Regional and landscape-scale pressures: Land clearing

2016

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 also lead to a variety of impacts on soils, including erosion and loss of nutrients. In addition to the negative impact on native vegetation and soil quality, historical land clearing and other colonial activity disrupted or destroyed traditional Indigenous land management practices.

Loss of native vegetation

Extensive historical clearing resulting in fragmentation continues to exert pressures on the land environment. Clearing rates have decreased over time, largely due to the reduced availability of forested land to clear over time (Figure LAN5). Approximately 44 per cent of Australian forests and woodlands have been cleared since European settlement; 39 per cent was cleared before 1972, although the proportion of any single community lost ranges from complete clearance to an increase in extent due to replacement of other vegetation communities (Figure LAN6). The 3 most heavily cleared communities (mallee with a tussock grass understorey, brigalow, and temperate tussock grasslands) together previously covered more than 170,000 square kilometres of Australia, and each has less than 20 per cent of its original extent remaining (Tulloch et al. 2015).

Note: Each National Vegetation Information System (NVIS) vegetation community is colour coded by (a) total loss of extent and (b) a fragmentation measure (change in proportion made up of patches of <5000 hectares).

Source: Adapted from Tulloch et al. (2015)

Figure LAN6 Continental pattern of Australia’s forest cover change

The Australian Government Department of the Environment and Energy uses detailed satellite data to track and report, on a national scale and by state and territory, the greenhouse gas emissions from land clearing and regrowth of vegetation as part of Australia’s national greenhouse gas accounting obligations. Annual forest clearing activity and associated emissions from 1989 are analysed and reported each year as part of the Australian Government’s National Inventory Report, in accordance with the United Nations Framework Convention on Climate Change and its Kyoto Protocol (Figure LAN7). Note that Figures LAN5 and LAN7 essentially use the same dataset produced by the department for the national inventory of greenhouse gas accounts. However, the estimates shown in these figures differ in how they define primary clearing (first clear) and regrowth clearing (reclear). The Full Carbon Accounting Model (FullCAM; Figure LAN7) applies a stricter test of reclearing. In FullCAM, reclearing includes more than one clearing and also first clearing of young forests that have grown on lands that were cleared before 1972.

The Australian Government Department of the Environment and Energy estimates a net loss of forest, from human-induced conversion of forest to other land uses and gains from human-induced revegetation, of 149,000 hectares in 2014. This is similar to the net loss recorded in 2009 (153,000 hectares), but higher than in 2011, when there was an estimated net gain of forest cover of 65,000 hectares. For woody vegetation that does not meet the forest thresholds, there was a net gain of 330,000 hectares in 2014, down from net gains estimated for 2009 (1,618,000 hectares) and 2011 (1,637,000 hectares). Drivers of change in woody cover are complex; they reflect a mix of factors, including climate signals, economic conditions, and changes in management practices and land management regulations.

Each state and territory has its own native vegetation legislation, and associated issues relating to agriculture and land-clearing rates. In Queensland, clearing has increased since 2011. Following the introduction of a ban on broadscale clearing that came into effect in 2006, and the extension of clearing controls to high-value regrowth in 2009, land clearing fell to a historical low of 78,378 hectares in 2009–10. High-value regrowth refers to nonremnant vegetation that was cleared more than 20 years earlier. Major reforms to the Vegetation Management Act 1999 were introduced in 2013 to allow landholders to clear vegetation not cleared since 31 December 1989 on land that is suitable for economically viable agricultural development. This change recognises that restrictions on how native vegetation could be managed were having an impact on agricultural productivity, a topic revisited in the recent Agricultural competitiveness white paper (Australian Government 2016). By 2013–14, clearing had increased to 296,324 hectares. This compares with the average annual rate of land clearing before the 2006 ban of 448,000 hectares per year. A recent report by the World Wildlife Fund (Taylor 2015) on clearing rates in Queensland found that:

  • clearing of nonremnant native vegetation increased from about 54,000 hectares in 2009–10 to about 183,000 hectares in 2013–14
  • clearing of remnant vegetation nearly doubled from about 52,000 hectares in 2012–13 to about 95,000 hectares in 2013–14, and has nearly quadrupled since 2009–10
  • about 700,000 hectares of high-value regrowth lost protection in 2013, and are currently being cleared
  • about 125,000 hectares of remnant vegetation, including about 12,000 hectares of endangered ecosystems, have been remapped as exempt from protection on regulatory maps since 2012.

