Eucalypt-acacia-eremophila shrubland plain pastures in the southern rangelands of Western Australia

Pastoral value – very low

Eucalypt-acacia-eremophila shrubland plain pastures have very low pastoral value.

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.

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

They are generally not impacted by grazing as there are usually few palatable species present. Palatable species associated with tree-based clumps may disappear under heavy grazing. Some similar pastures that once supported chenopod understoreys in the southern Goldfields have transitioned into this form of eucalypt woodland pasture due to overgrazing, leading to a proliferation of unpalatable broom bush. It is likely that disturbances such as historic grazing, woodcutting, hail storms and/or fires are responsible for disappearance of the saltbush seed source. This transition is likely to be a permanent state.

Pasture condition

Survey data show that these pastures in the southern rangelands are predominantly in fair condition.

Good

See Figure 1. The most reliable indication of grazing impact is the diversity and density of palatable low shrubs (including saltbushes, bluebushes, cotton bush). Tree-based clumps are important indicators of condition. There is good mix of shrubs among tree-based clumps, including palatable bird-dispersed berry-bearing plants such as ruby saltbush, tall saltbush, lake-fringe rhagodia and currant bush.

Fair

See Figure 2. A reduction in palatable low shrubs may be evident, with or without an increase in unpalatable species such as broom bush or cassias. Tree-based clumps are not well-developed and palatable bird-dispersed berry-bearing plants are sparse or absent.

Poor

See Figure 3. Palatable low shrubs are absent. Unpalatable species may dominate the mid to low shrub strata (for example, broom bush, grey cassia, tomato bush). Tree-based clumps are absent. Soil erosion is usually associated with a decline in condition.

Eucalypt-acacia-eremophila shrubland plain pastures condition photographs

Figure 1: A eucalypt-eremophila woodland community in good condition. There is good mix of shrubs among tree-based clumps, including palatable bird-dispersed berry-bearing plants. The interpatch understorey is not dominated solely by unpalatable eremophilas. The soil surface is intact with a sand sheet supporting perennial grasses and annual herbs. Erosion is restricted due to the presence of cryptogamic crusts and obstructed flow paths created by branches, leaf litter, grasses and herbage in favourable seasons. The site is a loamy plain in the Johnston land system.

Figure 2: A eucalypt-eremophila woodland community in fair condition. Tree-based clumps are not well-developed and palatable bird-dispersed berry-bearing plants are sparse or absent. The understorey is becoming dominated by unpalatable shrubs such as broom bush and tan wattle. While cryptogamic crust is still present, the decline in obstructions between eucalypts and shrubs is resulting in unobstructed through-flow and sheet erosion is beginning to occur. The site is a level, loamy plain in the Doney land system.

Figure 3: A eucalypt-eremophila woodland community in poor condition. Unpalatable broom bush dominates the understorey and there are no tree-based clumps. Extensive sheetwash erosion is causing soil loss, and rills are developing as flow paths coalesce. The site is a loamy plain in the Doney land system.

Vegetation structure and composition

Pastures range from scattered to moderately close low woodland, tall shrubland or eucalypt woodland, with projected foliar cover (PFC) ranging from 15–30%. Low shrub stratum is also well-developed, with a variably developed mid shrub stratum.

The variability in eucalypt species is influenced by regional distribution, geology or topographic location. The upper strata are often co-dominated by mallee eucalypts and tall shrubs (e.g. mulga). The combined tree-shrub layer is typically moderately close (20–25% PFC). In the southern Goldfields, tall trees (>15m) dominate the overstorey, and cover ranges from very scattered to moderately close (5–30% PFC).

The mid and low shrub stratum (2.5–15% PFC) is commonly dominated by acacias, cassias or eremophilas with few halophytic species. Halophytes may be present but these pastures do not support complex chenopod communities. Tree-based clumps are important microhabitats that indicate condition. These are characterised by suites of palatable low and mid-shrubs different to the acacias and other shrubs that predominate between tree-based clumps. Palatable shrubs may include saltbushes, ruby saltbush, bluebushes, cotton bush, tall saltbush and currant bush.

Perennial grasses are largely absent, but may include broad leaf wanderrie grass, greybeard grass or speargrasses when present.

Occurrence

These pastures are found on broad, level valley floors forming drainage tracts between extensive sandplains, very gently undulating to level plains draining into adjacent salt lakes and wide drainage tracts connected with large granite outcrops. They cover an estimated 4.53 million hectares (5.6% of the southern rangelands). They commonly lack a surface mantle, except where they grade into upland landforms. A variable mantle may be present. Soils are calcareous loamy earths, red loamy earths or red sandy earths, sometimes over hardpan or calcrete and generally receiving dispersed run-on.

Figure 4 Map of the estimated distribution of eucalypt-acacia-eremophila shrubland plain pastures

Associated plants