The millennium drought and subsequent floods in eastern Australia are recent examples of the high levels of variability that typify the Australian climate, in cycles ranging from seasons to centuries. Australia’s soils and native vegetation have evolved under, and are adapted to, the pressures of such climatic variability. However, the pressures exerted by climate change are expected to change both the distribution and occurrence of native and exotic plant species. This will progressively change Australian landscapes.29
The impacts of climate change on Australia’s native vegetation are expected to be profound.52 Some native vegetation communities will no longer exist, the extent and distribution of others will change, and novel ecosystems of variously native and exotic species will arise.53 The extent of likely change can be assessed as dissimilarity from the current condition,52 which is shown for each of the 23 MVGs in Figure 5.21. Figure 5.22 depicts the possible extent of new environments by 2070 under a medium-impact scenario;e it is evident that these may be very extensive.
Climatic changes will also alter the occurrence and distribution of diseases, pests and weeds, and their impacts on vegetation communities. Only preliminary work has been conducted to investigate these possible impacts. The case of buffel grass (Cenchrus ciliaris), a major weed in arid and semi arid zones, is illustrative. This weed may become less problematic in its current environment, and more problematic in southern and eastern Australia, where it does not currently occur or is just becoming established.52 Diseases and pests are likely to move south as temperatures increase, impacting on both native vegetation and production systems.54
Both favourable and adverse impacts on agricultural and forestry production systems are expected. For example, there may be growth benefits from additional carbon dioxide fertilisation, but adverse impacts from changes in temperature extremes, precipitation, pests and nutrient availability.54
Rainfall, evaporation and the water storage capacity of soils are finely balanced in some parts of Australia, and relatively small shifts in climate can have a large impact on the viability of various land uses. For example, in northern cropping regions, large areas of heavy-textured soils stand to benefit from an increase in summer rainfall. Conversely, light-textured soils in southern cropping lands have small water storage capacities, and cropping is only possible because rainfall is light but regular throughout the growing season. Any reduction in the frequency of cold fronts across southern Australia during the growing season will adversely affect yields in these southern regions.
Cropping and livestock production is expected to decline by 2030 over much of southern Australia, due to increased drought and limited availability of nutrients; heat and drought are also likely to reduce the quality of grain, grape, vegetable, fruit and other crops.54 Both native and plantation forests are likely to be at greater risk of fire, and productivity is likely to be diminished by decreased rainfall and soil water availability.