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Forest Ecology and Management 238 (2007) 347–364

Distinctiveness of wildfire effects on soil erosion in south-east

Australian eucalypt forests assessed in a global context

R.A. Shakesby a,*, P.J. Wallbrink b, S.H. Doerr a, P.M. English c, C.J. Chafer d,G.S. Humphreys e, W.H. Blake f, K.M. Tomkins e

a Department of Geography, School of the Environment and Society, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UKb CSIRO Land & Water, P.O. Box 1666, ACT 2601, Australia

c Geospatial & Earth Monitoring Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australiad Sydney Catchment Authority, P.O. Box 328, Penrith, NSW 2751, Australia

e Department of Physical Geography, Macquarie University, North Ryde, Sydney, NSW 2109, Australiaf School of Geography, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

Received 30 June 2006; received in revised form 26 October 2006; accepted 27 October 2006

Abstract

The premise of this paper is that continued improvement in the understanding of wildfire impacts on soil erosion and better prediction of

resulting hazards can be best achieved by adopting a concept of global regional variants and endemic factors that distinguish some regions in terms

of post-fire erosion characteristics. The need for such an approach is exemplified here based on the fire-prone eucalypt forests in south-east

Australia. Wildfire effects on vegetation, fauna, soil erodibility and erosion in this environment are evaluated and placed in the context of the global

state-of-the-art for forest environments. In addition to expected variation resulting from, for example, geology, topography and climate of the area,

it is argued that a distinctive post-fire behaviour is caused in these eucalypt forests by the interaction between specific characteristics of the

vegetation, litter, soil properties, faunal activity and micro-scale surface features. Soil erosion limited only by post-fire rainfall intensity and

quantity, or until bedrock is exposed, might be expected after wildfire on steep slopes in these forests given the non-cohesive character of the often

sandy soils and their universally water repellent character. That this scenario is not realised, except possibly under extreme rainfall conditions,

which rarely occur during the vulnerable post-fire period, can be attributed to a unique suite of features that disrupt or provide sinks for overland

flow, bind the loose in situ soil and trap mobilised sediment. These include mats of fine roots, litter dam–microterrace complexes and faunal activity

by small mammals and ants. In combination, these characteristics reduce post-fire hillslope-channel sediment transfer, at least under light to

moderate intensity rainfall typical of post-fire periods following recent wildfires. Evidence is discussed suggesting that the long-term

geomorphological role of wildfires in south-east Australia may be of relatively minor importance and confined largely to enhanced weathering

of exposed outcrops and redistribution of soil across existing erosional and depositional landforms. The soil fertility and downstream water quality

implications of widespread transfer of topsoil to watercourses resulting from frequent, often severe wildfires are nevertheless significant.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Wildfire; Australia; Soil erosion; Overland flow; Soil water repellency; Litter dams; Microterraces; Bioturbation; Radionuclides; Root mats

1. Introduction

Most knowledge of erosion in forests following wildfire has

been built on research carried out in the USA during the last 50

years, to which has been added in the last few decades an

increasing, though still smaller body of research from

elsewhere. Prominent amongst these other areas are the

* Corresponding author. Tel.: +44 1792 295236; fax: +44 1792 295955.

E-mail address: r.a.shakesby@swansea.ac.uk (R.A. Shakesby).

0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2006.10.029

Mediterranean and south-east Australia. It would be expected

that in the Mediterranean, where wildfires often affect

plantations of highly flammable, non-native trees located on

often stony, thin, degraded soils (e.g. Shakesby et al., 1993,

1994, 2002; Ferreira et al., 2000; Pardini et al., 2004), wildfire

patterns and post-fire erosion characteristics might differ in

some important respects from those described for many native

forests in the USA (e.g. very high fire frequency, forest stands of

different ages causing large differences in fire severity, stone

armours on thin degraded soils limiting post-fire erosion). On

the other hand, the more natural state of many eucalypt forests

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364348

in south-east Australia might logically be expected to be

characterised by essentially similar erosion patterns and

controls. There are, however, important differences. For

example, eucalypt forest soils tend to exhibit comparatively

high levels of water repellency without the intervention of fire

(e.g. Bond and Harris, 1964; Burch et al., 1989; Crockford

et al., 1991; Doerr et al., 2006; Howell et al., 2006). For most

wildfire-affected coniferous forests studied in the USA, pre-fire

water repellency is lower (Letey, 2001; MacDonald and

Huffman, 2004), or is assumed to be insignificant. In south-east

Australia, any wildfire-induced repellency impact is, therefore,

overprinted on an existing repellent substrate. Unless killed by

extreme fire conditions, post-fire recovery of eucalypt

vegetation tends to be more rapid than in native coniferous

forests. Comparison of wildfire regimes is difficult as fire

intensity, fire severity and area burnt need to be considered as

well as fire intervals (e.g. Whelan, 1995; Malamud et al., 2005).

Banks (1990), however, considered that fire frequency in dry

sclerophyll eucalypt forests in south-east Australia was

relatively high, and according to Vines (1974) a natural

recurrence interval of as little as 13 years might be appropriate.

This would place fire recurrence in these forests at the most

frequent end of the range for most other forest types worldwide

(Chandler et al., 1983). The geomorphological role of post-fire

faunal activity, rarely acknowledged outside Australia (cf.

Booker et al., 1993), is significant in terms of the supply and

downslope transfer of sediment and also possibly the provision

of infiltration routes for overland flow through the near-surface

water repellent soil (e.g. Humphreys and Mitchell, 1983;

Dragovich and Morris, 2002; Shakesby et al., 2003, 2006).

Some of the distinctive characteristics of post-fire conditions

in Australia outlined above are evident in mainstream as well as

the less easily accessible publications (e.g. Boughton, 1970;

Brown, 1972; Good, 1973; Humphreys and Craig, 1981;

O’Loughlin et al., 1982; Burgess et al., 1981; Blong et al., 1982;

Mackay and Cornish, 1982; Leitch et al., 1983; Mitchell and

Humphreys, 1987; Burch et al., 1989; Prosser, 1990; Zierholz

et al., 1995; Prosser and Williams, 1998; Dragovich and Morris,

2002; Shakesby et al., 2003, 2006; Blake et al., 2005, 2006;

Wallbrink et al., 2005; Doerr et al., 2005, 2006; Lane et al.,

2006), but the results and ideas are either often overlooked in

general reviews of wildfire impacts or they have been published

too recently for inclusion. In the case of oversight, this may

have been caused by one of the following points: the relative

inaccessibility of some of the Australian work; a focus on one

aspect of post-fire behaviour or on the implications to the

Australian environment; or the amount of research not having

reached some ‘critical mass’ such that common outcomes

became readily apparent. It is our view that such a ‘critical

mass’ has now been reached for the Sydney Basin and other

locations in south-east Australia (Fig. 1). The aim of this paper

is therefore to evaluate critically the distinctive characteristics

of post-fire patterns of erosion and their controls in these

eucalypt forests with respect to the global knowledge base

founded largely on research in the USA. We consider the

influence on post-fire erosion of: (1) changes to the vegetation,

litter and soil; (2) faunal activity; (3) hillslope resistance to

post-fire erosion; (4) the long-term significance of wildfires in

geomorphological change.

2. Eucalypt forests in south-east Australia: background

Eucalypt forests and woodlands in southern Australia

dominate areas receiving more than 600 mm annual rainfall

and are thus restricted largely to Tasmania, the south-western

corner of Western Australia and a belt a few hundred kilometres

wide in south-east Australia, which stretches inland from the

coast to the eastern uplands. Many eucalypt species produce

highly flammable bark and leaf litter, and particularly in the

drier regions of western and south-east Australia, forests are

well adapted to fire. The bulk of the current knowledge of fire

effects on soil erosion in Australia stems from work carried out

primarily in sandstone-dominated terrain of upland and coastal

New South Wales (Fig. 1). The climate is warm-temperate with

mean annual temperatures mostly between 10 and 18 8C.

