngest trees were on the slope.
In Mosaic 1b, both the oldest and the youngest tree were found at the bottom of a slope, near the
swale. The correlation of growth among trees was poor (MIC 0.120 and 0.150, Table 1), but
typical of C. verrucosa in this study.
Mosaic 3’s age structure shows a small number of similar-aged trees recruited around 1900, with
some recruitment continuing afterwards. The annual growth of trees within Mosaic 3 was more
correlated than the other sites (MIC 0.262), but was still poor. The site was characterised by a
gentle slope potentially providing similar environmental conditions for all sampled trees. The
oldest tree was recruited in 1899, suggesting the absence of severe fire for 111 years.
The age structure of Mosaic 4 showed the clearest evidence of a recruitment pulse in any of the
long-unburnt site surveyed, but the number of trees constituting this ‘pulse’ is small. The majority
of the recruitment occurred between 1935 and 1945. The site was characterised by a steep dune,
and sampling was undertaken on the slope (north-westerly aspect) and dune crest. This site was
near to an area burnt in 1932; the reconstructed recruitment pulse is approximately in agreement
with this date. MIC was up to 0.500 for individual tree ring series, although the overall correlation
was poor (0.200). That Mosaic 4 is also the youngest long-unburnt stand studied may indicate
increasing error in age estimation from tree rings with increasing tree age.
Mosaic 7 showed no evidence of a recruitment pulse. The oldest tree recruited in 1880 and the
youngest in 1931. There were no major topographic features at this site. MIC was 0.223, aboveaverage for this study, but still poor. The establishment dates of three trees could not be
determined because the tree centres were rotten. Interestingly, these three trees were within 50 m
of one another.
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Reconstructing Mallee fire history using Callitris verrucosa tree-rings
The majority of trees at the Bambill Road site established from 1850 to 1870, although the oldest
tree established in 1830. A much younger recruit was also sampled, 82 years old (recruited in
1928). Two trees could only be assigned minimum ages as their centres were rotten. This site was
characterised by a low dune; there was no apparent relationship between tree position and tree age.
Two trees at Pheenys Track Site 1, an ‘island’ of C. verrucosa on the flats, recruited in 1852 and
1853. We may assume that they recruited after the same fire. There were younger trees in the stand
also, recruited in 1938 and 1911.. The oldest tree recruited in 1879. There were no major
topographic features at this site, and annual growth correlation among trees was typically poor
(MIC .135).
Age structures for Mosaics 8, 9, 11 and Underbool track were composed of sites in areas of known
fire history (burnt post 1972; Zimmer et al. 2009). Each site showed evidence of a post-fire
recruitment pulse.
Rainfall data provided little insight into the recruitment patterns of long-unburnt stands, as
instrumental records did not commence until 1911. Mean inter-series correlations for all sites were
below that required for dendroclimatological analysis.
3.2
Age-diameter relationship
There was a positive linear relationship between age and diameter (R2 = 0.62) (Figure 5).
However, the deviation from this linear relationship increased with age and diameter.
Figure 5. Stem diameter vs. age estimated from tree rings.
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Reconstructing Mallee fire history using Callitris verrucosa tree rings
4 Discussion
4.1
Stand age structures and post-fire recruitment
We have defined age structures at 12 sites, including eight long-unburnt areas, based on estimates
of tree age from tree rings. The oldest trees in long-unburnt areas established in the 1830s, 1880s,
1850s and 1920s, indicating that fire has been sufficiently patchy to allow tree survival since these
dates. However, what proportion of the original population these surviving trees represent remains
unknown; we do not know if, or how many, trees have been killed by fire in the interim.
Furthermore, within many study mosaics we observed two or more cohorts (age classes) and
individuals apparently recruited asynchronously, suggesting fire survival and/or inter-fire
recruitment and/or error in tree-ring-derived age estimates; sample sizes are small. We had
assumed that, as an obligate seeding species, C. verrucosa would be sensitive to, and regenerate
after, fire. That our dendrochronological analyses show that C. verrucosa exists in multi-aged
stands suggests that trees are not killed by (or avoid being burnt by) low intensity fires and can
recruit between fires. Inter-fire recruitment of species with long-term canopy storage of seeds
(bradyspory) has been recorded in Banksia (Enright et al. 1998, Whelan et al. 1998, Griffith et al.