Land cleared in Queensland’s reef catchments increased by 229 per cent from 2008–09 to 2013–14, from 31,000 hectares per year to 102,000 hectares per year. A 113 per cent increase from 2010–11 to 2012–13 coincided with the policy change to reduce compliance activities. In a 2015 report, the Queensland Auditor-General noted that (Queensland Audit Office 2015):

… this result may lead to an increase in the extent of bare ground which, depending on the occurrence of storms and the amount of ground cover provided by the replacement land use, increases the risk of soil erosion within the catchment. Therefore, a rise in tree clearing rates can contribute greater sediment run-off.

In South Australia, although clearance of native vegetation has stabilised and remaining native vegetation is protected by legislation, the remaining extent is strongly related to previous land use: in the arid natural resource management (NRM) regions (South Australian Arid Lands and Alinytjara Wilurara), 99 per cent of native vegetation remains, while historical agricultural and urban developments in the southern NRM regions have left only about 25 per cent of native vegetation (South Australian Government 2014). This pattern is repeated in other states—that is, less tractable agricultural land often remains largely intact, while land close to settlements or with predictable water availability and fertile soils has been heavily cleared in the past (Figure LAN8).

Fragmentation of native vegetation

Fragmentation of remnant vegetation following land clearing may adversely affect the quality and persistence of that vegetation, because of the disruption to essential ecosystem processes such as pollination, seed dispersal and regeneration. Smaller fragments also have more edges in proportion to their total area, so opportunities may increase for weed encroachment, changed micro-environmental conditions, ingress of fire from outside the patch and other dynamic processes, further threatening the remnant patch. The National Connectivity Index (DoE 2014a), a nationally consistent approach to characterise fragmentation, has been developed as an instrument for monitoring and prioritising the maintenance and restoration of Australia’s heavily modified landscapes. The VAST (vegetation assets, states and transitions) assessment ‘classifies vegetation condition by degree of anthropogenic modification from a benchmark condition state’ (Lesslie et al. 2010; see Condition) and thus also provides continental-scale information, but only at a relatively coarse scale. In general, fragmentation impacts will be greatest where land clearing has been greatest, both recently (Figures LAN6b and LAN8) and historically (Figure LAN9).

Impacts on soils

Soils and vegetation have co-evolved across the Australian landscape over millennia. Vegetation has adapted to the frequently nutrient-poor and sporadically wet soils, and its rooting patterns and litterfall contribute to soil structure and fertility. 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.

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 is greater than under natural conditions. Soil carbon typically decreases to 20–70 per cent of the pre-clearing amount (Luo et al. 2010). Restoring this very large stock of carbon is now a key focus of programs for mitigating greenhouse gas emissions around the world and nationally (e.g. the Australian Government’s Carbon Farming Initiative).

Removal of native vegetation results in major changes to the hydrological cycle, including dryland salinity. The soil also experiences more rapid leaching (loss of water-soluble nutrients), and this can change soil properties and processes (e.g. clays may disperse and reduce permeability). Rising watertables and surface evaporation of soil water increase the salt content of surface soils. 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 loss of the micro-environments that are required by many species.

Metcalfe D, Bui E (2016). Land: Regional and landscape-scale pressures: Land clearing. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/land/topic/2016/regional-and-landscape-scale-pressures-land-clearing, DOI 10.4226/94/58b6585f94911