Average rainfall varies from 750 to 1450 mm with large annual

variations due to ENSO (El Nino-Southern Oscillation) effects

that cause anomalous warming (El Nino) and cooling (La Nina)

in the central and eastern tropical Pacific Ocean. Wildfires in

south-east Australia are strongly linked to El Nino events that

typically deliver below average rainfall. Conversely, higher

than average rainfall is experienced in La Nina events

(Skidmore, 1987; Kiem et al., 2006). High rainfall intensities

are often associated with storms that typically last 1–4 days,

with daily totals>50 mm. Snow may occur once or twice a year

at high altitudes, but is limited and hence snowmelt and rain-on-

snow runoff generation are absent or negligible (Lane et al.,

2006) and avalanching does not occur.

Most of the published studies of post-fire effects on soils and

erosion in this region have been carried out in the Sydney Basin

(Fig. 1A), with a smaller number located in the Southern

Tablelands and farther south in New South Wales and Victoria

(Fig. 1A and B). Sydney Basin is an area dominated by near-

horizontally bedded Permo-Triassic sandstones, covering

approximately 3.6 Mha. Erosion by coastal streams has created

a landscape of deep, cliffed gorges incised by up to several

hundred metres and remnant plateaux rising to more than

1000 m. Valley widening has occurred where incision has

reached the underlying Permian mudstones. The lower slopes

are mantled in colluvium derived from mass movement

(Tomkins et al., 2004). On upper slopes and ridge crests, the

sandstone produces shallow, nutrient-poor sandy soils (Char-

man and Murphy, 1991), though on less dissected plateau

remnants deeper soils occur (Wilkinson and Humphreys, 2006).

Despite being in an area of the earliest European settlement and

its proximity now to a large urban population, there remain

large areas of this sandstone terrain under indigenous

vegetation, preserved as State Forests, National Parks and

Nature Reserves or with restricted access in water supply

catchments. In all these locations, the terrain is covered in

highly flammable, eucalypt forest and understorey vegetation.

The forest varies from woodland on drier slopes to open forest

in moister, sheltered valleys (Fisher et al., 1995) with structural

changes also reflecting species changes in the highly diverse

Fig. 1. Location map showing sites of selected published studies concerned with post-fire soil erosion and soil water repellency. (A) Studies in and around the Sydney

Basin, and (B) in south-eastern New South Wales and Victoria.

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364 349

Fig. 2. Rapid regrowth of eucalypt forest vegetation near Blue Gum Creek,

Nattai National Park 5 months after wildfire. Note epicormic shoots on eucalypt

trunks and branches and well developed regrowth of grass trees (Xanthorrhoaea

arborea), which form most of the ground vegetation in this photograph (May

2002).

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364350

Myrtaceae (e.g. Eucalyptus, Angophora, Corymbia, Leptos-

permum) and Proteaceae families, which have a wide range of

adaptations to fire. Eucalypt leaves tend to be oil-rich. Bark,

twigs and branches are readily shed and the high-lignin, low-

nutrient character of the litter favours slow decay (Adamson

et al., 1983). These characteristics lead to the rapid build-up of a

fuel load on the forest floor with a relatively shallow

fermentation layer in the soil of <2 cm. As regards fauna,

numbers of both ground-dwelling vertebrates as well as soil

invertebrates, microfauna and insects may be drastically

reduced after fire, but some seem to have important

geomorphological roles both pre- and post-fire (e.g. Adamson

et al., 1983; Humphreys and Mitchell, 1983; Dragovich and

Morris, 2002; Shakesby et al., 2006) as is discussed below.

As is the case elsewhere, wildfires in south-east Australia are

strongly linked to drought conditions. For example, in large-

scale wildfires during the particularly dry summer of 2001–

2002, some 225,000 ha of eucalypt woodland and forest were

burnt in the Sydney Basin alone (Chafer et al., 2004). Prior to

these fires, controlled, fuel-reducing burns had been conducted

around the urban fringes on a 5–7 year rotation when fuel loads

were estimated to have exceeded 8–10 t ha�1 (Morrison et al.,

1996; Jasper, 1999). In some areas, by late 2001, however, fuel

loads of >40 t ha�1 had accumulated leading to extreme fire

intensity and severity. Even where fuel loads were <5 t ha�1,

fire intensity, a function of the rate of energy produced, and

severity, a function of the physical, chemical and biological

changes at a site, were still ranked as high or very high (Chafer

et al., 2004).

3. Wildfire effects on vegetation and litter

The most vivid changes seen in any burnt forest landscape

are to the vegetation and litter cover, which may undergo partial

or complete destruction depending on fire severity (Swanson,

1981; Chandler et al., 1983; Chartres and Mucher, 1989;

Robichaud et al., 2000; Ice et al., 2004). This loss of vegetation

and litter cover temporarily reduces or stops transpiration

together with rainfall interception and storage of rainfall by the

canopy as well as by the ground vegetation and litter

(Tiedemann et al., 1979; O’Loughlin et al., 1982; Imeson

et al., 1992; Loaiciga et al., 2001; Neary et al., 2003, 2005).

Overland flow is both increased in volume and speed as a result

of the increased percentage of rainfall available and the

reduction in surface roughness caused by the removal of this

vegetation and litter (Hibbert et al., 1974; Rice, 1974; Scott and

Van Wyk, 1990; Lavee et al., 1995). The resulting exposure of

the soil leaves it prone to rainsplash detachment (Terry and

Shakesby, 1993) as well as to fluctuations in temperature and

moisture (Shakesby and Doerr, 2006). Fire also generally

leaves a highly erodible soil surface (White and Wells, 1982;

Giovannini et al., 1983; Scott et al., 1998). These changes are

widely considered to be responsible for much of the sharp

increase in erosion reported after wildfire from the USA and

elsewhere (e.g. Anderson et al., 1976; Swanson, 1981; Dıaz-

Fierros et al., 1982; Calvo Cases and Cerda Bolinches, 1994;

Shakesby et al., 1994; DeBano et al., 1998; Inbar et al., 1998;

Scott et al., 1998; Robichaud and Brown, 1999). For example,

Benavides-Solorio and MacDonald (2001), carrying out

rainfall simulations on 1 m2 plots following wildfire in the

Colorado Front Range, USA, found that percentage cover

explained 81% of the variability in sediment yield at least at this

small scale. Their work is unusual, however, in making this link

and in most studies it has either not proved possible or no

attempt has been made to separate changes in the vegetation

and litter from those of the soil (see below) in explaining the

typically observed increases in runoff and erosion even when

monitoring at manageable small scales and certainly when

considering large scales (Shakesby et al., 2000). In addition to

the protection to the soil provided by the recovering vegetation,

many studies have highlighted the importance of the fall of leaf

litter from scorched vegetation (e.g. Connaughton, 1935; Grigal

and McColl, 1975; Megahan and Molitor, 1975; Wells et al.,

1979; White and Wells, 1982; Shakesby et al., 1993; Neary

et al., 2005).

Wildfire effects on the vegetation and litter in the eucalypt

forests of south-east Australia agree in many respects with

those found elsewhere. There are, however, three important

differences: (1) the particularly rapid recovery of the

vegetation; and the roles of (2) fine roots and (3) litter dam–

microterrace complexes. In fire-prone terrain, vegetation has

developed different strategies for regeneration following fire.

Most sclerophyllous shrubs and trees are obligatory root

resprouters. Perennial herbaceous plants are facultative

resprouters, but a number of tree species, including conifers,

rely on seed germination (Naveh, 1990). Much of the eucalypt

forest vegetation tends to lead to a rapid build-up of leaf cover

after fire through regrowth from epicormic and lignotuber

shoots (e.g. Morrison and Renwick, 2000) (Fig. 2). This

regrowth gives some protection to the soil soon after burning.