2004 Whelan et al. 2006), Leptospermum and Melaleuca (Griffith et al. 2004). Further research is
needed to quantify C. verrucosa inter-fire seed rain. Indeed, Cheal (1999) reported that 29% of
cones were open on C. verrucosa trees 35 years after fire, and photographed a C. verrucosa
seedling underneath an old growth C. verrucosa stand.
Notably, the younger stands of C. verrucosa (Figure 4, lower histograms) showed stronger
evidence for pulsed post-fire recruitment. This difference between young and long-unburnt stands
may be explained by more severe/less patchy fire impacts on the previous cohort, or increasing
tree ring age estimation error with tree age. Finally, it is interesting that no recruitment was
recorded in the long-unburnt stands since 1972, nor have these sites been burnt since 1972.
4.2
Fire severity and vegetation resistance to burning
Callitris verrucosa is relatively less flammable than mallee eucalypts as it produces significantly
less leaf litter and deters Triodia scariosa (a fire-propagating grass) (Bradstock and Cohn 2002a).
The ability of C. verrucosa to survive fire has been noted in previous studies; older Callitris have
increased fire resistance conferred by bark thickness and increased foliage height (Bradstock and
Cohn 2002a). Increasing fire resistance with age, coupled with fire-induced seed release, is
considered a paradox because of the likelihood that mass seed release occurs after fire (Bradstock
and Cohn 2002a). It appears that C. verrucosa is adapted to fire but can release viable seeds
without it. This suggests that adaptation towards serotiny may have occurred to aid survival in a
fire-prone landscape.
Maps of the recent fire history of Murray–Sunset National Park show variation within burnt areas,
indicating the patchiness of fires occurring within the last 38 years, and it is reasonable to expect
that pre-1970 fires were also patchy. This is logical because biomass in mallee ecosystems is
relatively small compared to other forest types (Grierson et al. 1992), and may provide a further
explanation for the variation of tree age within long-unburnt stands. Large, intense conflagrations
would have resulted in the loss of long-unburnt stands of C. verrucosa. On the other hand, lowseverity burns may have left C. verrucosa stands unaffected or ‘scorched’, causing the release of
seeds. Moreover, low-severity burns are more likely to have removed only the younger and
therefore more fire-sensitive age classes of C. verrucosa (Cohn and Bradstock 2000).
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4.3
Dune systems and variation in fire severity
The majority of old and young C. verrucosa trees sampled for this study in dune systems grew on
dune crests and slopes, where there were no individuals in adjacent swales. This relative
abundance of C. verrucosa in the dune systems of Murray–Sunset may be a result of a
predominance of low-severity, patchy fires on dune crests. Fire propagation and intensity is
moderated by topography, fuel and weather (Bradstock et al. 2009). Litter and nutrients are more
likely to accumulate in swales, while fuel may be discontinuous on dune slopes and crests. For this
reason, dunes are more likely to have localised unburnt patches, allowing survival of Callitris
(Pearson et al. 2001, Bradstock et al. 2006).
4.4
Grazing and rainfall limit post-fire recruitment
Grazing pressure from domestic stock and rabbits is thought to be a major limitation to recruitment
of Callitris in the Mallee (Zimmer et al. 1944, Leigh et al. 1989). Livestock were first introduced
into the Murray–Sunset in the 1860s, and they were removed in the 1990s when the area was
declared a national park (Parks Victoria 2009). As data on C. verrucosa recruitment pre-1860 is
limited, it is difficult to draw conclusions about the impact of livestock grazing. The influence of
goats, which are also present in Murray–Sunset, is also difficult to quantify, although some authors
have suggested it can be devastating to C. verrucosa in particular (Cheal 2004). The introduction
of these exotic grazers into the landscape may also have increased the incidence of pulsed
recruitment, i.e. during periodic releases from grazing pressure.
