Open Journal of Forestry
2012. Vol.2, No.4, 183-186
Published Online October 2012 in SciRes (
Copyright © 2012 SciRes. 183
Drought and Heat Triggers Sudden and Severe Dieback in a
Dominant Mediterranean-Type Woodland Species
George Matusick, Katinka X. Ruthrof, Giles St. J. Hardy
Centre of Excellence for Climate Change Woodland and Forest Health, Murdoc h Uni ve rsity, Perth, Australia
Received June 7th, 2012; revised July 10th, 2012; accepted July 25th, 2012
Ecosystems in Mediterranean climate regions are projected to undergo considerable changes as a result of
shifting climate, including from extreme drought and heat events. A severe and sudden dieback event,
occurring in regionally significant Eucalyptus gomphocephala woodland in Western Australia, coincided
with extreme drought and heat conditions in early 2011. Using a combination of remote sensing and field-
based approaches, we characterized the extent and severity of canopy dieback following the event, as well
as highlighted potential predisposing site factors. An estimated 500 ha of woodland was severely affected
between February and March 2011. Tree foliage rapidly discolored and died over this period. In the af-
fected portion of the woodland, approximately 90% of trees greater than 20 cm DBH were impacted,
while in the adjacent unaffected woodland 6% showed signs of damage. Tree density in the unaffected
area had approximately 4.5 times more trees than the affected woodland. Precipitation drainage patterns
are thought to explain the difference between affected and unaffected woodland. Dropping groundwater
levels, a relatively shallow soil profile, and extreme drought and heat in 2010-2011 are thought to predis-
pose water-shedding sites to drought-triggered canopy dieback during extended periods of dryness.
Tracking forest health changes in response to severe disturbance is an important key to deciphering past
and future vegetation change.
Keywords: Eucalyptus Gomphocephala; Forest; Western Australia; Die-Off; Climate Change
In Mediterranean climate regions, drought is one of the key
selection pressures on forest distribution and growth (Damesin
et al., 1998). Despite a range of adaptations to disturbance
events, trees in Mediterranean ecosystems can be vulnerable to
drought and heat-induced canopy dieback (Lloret et al., 2004)
and associated declines in growth (Jump et al., 2006) and car-
bon storage (Galiano et al., 2012). These forests will be further
challenged if drought periods become more frequent, as pre-
dicted by climate change projections (Giorgi & Li o ne l lo, 2008).
The Mediterranean climate of the south-west of Western
Australia (SWWA) is experiencing a sustained and substantial
shift to drier and warmer conditions (Bates et al., 2008). Spe-
cifically, the region has been experiencing a pronounced, long-
term decline in rainfall since the mid 1970’s (Bates et al., 2008).
Concurrently, the average temperatures have risen at a rate of
0.15˚C per decade over the same time period (Bates et al.,
2008). Drier conditions in SWWA have corresponded with
decreases in streamflow (Petrone, 2010) and groundwater levels
as a result of reduced precipitation (Croton & Reed, 2007) and
exploitation (Sommer & Froend, 2011).
Eucalyptus gomphocepha la DC. is an endemic medium to
tall tree with a highly restricted distribution along the Swan
Coastal Plain in SWWA. It generally occurs in pure stands on
coastal limestone derived soils and is one of the few eucalypts
adapted to calcareous alkaline soils (Eldridge et al., 1994).
Most E. gomphocephala-dominated woodlands have been
cleared for urban and agricultural development (DEC, 2010).
Indeed, only 33% of E. gomphocephala-dominated woodlands
remain. Eucalyptus gomphocephala has experienced low levels
of crown dieback throughout its range (Edwards, 2004). Addi-
tionally, sudden and severe dieback leading to mortality has
occurred (Fox & Curry, 1980; Cai et al., 2010). A variety of
causal factors have been proposed for these tree health changes
(Close et al., 2011; Curry, 1980), though extreme climate con-
ditions have not been co ns id e red previously.
Corresponding with the hottest period of 2010/2011 and in
the midst of a record dry summer, E. gomphocephala tree
crowns began suddenly collapsing in a regionally significant
population. We hypothesize the extreme heat and dry condi-
tions contributed to this observed dieback event. Understanding
within-site patterns of crown mortality can identify potential
site factors which predispose areas to dieback (Lloret et al.,
2004), which may assist efforts to develop predictive tools.
This research aims to estimate the incidence of damage, de-
scribe the symptomology and severity of the dieback, as well as
highlight potential site differences between affected and unaf-
fected woodland.
