Over 200 potential references were reviewed with many covering aspects of water quality, grazing lands and their effects on the Great Barrier Reef (GBR), and rehabilitation of degraded landscapes. There was little reported information on the mechanical rehabilitation of bare, D-condition grazing lands in the reef catchments. There is, however, literature on machinery suitable for soil surface disturbance, pasture technology for developing permanent perennial pastures and on grazing management for improving C-condition land. This literature review is complementary to the other two aspects of this project, the field experiments of mechanical disturbance, and the landholder surveys on their rehabilitation experiences.
Rehabilitation of degraded grazing lands of the GBR catchments has become a major focus of the Reef Rescue Programme primarily because the large area of land involved has the potential to contribute significant sediments and nutrients to the GBR lagoon. Poor water quality from grazing lands has the capacity to do serious damage to the reef ecosystems. Any reduction in sediment runoff from eroding land implies a worthwhile improvement in water quality entering the GBR lagoon. A limitation with this idea is that it assumes almost no net losses as the turbid water moves downstream and minimal ability of the marine and reef ecosystems to recover from short-term stresses. Recent published research indicates that neither assumption is correct. It also does not recognise the dynamic nature of natural ecosystems in the semi-arid tropics where there is perpetual shifting of species dominance and mixes in response to the perturbations in the surrounding environment, particularly grazing pressures, climatic extremes and high seasonal variability.
High levels of sediment, pesticides, nutrients, or fresh water does kill or weaken some reef species, however, other species benefit, either directly by better using the incurring resources or by expanding into the ecological gap left by the damaged suite of species. For example, the increase in algae due to extra sediment and nutrients at the expense of hard corals is well known, but if the perturbation is short-lived (like a flood plume) or not widespread, then recolonisation by the displaced species can occur, provided no ongoing trauma occurs. However there is good evidence to suggest that the amount of sediment and nutrients reaching the ocean from agricultural and urban land is well above the 18th century levels, and thus could be damaging the reef ecosystems and the fishing and tourism industries that depend on them being in good health. There is also data to show that the quality of pastures on grazing lands is often poorer than 150 years ago and that areas of land may be denuded more than is desirable, particularly by grazing in drought periods.
Thus it is beneficial to all if better pasture quality and cover on grazing lands is encouraged and achieved. The main questions are what should the pasture quality target be and how can this be achieved at a realistic social and economic cost. Because the relationship between runoff and sediment load against ground cover is strongly non-linear, logic says that the greatest benefit will accrue from revegetating the barest areas, such as D-condition land. This eroded land is often found close to major watercourses so that the payoff is greater for the investment made in improvements because there is little scope for deposition of entrained sediment between those eroding areas and the nearby fast-flowing channel water.
Published studies, plus this project’s research, have shown that the regeneration of healthy, perennial pastures on D-condition land is possible on many soil types provided several pre-conditions and favourable seasonal co-incidences are met. These include: grazing animals, including macropods, have to be excluded completely for some time; significant disturbance of the soil surface is needed for all soils that do not have a natural loose surface; and, good growing season rainfall has to be received shortly after the surface disturbance and pasture seed has been sown.
If good rains are not received soon after rehabilitation, resowing of pasture seed may be needed along with re-cultivation to loosen and roughen the soil surface. There is a broad range of appropriate cultivation implements available for rehabilitation work, but the options for perennial grasses and legumes to sow are limited, as is the availability of adapted native species seed.
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