Impacts of a single fire event on large, old trees in a grass-invaded arid river system

Abstract

Background
Large old trees are keystone structures of terrestrial ecosystems that provide unique habitat resources for wildlife. Their widespread decline worldwide has serious implications for biodiversity and ecosystem integrity. In arid regions, large trees are relatively uncommon and often restricted to areas with elevated soil moisture and nutrients. Introduced grasses, now pervasive in many dryland environments, also thrive in such areas and are promoting more frequent and intense fire, potentially threatening the persistence of large trees. Here we report on the impact of a single wildfire on large river red gums ( Eucalyptus camaldulensis Dehnh.) in arid riparian woodland invaded by buffel grass ( Cenchrus ciliaris L.), a serious invader of desert ecosystems worldwide. In 2018, 266 trees with > 80 cm equivalent trunk diameter were mapped at six sites to provide a ‘pre-fire’ baseline. Within a year, the sites were impacted by a large, unprecedented wildfire that burnt an area of 660 km ² ha in 15 days. Sites were resurveyed in February 2019 to assess the fate of the trees. Reference to fire severity, calculated from remote-sensed imagery, is provided for additional context.

Results
In total, 67 trees, 27% of all large trees at the sites were destroyed. If trees in unburnt patches are excluded, 54% of trees exposed to the fire were destroyed and the remainder lost on average 79% of their canopy.

Conclusions
This severe detrimental effect of a single fire, on trees estimated to be centuries old, is indicative of tree-loss occurring across remote arid Australia in habitats where fire is now fuelled predominantly by invasive grasses. Large volumes of novel grass fuels along creeklines in combination with extreme weather events were major factors driving the spread, extent and impacts of the wildfire we report on and are causing a shift from relatively uncommon and predictable, rainfall-dependent large wildfires to large, severe fires that can occur anytime. We predict further decline in the abundance of large trees from similar fires will occur widely throughout arid Australia over the next decade with substantial long-term impacts on multiple species. New strategies are urgently required to manage fire in invaded arid ecosystems to better protect large trees and the critical resources they provide.

Full text

F I E L D N O T E Open Access
Impacts of a single fire event on large, old
trees in a grass-invaded arid river system
Christine A. Schlesinger
*
and Erin L. Westerhuis
Abstract
Background: Large old trees are keystone structures of terrestrial ecosystems that provide unique habitat resources
for wildlife. Their widespread decline worldwide has serious implications for biodiversity and ecosystem integrity. In
arid regions, large trees are relatively uncommon and often restricted to areas with elevated soil moisture and
nutrients. Introduced grasses, now pervasive in many dryland environments, also thrive in such areas and are
promoting more frequent and intense fire, potentially threatening the persistence of large trees. Here we report on
the impact of a single wildfire on large river red gums (Eucalyptus camaldulensis Dehnh.) in arid riparian woodland
invaded by buffel grass (Cenchrus ciliaris L.), a serious invader of desert ecosystems worldwide. In 2018, 266 trees
with > 80 cm equivalent trunk diameter were mapped at six sites to provide a ‘pre-fire’baseline. Within a year, the
sites were impacted by a large, unprecedented wildfire that burnt an area of 660 km
2
ha in 15 days. Sites were
resurveyed in February 2019 to assess the fate of the trees. Reference to fire severity, calculated from remote-sensed
imagery, is provided for additional context.
Results: In total, 67 trees, 27% of all large trees at the sites were destroyed. If trees in unburnt patches are
excluded, 54% of trees exposed to the fire were destroyed and the remainder lost on average 79% of their canopy.
Conclusions: This severe detrimental effect of a single fire, on trees estimated to be centuries old, is indicative of
tree-loss occurring across remote arid Australia in habitats where fire is now fuelled predominantly by invasive
grasses. Large volumes of novel grass fuels along creeklines in combination with extreme weather events were
major factors driving the spread, extent and impacts of the wildfire we report on and are causing a shift from
relatively uncommon and predictable, rainfall-dependent large wildfires to large, severe fires that can occur
anytime. We predict further decline in the abundance of large trees from similar fires will occur widely throughout
arid Australia over the next decade with substantial long-term impacts on multiple species. New strategies are
urgently required to manage fire in invaded arid ecosystems to better protect large trees and the critical resources
they provide.
Keywords: Invasive alien grasses, Disturbances, Cenchrus ciliaris, Wildfire, Large trees, Arid riparian woodland,
Eucalyptus camaldulensis, Australia
© The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
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* Correspondence: christine.schlesinger@cdu.edu.au
Research Institute for the Environment and Livelihoods, Charles Darwin
University, Alice Springs Campus, Grevillia Drive, Alice Springs, NT 0870,
Australia
F
ire Ecolog
y
Schlesinger and Westerhuis Fire Ecology (2021) 17:34
https://doi.org/10.1186/s42408-021-00121-4

