Ecology
The biology of mulga (Acacia aneura)
Mulga can flower throughout the year, mainly from April to July and especially after good rains. Pods appear to mature only between September and December. However, mulga may not flower every year and their pods may not produce seed every year.
Mulga seed will germinate without fire, but a greater proportion of seeds will germinate after exposure to fire (or at least heat). Mulga seedling regeneration appears to be most spectacular after heavy summer rain, but occurs in most years if stocking rates (see carrying capacity) are low (less than 1 sheep per 5 hectares).
Queensland mulga have reported height-growth rates ranging from 22 to 41cm per year, with some mulga in south west Queensland reaching trunk diameters of 20cm in 20 years. Growth will be much slower in dry years. Individual mulga trees in South Australia have been estimated to live for around 250 years.
Mulga can replace itself without fire as, unlike many other wattle species, the mulga seed does not require fire to germinate.
In the absence of exotic grasses, mulga vegetation tends to have low levels of ground layer fuel and is unlikely to burn. Sufficient fuel to carry a fire can be created when grasses build up, which can occur following rare periods of high rainfall combined with low levels of grazing. Fires are also more likely to occur when exotic grasses are present.
Rainfall and temperature
Mulga is very drought resistant and moderately frost resistant, but is found throughout Australia in areas that have some probability of rainfall throughout the year.
Areas with regular summer or winter droughts appear to be unsuitable for mulga. Extreme drought can kill both young and mature mulga trees.
Grazing pressure
Grazing pressure from sheep, goats and kangaroos can slow and prevent the recruitment and growth of mulga, though young mulga plants are less likely to be eaten by cattle.
Sustained high grazing pressure can also lead to soil degradation, an effect that can be worsened by termites during drought, but only where the cover of mulga trees has been lost. While it can also result in dense stands of so-called ‘woody weeds’, especially low unpalatable shrubs (such as Eremophila species), these are unlikely to seriously inhibit mulga restoration.
Reducing total grazing pressure can reverse changes to mulga vegetation caused by grazing (such as reduced species richness), however once soils are eroded plant establishment and growth may be slowed, making restoration more problematic.
Stopping grazing on mulga lands has been linked to increases in soil water infiltration rates and water availability, which are likely to improve plant germination and survival.
Clearing
Although clearing trees and shrubs changes the structure of mulga vegetation, the plant composition of cleared areas that are allowed to regrow can be similar to remnant uncleared areas. However, clearing mulga and converting it to buffel grass pasture has been shown to cause significant decreases in soil carbon and nitrogen.
Removing mulga trees during drought can exacerbate the effects of grazing pressure, due largely to harvester termites, which feed on grass tussocks and build large nests. These nests are hard-topped structures 2m to 3m in diameter, which prevent water from penetrating into the soil and make it difficult for seedlings to get established. In dense colonies, termite nests can occupy up to 20% of the soil surface, with their effects persisting for more than 70 years.
Fire
Fire in mulga vegetation will kill some mulga and other fire-sensitive trees and shrubs, tending to encourage the survival and expansion of grasses. However, as mulga has some ability to resprout after fire, and germination of its seed can be triggered by fire, only intense or repeated fires are likely to remove it altogether.
As it does not need fire to germinate, mulga is likely to regenerate naturally if fires are prevented and both seed and adequate rainfall are available. Landholders have also observed that fire in mulga lands can harden the soil surface, reducing water infiltration and increasing runoff.
The recommended fire management approach for mulga vegetation is to prevent fire. Mature mulga vegetation, in the absence of exotic grasses, rarely produces enough ground fuel to carry a fire. In very wet years when grass fuel loads are sufficient to carry a fire within mulga, patch burning of surrounding fire-tolerant vegetation can prevent fire from spreading into the mulga.
When soil moisture is high, careful application of patchy, low to moderate severity burns within the mulga can reduce the chance of more severe and widespread fires.
Buffel grass (Cenchrus ciliaris) is an exotic species, widely distributed in Queensland but currently only common in more easterly mulga stands. High densities of buffel grass increase ground fuel loads and the frequency and intensity of fire in acacia woodlands such as mulga. Control of buffel and other exotic grasses is likely to be required to ensure mulga can be restored.
Nutrient cycling
Nutrient cycling within mulga vegetation appears to be helped by the distribution of mulga trees into clumps or ‘groves’. Nutrients can also be trapped when fallen timber and grass tussocks slow over-land water flow and accumulate litter and soil.
Soil from mulga groves has been measured to have higher levels of organic and exchangeable nutrients (and plant cover) than soils in clear areas, indicating that nutrients and organic matter are trapped and tightly cycled by these groves. Surface soil beneath mulga has been found to have higher quantities of exchangeable calcium, magnesium and potassium than deeper soils, suggesting that these minerals may be absorbed by mulga roots at depth then released at the soil surface when mulga litter decomposes.
When perennial grasses (grasses that live for more than 2 years) are removed by grazing, this nutrient cycling appears to be disrupted, increasing erosion and reducing water infiltration. This can lead to increased rates of tree death during droughts.
Although only 10% to 20% of mulga distribution in Queensland tends to form obvious groves, similar processes are likely to be supported by fallen timber and perennial grass tussocks in mulga vegetation of any configuration.