Hardpan mulga shrub pastures in the Pilbara, Western Australia

Pasture potential

Hardpan mulga shrub pastures have low to moderate pastoral value. The large intergrove areas support only very sparse desirable low shrubs and sparse annual grasses and herbs in season. The relatively small areas of groves are more productive, with a range of desirable shrubs (and a few grasses) which provide high quality feed. However, because of the overall sparsity of desirable plants, hardpan mulga shrub pastures have only limited durability in dry times.

Suggested levels of use (per annum)

Hardpan plains and intergrove plains

• Good condition: 120 hectares per cattle unit (ha/cu).
• Fair condition: 145 ha/cu.
• Poor condition: 170 ha/cu.

Groves

• Good condition: 60 ha/cu.
• Fair condition: 84 ha/cu.
• Poor condition: 120 ha/cu.

Managing hardpan mulga shrub pastures in the Pilbara

Grazing

Hardpan mulga shrub pastures can be grazed continuously provided stocking rates are low enough. If the pastures are in good or fair condition, and the stocking rate is low enough, they will supply feed of sufficient quality for breeding livestock. These pastures can also have relatively heavy, short-term opportunistic grazing of the ephemeral grasses and herbs in good seasons. Control the grazing pressure to minimise damage to desirable shrubs.

As with all pastures, stocking rates can be increased in favourable seasons, but should be rapidly reduced before desirable perennials are killed and pasture condition declines. Spell these pastures for a full growing season to improve the cover and vigour of shrubs.

Fire

Hardpan mulga shrub pastures should be protected from burning, as mulga and many associated shrubs are fire-sensitive. In average years, these pastures will not generally carry a fire, but in wetter seasons, wind grass (Aristida contorta) and other herbage will supply sufficient fuel. Control fire fuel with grazing.

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Pasture condition

Traverse data (620 observations) during the Pilbara rangeland survey 1995–97 recorded:

Pasture condition:  very good 6%, good 26%, fair 29%, poor 30%, very poor 9%.
Soil erosion:  nil 91%, slight 1.5%, minor 3%, moderate 2%, severe 2%, extreme 0.5%.

The condition of hardpan mulga shrub pastures is highly variable. Extensive parts are degraded with moderate to substantial losses of desirable understorey shrubs. Disrupting overland water flow can kill the mulga overstorey.

This pasture type, with its level topography and well developed cryptogamic soil crusts, is relatively resistant to erosion. However, sheet erosion can occur on hardpan plains and tracts subject to more concentrated through-flow if the soil crust is broken by trampling or other disturbances. Some localised but significant areas are severely degraded and eroded.

Very good–good condition

Hardpan mulga shrub pastures in good condition (Figures 1, 2) support isolated to very scattered decreaser low shrubs such as horse mulla mulla (Ptilotus schwartzii) and cotton bush (Ptilotus obovatus) in the intergroves, with projected foliage cover (PFC) of up to 5%.

A wider range of desirable plants occur under the dense mulga shrubs in groves. The groves receive and trap water being shed off adjacent hardpan intergrove plains, and are areas of high biological activity, with deep soils, dense vegetation and plentiful litter. They are areas preferred by livestock and other animals. There is no soil erosion.

Figure 1 A hardpan plain of the Cadgie land system supporting mulga shrub hardpan pasture in good condition. There is a scattering of desirables such as horse mulla mulla (Ptilotus schwartzii), creeping mulla mulla (Ptilotus roei), tall saltbush (Rhagodia eremaea) and tall sida (Sida calyxhymenia). The site has very shallow soil over hardpan and does not have the potential to carry many more shrubs.

Figure 2 Hardpan mulga shrub pasture in a grove that is favoured by additional water running on from adjacent intergrove surfaces. The mulga (Acacia aneura) trees are close (PFC 30–50%) and there is a good range of desirable decreaser shrub species in the understorey. The site is on the Pindering land system and condition is good.

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Fair condition

In fair condition (Figure 3), hardpan mulga shrub pastures have fewer desirable shrubs – PFC is generally less than 2.5% – in the hardpan intergrove areas, but desirable species are still relatively common in the groves.