Resumen
Antecedentes: Los árboles grandes y añosos son estructuras claves de los ecosistemas terrestres ya que proveen
recursos y hábitat para la vida silvestre. Su extendida declinación a nivel mundial tiene serias implicancias para la
biodiversidad y la integridad de los ecosistemas. En regiones áridas, los árboles grandes son poco comunes y
frecuentemente restringidos a áreas con elevada humedad del suelo y nutrientes. Los pastos introducidos, ahora
generalizados en muchos sistemas de secano, también prosperan en esas áreas, y promueven fuegos más intensos
y frecuentes, amenazando la persistencia de agrandes árboles. Reportamos acá el impacto de un incendio sobre
rodales de eucaliptus rojo (Eucalyptus camaldulensis Dehnh.) ubicados a la vera de un largo río cuyas riveras estaban
invadidas por pasto búfalo (Cenchrus ciliaris L.), un serio pasto invasor en ecosistemas de desierto en todo el
mundo. En 2018, 266 árboles con un diámetro equivalente > 80 cm fueron mapeados en seis lugares para proveer
una línea de base pre-fuego. Durante ese año, los sitios fueron impactados por un gran incendio que no había
tenido precedentes y que quemó 660 km
2
en 15 días. Los sitios fueron relevados en febrero de 2019 para
determinar el destino de los árboles. La referencia sobre la severidad del fuego, calculada mediante imágenes de
sensores remotos, fue provista como un contexto adicional.
Resultados: En total, 67 árboles, el 27% de todos los árboles grandes de estos sitios fueron totalmente destruidos
por el fuego. Excluyendo del análisis los árboles en los parches no quemados, el 54% de los árboles expuestos al
fuego fueron destruidos y del resto, un 79% promedio perdieron sus doseles.
Conclusiones: Este efecto perjudicial de un fuego individual en árboles que estimativamente tienen cientos de
años, es indicativo de las pérdidas árboles ocurridas a lo largo de áreas remotas de las zonas áridas de Australia,
donde ahora el fuego es estimulado predominantemente por pastos invasores. Un gran volumen de pastos nuevos
y combustibles a la vera de arroyos, en combinación con eventos meteorológicos extremos, fueron los mayores
factores conducentes a la propagación, extensión, e impactos que reportamos, y están causando un desvío de
grandes incendios relativamente poco frecuentes y predecibles, dependientes de la lluvia, a incendios grandes y
severos que pueden ocurrir en cualquier momento. Podemos predecir que, en la próxima década, en lugares áridos
de toda Australia habrá una mayor declinación en la abundancia de grandes árboles debido a fuegos similares al
reportado, con impactos a largo plazo en múltiples especies. Nuevas estrategias son urgentemente requeridas para
manejar el fuego en ecosistemas áridos invadidos para proteger los árboles grandes y añosos, y los recursos críticos
que ellos proveen.
Introduction
Large old trees are keystone habitat features in ecosys-
tems world-wide, with importance for wildlife. They pro-
vide unique and diverse microclimates, foraging
substrates, and nesting sites (Dean et al. 1999; Haworth
and McPherson 1995; Law et al. 2016; Lumsden et al.
2002a; Whelan and Maina 2005). Of particular value are
the hollows and cavities of a variety of shapes and sizes
that typically occur on large trees, some of which can
take decades or even centuries to develop (Haslem et al.
2012; Parnaby et al. 2011). These attributes are import-
ant to diverse groups of fauna including birds (Aumann
2001; Dean et al. 1999; Goldingay 2009), bats (Clews
2016; Lumsden et al. 2002b; Rhodes and Wardell-
Johnson 2006; Goldingay 2009), mammals (Dickman
1991; Goldingay 2011), reptiles (McElhinny et al. 2006)
and invertebrates (Majer et al. 1997). Large old trees are,
however, declining in many parts of the world (Linden-
mayer et al. 2012). Given the high habitat value of large
trees and the length of time they take to grow, the loss
of even a single tree can have significant and long-
lasting consequences for local biodiversity, especially
where few such trees remain (Lindenmayer and Laur-
ance 2016). While land clearing for forestry and agricul-
ture continues to threaten the persistence of large old
trees in many regions, fire has increasingly become an-
other major risk (Lindenmayer and Laurance 2016), par-
ticularly as current climatic patterns are supporting
unprecedented severe fire events (Nolan et al. 2020)
even in ecosystems with minimal recent history of fire
(Marris 2016).
Distribution of large trees in arid Australia and the threat
from fire fuelled by invasive grasses
In the Australian arid zone, large trees are naturally
scarce and completely absent in some regions. The high-
est densities occur in open woodlands associated with
drainage lines and flood-outs. Although the inland
creeks of central Australia are ephemeral, and flow only
for short periods following rain, permanent sub-surface
water is accessible to deep-rooted trees growing along
the creek beds, banks and adjacent floodplains (Reid
2015). The persistence of large trees in these arid river-
ine woodlands is, however, increasingly threatened by
Schlesinger and Westerhuis Fire Ecology (2021) 17:34 Page 2 of 13