For example, Blong et al. (1982) reported Xanthorrhoaea

arborea stalks resprouting just 4 days after a ‘moderately

Fig. 3. Strips of bark from eucalypt trees near Blue Gum Creek, Nattai National

Park 5 months after wildfire. Bark and leaves fallen from scorched vegetation

can quickly provide ground cover after wildfire and the bark strips can also trap

sediment effectively, as in this photograph (May 2002).

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364 351

intense’ wildfire in late 1979 on Hawkesbury Sandstone near

Narrabeen Lagoon in Sydney’s northern suburbs. Zierholz

et al. (1995) estimated that just 7 weeks after fire on the

Woronora Plateau in the Royal National Park south of Sydney,

vegetation regrowth gave a 5–25% cover, with leaf litter and

bark strips providing as much as a 10–25% cover on the soil

(Fig. 3). Prosser and Williams (1998) reported an even higher

percentage ground cover of 60% from leaves fallen from the

scorched eucalypt canopy. Considering the longer term, Blong

et al. (1982) found a ground vegetation cover of 20–55% on

three 8 m2 erosion plots 1 year after fire. In addition, trapping

of seeds behind litter dams (see below) potentially provides

conditions for rapid regrowth of ground cover vegetation,

which is important for reducing post-fire erosion. Although

Mitchell and Humphreys (1987), working at sites in the

Woronora and Hornsby Plateaux in the Sydney Basin, found

that such growth provided only 6% cover at the end of their

113-day monitoring period, the low figure was attributable to

the particularly dry post-fire conditions. Prosser and Williams

(1998) similarly noted a slow recovery of ground vegetation

25 km south-west of Sydney following wildfires in 1994: the

vegetation cover reached less than 20% after 100 days. Under

wetter conditions, Mitchell and Humphreys (1987) argued,

the trapped seed bank could be expected to provide a

comparatively rapidly-forming ground cover, supplementing

and then replacing, the protective role initially provided by

litter dams and microterraces under light to moderate rainfall

intensities.

Generally, in forest areas, it is only the large roots of trees in

post-fire situations that are seen as in any way significant from a

geomorphological point of view. They anchor soil to prevent

shallow landslides, unless or until decay weakens or destroys

them (e.g. McNabb and Swanson, 1990; Meyer et al., 2001).

Both living and burnt-out large roots are also implicated in

terms of providing by-pass routes through water-repellent

layers for potentially erosive overland flow (e.g. Imeson et al.,

1992; Ferreira et al., 1997, 2000). DeBano et al. (2005)

considered that provided the soil is dry, and therefore a poor

conductor of heat, roots deeper than 2 cm in the soil are unlikely

to be damaged by fire, but any geomorphological role of fine

roots in this position has not been identified outside of

Australia. Although Prosser (1990), working some 100 km

south of Canberra, considered that severe fire would destroy

root mats, investigations of dense proteoid root mats of Banksia

serrata L.f. on Hawkesbury Sandstone in the Sydney Basin

have indicated that they may be very effective in reducing post-

fire erosion. Gould (1998), working in a newly-burnt bushland

reserve near Lane Cove National Park north of Sydney, found

that subsurface roots were widespread, covering up to 39% of

surfaces in the 1 m2 plots used. The roots survived close to the

surface immediately below the upper burnt layer and helped to

protect the sandy subsoil from surface wash and from rill and

gully development despite the highly water repellent nature of

the mat. Even when killed by the fire, Gould (1998) found that

the root mats remained strong and provided cohesion for the

soil in many locations until a new network developed. Similar

reports of post-fire survival of roots were also made for

sandstone soils on the Hornsby Plateau north of Sydney by

Koop (2000) and in the Royal National Park, south of Sydney

by Zierholz et al. (1995). The latter authors found that erosion

of the weakly cohesive sandy subsoil was restricted largely to

fire tracks and walking trails where roots were absent or had

been partially destroyed by the fire.

One of the site types investigated by Gould (1998) for

proteoid root mat characteristics was the litter dam–micro-

terrace complex commonly seen in burnt terrain on low-angled

sandstone terrain in the Sydney Basin. These complexes form

on slopes <108 where surface wash is widespread (Bishop

et al., 1980; Mitchell and Humphreys, 1987; Eddy et al., 1999;

Howell et al., 2006). Although similar features have been noted

elsewhere (e.g. Schiff and Yoder, 1941; Gayel and Plachinda,

1958; Emmett, 1978; Dıaz-Fierros et al., 1994), their

geomorphological role has not been evaluated and only in

eucalypt forests of south-east Australia has an important role as

regards limiting soil erosion been highlighted. These features

develop around surface roughness elements such as the upright

scorched stems of grasses and other plants. Such obstacles can

very effectively trap elongated leaves and twigs fallen from the

scorched tree canopy after wildfire and other organic litter all of

which is usually highly susceptible to transport by surface

wash. During rain, an ephemeral pond forms on the upslope

side of the litter dam walls, which are aligned approximately

along the contour, and this pond fills with transported sediment

to form a microterrace. Litter dams measured by Mitchell and

Humphreys (1987) at sites on the Hornsby and Woronora

Plateaux were found to be up to 12 m long, with heights of 2–

5 cm on their downslope sides and they were spaced at about

0.2–2.0 m intervals (Fig. 4). These authors reported the features

being stable for several months and that even though the total

amount of sediment mobilised during a storm could be large,

much of it was trapped at the next downslope dam. Stabilisation

was helped by the growth of an algal mat, which developed on

all bare surfaces by the end of the monitoring period (113 days

after fire). Subsequently, it has been shown that in some cases

these systems are still traceable after several years (Humphreys,

Fig. 4. Litter dam, Nattai National Park (May 2002). Credit card at right-centre

of the photograph for scale.

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364352

1994a; Koop, 2000). The potential importance of these

sediment sinks in restricting post-fire hillslope erosion in

similar terrain has been noted by other researchers studying soil

erosion in south-east Australia (e.g. Adamson et al., 1983;

Zierholz et al., 1995; Shakesby et al., 2003, 2006) in addition to

observations made by Mitchell and Humphreys (1987). Root

mats may also help to stabilise other micro-geomorphological

features as well as litter dams (e.g. boulder dams, ephemeral

drainage lines, rock overhangs; Gould, 1998).

4. Wildfire effects on soils

Both the erodibility of a soil and its effect on infiltration have

been found to be affected by wildfire. Although some

researchers have noted that aggregation of fine particles,

protection of aggregates with hydrophobic compounds and

fusing of minerals can occur (Giovannini et al., 1987;

Ketterings et al., 2000; Andreu et al., 2001; Wondzell and

King, 2003; Mataix-Solera and Doerr, 2004; Blake et al., 2005),

there is usually a decrease in both soil organic matter

(Fernandez et al., 1997; Litton and Santelices, 2003;

Gonzalez-Perez et al., 2004) and aggregate stability (e.g.

McNabb and Swanson, 1990; Neary et al., 1999; Badıa and

Martı, 2003; Ice et al., 2004). The sealing of soil through pores

becoming clogged with ash and freed clay minerals (Wells

et al., 1979; Durgin and Vogelsang, 1984; Mallik et al., 1984;

Mills and Fey, 2004) and the collapse of organo-mineral

aggregates (Giovannini et al., 1988) have also been reported.

The most often researched change to the soil after fire, which

has potentially important hydrological and erosional conse-

quences, however, is its wettability. Fire-induced or enhanced

soil water repellency has often been cited as one of, if not the,

major cause of post-fire enhanced runoff and erosion at both

small and large scales (e.g. Sartz, 1953; DeBano et al., 1970,

1977, 1998; Swanson, 1981; Morris and Moses, 1987; Shahlaee

et al., 1991; Scott and Van Wyk, 1990; Inbar et al., 1998).

