Bushfire

Australia’s fire regimes—the frequency, intensity and timing of bushfires—have major consequences for vegetation distribution, composition and condition, and soil bareness and erosion.

The pattern of fire occurrence over the Australian continent (illustrated by Figure 5.23 for 1997-2009) is generally of:

• frequent fires in the tropical savannas, half or more of whose area may burn in any one year ⁵⁵
• regular fires in the arid and semi-arid grasslands following years of above-average rainfall
• major fires in the south-east and south-west of the continent and in Tasmania associated with major drought cycles.^41,55

These characteristic patterns of fire occurrence in different parts of the continent are evident in the decadal record of vegetation, which is available on the SoE website.^g

A review of bushfires in Australia over the seven years to 2004 demonstrates these patterns, and the much greater average area burned annually by bushfires across northern Australia outside the wet tropics (Figure 5.24).⁵⁶ Over 1997-2004, the average extent of fire-affected area in the three northern zones ranged from 19% to 35%, while elsewhere in Australia the extent ranged from 1% to 5%. Conversely, the greatest impacts on life and property of intense wildfires occur in southern Australia, tragically exemplified by the 2009 Victorian fires.¹³

Fire regimes in all parts of Australia impact on environmental values.⁵⁷ For example, frequent high-intensity fires impact on the recruitment and persistence of obligate seeder plant species—that is, plants with large, fire-activated seed banks that germinate, grow, and mature rapidly following a fire. This happens in environments as diverse as the Arnhem Land Plateau savannas⁵⁸ and the tall, wet eucalypt forests of south-eastern Australia.⁵⁹ One consequence of the millennium drought in south-eastern Australia was the frequent occurrence of high-intensity wildfires in parts of Victoria’s alpine region. This led to the repeated burning of some areas of Eucalyptus delegatensis and E. regnans that were too young to have seed crops adequate for stand replacement.^60-61 Conversely, an insufficient frequency of low-intensity fires will also impact adversely on the environmental values of ecosystems adapted to relatively frequent low-intensity fires.

Weather conditions favouring more severe bushfires appear to be becoming more frequent. The past 30 years have seen an upward trend in the cumulative forest fire danger index (a measure of predicted fire severity), exemplified by the change at one recording station in Victoria (Figure 5.25). This reflects the effects of both progressively increasing temperatures and, in the latter period, the millennium drought. This trend is expected to continue under predicted climate change conditions; the average number of ‘extreme’ fire danger days in 2020 is predicted to increase by 5-25% and 15-65% compared with 1990 levels, for 0.4°C and 1.0°C temperature increases, respectively.⁵⁴

Land clearing

Land clearing represents a fundamental pressure on the land environment, causing the loss and fragmentation of native vegetation. Depending on subsequent management, land clearing can lead to a variety of impacts on soils, including erosion and loss of nutrients.

Loss of native vegetation

The pattern of forest cover change over 2002-06 (the most recent years for which full continental data are available) shows woody vegetation loss concentrated in the north of the Northern Territory, southern Western Australia, northern and eastern Tasmania, and inland central and northern Queensland (Figure 5.26). This general pattern continued in the latter part of the decade—small number of surveyed bioregions lost more than 10% of their extent of woody vegetation from 2000 to 2010. These regions were along the coastal plain of Western Australia, from Geraldton to Cape Naturaliste; in the Northern Midlands of Tasmania; and in the northern Brigalow Belt of central Queensland. These changes primarily reflect urban and peri-urban expansion in the coastal areas, and clearing for agriculture in others.

The best nationally consistent data on land clearing are available from the Australian Government Department of Climate Change and Energy Efficiency’s monitoring of change in Australia’s forest vegetation for international reporting under the United Nations Framework Convention on Climate Change. There are some caveats to these data: they do not correspond exactly to vegetation clearing in toto, because the monitoring system is directed at measuring changes in the carbon stocks of forests and woodlands, which are defined as woody vegetation with a minimum of 20% canopy cover, a minimum height of 2 metres, and a minimum area of 0.2 hectares.⁶² The monitoring system does not assess change in nonwoody native vegetation, and may report the impacts of events such as intense fires or cyclones as loss of forest, and of natural regeneration and recovery following those events as expansion (regrowth) of forest.