introduced grasses that significantly alter fire regimes,
especially buffel grass (Cenchrus ciliaris L. = Pennisetum
ciliare L.). A high biomass and drought-tolerant grass,
buffel grass has invaded diverse habitats in dryland
Australia but is especially pervasive along creek lines
and adjacent lowland plains (Duguid et al. 2008; Friedel
et al. 2014; Schlesinger et al. 2013; Fig. 1).
Increased frequency, intensity and extent of fire pro-
moted by introduced grasses is one of the most serious
contemporary threats to biodiversity in arid ecosystems
(D'Antonio and Vitousek 1992), and buffel grass, a
perennial native of south and east Africa and southern
Asia, is one of the worst of the fire-promoting invaders
(Godfree et al. 2017; Grice 2006; McDonald and
McPherson 2013; van Klinken and Friedel 2017). In
Australia, buffel grass has been ranked with the most
serious environmental threats for projected future im-
pacts on dryland biodiversity (Read et al. 2020) although
it is less well-recognised and has received much less re-
search attention compared to other threats. In buffel-
invaded arid regions worldwide, there has been increas-
ing concern expressed about the impacts of buffel-
fuelled fire, particularly on large trees and other long-
lived keystone plants (Abella et al. 2012; Clarke et al.
2005; McDonald and McPherson 2011; Schlesinger et al.
2013; Rodríguez-Rodríguez et al. 2017).
The river red gum (Eucalyptus camaldulensis subsp.
arida Brooker & M.W. McDonald), although restricted
to the banks and floodplains of creeks, is the most wide-
spread arid-zone tree species in Australia. It has special
significance, therefore, with respect to the provision of
wildlife habitat. The largest individual trees are especially
important for cavity dependent fauna because they have
the most hollows (Westerhuis et al. 2019). River red
gums are susceptible to cambial injury when exposed to
fire or other disturbance, which predisposes the tree to
fungal and insect attack, and thereby promotes hollow
development (Gibbons et al. 2010), although it can take
decades to produce hollows suitable for occupation by
fauna through these processes. Because hollow trees are
also more susceptible to subsequent fire, the develop-
ment of large old trees with numerous hollows is only
supported in a system when fire intensity and frequency
are relatively low (Haslem et al. 2012). In recent decades,
in those regions where buffel grass has invaded, increas-
ing exposure to frequent and severe fire appears to be
threatening the persistence of large hollow bearing river
red gums. Across Australia, the species is already esti-
mated to have declined by 29% since European settle-
ment and is listed as Near Threatened (Fensham et al.
2019). The threat caused by interactions between inva-
sive grasses and fire on the arid subspecies is, therefore,
of particular concern.
Research aims
Although increasingly referenced, especially in the grey
literature (e.g. Day 2020), to date there have been a lack
of published empirical data documenting the extent of
damage to river red gums from fire fuelled by invasive
grasses. We aimed to contribute to filling this gap. In
June 2018, baseline data on the location of large river
red gums were collected along a heavily invaded ephem-
eral creek that had been minimally impacted by fire
since buffel grass had become dominant.
Fig. 1 aLarge river red gums amid a dense monoculture of buffel
grass on the banks of a sandy creek line following recent rain and b
during more frequent dry conditions (photos, Christine Schlesinger)
Schlesinger and Westerhuis Fire Ecology (2021) 17:34 Page 3 of 13

