Acacia hardpan pastures in the southern rangelands of Western Australia

Pastoral potential – moderate to low

The patches between groves of acacia hardpan pastures typically only support a very sparse range of desirable shrubs, with some annual grasses and herbs in favourable seasons. The pastoral potential in good condition is moderate to low. The groves are more productive with greater floristic diversity including desirable shrubs that provide high quality feed, but durability is limited in dry times. The soil surface infiltration capacity of the interpatch areas is significantly lower than within the groves.

The groves of acacia hardpan pastures are preferentially grazed. Acacia hardpan pastures in good condition are now typically rare. Short-term opportunistic use of the ephemeral plants in good seasons is possible, provided that the period of grazing is limited so that damage to desirable shrubs is avoided. Periods of complete spelling over two or more growing seasons are recommended to improve the abundance and vigour of shrubs.

Suggested levels of use (per annum)

Table 1 provides a rough guide to the range of pastoral values for good condition pastures, which must be checked against conditions in each region and paddock. Carrying capacities for fair condition pastures might be 75% to 50% of good, and poor condition pastures less than 50% of good.

See Introduction to pastures in the southern rangelands of Western Australia for an explanation of how carrying capacities are estimated. The carrying capacity expressed as 105/140 is for moderate/poor pastoral potential.

¹ DSE is based on the feed energy required to maintain a 45 kilogram liveweight Merino wether with zero weight change, no wool growth additional to that included in maintenance, and walking 7 km/day. 1 DSE has an energy requirement of approximately 8.7 MJ ME/day.
² CU in the southern rangelands is based on a 400 kg steer at maintenance and equivalent to 7 DSE.
³ AE is based on the feed energy to maintain a 450 kg Bos taurus steer 2.25 years of age, walking 7 kilometers each day. 1 AE has an energy requirement of approximately 73 MJ ME/day and equivalent to 8.4 DSE.

Management

Grazing management

Acacia hardpan pastures in good condition are now typically rare. Short-term opportunistic use of the ephemeral plants in good seasons is possible, provided that the period of grazing is limited so that damage to desirable shrubs is avoided. Periods of complete spelling over two or more growing seasons are recommended, to improve the abundance and vigour of shrubs.

Fire management

Mulga hardpan pastures will not generally carry a fire in average seasons but in good seasons, grasses and herbs can supply sufficient fuel to carry a fire. These pastures should be protected from burning as mulga and many associated shrubs are fire-sensitive.

Pasture condition

Good

See Figure 1. These pastures support isolated to very scattered palatable low shrubs such as horse mulla mulla and cotton bush in the intergroves in good condition. A diverse range of palatable plants occurs in the groves under the acacia trees and tall shrubs. Healthy groves have good soil-moisture holding and resource retention characteristics and therefore have deeper soils, dense vegetation and thick accumulations of organic matter from decomposing leaf litter. Healthy groves typically do not exhibit browse lines. There is no soil erosion; where there is no leaf litter cryptogamic soil crusts are well-developed and extensive.

Fair

See Figure 2. Palatable shrubs in the intergrove areas are reduced but palatable species are still relatively common in the groves. Unpalatable species such as crinkle leaf cassia may increase marginally in the intergroves, and intermediate species such as Wilcox bush are common but do not form dense stands. Browse lines may be evident. There is no soil erosion.

Poor

See Figure 3. Palatable shrubs are absent where acacia hardpan pastures are in poor condition. Hardpan intergrove plains support only isolated to very scattered intermediate or undesirable species. Unpalatable large shrubs such as turpentine bush and royal poverty bush have increased in these pastures as they are not grazed. Groves may still be intact with relatively dense tall shrublands or woodlands of mulga but browse-lines are common and palatable low shrubs are sparse to absent. In some instances the grove structure is breaking down as herbivores push further into them to search for feed; browsing can eventually lead to canopy death. The shrubs beneath the mulgas consist of a few undesirable or intermediate species such as crinkle leaf cassia and Wilcox bush. Changes in hydrological processes, due to erosion or poorly located infrastructure such as tracks, can cause water starvation which may result in the grove association dying from dehydration. As pasture condition deteriorates rates of soil infiltration also decline.

Acacia hardpan pastures condition photographs

Figure 1 A mulga grove community in good condition with mixed-aged shrubs, a dense canopy and abundant mix of perennial and annual grasses. The foreground shows the upslope intergrove. The site is on a level plain in the Frederick land system.

Figure 2 A mulga hardpan community in fair condition. Desirable species including green mulla mulla and tall sida remain and occasional bluebush is present under bush clumps. The site is on a level plain in the Tindalarra land system.

Figure 3 A mulga hardpan community in poor condition. Weedy juvenile needlebush dominates the low shrub layer. The stumps from dead mulga trees are evident and sheet erosion is occurring. The site is on a level plain in the Belele land system.

Vegetation structure and composition

The interpatches of Acacia hardpan pastures are typically isolated to very scattered low to tall shrublands with projected foliar cover (PFC) ranging from <2.5–10%. Common variants within this broad group are dominated by mulga, snakewood and gidgee. The most common low shrubs are cotton bush, horse mulla mulla and Eremophila species. Tall and mid-height shrubs include mulga, snakewood, gidgee and curara. Annual grasses and herbs occur as a ground layer in favourable seasons.

Acacia hardpan pasture groves consist of moderately close to closed (25≥50% PFC) tall shrublands or woodlands. Trees and tall shrubs are mulga and gidgee. Common mid to low shrubs are Wilcox bush, cotton bush, tall sida, tall saltbush, ruby saltbush and cassias. A few perennial grasses may be present.

Acacia-dominated tree groves and bush clumps act as fertile patches. Branches and leaf litter build up within groves, obstructing ground surface winds and water flow. Leaf litter, seeds, animal scats and general debris accumulate within and immediately upslope of the grove or clump. This enriches soil with nutrients, increases microbial activity, contributes to greater soil moisture and creates improved conditions for germination and establishment.

Occurrence

These pastures are on level or very gently sloping plains underlain by red-brown hardpan, and cover an estimated 9.49 million hectares (11.7% of the southern rangelands). The acacia hardpan pastures typically occur as groves or sandy banks with dense vegetation with broad interpatches between the groves or banks. The groves and banks are more or less arranged on the contour of the land. The largest groves and banks have soils which are deep loams or loam over clay. The interpatches with sparse vegetation are shallow, slightly acidic, loam soils over hardpan. Soil surface mantles vary from very few to abundant pebbles of ironstone or quartz. The plains are subject to sheetwash water flow after rainfall.

Figure 4 Map of the estimated distribution of acacia hardpan pastures

Associated plants