The observed variability in the density of C. verrucosa populations has been said to be a
consequence of variation in seedling survival and establishment following post-fire rainfall
(Bradstock and Cohn 2002a). Long periods of above-average rainfall have been shown to be
critical to the recruitment of the non-bradysporous, arid-dwelling C. glaucophylla (Read 1995). A
cool summer with above-average rainfall may be particularly important in ensuring C. verrucosa
seedling survival. Fire increases the probability of seedling establishment because of the increased
seed rain. But fire during drought, or two fires within a short period, is likely to reduce seedling
and sapling survival (Cohn and Bradstock 2000), and C. verrucosa may even become locally
extinct (Bradstock and Cohn 2002a). Furthermore, if a fire was to occur during drought,
C. verrucosa seedlings may be out-competed by fast-growing lignotuberous eucalypts. Survival
probability is thought to increase substantially after seedlings are one year old, the main limiting
factors then being fire and grazing.
4.5
Maintenance of long-unburnt vegetation in a fire prone landscape
Unburnt vegetation provides critical habitat for a large range of taxonomic groups, especially
immediately after adjacent areas are burnt. It acts as a resource for animals, which will then
disperse as the burnt area regenerates. Long-unburnt vegetation is perceived to have particular
benefits, specifically hollows that form only in old trees and are used by arboreal mammals, birds,
bats and reptiles. However, it is difficult to find specific information on long-unburnt mallee
habitat use, perhaps because the nature of long-unburnt mallee habitats is poorly known as a result
of the lack of pre-1970s fire history information. Some authors have suggested that the endangered
Malleefowl Leipoa ocellata uses long-unburnt (‘relatively old’) mallee, in particular old-growth
C. verrucosa (e.g. Benshemesh 1990, cited in Bradstock et al. 2009). Other studies have
highlighted specific species or groups and their use of long-unburnt habitat, e.g. the Mallee Emuwren Stipiturus mallee (Brown et al. 2009), Eastern Bristlebird Dasyornis brachypterus (Baker
2000), Black-eared Miner Manorina melanotis (Clarke et al. 2005) and other small vertebrates (see
Friend 1993 for a review).
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4.6
Utility of Callitris verrucosa for fire history reconstruction
Several recent publications have demonstrated the utility of Callitris species for fire history
reconstruction. In particular, O’Donnell et al. (2010) successfully cross-dated Callitris preissii,
showing that the majority of recruitment occurred within one year of fire and that the maximum
within-stand age range was four years. The development of a cross-dated chronology for their
study was assisted by knowledge of the growth response of local C. columellaris to variability in
autumn–winter rainfall (Cullen and Grierson 2009; see also Sgherza et al. 2010).
In contrast, we had some difficulty in cross-dating samples from long-unburnt sites; there was no
strong stand-level growth signal, and mean inter-series correlations were below acceptable levels
for rigorous dendrochronological study (MIC > 0.4). The poor correlation of annual growth among
trees may be due to varying opportunistic (intra-annual) growth of trees within stands (Pearson and
Searson 2002); the presence of unclear wood anatomy, such as scarring and intra-annual false
rings, was not uncommon (Figure 6).
Figure 6. A problematic increment core. Three double rings (assumed to represent single years,
left) and a false ring (right) are indicated by arrows.
Further uncertainty was introduced with the estimation of ‘distance to pith’ (Duncan 1989) and
‘years to grow to coring height’ (O’Donnell et al. 2010). In addition, the small sample sizes
allowed under collecting permits could not adequately capture the unexpectedly high variability in
tree ages encountered. Finally, because C. verrucosa is not killed by all fires, and seedlings are
recruited between fires, there must be a qualifier on our assumption that the age of C. verrucosa
indicates the age of surrounding vegetation; this would be true only if successful recruitment
occurred immediately after a severe fire. Currently, C. verrucosa age structures may provide a
guide to tree age, and may be best used in conjunction with other proxies such as bomb pulse
dating (Hua 2009) and age–diameter models for mallee eucalypts (Clarke et al. 2010). It is
pertinent to note here that the linear relationship between age and diameter found in this study
describes only about 60% of the variation; diameter-based age estimates of C. verrucosa should be
treated with caution (see also Gibson et al. 2008).
