State of the Environment 2011 Committee. Australia state of the environment 2011.
Independent report to the Australian Government Minister for Sustainability, Environment, Water, Population and Communities.
Canberra: DSEWPaC, 2011.
Given enough time, the land (i.e. landforms, soils, drainage networks of streams and rivers, vegetation and other biota) comes into equilibrium with the climate and the regime of land management. This rarely happens, because land management is always being adjusted and shifts in climate occur before the land has had time to equilibrate. As a result, the land is always changing. The rates of change are often difficult to detect if we rely on unaided human observation and memory. Furthermore, the slow changes (e.g. habitat loss, soil acidification and erosion) often present the biggest environmental problems in the long run.
The degree to which the land provides a long-term, optimal mix of ecosystem services depends on its overall quality and resilience to these changes. In general, good-quality and resilient land has these related features:
- Leakage of nutrients is low.
- Biological production is high relative to the potential limits set by climate.
- Levels of biodiversity are relatively high.
- Rainfall is efficiently captured and held within the root zone.
- Rates of soil erosion and deposition are low, with only small quantities transferred out of the system (e.g. to the marine environment).
- Contaminants are not introduced into the landscape, and existing contaminants are not concentrated to levels that cause harm.
- Systems for producing food and fibre for human consumption do not rely on large net inputs of energy.
At a glance
Australia has large areas of ancient and weathered soils. Their resilience to change is lower than the younger landscapes of parts of Europe and North America.
Soils are prone to serious degradation when they have insufficient resilience and when particular thresholds are exceeded. Unless agricultural management practices change, these thresholds may soon be exceeded for soil acidity and organic carbon. Thresholds for salinity have already been exceeded in significant areas from Western Australia to Queensland.
Lands managed for nature conservation and for minimal use are generally resilient. In contrast, resilience of vegetation is likely to be poor and often diminishing in much of the intensive land use zone—increasingly so in peri-urban and coastal zones, where most vegetation occurs as disconnected remnants.
Principles to restore resilience in native vegetation are well established. They focus on maintaining or restoring connectivity across the landscape; maintaining vegetation composition, structure and regeneration processes; managing key and threatened plant and animal species; and managing threatening processes such as invasive species and inappropriate fire regimes.
The resilience of land to disturbances such as intense storms, clearing of vegetation or changes in land management depends on the age and character of the soils and landforms. In general, ancient, strongly weathered landscapes have different responses to disturbance from young landscapes.124 When young landscapes are disturbed, they tend to return to something similar to their previous state, because there are sufficient nutrient reserves for vegetation to re-establish. Old, weathered landscapes are unlikely to return to the pre-disturbance state or to a functionally similar variant. Their smaller reserves of nutrients mean that any loss through leakage (e.g. leaching and erosion after clearing) will constrain biomass production when vegetation re-establishes. Australia has large areas of ancient and weathered soils with less resilience to change than young landscapes such as Europe and North America.
Other factors can also affect the resilience of soils. The presence of clays that shrink and swell with drying and wetting allows soils (e.g. Vertosols) to recover from compaction and physical degradation. Soils without this capacity are less resilient and can be permanently compacted. Some of Australia’s best soils for horticulture and intensive agriculture (e.g. Ferrosols on the east coast) have excellent physical and chemical fertility but, because they do not shrink and swell, once compacted by poor land management they are extremely difficult, if not impossible, to repair. Some soils are naturally fertile because of the parent material type (e.g. basalt, most forms of river alluvium), and the reserves of nutrients generated by this material confer resilience.
Soils are prone to serious degradation when they have insufficient resilience and when particular thresholds are exceeded. The most important thresholds highlighted in this report are described below:
- Organic carbon declining past a threshold below which physical and chemical fertility effectively collapse. The precise carbon content varies with soil type, but is usually between 1% and 2% for surface layers. Management is improving in most regions, but some are likely to pass below this threshold unless more conservative forms of land management are implemented (see Section 2.2.4).
- Soil pH decreasing below 4.2 (increasing acidity). This is a critical threshold, because it triggers aluminium toxicities and the soil becomes very difficult to remediate. Many plants are affected before this threshold is reached. Unless land management changes in Australia, the time before this critical pH is reached across large areas used for agriculture is only two or three decades and, in some regions, only a matter of years (see Section 2.2.5).
- Surface cover declining past a threshold below which storm rainfall causes hillslope erosion or wind erosion. Most regions have locally appropriate targets for surface cover, based on rainfall intensity, soil erodibility and land-management practices (see Section 2.2.6).
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