A large wildfire occurred in the region approximately
6 months later, and here, we report on the impact of this
fire on the mapped trees. Fire severity, calculated from
remotely sensed imagery, is provided to assist with inter-
pretation and the conditions under which the fire oc-
curred and spread are discussed. Conclusions are drawn
with respect to future fire patterns and the persistence
of large old trees in riparian woodlands invaded by buffel
grass in arid Australia.
Methods
Study area and site characteristics
The MacDonnell Ranges form part of a mountainous re-
gion in central Australia with complex topography, includ-
ing tall ranges, rocky hills and extensive lowland plains, and
are the source of several major inland rivers. Throughout
the region, the creekbanks, floodplains and other lowland
areas are heavily invaded by buffel grass. The other com-
monly present introduced grass is couch grass (Cynodon
dactylon L.) which has a more restricted distribution along
some creekbanks. These high biomass grasses commonly
form dense swards and have substantially replaced what
was a comparatively sparse native understory of mixed
grasses, chenopods and other forbs (Clarke et al. 2005).
The overstorey in the lowland areas includes river red gum
woodlands (near the creeklines) and mixed open woodland
or shrubland elsewhere (incl. Corymbia aparrerinja
K.D.Hill & L.A.S.Johnston, C.opaca (D.J.Carr & S.G.M.Carr)
K.D.Hill & L.A.S.Johnson, Grevillea striata R.Br. Hakea sp.,
Acacia estrophiolata F.Muell.,andA. victoreae Benth.).
Other major plant communities in the MacDonnell ranges
bioregion are Acacia shrublands (dominated by Acacia
aneura F.Muell. ex Benth. and Acacia kempeana F.Muell.)
which often occur on the lower slopes, and native spinifex
(Triodia sp.) hummock grasslands that dominate the
steeper rocky hillslopes and often co-occur with a sparse
overstorey of Eucalypts, especially mallee-form species.
Buffel grass has been slower to invade the Acacia shrub-
lands and spinifex grasslands, and although this is changing
in some areas, many of these upland plant communities still
feature a predominantly native understory.
Oursurveyareawasapproximately150kmfromthe
town of Alice Springs (latitude −23.655, longitude 132.729)
within the Tjoritja / West MacDonnell Ranges National
Park (hereafter—Tjoritja). The park extends for 161 km
westward from Alice Springs, incorporating many of the
tallest ranges and sheltered gorges within the bioregion;
covers an area of 2568 km
2
; and is recognised internation-
ally for its natural environment and as a place of cultural
importance (Day 2020). Before Tjoritja was established, the
area was used extensively for cattle grazing but domestic
stock have been excluded for approximately 30–50 years as
different areas became incorporated into the park. The six
survey sites were originally established in 2016 along a
5.5kmsectionofOrmistonCreek, a tributary of the Finke
River, as part of ongoing research on the local avifauna.
Each site measured 500 m along the creek (Fig. 2)andin-
cluded trees in the creek bed and on the banks. The buffel-
invaded creekbanks and floodplains (which were of variable
width) were bordered by predominantly buffel-free rocky
hillslopes which retained a native understory dominated by
spinifex grasses. At three sites (1, 3, 5), river red gums grew
only along the banks with few or no trees or shrubs in the
creek bed (‘simple’sites). The other three sites (2, 4, 6) had
river red gums and shrubs (predominantly inland tea tree—
Melaleuca glomerata F.Muell.) lining the banks and grow-
ing on sand islands between parallel smaller channels, and
sometimes in the creek bed (‘complex’sites).
Ground cover along the banks of all sites was domi-
nated by buffel grass with discontinuous patches of
couch grass also occurring in some areas. Cover in the
creek was much more variable with some of the vege-
tated sand islands of complex sites heavily invaded, while
others had no introduced grasses present. The creek bed
was either sandy or rocky and tended to have very low
ground cover, and no buffel or couch grass present.
Based on estimates made when the sites were first estab-
lished, ground cover within a 5-m radius under a
Fig. 2 Location of the six sites along Ormiston Creek. Inset shows
the location of the area within the Northern Territory, Australia
Schlesinger and Westerhuis Fire Ecology (2021) 17:34 Page 4 of 13

random sample of river red gums averaged 56% (± 27)
beneath trees on the bank (n= 310, across six sites) and
21% (± 23) in the creek bed and sand islands of complex
sites (n= 149, across three sites) and average modal
height was 66 (± 29) cm on the banks and 59 (± 29) in
the creek bed. We did not measure fuel loads; however,
it has been previously documented that buffel grass inva-
sion is associated with at least a doubling or tripling of
fuel loads in central Australian open woodland commu-
nities. Miller et al. (2010) recorded 1804 ± 154 (SE) kg
ha
−1
above-ground biomass at buffel-free sites compared
to 6679 ± 969 (SE) kg ha
−1
at sites where buffel grass
was non-continuous and invasion was ongoing. Fuel
loads on the banks at our study sites where buffel grass
was long established and had achieved near-continuous
dense cover were likely considerably higher.
Assessment of large old trees
River red gums were visually assessed from the ground in
June 2018. All trees not obviously too small were measured
to check their size against the thresholds that we used to
define large trees: trunks ≥80 cm DBH (diameter at breast
height) for single-stemmed trees, and at least one stem ≥
50 cm DBH for multi-stemmed trees. Trees with trunks at
or above these diameters have basal areas greater than ~
0.4 m
2
and characteristically have multiple hollows present
(Westerhuis et al. 2019). The locations of 266 large river
red gums (excluding stumps)—all those detected that met
the criteria—were recorded with a handheld GPS.
On 12 January 2019, a fire was ignited by lightning at a
site 65 km to the east of Ormiston Creek and burnt gener-
ally westward covering an area of 660 km
2
(approximately
22% of the park area, including our survey area) over 2
weeks (Day 2020). Approximately a month after the wild-
fire, in February 2019, we attempted to find all the
mapped trees again. Once found, trees were categorised as
follows: A. unaffected—no evidence of fire impact to main
tree structure or canopy, or no evidence the fire reached
the area where the tree was located; B. affected—clear evi-
dence of scorching on the trunk, or canopy partially or
completely scorched or burnt; and C. destroyed—main
stem of tree reduced to a stump or tree completely incin-
erated and evident only as ash and a hole in the ground
(see Fig. 3for examples of each category). Although some
Fig. 3 Example of post-fire tree types. AUnaffected trees on unburnt bank (top) and in creek bed (bottom). BAffected trees with partial canopy
scorch (top) and full canopy scorch (bottom). CDestroyed trees, completely combusted (top) and stump remaining (bottom) (photos,
Erin Westerhuis)
Schlesinger and Westerhuis Fire Ecology (2021) 17:34 Page 5 of 13