Materials and Methods
The study site is located 37 km south of Perth, Western Aus-
tralia (Figure 1) in the Rockingham Lakes Regional Park
(32˚17'00.36''S, 115˚47'17.71''E). This area is significant for its
geomorphic landforms because the distinct parallel sand ridges
indicate the positions of former shorelines, providing a record
of sea level changes over the past 7000 years. Lakes have
formed in between the sand ridges, and these are also signifi-
cant because they form part of an evolutionary time sequence
and support unique vegetation communities (DEC, 2010). The
Figure 1.
The location of the study site in the south west of Western Australia
within the range of Eucalyptus gomphocephala (shaded). Inset: the
native vegetation (light and dark grey shading), surrounding Lake
Cooloongup (black), affected by crown dieback (dark grey shading).
Sampled unaffected (light grey hatched) and affected woodland (dark
grey hatched).
wetlands have historically formed from precipitation runoff
from small catchments that are bounded by a cemented dune
system to the east. The study area represents the catchment for
Lake Cooloongup and it is highly fragmented within the urban
matrix and, in places, is highly disturbed. The dominant canopy
species, E. gomphocephala, occurs in open woodland across a
flat plain. Precipitation intercepted by the plain drains into Lake
Cooloongup to the south through one large water-gaining area
to the east. This historical drainage contains a mixture of Me-
laleuca species in the wettest areas and closed canopy E. gom-
phocephala in the slightly dryer areas. Early field observations
were made in 29 March 2011, when trees crowns were found to
be dying. High resolution ortho-rectified photographs obtained
monthly by NearMap (NearMap Pty Ltd., Perth, Australia)
were used to detect crown changes from the period of January
2011 to June 2011. Crown sympotomology, which peaked in
March 2011, was used to delineate the extent of severe canopy
collapse using ArcGIS 10 software (ESRI, Redlands, CA).
A field survey of affected and adjacent unaffected wood-
land was conducted in December 2011 and was restricted to the
state managed Rockingham Regional Park. Six points were
randomly selected in each of the affected and unaffected areas
using fGIS forestry cruise software on a 100 × 100 m grid
(Wisconsin DNR-Division of Forestry). The point-centered
quarter method was used (Mitchell, 2001), including 150 m
long N-S transects with points at 50 m spacing. The nearest tree
> 20cm DBH from each quadrant (NE, NW, SE, SW) was se-
lected for measurement. The total number of trees sampled
within the study area was 144 (72 affected/72 unaffected).
Measurement included distance to centre point, DBH, height
using digital clinometer (Haglof HEC, Langsele, Sweden),
defined as maximum height of foliage, and canopy cover using
a spherical densiometer 2 meters from the stem at N, S, E, W.
Total percent of canopy dieback, based on presence of branches
free of foliage, and was visually estimated, as was the percent-
age of total foliage composed of epicormic resprouts formed
since the disturbance. Finally, the percentage of total crown
with recent flagging, defined as yellow and dead foliage, was
Crown dieback (Figure 2) corresponded with a prolonged
heat wave, characterized by nine days greater than 35˚C (Fig-
ure 3). These climate conditions contributed to the record hot
summer period 2010-2011, including the third hottest February
on record (BOM 2011). Precipitation in 2010 preceding the
collapse was 40% - 50% lower than average, resulting in the
second driest year on record (BOM, 2011) (Figure 3).
Tree health was found to be substantially different between
the affected and unaffected woodland. Approximately 90% ±
5% of measured trees were impacted in affected versus only 6%
± 6% in unaffected areas. Although affected trees lost most of
their original foliage, mortality was low (3%) due to prolific
epicormic re-sprouting from the stem and lower branches.
Re-sprouts represented 50% ± 7% of tree foliage in affected
areas compared with 5% ± 2% in the unaffected. Ten months
following the event, total crown dieback (crown retraction) was
50% ± 7% in the affected area and 10% ± 2% in the unaffected.
A low number of trees were continuing to experience crown
dieback in December 2011, indicated by 5% ± 2% branch flag-
ging across the affected area. Both sites had trees of similar
sizes (66.4 ± 3.4 cm and 69.9 ± 5.4 cm DBH for affected and
unaffected sites respectively). Trees in the affected site are now
substantially shorter (13.4 ± 3.3 m) compared with the unaf-
fected trees (23.1 ± 2.5 m). Due to the presence of epicormic
re-sprouts near the base of the trees, canopy cover was found
not to be different between the sites (30% ± 3% and 30% ± 2%
for unaffected and affected sites, respectively).
The affected and unaffected sites were also different in their
Figure 2.