Repellency in most unburnt areas is usually considered to be

weak and discontinuous, and geomorphologically-effective

repellency in densely vegetated environments is only present

after wildfire (Huffman et al., 2001). Fire-induced repellency is

thought to be caused by: (1) volatilisation and subsequent

condensation onto soil particles of certain organic compounds

(DeBano and Krammes, 1966); (2) polymerisation of organic

molecules to more repellent versions (Giovannini et al., 1983);

(3) improved bonding of such molecules to soil particles

(Savage, 1974); and (4) the melting and redistribution of waxes

onto soil aggregates and mineral grains (Franco et al., 2000).

Laboratory studies have shown that repellency responds in a

complex way to the temperatures reached and to the duration of

the heating. DeBano and Krammes (1966) were the first to

show this complexity. They found that when slightly repellent

Californian chaparral soil was heated for 5–20 min, repellency

remained essentially unaltered at temperatures <175 8C,

became enhanced at 175–200 8C and destroyed at 250–

300 8C, depending on heating duration. Although the changes

reported at lower temperatures are now known to vary,

destruction of repellency at 250–400 8C has been firmly

established for coniferous and eucalypt forest soils in diverse

locations (Savage, 1974; Scholl, 1975; DeBano et al., 1976;

Robichaud and Hungerford, 2000; Garcıa-Corona et al., 2004),

including sandy soil from south-east Australian eucalypt forests

(Doerr et al., 2004), provided sufficient oxygen is available for

oxidation of the hydrophobic compounds (Bryant et al., 2005).

Studies of different forest soils have shown that where

conditions are particularly conducive to high soil temperatures,

as in chaparral scrub in the western USA (DeBano et al., 1979),

post-fire repellency tends to be induced or markedly enhanced

in a subsurface layer overlain by wettable (i.e. non-repellent)

surface soil up to 5 cm deep (Fig. 5). In most other forest types

reported in the literature, however, repellency is consistently

induced or enhanced at the soil surface when exposed to high

fire severity conditions (e.g. Reeder and Jurgensen, 1979;

Huffman et al., 2001; MacDonald and Huffman, 2004).

Repellency is also thought spatially to become increasingly

more homogeneous with increasing fire severity. That water

repellency becomes induced or enhanced by fire has become a

widely accepted axiom, such that where detailed measurements

are not actually carried out or even where there is evidence of

pre-fire levels of repellency matching post-fire ones, there is

often a reluctance to view repellency as being produced other

than by fire. Research during the last 10 years has

demonstrated, however, that repellency can be just as severe

in long unburnt as in adjacent burnt forest terrain (Doerr et al.,

1998), and, moreover, that long unburnt eucalypt soils exhibit

some of the highest levels of repellency recorded worldwide

(Burch et al., 1989; Crockford et al., 1991; Shakesby et al.,

1993; Doerr et al., 1998; Scott, 1993, 2000). Studies continue to

show that repellency may indeed be intensified following fire in

certain forest types and situations, but post-fire intensification

of repellency in some terrains may be more apparent than real.

In the light of work on eucalypt and pine plantations both before

and after fire in Portugal, Shakesby et al. (2000) concluded that

repellency in unburnt terrain was essentially a hydrologically

and geomorphologically ‘dormant’ characteristic, which only

became ‘activated’ after fire because of a range of other

Fig. 5. Soil water repellency changes following fire of moderate or high severity for chaparral (A), typical coniferous forest (B) and eucalypt forest in south-east

Australia (C). Darker shading denotes more intense repellency.

Fig. 6. Water droplet on highly water-repellent subsurface sediment, Nattai

National Park (May 2002).

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364 353

changes (e.g. litter and vegetation destruction, reduction in soil

aggregate stability) that enabled repellency to have an impact.

Researchers have reported high water repellency in eucalypt

forest soils on different lithologies in south-east Australia both

before and after fire for a range of soil textures (e.g. O’Loughlin

et al., 1982; Mitchell and Humphreys, 1987; Burch et al., 1989;

Crockford et al., 1991; Zierholz et al., 1995; Prosser and

Williams, 1998; Doerr et al., 2004; Howell et al., 2006). They

show that repellency is widespread and usually attains high

intensities not only in sandy and sandy loam soils but even in

sandy clay loams with up to 40% clay from south-east New

South Wales in the Upper Yass Representative Basin

(Crockford et al., 1991; Fig. 1). Shakesby et al. (2003)

presented preliminary field measurements and Doerr et al.

(2006) detailed laboratory results of surface and subsurface

repellency characteristics on eucalypt sandstone soils in Nattai

National Park in the Sydney Basin, an area which was burnt in

late 2001, and in the Cataract River basin some 20 km north-

west of Wollongong, burnt in early 2003. Laboratory tests of

repellency levels showed that in neighbouring long-unburnt

locations, repellency persistence (i.e. the period that repellency

can be sustained beneath a water droplet) was dominantly

severe to extreme at the surface but only slight to moderate

below it. At the burnt sites, in contrast, repellency at the surface

was mainly absent, but generally intensified 0.5–5 cm below

the surface (Fig. 6). Given the established temperature

thresholds of 250–400 8C for repellency destruction discussed

above, these temperatures must have been reached in surface

soil at many of the burnt sites investigated by Doerr et al. (2006)

in the Sydney Basin. This view is supported by evidence from

some locations in the region of high temperatures being reached

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364354

(e.g. fire-induced spalling of exposed sandstone rock, red-

dening of soils below charred logs; Humphreys et al., 2003;

Shakesby et al., 2003) and observations by us of melting or

drooping of aluminium stakes. Soils in chaparral scrub are

similarly subject to ‘hot’ fire temperatures, which can lead to

high levels of water repellency in a sub-surface layer (Fig. 5). In

long-unburnt chaparral soil, repellency is present near the

surface, but unlike the eucalypt soils, it is usually relatively

weak. For example, Brock and DeBano (1990), working in

Arizona, reported an average water drop penetration time

(WDPT) of 70 s for the upper 2 cm of soil in unburnt terrain,

which according to a verbal description of repellency rates as

‘strong’, compared with predominantly ‘severe’ to ‘extreme’ in

long-unburnt eucalypt forests of south-east Australia (Doerr

et al., 2006). These results contrast with most others from burnt

forest soils outside Australia where surface repellency is

generally reported as being enhanced by high fire severity (e.g.

Reeder and Jurgensen, 1979; Huffman et al., 2001; Wohlge-

muth et al., 2001; MacDonald and Huffman, 2004).

While it is possible that a much higher temperature than

400 8C is needed to destroy repellency where oxygen levels

become depleted, possibly explaining the lack of destruction by

fires of high severity (Bryant et al., 2005), there are two other

more prosaic explanations for the difference between the

Australian results and those reported from forests elsewhere,

both of which highlight methodological problems. Following

fire, soil surfaces are covered in varying thicknesses of ash and

charred woody material, all of which is wettable. A standard

definition of the soil ‘surface’ under such circumstances,

therefore, becomes critical for measuring surface water

repellency, yet this can be difficult to define rigorously in all

circumstances. Different surface repellency results could

simply reflect the varying degrees to which the wettable

surface material is removed prior to carrying out measure-

ments. This is most likely to cause discrepancies where there is

a gradual rather than sharp junction between minerogenic and

organic material. A second reason is that a typical fire severity

classification based on the degree of destruction of the

vegetation does not seem to be a good proxy for soil heating

characteristics during the fire (e.g. Hungerford, 1996; Doerr

et al., 2004). There have been recent attempts to develop

classification systems focusing on soil and litter in addition to

vegetation effects (e.g. USDA, 2000; Ice et al., 2004), but these

link fire-induced surface water repellency to high rather than

low severity fires, which is clearly inapplicable to the situation

in south-eastern Australian eucalypt forests, as indeed as it is

also to chaparral scrub and, in addition, possibly to some forest

environments in North America.