of the trees reduced to stumps are likely to resprout in the
future, we considered their value as large trees to have
been permanently lost (if current fire-regimes persist) or,
otherwise, lost for over a century. For trees that were af-
fected but not destroyed, we also visually estimated the ex-
tent of scorching of the canopy (i.e. leaves consumed or
still in place, but dead) to the nearest 10%, and the max-
imum height at which scorching was apparent.
Assessment of fire severity
Fire severity at the sites and surrounding region was
assessed using the delta normalised burn ratio (ΔNBR)
calculation on pre- and post-fire Sentinel-2 images of
the study area (Zone/ Path 53KKP). To minimise differ-
ences between images relative to moisture and plant
phenology (Lutes et al. 2006), images from 15 December
2018 and 28 February 2019 (those closest to before and
after the fire) were used. Buffel grass in Tjoritja was fully
cured by extreme heat in December and January (Day
2020) and only 10 mm of rainfall was recorded at
Ormiston Gorge (< 1 km from site 1, Bureau of Meteor-
ology 2021) between the dates when the images were re-
corded. Any herbaceous plant growth during this
interval was therefore assumed to be insignificant. Im-
ages were downloaded and converted to at-surface-
reflectance using the SCP plugin (Congedo 2016)in
QGIS version 3.16.0.
The delta NBR was calculated as:
ΔNBR ¼NBRpre fire−NBRpost fire ;
where NBR = (band 8 −band 12) / (band 8 + band 12).
This uses the Sentinel-2 bands which respond strongly,
but in opposite ways, to changes to the landscape follow-
ing a fire. Band 8 of Sentinel-2 images acquires wave-
lengths in the near-infrared spectrum (0.785–0.900 μm)
and is responsive to biomass, while band 12 acquires far
short-wave infrared reflectance (2.100–2.280 μm) and is
responsive to the moisture content of soil and vegeta-
tion. The difference between the two bands for each
image is normalised by the sum of the two bands. This
removes some of the effects of topography and differ-
ences in solar illumination between dates. The accuracy
of the delta NBR was checked against high temporal-
resolution hotspots recorded in the area between 12 and
27 January 2019 using the Northern Australian Fire In-
formation (NAFI) plugin (Lynch 2020) for QGIS version
3.16.0. Finally, the delta NBR was used to generate a
polygon of the fire scar at the sites and in the surround-
ing areas using tools in QGIS.
To compare remotely sensed and ground-based mea-
sures, the delta NBR values for a 30 × 30 m
2
surround-
ing each tree (9 pixels) were extracted. The use of nine
pixels provided a less precise but more accurate
indication of the fire severity trees were exposed to by
allowing for variability in the accuracy of GPS coordi-
nates of trees. The average delta NBR value was calcu-
lated for the 9 pixels surrounding each tree point and
the mean delta NBR value and quartile ranges were cal-
culated for each post-fire tree category.
Results
Of 256 trees re-identified after the fire, 67 (27%) were
destroyed, 98 (40%) were affected but not destroyed and
81 (33%) were unaffected by the fire. Ten other trees,
mapped prior to the fire, could not be found, possibly
due to inaccuracies in their GPS coordinates. Unaffected
trees were, predictably, associated with low delta NBR
values (<50) (Fig. 4) and were mostly growing within a
few larger patches where the fire did not reach, includ-
ing the entire western bank of site 2 and part of the
western bank of site 3 (Fig. 5). Others were growing
within the creek bed at sites 2, 4, and 6. Destroyed and
affected trees were, conversely, in areas with higher delta
NBR values with destroyed trees tending to be associated
with the highest delta NBRs (Fig. 4).
There were some exceptions, however, where trees
were destroyed or affected in areas where the fire sever-
ity index was relatively low. This was especially evident
at site 6, where some trees within the creek bed were
destroyed, and many others were affected despite overall
fire severity being much lower compared to the banks
(Fig. 5). Ground-based observations post-fire confirmed
that some of these trees were surrounded by bare sand
and that the fire had spread directly from canopy to can-
opy, even when the nearest tree was tens of metres away,
an indication of the intensity of the fire at this site. Ex-
cluding site 6, almost all the destroyed trees were on the
banks which were dominated by buffel grass. Of the 156
trees that the fire reached, 54% were destroyed and those
remaining had an average of 78.7 % (± 20.93 SD) of their
canopy burnt with fire damage evident on average up to
11.0 m high (± 3.0 SD). Approximately a third of the af-
fected but not destroyed trees lost 100% of their canopy.
Discussion
Impacts of fire on large trees and wildlife habitat
The severe detrimental effect of the 2019 fire at Tjoritja,
here documented, on trees estimated to be centuries old,
is indicative of tree-loss occurring across remote arid
Australia in habitats where fire is now fuelled predomin-
antly by invasive grasses. The 2019 fire, to our know-
ledge, is only the second to affect the section of
Ormiston Creek where our sites were situated since buf-
fel grass began to dominate riparian woodlands in the
West MacDonnell Ranges in the 1990s. This is likely be-
cause this portion of the creek is in a relatively isolated
valley which has been afforded some degree of
Schlesinger and Westerhuis Fire Ecology (2021) 17:34 Page 6 of 13