Aerial photograph looking north from Lake Cooloongup, show-
ing Eucalyptus gomphocephala crown symptoms in affected
study area to the left and healthy crowns in the unaffected study
area to the right. Photo credit: Brett Glossop.
Copyright © 2012 SciRes.
Copyright © 2012 SciRes. 185
Figure 3.
Fifty-year mean maximum temperature and mean precipitation (mm) for January and De-
cember and mean maximum and total precipitation for January 2010-December 2011. High-
light represents period of Eucalyptus gomphocephala canopy dieba ck in stu dy area.
site and stand characteristics. Tree density in the affected
woodland site was 4.5 times lower than the unaffected site (39
± 16 vs 174 ± 48 trees/ha). This translates to a basal area for the
affected site of 16 ± 6 m2/ha versus 58 ± 13 m2/ha for the unaf-
fected. Observations of fallen trees in both sites, coupled with
anecdotal information obtained from neighboring land owners
suggest that root systems are restricted by a limestone base-
The primary difference between affected and unaffected sites
was with respect to their drainage patterns. Whereas the af-
fected woodland occurs in a slightly raised, water shedding flat
plain, the unaffected occupies a water-gaining site with notable
past erosion. As a result, the unaffected site is approximately
2m lower in elevation on average. The unaffected site is situ-
ated at the base of a cemented dune (approximately 30 m in
elevation) and has historically facilitated drainage into the
nearby Lake Coolo on gu p .
Here we conclude that a severe and sudden dieback event in
E. gomphocephala woodland coincided with extreme drought
and heat conditions in early 2011. This represents the first
documented episode of this type associated with extreme cli-
mate conditions in E. gomphocephala woodland. A similar,
though more severe, dieback event of Eucalyptus margi-
nata-dominated forest in the SWWA occurred during the same
time; affecting an estimated 16,500 ha (Matusick et al., unpub-
lished data). Forest crown dieback events have been recorded in
other Mediterranean and semi-arid systems following extreme
drought (Breshears et al., 2005; Lloret et al., 2004). The Iberian
Peninsula in NE Spain has experienced similar severe drought
in 1994, for example, which has resulted in a massive Quercus
ilex L. forest dieback (Lloret et al., 2004). Sudden and severe
drought in 2002/2003 in SW North America resulted in tree
die-off extending 12,000 m2 in Pinus edulis Engelm.-dominated
woodlands (Breshears et al., 2005). These events are likely to
become more frequent in the SWWA and other similar regions
with continued climate change.
The dominance of E. gomphocephala in the overstorey along
coastal SWWA suggests it is highly adapted to the region. It is
a facultative phreatophyte and has the ability to grow in areas
with and without access to groundwater. However, changes in
the quantity of precipitation have resulted in a significant low-
ering of groundwater and lake levels in the entire study area
(DOW, 2008). As a result, differences in precipitation drainage
patterns between the affected (water-shedding) and unaffected
(water-gaining) may explain the strong contrast in woodland
canopy impacts. Trees in water-limited forest ecosystems are
highly dependent on soil type, slope, and drainage patterns
(Costa et al., 2008). We therefore hypothesize the cumulative
effects of long-term rainfall reductions (leading to dropping
groundwater levels), inadequate winter precipitation in 2010/
2011 (leading to inadequate soil recharge), and severe heat
conditions in February 2011(leading to high canopy water
stress) led to canopy dieback in the water-shedding E. gom-
phocephala woodland. During extreme periods of dryness,
eucalypts located on water-shedding sites, with low soil water
holding capacity are susceptible to rapid drying and dieback
(Pook et al., 1966). We suspect the neighboring unaffected E.
gomphocephala woodland, although transpiring more water,
receives higher rates of precipitation runoff and therefore is
capable of withstanding stressful summertime conditions.
Recent climate change modeling suggests that Mediterranean
ecosystems will exhibit a range of responses to future climate
changes (Klausmeyer & Shaw, 2009). Specifically, this model-
ing has predicted that E. gomphocephala-dominated ecosys-
tems from the study area north will contract, while areas south
of the study are expected to stay stable or expand (Klausmeyer
& Shaw, 2009). Observations from this study are able to con-
firm these modeled projections are correct for the study area.
Additionally, they provide important field evidence for how
future contractions in E. gomphocephala ecosystems may occur.
An accumulation of this type of data is required for field vali-
dation of modeling tools as well as understanding the cascade
of immediate, short- and long-term changes in ecosystem
structure and functioning following dieback (Ellison et al.,
2005). Ultimately, this information will be required to facilitate
mitigation strategies.
The authors would like to thank the West Australian State
Department of Environment and Conservation and the City of
Rockingham for their support.
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