As regards the longevity of changes to soil wettability

following fire, surprisingly little is known. For North American

pine stands, Dyrness (1976) reported that fire-induced water

repellency persisted for 6 years after a severe fire in an Oregon

pine forest whereas Huffmann et al. (2001) found that fire-

induced repellency persisted for at least 22 months following a

severe fire in Colorado. In contrast, examining the effects of a

severe fire in eastern Spain, Cerda and Doerr (2005) reported

that water repellency, which had been destroyed during

burning, returned within 3 years under pine vegetation. In

Nattai National Park, Sydney Basin, Doerr et al. (2006) found

that, 2 years after burning, repellency showed little sign of

recovery where it had been destroyed, but a slight reduction was

observed in surface and subsurface soils where it had been

enhanced. It was concluded that the fire-induced changes may

have been too severe to allow recovery to pre-fire soil

conditions in only 2 years. Other authors (e.g. Leitch et al.,

1983; Prosser and Williams, 1998) have reported continued soil

water repellency after fire but there are no other published

measurements of change in repellency through time in burnt

and long unburnt soils in south-east Australia known to the

authors.

5. Wildfire effects on fauna and its geomorphological

implications

Despite symposia and reviews focusing on ecological

aspects of fire (e.g. Mooney et al., 1981; Goldammer and

Jenkins, 1990; Brown and Smith, 2000), they have concerned

the effects of fire on vegetation and/or on its recovery with no

reference in these cases to fauna. Attention given to fauna has

usually emphasised the degree of destruction and subsequent

recovery of soil micro-organisms and invertebrates caused by

fire (e.g. DeBano et al., 1998; Prieto-Fernandez et al., 1998;

Busse and DeBano, 2005; Certini, 2005) with occasional

reference to macro-fauna, similarly stressing the substantial

reductions in the numbers of, for example, birds, mammals,

amphibians, reptiles, insects and aquatic biota (e.g. Campbell

et al., 1977; Smith, 2000; Bowman and Boggs, 2006). Almost

with no exception, reviews of the geomorphological con-

sequences of fire make no reference to any faunal impact on

post-fire hydrology and geomorphology (see, for example,

Tiedemann et al., 1979; Swanson, 1981; Wells et al., 1987;

McNabb and Swanson, 1990; Robichaud et al., 2000; Neary

et al., 2005). Indeed only one study known to the authors

outside of Australia proposes any post-fire biogeomorpholo-

gical role for macro-fauna (Booker et al., 1993). This omission

is not surprising when the state of knowledge is developed

largely from North American experience and, to a lesser extent

from other areas including particularly Mediterranean fire-

prone forests, where identifiable post-fire faunal effects are less

apparent. In contrast, in the eucalypt forests of south-east

Australia, faunal activity can have appreciable hydrological and

geomorphological effects with an impact on soil erosion even at

the hillslope scale.

Following widespread moderate or severe fire in eucalypt

forests, numbers of many ground-dwelling vertebrates are

substantially reduced (Newsome et al., 1975) as are those of the

usually abundant lyrebirds, capable of considerable localised

disturbance to unburnt forest floors (Adamson et al., 1983;

Humphreys and Mitchell, 1983; Humphreys, 1985; Heimsath

et al., 2001). Low numbers of smaller mammals, notably

bandicoots, are present following fire and disturb the forest

floor by excavating shallow depressions (Dragovich and

Morris, 2002). The most important faunal influence on

hydrology and soil erosion in sandy to loamy eucalypt soils,

Fig. 7. Nests of the ant species Aphaenogaster longiceps in a footslope zone

near Blue Gum Creek, Nattai National Park 5 months after wildfire. This species

is particularly active on low-angled slopes affected by moderately severe fire

(May 2002).

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364 355

however, seems to be associated with the activities of ants,

especially the funnel ant (Aphaenogaster longiceps) (Hum-

phreys, 1981, 1994b; Humphreys and Mitchell, 1983). This

species produces mounds 5–10 cm high and 20–30 cm in

diameter (Humphreys, 1981) surrounding vertical tunnels up to

4 cm wide and 30 cm deep, which lead to extensive lateral

gallery systems (Fig. 7). Such ants can not only survive in the

deep tunnel systems since even extreme temperatures reached

beneath piles of smouldering logs may not induce lethal

temperatures deeper than 22 cm in the soil (Roberts, 1965), but

also their mounding activity generally increases following fires

of moderate severity despite reduced populations (Andersen

and Yen, 1985; Dragovich and Morris, 2002). Only 2 weeks

after the 1994 fires south of Sydney, ant mounds reportedly

covered 5% of the entire soil surface (Prosser and Williams,

1998). Their impact on sediment supply has been documented.

Humphreys and Mitchell (1983) reported that quantities of

surface mounding, mainly by ants, generally exceeded the

amount of sediment moved by slopewash by 2–30 times. This

view was supported by the work of Dragovich and Morris

(2002) who installed small erosion plots on sandstone hillslopes

in eucalypt forest at Faulconbridge in the Blue Mountains east

of Sydney. They found not only that ants transferred sediment

to the surface, which was available for subsequent removal by

surface wash, but they also moved large quantities downslope in

their own right (so-called ‘bio-transfer’). In fact, the authors

attributed 36% of the total sediment collected on their plots to

this bio-transfer process, and found that it was ten times more

effective on terrain affected by moderate fire severity than on

terrain affected by either low or high severity fire. Moreover,

more than 90% of this bio-transferred sediment was contributed

by ant mounding with only 9% represented by small mammal

scrapings.

In Nattai National Park south-west of Sydney, following the

2001–2002 fires, Shakesby et al. (2006) found a contrasting,

though nonetheless important post-fire role for ants and

mammals. Rates of bioturbation (i.e. the vertical and lateral

redistribution of sediment) mainly by small mammal and ant

activity were monitored on plots throughout February 2003, a

little over 1 year after fire. The results indicated extrapolated

annual rates of 0.5–7.8 t ha�1 (with 71% of bioturbation

attributed to shallow burrowing by small mammals) compared

with net soil losses estimated from ground-level changes during

the second year after fire of ca. 4–46 t ha�1. Although both sets

of figures are approximate and concern relatively few small

plots and transects, respectively, they nevertheless suggest a

more minor role in supplying sediment for downslope sediment

transfer or in any direct bio-transfer than previously proposed,

at least during the second year after fire. Instead, ant mounding

may actually have been more important in limiting rather than

in promoting post-fire erosion (see below).

6. Hillslope resistance to post-fire erosion

In addition to factors such as geology, soil type and slope,

measured post-fire hillslope erosion is related to the magnitude

of the disturbance by fire (as discussed earlier), to the length of

the recovery time (or ‘window of disturbance’; Prosser and

Williams, 1998), to the magnitude of rainfall events and

resulting overland flow, and to the scale of measurement

(Shakesby and Doerr, 2006). Within the window of disturbance,

the post-fire soil erosion pattern takes the form of a sharp peak,

corresponding to early large rainfall events when the protective

cover is minimal and large quantities of highly erodible ash and

soil are available for removal (Swanson, 1981). This early peak

is followed by a decline over subsequent weeks, months or

years until levels similar to those typical of pre-fire conditions

are restored (e.g. Helvey, 1980; Robichaud and Waldrop, 1994;

Shakesby et al., 1994; DeBano et al., 1996; Inbar et al., 1998).

