Drought and Heat Triggers Sudden and Severe Dieback in a Dominant Mediterranean-Type Woodland Species

doi:10.4236/ojf.2012.24022

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Open Journal of Forestry

2012. Vol.2, No.4, 183-186

Published Online October 2012 in SciRes (http://www.SciRP.org/journal/ojf) http://dx.doi.org/10.4236/ojf.2012.24022

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

Email: [email protected]

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

Introduction

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

G. MATUSICK ET AL.

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

estimated.

Results

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.

184

G. MATUSICK ET AL.

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-

ment.

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 .

Discussion

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

G. MATUSICK ET AL.

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.

Acknowledgements

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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