Before 2007, monitoring was conducted for the whole continent; since 2007, monitoring has focused on mapping the intensive land-use zone shown in Figure 5.27. Approximately 80% of Australia’s forest vegetation, and most human-induced forest cover change, occur in this zone. Forest loss, forest regrowth and net forest cover change for those regions assessed continually since 1972 are shown in Figure 5.28. Annualised data for the decade to 2010 are shown in Figure 5.29.

The annual rate of forest loss in the mapped intensive-use zone over the decade to 2010 averages 1.1 million hectares (range 0.7 million - 1.5 million hectares) (Figure 5.27). This loss has been offset by forest expansion averaging 1 million hectares annually (range 0.6 million - 1.3 million hectares). As a consequence, there was a small net gain of forest in Australia in 2007-10, for the first time since the early 1990s. The overall average net rate of forest change in the area mapped over the decade to 2010 was a loss of around 160 000 hectares annually. As the 2006 SoE report³ noted, ‘regrowth’ vegetation and its environmental values are generally different in many respects from the vegetation that has been cleared.

Fragmentation of native vegetation

The fragmentation of remnant vegetation that follows land clearing may impact adversely on the quality and persistence of that vegetation, because of the disruption to essential ecosystem process such as pollination, seed dispersal and regeneration. This is most acute in the case of paddock trees and small remnant patches of vegetation.^63-64 As discussed by the Assessment of Australia’s terrestrial biodiversity 2008⁴⁶, no nationally consistent data to characterise fragmentation are yet available, although analyses have been conducted for some states (e.g. Tasmania⁶⁵) and are under way elsewhere. The VAST assessment reported in Section 2.3 currently provides the best continental-scale information, but only at a relatively coarse scale. In general, fragmentation impacts will be greatest where land clearing has been greatest, both historically (Figure 5.18) and recently (Figure 5.26).

Fragmented vegetation is also subject to ‘edge effects’, which are a diverse range of ecological changes occurring at the abrupt artificial margins of uncleared and cleared land. Increased light levels and wind at the edges can change the local habitat, and the fragmented vegetation is usually more prone to colonisation by invasive species.

Impacts on soils

Soils and vegetation have co evolved across the Australian landscape over millennia. Clearing of the predominantly deep-rooted native vegetation has many impacts on soil, changing the cycling of water, nutrients, sediments and solutes. Soils take decades, and in some cases centuries, to adjust to the new conditions. Many soils across Australia are therefore still equilibrating to European land use.

The initial disruption to soil usually results in a significant loss of nutrients. Organic matter is oxidised, and the removal of surface cover (litter and protective vegetation) makes the soil more prone to erosion. Stores and cycles of nutrients adjust under the new land use, but in most cases the net loss of nutrients and leakage are greater than under natural conditions. As noted in Section 2.2.4, soil carbon typically reduces to 20-70% of the pre-clearing amount. Restoring this very large stock of carbon is now a key focus in programs for mitigating GHGs around the world, and nationally (e.g. the Australian Government’s Carbon Farming Initiative¹⁴).

The removal of native vegetation also results in major changes to the hydrological cycle, including dryland salinity (see Section 2.2.3). The soil also experiences more rapid leaching, and this can change soil properties and processes (e.g. clays may disperse and reduce permeability). A less widely appreciated effect of clearing is that the land surface becomes more uniform—the patchiness of the native system is lost. For example, removing mounds of litter, grass tussocks and rough surfaces leaves a relatively smooth soil surface. This almost invariably leads to more rapid run-off and erosion, less effective water infiltration, and a loss of the micro-environments necessary for many species.