The accepted explanation for the shape of this curve is that soil

erosion is initially transport-limited while the protective cover

is least and large quantities of fine, easily removed material are

available for entrainment (Moody and Martin, 2001). Through

time, the system becomes increasingly supply-limited with

coarse, less easily entrained sediment being left behind (Morris

and Moses, 1987; Thomas et al., 1999). The surface becomes

better protected by an increasing cover, first, of post-fire

litterfall from scorched trees and bushes in areas affected by

less than extreme fire severity, second, of resprouting and

regenerating vegetation (Connaughton, 1935; Grigal and

McColl, 1975; Megahan and Molitor, 1975; Wells et al.,

1979; White and Wells, 1982; Shakesby et al., 1993) and, third,

in some locations, of surface stones forming an armoured layer

(e.g. Morris and Moses, 1987).

In the light of this evidence, south-east Australian eucalypt

forests in sandstone terrain might be expected to be subject to

catastrophic post-fire soil loss on slopes following widespread

destruction of the litter and vegetation cover by wildfire even

with moderate rainfall in view of the following characteristics.

Certainly, data available on post-wildfire soil erosion amounts

at plot- and catchment scales in Table 1 suggest that soil losses

for south-east Australia are similarly high to, or higher than, the

relatively few data available from elsewhere, with single

intense storms able to produce considerable erosion even when

measured at the catchment scale. Observations suggest that

Table 1

Selected plot-scale soil losses and catchment-scale sediment yields for part or the whole of the first year after wildfire from locations in south-east Austra a (see Fig. 1 for locations) and the rest of the world

Location Rock type Tree type(s) Rainfalla (mm) Soil loss/sediment

yield (t ha�1 year�1)

easurement details Author(s)

(A) Plot-scale (Australia)

Nr. Narrabeen Lagoon, N.

Sydney, NSW

Sandstone Eucalypt 736 2.5–8.0 m2 plots Blong et al. (1982)

Wangrah Creek, Southern

Tablelands, NSW

Metamorphosed sandstone

and siltstone

Eucalypt 152b 32 m2 plots Prosser (1990)

Grays Pt, Royal National

Park, NSW

Sandstone Eucalypt 320 (including two

1-in-10-year events)

30–48 m2 plots Atkinson (1984)

Nr. Warburton, Central

Highlands, Victoria

Metamorphic sandstone

and siltstone

Eucalypt 17c (with 50 mm h�1 intensity) 22 stimated areas of

on-eroded, eroded and

eposited material

Leitch et al. (1983)

(B) Plot-scale (rest of the world)

Galicia, Spain Granite, schist, quartzite

and amphibolite

Pine plantation 1400 15–170 0 m2 plots Dıaz-Fierros et al.

(1982, 1987)

Western Cape Province,

South Africa

Mainly sandstone with tillites,

shale and quartzite

Pine plantation ca. 1500 10–26 4 m2 plots Scott and Van Wyk (1990),

Scott et al. (1998)

San Gabriel Mts,

California, USA

Mostly Mesozoic granite with

Precambrian igneous and

metamorphic rocks

Chaparral scrub 559–773 19–197 6 m2 and 80 m2 plots Krammes and Osborn

(1969), Wells (1981)

(C) Catchment scale (Australia)

Tumut Valley, Snowy Mts, NSW Mainly metamorphic sandstone

and siltstone

Eucalypt forest – open

alpine woodland

16 (in 2 h) 0.25d 8 km2 catchment Brown (1972)

Slippery Rock Creek, Victoria Gneiss, quartz diorite and

granodiorite

Eucalypt woodland 1825 2.96 .36 km2 catchment Lane et al. (2006)

(D) Catchment scale (rest of the world)

Rattle Burn, Arizona, USA Limestone Ponderosa pine 737 4.3e .081 km2 catchment Campbell et al. (1977)

Washington, USA Granodiorite and quartz diorite Mixed conifer 580 0.12 .514 km2 catchment Helvey (1980)

a Annual rainfall unless otherwise indicated.b Rainfall over ca. 3 months.c Single rainfall event.d Sediment yield in a single 16 mm rainfall event.e Sediment yield measured during 6 months.

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R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364 357

much of the early soil loss in south-east Australia is in the form

of ash, charcoal and burnt soil (e.g. Leitch et al., 1983;

Atkinson, 1984). Over much of the Sydney Basin, the

subsurface soil is sandy, usually loose-textured and therefore

very erodible and highly water repellent, so that considerable

erosion would be anticipated through enhanced rainsplash

detachment (Terry and Shakesby, 1993) and increased amount

and speed of delivery of overland flow to stream channels (e.g.

Hibbert et al., 1974; Rice, 1974; Scott and Van Wyk, 1990).

Aggregate stability in the topsoil is generally reduced and up to

several centimetres of easily eroded ash and charred debris

remain on the surface (Atkinson, 1984; Zierholz et al., 1995).

Various sources report widespread loss of the burnt topsoil

material during early rainfall events after wildfire (e.g. Leitch

et al., 1983; Atkinson, 1984; Zierholz et al., 1995; Shakesby

et al., 2003, 2006) delayed only where rainfall is insufficient in

quantity or intensity to exceed the storage capacity of the

surface ash, charcoal and wettable soil layer (Doerr et al.,

2006). Gauging the quantity of burnt topsoil removed soon after

fire has proved problematic because of inevitable difficulties of

monitoring. Leitch et al. (1983, p. 177), studying the effect of

the Ash Wednesday wildfires in eucalypt forests in Victoria,

considered that ‘‘about 50% of the ash and loose soil was

transported from the catchment’’ during a single intense

thunderstorm 6 days after the fire, which amounted to the

estimated loss of 22 t ha�1 shown in Table 1. Shakesby et al.

(2003) estimated that potential topsoil loss represented a total

of 140 t ha�1 assuming a thickness of 2 cm and bulk density of

1.0 g cm�3, though the amount exported from the hillslopes

would have been inevitably less because of probable thinner

topsoil on steep slopes, some convective loss of organic matter

during the fire and topsoil retention on the slopes. Nevertheless,

most studies report widespread hillslope-channel transfer of

this burnt topsoil, ash and charred debris (e.g. Leitch et al.,

1983; Atkinson, 1984; Shakesby et al., 2003; Blake et al., 2005,

2006).

The underlying sandy subsurface material in sandstone

terrain of south-east Australia is also highly erodible. Although

the soil is stony, the cover is not usually complete (cf. Adamson

et al., 1983). The cover is no more than ca. 30–40%

(discounting bedrock outcrops) according to Shakesby et al.

(2003) for their Nattai National Park study site. Substantial

erosion would therefore be anticipated even under moderate

post-fire rainfall intensities. Much of the scorched topsoil, ash

and charred debris was mobilised during early storms after fire,

reaching channels under low-intensity as well as moderate-

intensity rainfall conditions. Amounts of sandy, highly water

repellent material eroded from hillslopes, however, have

reportedly remained low, except under particularly high rainfall

intensities (Atkinson, 1984; Zierholz et al., 1995; Shakesby

et al., 2003). Shakesby et al. (2003, 2006) reported for Nattai

National Park clear evidence of post-fire detachment and

redistribution of the sandy subsurface from the evidence of: (1)

observations 5 months after fire of stone-capped pedestals up to

several centimetres high formed of this material, which

represents ca. 50–100 t ha�1 assuming conservatively a soil

bulk density of 1.0 g cm�3 and (2) patches of newly-deposited

sandy material, particularly in footslope zones. Confirmation of

the relatively limited export of this sandy material from the

hillslopes was provided by two independent pieces of evidence.

First, about 5 months after the fire only relatively small

quantities of freshly-deposited sandy material were found in

drainage ditches on the upslope side of a forest road cut into the

base of the subcatchment footslopes and across gullies. Second,7Be, 210Pbex and 137Cs budgets confirm widespread loss from

the hillslopes of topsoil but only small losses of the sandy

subsurface soil (English et al., 2005; Wallbrink et al., 2005).