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5 Conclusions
This study has revealed new information about the age structures of long-unburnt C. verrucosa
stands. The discovery of many multi-aged stands, instead of the expected same-aged stands,
indicates that some C. verrucosa trees are able to survive fire and recruit between fires. Although
these findings made the initial aim of fire history reconstruction difficult to achieve, age structures
do indicate the period for which the trees studied have not experienced severe stand-destroying fire
(up to 180 years). A stand-destroying fire may have occurred immediately prior to the
establishment of the oldest tree, or it may have occurred sometime before then if the tree was an
inter-fire recruit. The presence of trees with a range of ages within one stand may indicate the
influence of low-severity fire that scorches but does not kill trees and causes the release of seeds,
or recruitment in the absence of fire. This study provides evidence supporting the hypothesis that
C. verrucosa has a broader regeneration niche than expected, i.e. it is not strictly serotinous. Sameaged stands are most likely to be a result of severe fire, and old growth, multi-aged stands can
develop in the absence of severe fire. Because of the small sample size in this study, our
conclusions about the prevalence of fire survival and inter-fire recruitment of C. verrucosa can
only be tentative.
Further investigations are planned for late 2010 to assess the accuracy of C. verrucosa age
estimates derived from tree rings using bomb pulse dating (Hua 2009).
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6 Fire policy and management considerations
Victorian legislation and policy require land managers to protect life and property as well as
maintain ecosystem function and biodiversity. The protection of biodiversity values involves
understanding appropriate fire regimes for different ecosystems. This in turn involves examination
of how fire influences growth stages of vegetation over time and space, and the needs of
associated native fauna.
Important policy and management issues associated with this project include:
 how to manage ‘long-unburnt’ vegetation in the absence of more detailed fire history
information
 how to maintain old growth C. verrucosa vegetation, and its associated fauna, in the landscape
 how to maintain ecosystem processes that enable the recruitment of C. verrucosa.
While precise fire history information is a desirable input to planning for appropriate fire regimes,
it is frequently not available. In the absence of this information, fire managers need to take
additional care to manage risk associated with applying fire to the landscape. This includes
understanding the values represented in the landscape and ensuring that management approaches
reduce risks to important values. Dendrochronological analysis has not revealed precise fire
history information for long-unburnt areas in this study, so additional care is appropriate for
managing the values associated with these areas.
Callitris verrucosa is a long-lived species. Data from studies on other Callitris species indicate
individuals may reach an age of more than 250 years (Bowman and Harris 1995, Bradstock and
Cohn 2002a). Observations made during this project suggest that dune systems are important
refugia for this species, providing protection to individuals that can survive the low-intensity,
patchy fires that occur in dune systems. For this reason we suggest that old C. verrucosa
populations should be protected where possible from high-severity bushfire.
This study indicates that C. verrucosa trees up to 180 years of age are present in the study area.
Mature individuals probably play an important role in recruitment immediately after fire events
and between fires. This study suggests that long-unburnt C. verrucosa trees can produce
recruitment between fires. Reconstructed age structures of stands burnt since 1972 indicated that
recruitment of most individuals could be associated with a particular fire event. Hence, recruitment
of C. verrucosa may occur both between fires and immediately post-fire. Fire is a major
determinant of vegetation patterns in the Mallee. Where fire needs to be applied for management
purposes, mature C. verrucosa trees should be able to survive low-severity fire.
In addition to fire effects, managers should also consider the potential impacts of grazing and
drought, as successful recruitment of C. verrucosa is likely to depend on rainfall in the immediate
post-fire season.
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Appendix: Site photos
Mosaic 1
Mosaic 3
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Mosaic 4
Mosaic 7
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Mosaic 8, burnt in 1980 (28 years since fire).
Mosaic 9, burnt in 1980 (28 years since fire).
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Mosaic 11, burnt in 1975 (33 years since fire).
Underbool Road, burnt in 1975 (33 years since fire).
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Bambill Road site
Pheeny’s Track site 1
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Reconstructing Mallee fire history using Callitris verrucosa tree-rings
Pheeny’s Track site 2
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Arthur Rylah Institute for Environmental Research Technical Report Series No. 215
ISBN 978-1-74242-969-4 (print)
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ISSN 1835-3827 (print)
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