Sandy granitic acacia shrub pastures in the southern rangelands of Western Australia

Pastoral potential – low to very low

The pastoral value of the sandy granitic acacia shrub pastures is low to very low. Even in good condition, vegetation will not support high stocking rates. Gnamma holes in the granite can hold fresh water temporarily, attracting animals to these pastures, which may lead to over grazing of palatable plants.

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.

Managing sandy granitic acacia pastures in the southern rangelands

The abundance of outcrop, shallow soils and low water holding capacity makes the vegetation of these granitic pastures more vulnerable to water stress than surrounding pasture types during dry periods. The gently inclined gradients also allow run-off after even the lightest of rain which can stimulate the rapid growth of annual species, though how long they persist is dependent on seasonal conditions. Fresh water run-off can also be captured and pooled in gnamma holes in the granite, which can temporarily attract animals to these pastures while the water persists and may result in over grazing of palatable plants.

Kite-leaf poison occurs around the granite domes in some areas and has resulted in poisoning of sheep, cattle, goats and horses. The leaves are usually eaten when fresh green shoots are present but no other attractive forage is available.

Pasture condition

Sandy granitic acacia shrub pastures are usually stable, but erosion may be evident in degraded areas.

Good condition

See Figure 1. Dominant plants are acacias, poverty bushes, cotton bush and cassias. A good indicator of range condition is the species density and the abundance and variety of the palatable plants. Concentrating on one species only (e.g. cotton bush) can be misleading. Some areas within these pastures may be unusually productive for cotton bush, and soil fertility can vary over short distances irrespective of range condition. Not all palatable species grow in the same area, but four to nine palatable species are expected (Table 2), and there is a range of different-aged plants. These include cotton bush, Wilcox bush, felty fuchsia bush, tall saltbush, horse mulla mulla, warty-leaf eremophila, sidas and small bluebushes. The density of palatables is at least 1000 plants per hectare. A few unpalatable species such as cassias may be present but are not dominant in any of the vegetation layers. There is no accelerated soil erosion. Soil surface crusts are well-developed and plentiful.

Fair condition

See Figure 2. The number of palatable low shrubs falls to between 500 and 1000 plants per hectare and the number of species is about four to six. Key decreaser species may become confined to protected areas under taller shrubs where grazing is restricted. Some unpalatable species are present but not usually dominant. There is no accelerated soil erosion and soil surface crusts are still obvious and largely intact.

Poor condition

See Figure 3. Few palatable species are present and areas of bare ground are evident. The number of different palatables will fall to below four species and the total number of palatable plants to less than 300 per hectare. Cotton bush, Wilcox bush, curara and mulga are likely to be absent. Tall sida and tall saltbush are very resilient to overgrazing and may be the last remaining palatables. Unpalatable species such as turpentine bush, grey turpentine bush, spoon-leaf eremophila, straight leaf cassia, banana-leaf cassia and crinkle leaf cassia are more frequent and dominate the low and/or medium shrub layers. There may be slight or minor soil erosion in the form of small scalds up to 5 metres in diameter with redistribution of soil surface material and litter. Soil surface crusts are still present but patchy.

Sandy granitic acacia shrub pastures condition photographs

Figure 1: A sandy granitic acacia shrub community in good condition. Cotton bush is dominant and abundant; other desirable plants include Wilcox bush, tall sida and tall saltbush. Desirable palatable shrub density is around 2,750 plants per hectare (28/100 m2). The site is on the mid-slope of an undulating rise in the Challenge land system.

Figure 2: A sandy granitic acacia shrub community in fair condition. Flat leaf bluebush and silky bluebush are absent, but mulga bluebush remains. Cotton bush and horse mulla mulla are present but not abundant and palatable shrub density is about 620 plants per hectare (6/100 m2). Wilcox bush is desirable in these pastures. Unpalatable fine-toothed poverty bush and annual grasses are present. The site is on the mid-slope of an undulating rise in the Challenge land system.

Figure 3: A sandy granitic acacia shrub community in poor condition. Unpalatable pink poverty bush dominates. Low shrub diversity and density are reduced, though cotton bush and horse mulla mulla are present. Bare ground is evident and erosion is occurring. The site is on the upper slope of a gently undulating plain in the Challenge land system.

Vegetation structure and composition

Sandy granitic acacia shrub pastures vary with location: the dominant plants are generally acacias, eremophilas, cassias or Ptilotus species (cotton bush and mulla mullas).

Trees and tall shrubs are mostly sparse – usually only 100 to 200 per hectare – but include granite wattle, mulga, curara and minniritchie. Tall saltbush can also be a feature. Palatables include compact poverty bush, flat leaf bluebush, horse mulla mulla, lax bluebush, warty-leaf eremophila, silky bluebush, cotton bush and Wilcox bush.

The characteristic grasses are short annuals such as wind grass and Murchison red grass. Lemon-scented grass is widely distributed through the Murchison, Gascoyne and northern Goldfields; other perennial grasses such as broad leaf wanderrie grass and creeping wanderrie are uncommon.

Occurrence

Sandy granitic acacia shrub pastures occur on low granite pavements (Figure 4) and are often adjacent to hardpan plains, and cover an estimated 1.85 million hectares (2.3% of the southern rangelands). The soils are generally shallow sandy loams. Granite outcrop is common. Soil surfaces are covered with coarse sandy grit plus some larger stones and pebbles of quartz and granite. The soils tend to be a paler red than the hardpan plains below. These pastures grade into stony hardpan in some northern areas. Both landscapes are similar, but the soil surfaces are sandier in the granitic shrubland. The soils of sandy granitic acacia shrub pastures are typically nutrient poor and their shallowness reduces their capacity to hold water. The gritty-surfaced plains supporting these pastures occur on most granite-based land systems, and the largest areas are on the Challenge, Sherwood, Norie, Bandy, Charlina and Wyarri land systems.

Figure 4: Map of estimated distribution of sandy granitic acacia shrub community pastures in the southern rangelands.

Associated plants

D = desirable, U = undesirable, I = intermediate, N = no indicator value