This interpretation is based on the estimated high export from

the hillslopes of the 210Pbex and 7Be tracers, which tend to be

preferentially retained in the uppermost, organic-rich 20 mm of

the soil profile (Wallbrink and Murray, 1996). This contrasts

with negligible loss of 137Cs, which tends to lie deeper in the

soil. In addition, the tracer budgets show that much of the

topsoil material was derived from the upper slope and that high

proportions of mobilised sediment were retained on footslopes.

Factors regarded as critical in explaining the limited

hillslope-channel transfer of the sandy material determined

in these Sydney Basin studies have already been described.

They include, first, the litter dam–microterrace complexes on

low-angled slopes. With reported thicknesses of 1–3 cm of

trapped sediment (Mitchell and Humphreys, 1987), these

micro-forms can preferentially retain large quantities of the

sandy material while allowing much of the lighter organic-rich

material to move downslope or add to the dam barrier

(Adamson et al., 1983; Mitchell and Humphreys, 1987;

Humphreys, 1994a,b; Howell et al., 2006). Second, the root

mats, even where killed but not destroyed by the fire, remain

strong and can help to bind the loose, sandy subsurface material

(Zierholz et al., 1995). Third, the proliferation of ants’ nests

after fire may act in some cases to promote the downslope

transfer of soil, but can also limit it by increasing surface

roughness and by providing by-pass drainage routes for

overland flow to reach wettable soil below the water repellent

layer. A simple field test, involving rapidly pouring several

litres of water into a nest entrance and subsequently excavating

the nest, indicated that the water rapidly reached depths of at

least 50 cm, well below the repellent soil layer. Significantly,

ants’ nests tend to be common in footslope zones where,

according to the radionuclide and mineral magnetic evidence

from Nattai National Park (English et al., 2005; Wallbrink et al.,

2005; Blake et al., 2006), much of the mobilised sandy

sediment from upslope seems to have been deposited in large

quantities. This is indicated by the greater thickness of deposits

here (>2 m) compared with farther upslope (<0.5 m) (Tomkins

et al., 2004).

This range of erosionally protective features appears to limit

the export of soil from the hillslopes to the channels in these

eucalypt forests, at least while rainfall intensity and quantity

remain below some unknown threshold. Atkinson (1984),

studying the aftermath of wildfires on sandstone soil in the

Royal National Park south of Sydney in January 1983, reported

widespread losses of organic-rich topsoil but limited erosion of

sandy subsurface soil during brief events of relatively intense

rainfall. The total rainfall (16.5 mm) was modest but occurred

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364358

in only 45 min (equivalent to 22 mm h�1). However, later

during 1983, some 120 mm of rain fell followed 3 days later by

>200 mm in only 31 h, both representing 1-in-10-year storm

events according to Atkinson (1984). These events led to litter

dams being breached and the sandy soil reportedly ‘‘flowed like

water’’ (p. 7). Atkinson observed large quantities of sand not

only reaching the streams but also being exported from the

catchment. The extent to which this significant change in the

severity of erosion with increased rainfall follows a smooth

upward trend or undergoes a sudden increase beyond some

rainfall threshold, and its comparison with post-wildfire

scenarios outside of Australia, are unclear. What is evident

is that understanding the relationship between post-fire rainfall

characteristics and soil losses is critical in mitigating the

damaging on-site and off-site effects of erosion.

7. Long-term geomorphological significance of wildfire

Not surprisingly, given both the number of studies and limited

geographical coverage, knowledge about the geomorphological

impacts of wildfire over long time spans is sparse relative to data

collected on the short-term impacts of recent individual fires

(Shakesby and Doerr, 2006). Nevertheless, the consensus

reached for a number of forest environments is that wildfire

has been an important, sometimes the most important

disturbance event bringing about geomorphological change

during historical and prehistoric times (e.g. Morris and Moses,

1987; DeBano et al., 2005). A distinction can also be made

between our understanding of the contribution of long-term fire-

induced erosion occurring over centennial and millennial

compared with decadal timescales, Arguably, albeit still weak,

our understanding is actually better for the former two longer

periods in some areas than for the decadal scale. The latter scale is

affected by both the logistical difficulties of monitoring events

with recurrence intervals measured on the scale of a human

lifetime, and the changing frequency and severity of wildfires in

recent decades in response to changes in forest management (cf.

Kirchner et al., 2001), and also possibly to changes in climate

(Meyer et al., 1992; Whitlock et al., 2003; Pierce et al., 2004).

In one of the few detailed studies of long-term effects of

wildfire on landforms and geomorphological processes, Meyer

et al. (1992, 1995) considered that it had a considerable impact

in north-east Yellowstone Park during the late Holocene. They

found that fire-related sedimentation associated with high fire

severity in cool, high elevation coniferous forests had occurred

approximately every 300–450 years and that fire-related debris

flow sediments contributed ca. 30% of the volume of alluvial

fan sediment during the Holocene, with much of the remaining

amount probably also attributable to fire. In the relatively warm,

low-elevation South Fork Payette River area, central Idaho,

Meyer and Pierce (2003) found more frequent, fire-related

events, particularly during the Medieval Warm Period (or, as it

is better termed, the Medieval Climatic Anomaly (10,050–650

cal. year BP)) and Pierce et al. (2004, p. 89) considered that

‘‘�50% of the total measured thickness of fan sediments

deposited over the last 2000 years in this area is probably or

possibly related to fire’’. Millspaugh et al. (2000) found for

central Yellowstone National Park that the fire recurrence

interval had been lower (every 100 years) in the earlier part of

the Holocene, but in the last 2000 years it had more or less

matched that suggested by Meyer et al. (1992, 1995). Moody

and Martin (2001), noting the discrepancy between relatively

short soil erosion recovery times compared with fire frequency,

stressed the length of residence time of fire-related redistributed

sediment within catchments in the Colorado Front Range. They

estimated that refilling times for sediment sinks might be as

much as 1000–10,000 years.

The decadal-scale geomorphological impact of fire at

hillslope and small catchment scales in contrast to the longer

term has been based largely on estimation. Published estimates

vary, but usually suggest a substantial contribution by wildfire.

For example, Swanson (1981) estimated that major forest

disturbance in a small catchment in the western Oregon Cascades

could account for as much as 25% of the long-term sediment

yield and DeBano et al. (2005) argued for an even higher

contribution, suggesting that perhaps>60% of the total sediment

production in certain fire-prone areas might be fire-related.

Evidence of longer term geomorphological impacts of

wildfire in south-east Australia is very limited, so that judging

its significance is difficult. Nevertheless, recent opinion is that

its role may be small. Prosser (1990), considering fire impacts

in the Southern Tablelands ca.100 km south of Canberra

(Fig. 1), reported no increase in the rate of denudation or

alluviation in response to the increase in fire frequency

beginning 3000–4000 years ago as a result of intensified

burning by Aborigines. Similarly, Dodson and Mooney (2002)

considered that the late Holocene in south-east Australia had

been characterised by sedimentary regimes in relative

equilibrium with Aboriginal fire management. In a study

based on a range of evidence in sandstone eucalypt forests in

the south-west Sydney Basin, Tomkins et al. (in press) compare

long-term erosion estimates (derived from cosmogenic radio-

nuclides, apatite fission-track thermochronology and post-

basalt flow valley incision) with soil erosion rates on hillslopes

after the 2001–2002 fires based on ground-surface change

measurements and radionuclide data presented by Shakesby

et al. (2003, 2006) and Wallbrink et al. (2005), respectively.