protection from management of fuel loads in the sur-
rounding spinifex dominated rocky hills. The area was
burnt in 2001 but there are no definitive records of the
extent to which buffel grass contributed to fuel loads at
that time. The creek area was used for pastoralism until
1992 when it was incorporated into the National Park
(Gary Weir pers. comm.) and anecdotal reports suggest
it took years before all the cattle were removed, and that
ongoing grazing may have slowed the accumulation of
high fuel loads. When our monitoring sites were estab-
lished in 2016, the trees appeared to be relatively intact,
with no evidence of recent fire damage. This contrasted
with many of the other rivers and creeks in Tjoritja, situ-
ated in less protected areas, where multiple fires have
occurred since buffel grass became dominant. Our re-
sults for Ormiston Creek, therefore, provide unique in-
sights into the ongoing impacts of fire, anecdotally
observed, on river red gums in other catchments in the
region over recent decades.
The 2019 fire and other buffel-fuelled fires in Tjoritja
are causing long-lasting impacts on the structural com-
position of river red gums within the park and the habitat
they provide. The significance of this impact is especially
apparent when the age of the trees that were destroyed at
Ormiston Creek and, consequently, their previous (pre-in-
vasion) resilience to multiple fires is considered. River red
gums are estimated to take well over 100 years to grow to
large size; therefore, the size structure of trees in the cen-
tral Australian riparian woodlands will shift markedly
downwards under repeated severe wildfires. The loss of
large river red gums will substantially reduce available
habitat for the many species that rely on large canopies
for foraging and shelter. Even if younger and resprouting
river red gums persist, the number and variety of hollows,
critical to the reproduction of multiple wildlife species, is
expected to become progressively more limited because
smaller trees typically have few, small hollows if any (Wes-
terhuis et al. 2019).
Although the importance of large hollow-bearing trees
for fauna is globally recognised (Lindenmayer and Laur-
ance 2016), the use of hollows by wildlife in central
Australia has been less-well researched compared to
many other regions, with targeted investigation limited
to a few species (Princess Parrot, Polytelis alexandrae
(Pavey et al. 2014); Nankeen Kestrel, Falco cenchroides
(Aumann 2001); Owlet Nightjar, Aegotheles cristatus
(Doucette et al. 2011)). Community-level studies indicate
the value of river red gum woodlands more generally in
supporting a suite of locally occurring nectivorous and
insectivorous canopy foraging birds (Reid 2015; Wester-
huis et al. 2020a). The abundance of large hollow-
bearing trees has been identified as an important pre-
dictor of bird assemblages in arid riparian woodlands
(Pavey and Nano 2009) and is also correlated with in-
creased levels of bat activity, especially in dry and hot
conditions (Westerhuis et al. 2020b). Ten of the 12 lo-
cally occurring insectivorous bat species require trees for
roosting and breeding. Although further research is re-
quired to better understand the impact on individual
species, a suite of fauna are expected to be adversely im-
pacted by the loss of large trees under repeated buffel-
fuelled fire, with long-term effects on regional diversity
and ecosystem function.
Changing fire regimes in invaded Australian arid
ecosystems
The extent to which the Tjoritja fire spread through the
creek lines (e.g. see Fig. 6) represents a departure from
Fig. 4 Box plot of average delta NBR values for destroyed, affected and unaffected river red gums calculated from nine pixels closest to the tree
location (equivalent to 30 × 30 m
2
)
Schlesinger and Westerhuis Fire Ecology (2021) 17:34 Page 7 of 13

historical fire patterns which were more likely to be con-
fined to native spinifex communities, except following
high rainfall periods. This novel pattern of spread en-
abled the 2019 fire to stay alight for many days and
cover an extensive area. The government report on the
fire states that ‘buffel grass has played a significant role
in the extent and severity of this wildfire. This grassy
fuel helped transfer the fire across the park via the many
creeks and rivers that it infests.’(Day 2020, p5). This is
illustrated, for example, during the final days of the
event (~ 23 and 24 January) when the wildfire spread
from spinifex hills to the east of Ormiston Creek into
the creek-line (our study area) and then from the south-
east to the north-west along the creek and through the
adjacent hills (Day 2020). Meanwhile, the fire spread
into the Finke River and Davenport Creek (that joins
Fig. 5 Fire impact to trees at six sites along Ormiston Creek relative to ΔNBR calculated from Sentinel 2 imagery. Data were categorised
according to equal intervals (low = 51–137, medium = 138–225, high = 226–312, extreme = 313–400) with darker colours indicating higher
severity fire. Values below 50 were not displayed as examination of unburnt areas across the entire Sentinel image indicated this was the
threhold above which change due to fire (as opposed to variation not related to fire) was observed. Inset shows entire fire scar relative to
the study sites
Schlesinger and Westerhuis Fire Ecology (2021) 17:34 Page 8 of 13