Invasive species

Invasive species impacting on the land environment comprise disease–causing organisms such as, fungi and parasites, insects and other invertebrates, pest animals and weeds.⁶⁶ Invasive species put pressure on land environmental values in a variety of ways, as well as impacting on biodiversity (see Chapter 8: Biodiversity). Some invasive species, such as rabbits, are long-established pressures. Others, such as foxes, are long established in some environments but new to others; yet others, such as the fungus that causes myrtle rust, are only recently established. The risk of invasive species incursions into Australia is increasing with the growth of international travel and trade.⁶⁷

Fungi

Among the numerous introduced fungi affecting plant health, two are of particular concern at a national scale.

The first is Phytophthora cinnamomi, which causes root rot that can significantly alter plant communities and lead to local and perhaps complete species extinctions.⁶⁸ Its impacts have been recognised since the 1960s. P. cinnamomi now occurs widely across Australia, but its most severe impacts are in vegetation communities in the south-west and south-east of the country. Many genera of endemic taxa have a high proportion of susceptible species—many of which are rare and threatened.⁶⁹ Consequently, the disease caused by P. cinnamomi is listed as a key threatening process under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act).⁶⁸

In contrast, the fungus that causes myrtle rust, Uredo rangelii, is a recent introduction. It was first detected in 2010 in central coastal New South Wales, and is now established in most of coastal New South Wales⁷⁰ and in south-east Queensland.⁷¹ Myrtle rust affects trees and shrubs in the Myrtaceae family of plants. This family includes many iconic Australian plant genera, including bottlebrush (Callistemon.), lilly pillies (Eucalyptus.) and affiliates (e.g. Corymbia.), lillypillies (Syzygium.) and tea tree (Melaleuca.). Because of the dominance of the Myrtaceae in the Australian flora and the high mobility of plant rusts, the potential impacts of myrtle rust are profound. However, little is known of the behaviour and impacts of myrtle rust under Australian conditions.⁷²

Pest animals

Some 73 invasive pest animal species (amphibians, birds, fish, mammals and reptiles) have established populations in Australia. In many cases—such as feral cats, foxes, rabbits and wild dogs—these populations are long established and distributed over much of the continent. In other cases, such as foxes in Tasmania, introductions are recent, and populations are still small. The highest concentration of significant pest animal species is along the eastern seaboard, and many coastal and offshore islands suffer significant impacts.⁷³ Some newly established pests, such as the Asian honeybee, may ultimately have significant impacts on ecosystem processes and thus vegetation.⁷⁴

Pest animals with the greatest impacts on the land environment, in terms of damage estimates, are foxes, feral cats, rabbits, feral pigs, wild dogs, house mice, goats, cane toads, wild horses and camels.⁷⁵ Their impacts are expressed as environmental damage, such as that caused to soil and vegetation by pigs or camels; as loss of production in agricultural systems; and as loss of biodiversity. Land degradation by goats, pigs and rabbits, and the impacts of cane toads, are formally listed as threatening processes under the EPBC Act.

Weeds

Invasive weeds present serious threats to Australia’s environmental values. They displace native species, contribute significantly to land degradation, and reduce farm and forest productivity.⁷⁶ The Australian and state and territory governments have identified the worst of these (currently, 20 species) as Weeds of National Significance for coordinated management action (Table 5.8). The impacts of gamba grass and four other introduced grasses in northern Australia, and of escaped garden plants nationally, have been listed as threatening processes under the EPBC Act. A further 28 species have been identified as being at an early stage of establishment, but with the potential to become significant threats.⁷⁷ The potential of weeds to transform the land environment is very significant; for example, ‘eleven plant species have the capacity to permanently alter ecosystems across Australia’s rangelands’.⁴¹

^a May also refer to Asparagus declinatus
^b The Weeds of National Significance listed show bitou bush and boneseed separately—these two taxa together are treated as one of the 20 Weeds of National Significance

Source: www.weeds.gov.au/weeds/lists/wons.html