They also include river sediment load data following wildfires

in 1968 and 2001–2002. They estimate that the contribution of

recent wildfire events accounts for only a very small proportion

(<0.5 mm kyear�1, or ca. 5%) of the contemporary estimated

rate of denudation of 5.5 � 4 mm kyear�1. These figures are

well below the 21.5 � 7 mm kyear�1 averaged from the erosion

estimates indicated above relating to the long-term (10 kyear to

10 Myear), which are broadly in line with rates determined

from cosmogenic nuclide evidence elsewhere in south-east

Australia (Heimsath et al., 2001; Wilkinson and Humphreys,

2005). Tomkins et al. infer that the contemporary geomorpho-

logical role of wildfires in hillslope-channel sediment transfer

in this location is largely one of reworking and redistributing

sediment on the hillslopes. More important in explaining long-

term denudation on this passive plate margin, they suggest, are

infrequent catastrophic floods and mass movement events

unrelated to wildfire.

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364 359

At the decadal scale, wildfires in south-east Australia often

occur during low-rainfall conditions associated with El Nino

events (Kiem et al., 2006). As a result, there is a low chance of

extreme rainfall and, by implication, also of extreme erosion

occurring during the post-fire window of disturbance (Prosser

and Williams, 1998). This pattern, however, might not always

apply. Conceivably, contemporary wildfires could under-

represent wildfire-related erosion resulting from the few

occasions when high-magnitude events do occur in this critical

post-fire period (cf. Meyer et al., 2001). However, even when a

relatively high-magnitude rainfall event (150 mm of rain fell in

1 h) occurred following severe fire in the Victorian uplands,

south-east Australia in 2003, large quantities of topsoil were

redistributed, but no major geomorphological change occurred

(EPA, 2003).

Consideration also needs to be given to the largely unknown

long-term impact of fire on rock weathering. Humphreys et al.

(2003) estimated that if just 1% of exposed sandstone bedrock

surfaces in the Sydney Basin were subject to spalling as a result

of heating by fire once every 20 years, it would represent a

denudation rate of ca. 2 mm kyear�1. This represents as much

as half the long-term weathering rate estimated from

cosmogenic nuclide evidence on exposed tors (albeit granitic)

80 km south-south-east of Canberra (Heimsath et al., 2001).

This interpretation of a comparatively minor geomorpho-

logical role for wildfire in south-east Australia clearly requires

further research. This perspective does not, however, diminish

the forest management and water resource implications of

large-scale removal of topsoil and nutrients and their down-

stream contamination of streams and reservoirs.

8. Conclusions

Investigations published in mainstream and less accessible

outlets concerning post-fire effects on vegetation, soil char-

acteristics and the erosional consequences in south-east

Australian eucalypt forests demonstrate that there are distinctive

interactions between flora, fauna and soils that can substantially

affect post-fire soil erosion by water. Although there are broad

similarities with post-fire behaviour reported in general reviews

of wildfire impacts (e.g. vegetation and litter destruction,

modification of soil water repellency and enhanced post-fire

erosion), the differences caused by these interactions are thought

to be important in accounting for certain characteristics of the

reported post-fire soil erosion behaviour in south-east Australia

in contrast to the globally-accepted view. Four main distinctive

characteristics have been identified in this paper.

First, post-fire vegetation and litter recovery can be rapid,

such that a relatively high percentage cover is added to, and can

protect, the vulnerable soil surface relatively soon after fire,

except in areas where the extreme severity of a wildfire

consumes all leaves and small twigs and kills the vegetation.

Second, soil water repellency is modified in south-east

Australian eucalypt forests rather than induced by wildfire, as is

often viewed as the norm for forest soils. Moreover, destruction

of the surface soil water repellency usually takes place to

varying depths in moderate to severe fires. The resulting

wettable surface layer provides storage for rainfall which can

possibly delay the onset of overland flow and thus soil erosion,

at least during rainfall events of low intensity and/or short

duration. Repellency in the underlying subsurface material may

be intensified, but given the already strongly repellent nature of

soils in eucalypt forests, the geomorphological significance of

this change may actually be slight. Under these circumstances,

other fire-induced changes (e.g. destruction of vegetation and

litter, provision of highly erodible, surface material) may

trigger the hydrological and geomorphological importance of

the repellent nature of the soil.

Third, most reports of hillslope-channel transfers of topsoil

and burnt organic matter following wildfire in south-east

Australia have indicated moderate to relatively high losses

compared with those reported elsewhere in the world. These

losses, however, are not matched by similarly high hillslope

exports of subsurface material, even though it is highly erodible

where it comprises sandy water-repellent sediment of low

cohesion. The potentially high post-fire erosion rates resulting

from water-repellent soils of low cohesion on steep slopes are

constrained by a suite of unusual, small-scale features (dense

mats of fine roots, litter dam–microterrace complexes, faunal

activity) that appear to limit very effectively the hillslope-

channel transfer of sandy subsurface material as long as rainfall

events are not extreme. Under these non-extreme conditions,

movement of the sandy material is largely restricted to

downslope redistribution and storage. Only with extreme

rainfall is there evidence (observational only) of large-scale

transfer of sandy material to the channel system.

Fourth, in contrast to evidence from North America,

comparison of long-term and recent post-fire erosion in eucalypt

forests in south-east Australia suggests a relatively minor

geomorphological role for wildfires during the period spanning

pre-European settlement to the present-day. There is some

evidence to suggest that contemporary wildfire-induced change

is confined essentially to accelerated weathering of exposed

outcrops of sandstone and redistribution of soil. The combined

denudational effect of these processes is small by comparison

with long-term denudational rates (10 kyear to 10 Myear). It is

suggested that the discrepancy between present-day low and

long-term high rates may be accounted for by changes resulting

from high-magnitude, low-frequency catastrophic floods and

large-scale mass-movement processes during the Holocene.

The overriding message arising from these points is that,

except under extreme post-fire rainfall conditions, present-day

wildfires affecting south-east Australia seem to be less potent in

geomorphological terms than might be expected given the

severity and frequency of the wildfires and the often highly

erodible and water repellent status of the soil after fire. It should

be stressed, however, that the post-fire transfer of large

proportions of the topsoil and its nutrients to watercourses has

deleterious effects on downstream water quality and soil

fertility. In addition, repeated wildfire disturbance may well

have lasting impacts on floral and faunal compositions.

In the light of our evaluation of wildfire effects on soil

erosion in south-east Australia, it is our premise that further

understanding of the nature of wildfire-induced accelerated

R.A. Shakesby et al. / Forest Ecology and Management 238 (2007) 347–364360

erosion and the mitigation of its detrimental on-site and off-site

effects in this region can best be achieved by not only

acknowledging the importance of the distinctive interactions

between flora, fauna and soil physical and chemical properties,

in addition to geological, soil textural, topographic and climatic

characteristics, but also by incorporating their impacts in any

predictive soil erosion models. By implication, progress in

understanding wildfire-induced soil erosion elsewhere might be

best achieved by shifting to a paradigm of large-scale

bioregional variants rather than continuing to try to apply a

‘one-size-fits-all’ post-fire soil erosion model to all regions.

Acknowledgements

This research was made possible by funding from NERC

(Urgency Grant NER/A/S/2002/00143 and Advanced Fellow-

ship NER/J/S/200200662 {S.H.D.}) and through the Sydney

Catchment Authority (SCA) Collaborative Research Project

Grants (#91001289 {P.J.W.} and 2003/28 {G.S.H.}). G.S.H.

also acknowledges the assistance of Macquarie University

(MU) for various research grants. We wish to thank the

following: SCA staff at the Warragamba Office, in particular

Glen Capararo for logistical assistance and James Ray for

accessing data; Danny Hunt and Chris Leslie of CSIRO Land &

Water who helped with fieldwork and radionuclide analysis;

Russell Field (MU) for assistance with fieldwork logistics;

Patrick Lane (University of Melbourne) and Peter Hairsine

(CSIRO) for reading earlier versions of the manuscript; Nicola

Jones and Anna Ratcliffe for drawing the illustrations; and the

two anonymous referees for their useful comments.

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