Undesirable species such as crinkled cassia (Senna artemisioides subsp. helmsii) may increase marginally in the intergroves and stability desirables such as poverty bushes (Eremophila spp.) are common but do not form dense stands. There is no soil erosion.

Figure 3 A hardpan plain on the Zebra land system with hardpan mulga shrub pasture in fair condition. The unpalatable stability desirable Eremophila sp. ‘Jigalong’ dominates the low shrub layer, and only a few palatable decreaser shrubs are present. The background spinifex is on a different unit (low sandy banks) of the Zebra system and is soft spinifex plain pasture.

Poor–very poor condition

In poor condition (Figures 4, 5), hardpan mulga shrub pastures have few or no palatable desirable shrubs. Hardpan intergrove plains support only isolated to very scattered stability desirables such as Acacia and Eremophila species (PFC up to 5%) .  Groves have good coverage with tall shrublands or woodlands of mulga (PFC 30–50%) but palatable undershrubs are virtually absent.

The shrubs beneath the mulgas consist of a few unpalatable stability desirables or undesirables, such as hop bush (Dodonaea petiolaris) or wild tomato (Solanum sturtianum). In extreme situations, where groves have been starved of water by alterations to run-off and run-on processes, all plants may die and the grove structure collapses.

Figure 4 An intergrove hardpan plain on the Zebra land system with hardpan mulga shrub pasture in poor condition. There are virtually no perennial plants remaining and the pasture produces only sparse annual grasses and herbs in season.

Figure 5 A collapsed grove in a hardpan mulga shrub pasture. The normal water recharge system to the grove has been disrupted and the mulga and associated shrubs have died. There is active soil erosion and condition is very poor.

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Vegetation structure and composition

Hardpan intergrove and interbank plains

Hardpan mulga shrub pastures on hardpan intergrove and interbank plains are typically isolated to very scattered low or tall shrublands (PFC up to 10%), or less frequently, scattered (PFC 10–20%) shrublands. The most common low shrubs are horse mulla mulla (Ptilotus schwartzii), cotton bush (Ptilotus obovatus) and Eremophila species.

Tall and mid-height shrubs include the ubiquitious mulga (Acacia aneura), gidgee (Acacia pruinocarpa) and curara (Acacia tetragonophylla). Annual grasses, mainly wind grass (Aristida contorta) and herbs occur as a ground layer in season.

Groves

Hardpan mulga shrub pastures in groves consist of moderately close to closed (PFC 25 to more than 50%) tall shrublands or woodlands. The trees and tall shrubs are mulga (Acacia aneura), gidgee (Acacia pruinocarpa) and Acacia catenulata.

Common mid height and low shrubs are Wilcox bush (Eremophila forrestii), cotton bush (Ptilotus obovatus), flannel bush (Solanum lasiophyllum), tall sida (Sida calyxhymenia), ruby saltbush (Enchylaena tomentosa), silky bluebush (Maireana villosa) and cassias (Senna spp.). A few perennial grasses may be present. 

Occurrence

This pasture type is found in southern and south-eastern parts of the Pilbara on level or very gently sloping plains underlain by red-brown hardpan or ‘Murchison cement’. The mulga shrublands on these plains consist of groves or sandy banks with dense vegetation and broad intergrove and interbank plains on land systems such as Cadgie, Jamindie, Nooingnin, Spearhole, Three Rivers, Wannamunna, Washplain and Zebra.

The groves and banks vary in size from about 10 to100m wide by 50 to 5000m long, and are arranged more or less on the contour. The largest groves and banks are found on the Nooingnin and Zebra systems and have deep loam or loamy clay soils.

The broad intergrove and interbank areas which support much sparser vegetation than the groves or banks have shallow, slightly acid loam soils over hardpan. Soil surface mantles vary from very few to abundant pebbles of ironstone or quartz. The plains are subject to broad sheetwash water flow after rainfall.

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Associated plants

* Only in groves.

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Other resources

van Vreeswyk, AM, Leighton, KA, Payne, AL, & Hennig, P 2004, An inventory and condition survey of the Pilbara region, Western Australia, Department of Agriculture and Food, Western Australia, Perth. Technical Bulletin 92.