Ormiston Creek at the head of the Finke) (Fig. 7). At
Davenport Creek alone, more than 40 km of riparian
vegetation burnt.
Prior to buffel grass invasion, spinifex was the most
flammable vegetation community in arid Australia.
Spinifex grasses are highly flammable, but produce dis-
continuous fuels, especially during the first few years fol-
lowing fire. Fire return times are typically between 5 and
20 years depending on region and rainfall (Edwards et al.
2008; Verhoeven et al. 2020) and coexisting overstorey
plants are adapted to relatively frequent fire (Marsden-
Smedley et al. 2012). Consequently, in Tjoritja and else-
where across the inland, contemporary management to
prevent large-scale wildfire has focussed predominantly
on prescribed burning of spinifex communities to create
a mosaic of different fire ages, an approach designed, in
part, to replicate traditional burning practices of Austra-
lian Indigenous people.
Fig. 6 Fire burning along floodplains on either side of a creek line during the 2019 Tjoritja fire (photo, Grant Allan). Dense areas of buffel grass
appear grey. At this location adjacent areas lacked sufficient fuel to sustain fire spread. Trees along the drainage line are river red gums and
shrubs in adjacent areas are mixed Acacias
Fig. 7 Fire scars derived from Sentinel-2 imagery, at Ormiston Creek and in association with river and creek lines to the west during the final days
of the fire event. Inset shows entire fire scar relative to the Ormiston Creek area
Schlesinger and Westerhuis Fire Ecology (2021) 17:34 Page 9 of 13

In contrast, riparian woodlands have not, until re-
cently, been targeted for fuel-reduction burning. Indeed,
prior to invasion by introduced grasses, the biomass pro-
duced by native grasses of the lowlands was insufficient
to support intense or widespread fire except following
infrequent periods of prolonged high rainfall (Duguid
et al. 2008; Edwards et al. 2008). Even following high
summer rainfall, when the growth of native grass is pro-
moted, the impact of native grass-fueled fire on trees is
substantially reduced compared with areas where buffel
grass dominates (Miller et al. 2010; Schlesinger et al.
2013). The long-lived overstorey species of the rivers
and floodplains are not, consequently, well adapted to
the severe fires that can now occur in buffel-invaded
areas multiple times within a decade (Schlesinger et al.
2013; Read et al. 2020). Buffel grass is now recognised as
one of the main wildfire risks for central Australia (Day
2020) and fuel-reduction burning or alternative manage-
ment (e.g. mowing) of invaded areas has become in-
creasingly necessary to protect infrastructure. It is
recognised that burning of grass-invaded river red gum
and other open woodland communities is not desirable
from an ecosystem management perspective because of
the damage caused to long-lived trees (Day 2020), but
there are currently few viable alternatives.
Even considering the increasing regularity of buffel-
fuelled fires, both prescribed and unplanned, over the
past two to three decades, the scale and timing of the
2019 Tjoritja wildfire was unprecedented. We propose
this fire marks a significant point in a transition away
from rare but relatively predictable, rainfall dependent
large wildfires that were formerly typical of central
Australia. Previously, large fires occurred in the warm
season following two or more consecutive years of
higher-than-average rainfall (Edwards et al. 2008;
Marsden-Smedley et al. 2012; Verhoeven et al. 2020).
But the Tjoritja wildfire was not preceded by high ante-
cedent rainfall. The rapid re-accumulation of fuel loads
in regions where buffel grass has heavily invaded, with
much less rainfall, and the persistence of buffel fuels in
the landscape, even during long-dry periods means large
fires can now happen at almost any time. This is espe-
cially so as extreme weather that promotes fire and hin-
ders containment efforts becomes more commonplace
due to continuing anthopogenic climate change (Abatzo-
glou and Kolden 2011).
More frequent extreme weather patterns, including
heat waves and extended droughts, have been pre-
dicted for arid Australia (Watterson et al. 2015), and
this is supported by recent climate observations (Bur-
eau of Meteorology 2021). Indeed, the Tjoritja wild-
fire occurred toward the end of a record heat wave in
January 2019 which, combined with high wind speeds
experienced during the two-week event, produced
significant challenges for fire containment and
undoubtably contributed to the extent of damage
caused to large trees. Fuel continuity within spinifex
communities was relatively low (too low to support
control burning in the preceding cool season (Day
2020) but the record high temperatures in the preced-
ing weeks would have reduced the moisture content
of spinifex to extremely low levels which, under
windy conditions, can enable the spread of fire even
through such relatively ‘young’spinifex (Burrows
et al. 2018). Coupled with the spread of fire through
the creeklines, where the continuous fine fuels pro-
duced by buffel grass promote fire spread even when
temperatures and windspeeds are low (Schlesinger
et al. 2013), these extreme conditions set the scene
for an extraordinary event. We predict, however, that
similar interactions between novel and native fuels
and severe fire weather will promote more large and
unpredictable fires of a comparable nature in the ex-
tensive areas now dominated by buffel grass across
the Australian arid zone. Such fires, especially when
combined with repeated more localised fires, will
cause significant further decline of the large trees
remaining in invaded areas throughout Tjoritja and in
other invaded ecosystems across arid Australia within
the next decade.
Management implications
Invasive grasses, including buffel grass, are usually ex-
tremely fire tolerant and regrow quickly after fire
(Marshall et al. 2012;Pyšek et al. 2012;Rossiteretal.
2003; Setterfield et al. 2010)suchthatbenefitsof
fuel-reduction burning in invaded ecosystems are
brief. Furthermore, adverse impacts on trees from
buffel-fuelled fire occur even in cool, still weather
(Schlesinger et al. 2013;Day2020). This makes fuel-
reduction burning in areas where biodiversity conser-
vation is a primary objective problematic, except at
very small scales at which individual trees can be pro-
tected. In our region, prescribed fire in native spinifex
communities is effective at preventing the spread of
wildfire through the wider landscape and, especially
when adjacent to invaded areas, may reduce the risk
of fire in buffel-grass dominated woodland communi-
ties. For example, the western side of Ormiston creek
at site 2, which remained unburnt, had low fuel loads
in January 2019 due to recent prescribed burning
undertaken to protect infrastructure in the rocky hills
west of the road (Grant Allan, pers. com; Fig. 4). This
was likely a factor in preventing the fire in the creek-
line from jumping to the western bank, resulting in
many trees at this site remaining unaffected. Add-
itional resources that enable conservation managers to
extend burning programmes in these fire-tolerant
Schlesinger and Westerhuis Fire Ecology (2021) 17:34 Page 10 of 13

