Australia: The Land Where Time Began
Acacia aneura - the 'real' Mulga. Other species are often called Mulga. Fauna of the Mulga Country
Mulga shrubland is found mostly in areas with red, infertile soils. Unexpectedly, the highest densities of woody shrubs in Australia occur in these areas. Being an Acacia, mulga has symbiotic bacteria associated with its roots that convert atmospheric nitrogen to forms that can be used by the mulga, as is also done by other legumes such as beans. Being deep-rooted, it can access nutrients from deep in the soil, allowing it to thrive in what are low-nutrient soils. The leaf litter collecting in these areas add nutrients to the soil. Mulga areas have the world's most unpredictable climate and unreliable rainfall. Mulga only grows in areas where the rainfall is unreliable.
Mulga has adapted extremely well to life in arid conditions. Instead of the usual leaves of most plants it has phyllodes, leaf stems that have thickened to do the job of leaves, with less surface area that reduces the amount of water lost to the atmosphere, as well as lower surface to volume ratio that reduces the amount of heat absorbed compared to that of standard flat leaves. Other adaptations for desert life are the phyllodes of a silvery greenish colour, increasing their reflectivity, reducing the amount of heat absorbed. They reduce water loss by having a high oil content, sunken stomates, small size of the phyllodes and a covering of hairs. The presence of hairs on the surface slows wind speed near the surface, and combined with the sunken stomates reduces the amount of water blown away when the stomates are open to allow gas exchange. The higher the water vapour content of air above the stomate, the lower the transpiration rate from the open stomata.
Mulga also drought-proof themselves further by gathering what rain they receive as efficiently as possible. In the driest parts of their range their phyllodes are longer and narrower to increase their efficiency as water gatherers. The shape of their crown also maximises the channelling of rain down to the soil around their stems where roots proliferate to absorb it. Because the rain they gather is concentrated in a small area around their stems it moves deeper into the soil were the soil is cooler, so evaporation is reduced, and close to the tree so it can be utilised as needed. And it is deep-rooted, the roots penetrating deep enough to reach damp soil, even when the surface layers are dry. It also drops its phyllodes in really dry times, so conserving water even further, the litter surrounding the base of the tree stores nutrients for when the rains return and it sprouts new phyllodes, the nutrients from the litter being recycled.
A aneura is the most variable of the Acacia species commonly called Mulga. Some think the varieties of this species should probably be classified as separate species.
There are 3 different shapes of mulga in the Great Sandy Desert, a shape for a habitat type. In rocky parts with water retaining problems the form taken is horizontal branches. In this habitat it is stunted and grows in open stands. The other 2 forms occur together, with the weeping form favouring areas of flat, loose ironstone gravel. Many other plants are found as understory in mulga woodlands.
It forms vegetation types that range from open forest up to 15 m tall in Queensland, to sparse shrublands in the arid areas. The most common vegetation type it forms is tall shrubland or low woodland.
It has been found that seed production, germination and seedling establishment are all synchronised with La Nina events when there is usually an assured water supply long enough for the seeds to become established. So nutrients are not wasted on producing seeds that are doomed to die before water returns. It survives through fire, drought and flood. Mulga is superbly adapted to survive in areas of Australia that are subjected to the most extreme diurnal and seasonal temperatures range, from the frosts that are common in winter to scorching temperatures in summer. Even in summer there can be a large range between the daily minimum and maximum.
Other Acacia species are also commonly referred to as mulga - bowgada (Acacia linophylla), horse mulga (A. ramulosa), umbrella mulga or turpentine (A. brachystachya), bastard mulga (A. stowardii), bendee (A. catenulata). They share the same adaptations to the unpredictable climatic conditions of the arid zone as the real mulga.
Mulga doesn't often grow on clay spoils, preferring soil types such as red sands, red hardpan soils, red earths, some red duplex soils and calcareous soils.
Mulga, along with the other arid land vegetation, evolved to withstand light grazing by the native animals and it not designed to withstand heavy continuous grazing as happens with cattle and sheep. Arid land vegetation only regenerates after good rain, when there is a flush of vegetation, most of the year, and often for years of drought, when there is no or little rain, they don't regenerate. Because cattle continue the heavy grazing on this flush of vegetation the plants have no time to seed before being eaten so each year fewer seeds are produced until the understorey often disappears completely. Once this happens the land is degraded, dropping its carrying capacity for cattle.
For thousands of years the firestick farmers used patch-burning techniques, with low-intensity fires, to sustainably maintain the mulga land vegetation. Once European settlers arrived in the area with their cattle things changed. As well as the cattle grazing on the flush, as the vegetation regenerated, reducing the seed bank after each wet season, they cut down the mulga to feed the cattle when the grasses and other edible plants weren't available during droughts. The combination has led to the degradation of once productive country. The stopping of burning meant that after good La Nina years the accumulation of litter meant that when fires did happen they were high-intensity and often burnt out very large areas in one go, in some cases more than 5,000 km2.
Mulga is not fire-tolerant, being killed by high-intensity fires that can also destroy the seeds it depends on for regeneration. Mulga is most susceptible to high-intensity fire damage in the drier areas, that are more prone to drought.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|