communities would be beneficial. However, alterna-
tive, effective methods to manage fire within invaded
areas are also urgently required to reduce the risk of
these areas acting as conduits for the spread of large
wildfires, and to prevent further loss of keystone arid
zone trees.
Pathogen or insect mediated biocontrol offers the
best long-term prospect for landscape-scale manage-
ment of buffel grass and associated fuel loads but
because of the risks to buffel-dominated grazing
lands, there has not been sufficient widespread
socio-political support to enable exploration of this
option. This may be changing, however, with in-
creasing recognition of the level of threat buffel
poses to ecosystems, biodiversity and associated so-
cial and cultural values, especially those of Aborigi-
nal people who are the traditional custodians and
majority of residents of semi-arid Australia (Read
et al. 2020). Fire risk associated with buffel grass is
greatest in un-grazed areas—conservation reserves
and Aboriginal lands—and in the short to medium
term, the management of fire in these areas is neces-
sarily reliant on local-scale interventions.Integration
of multiple management tools is required. Targeted
application of herbicides and manual and mechanical
removal are preferable to burning, especially where
the persistence of large trees is already threatened,
but more limited in the scale at which they can be
implemented. Ideally, the outcome of management
should be the restoration of areas of native vegeta-
tion, rather than simply the reduction of biomass in
the short term. Although this requires greater effort
initially, the level of maintenance long term will be
reduced, and the resulting natural green breaks (Hul-
vey et al. 2017;Porenskyetal.2018) can provide
multiple additional, measurable benefits for wildlife
(Schlesinger et al. 2020). In Australia, and elsewhere
where ecosystems have evolved without heavy graz-
ing by large herbivores, the use of livestock grazing
as a conservation tool is controversial, even (or espe-
cially) at sites that were previously heavily grazed—
like Tjoritja. But a small number of reserves in east-
ern Australia are using grazing to minimise the im-
pact of buffel-fuelled fires on fire sensitive vegetation
communities; Meltzer (2015) and Lebbink et al.
(2021) provide a comprehensive assessment of the
practical challenges, benefits and risks. The consen-
sus from these studies and a broader analysis by
Lunt et al. (2007) is that grazing should only be used
when there are no other effective options and that
the costs to biodiversity and ecosystem integrity of
reintroducing livestock into reserves are likely to
outweigh benefits in many circumstances, especially
in less productive dryland areas.
Managing fire to maintain biodiversity values in
buffel grass-invaded areas is certain to remain a major
challenge over the coming decades. To meet this
challenge, it is crucial that conservation managers are
adequately resourced to apply a range of management
tools, assess the outcomes, and adapt their manage-
ment accordingly.
Acknowledgements
We gratefully acknowledge the environmental science and management
students who helped collect the pre-fire field data during CDU’s 2018 Desert
Field Ecology field programme. We thank Grant Allan for information about
the fire, permission to use the image in Fig. 6, and comments on an early
draft of the manuscript. We also thank Marg Friedel, Izak Smit and an an-
onymous reviewer for comments on the manuscript.
Authors’contributions
CAS and ELW contributed equally to the conception of the work,
interpretation of the data and drafting the manuscript. ELW led the data
collection, remote-sensing and analyses. CAS led the revision of the manu-
script. All authors read and approved the final manuscript.
Authors’information
CAS is a wildlife ecologist and senior lecturer at Charles Darwin University
specialising in conservation ecology in desert environments and especially
the impacts of introduced species on wildlife.
ELW is a post-graduate student at Charles Darwin University studying the
ecology of arid riparian woodlands and importance for wildlife.
Funding
The research was funded by Charles Darwin University
Availability of data and materials
The datasets used and/or analysed during the current study are available
from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate
Not applicable
Consent for publication
Not applicable
Competing interests
The authors declare that they have no competing interests.
Received: 5 May 2021 Accepted: 21 September 2021
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