Journal of the Royal Society of Western Australia, 87(2), June 2004 
study area (e.g. Start 1986; van Leeuwen et al. 1995) and 
personal observations suggest that this density is 
controlled by fire, with the relatively fire-sensitive mulga 
increasing in density with time without fire. Structurally, 
therefore, this community varies from a hummock 
grassland (with emergent mulga) to an open woodland 
of mulga. Other reasonably common tree and shrub 
species include: A. pruinocarpa, Eremophila forrestii and C. 
deserticola. This community occurs on the very slight 
slopes of the bajadas and other areas of alluvial fans. The 
soils are relatively shallow (20-40cm deep) and are 
reported to be underlain by a siliceous hardpan (Lorimer 
1991). 
Subtype 6: Woodland of Acacia aneura on broad drainage 
flats and lines. 
Community 6A: Acacia aneura woodland/open 
woodland with tussock grass understorey (on alluvial 
flats and broad drainage lines; 7 sites). 
The woodland is relatively dense (> 15% cover) and is 
dominated by mulga, although it is sometimes mixed 
with sparse eucalypts such as £. victrix (on broad alluvial 
flats and basins) and £. xerothermica (on broad drainage 
lines). Tussock and bunch grasses, most of which are 
perennial, dominate the ground layer. Typically many 
grass species are found at each site, with the number 
increasing following large rainfall events. Examples of 
tussock grass include Chrysopogon fallax, T. triandra, P. 
muelleri and Eulalia aurea. Subshrubs are also common 
and average 13% coverage. Common subshrubs include 
M. villosa, Abutilon aff. exonemum, A. otocarpum , Erempohila 
'lanceolata ' ms (low woody shrub), P .obovatus and 
Rhagodia eremaea. Many of these have high indicator 
values (Table 2). Species richness and diversity are 
significantly higher than in most other communities, as 
are numbers of common and uncommon species (Table 
3). 
Community 6B: Acacia aneura woodland/open woodland 
with Triodia melvillei - tussock grass.understorey (on 
outwash plains, lower bajadas and broad drainage 
lines; 12 sites). 
This community is similar in several respects to 6A; 
the main differences being the dominance of hummock 
grass (T. melvillei) in the ground layer and the occurrence 
of A. citrinoviridis and A. pruinocarpa in the tree layer. The 
community is typically found on lower parts of bajadas 
and outwash fans. The community appears floristically 
and structurally intermediate between types 5 and 6A 
and may well represent a transitional zone or ecotone 
between them. On extensive outwash plains this 
community often exhibits a grove-intergrove vegetation 
pattern (Mabbutt & Fanning 1987), known in other 
countries as tiger bush or vegetation arcs. 
Subtype 6: Woodland of Eucalyptus spp. on confined 
drainage lines. 
Community 7A: Eucalyptus xerothermica open woodland 
with Themeda triandra — Eulalia aurea / Triodia pungens 
understorey (within drainage lines; 10 sites). 
Scattered £. xerothermica trees of 5-10% cover 
characterise the overstorey of this community, with the 
occasional Corymbia semiclara, A. aneura and £. victrix also 
found. It is typically found within distinct drainage lines 
on subdued topographies. The understorey typically 
consists of either a mix of two tussock grasses (T. triandra 
and £. aurea) or open hummock grasses (T. pungens and, 
less commonly, T. iviseana). Where hummock grasses 
dominate, tree cover is appreciably lower. Such sites also 
form a distinct group in the classification. Several 
indicator species occur in the shrub layer; these include 
Acacia dictyophleba, A. colei and A. validinervia 
Community 7B: Eucalyptus victrix - £. camaldulensis - 
Acacia citrinoviridis woodland with Themeda triandra - 
Triodia pungens grassy understorey (along major 
creeklines; 4 sites). 
A mixture of £. victrix (coolabah) and £. camaldulensis 
(river redgum) form a tall woodland formation along the 
major creeklines of the study area. A. citrinoviridis is 
another relatively common tree in these areas. The 
ground layer is dominated by perennial grasses such as 
T. triandra, T. pungens and, to a lesser extent, Eriachne 
benthamii, E. mucronata, P. clementii and P. muelleri. Shrubs 
comprise around 5% cover. A large number of shrub 
species were found in this community; the more common 
ones are Dicladanthera forrestii (indicator species), 
Dipteracanthus australasicus, A. bivenosa, A. pyrifolia, A . 
sclerosperma, Solatium lasiophyllum and I. monophylla. 
Several other indicator species identified for this 
community include Rulingia kempeana, Phyllanthus 
maderaspatensis and Dodonaea lanceolata. Overall species 
richness and diversity is significantly higher than at most 
other sites (Table 3). 
Community 8 (also Subtype 8): Eucalyptus victrix - Acacia 
aneura open woodland over perennial tussock and 
bunch grasses (in broad drainage basins; 2 sites). 
There are several internal drainage basins in the study 
area which collect water and sediment from the 
surrounding hills and ranges. Where the basin is 
extensive, an open woodland of coolabah (£. victrix), 
variously mixed with smaller mulga trees (A. aneura), is 
found on the very fine, deep alluvium at the lowest parts 
of the basin. This community is floristically poor (Table 
3) with only tussock grasses (such as £. aurea, T. triandra, 
C. fallax), bunch grasses (such as A. contorta) and the 
woody, much-branched shrub Muehlenbcckia florulenta (a 
characteristic species) being recorded in the understorey. 
As this community was only sampled in the middle of a 
dry period, the species richness recorded is likely to be 
below its potential maximum. This community is the 
most distinct in floristic terms (Fig 2). 
The non-hierarchical classification procedure (ALOC; 
Belbin 1987), when forced to produce 16 groups, resulted 
in more-or-less the same groups, each with a very similar 
(sometimes identical) composition of sites to the groups 
resulting from the hierarchical procedure (UPGMA). The 
major differences between the two classifications were 
that ALOC: 1) divided the T. wiseana/T. basedowii 
hummock grasslands on slopes into upper slope/plateau 
sites and lower slope sites rather than into groups linked 
to geological substrate; 2) community 3B was divided 
into two groups, one dominated by A. xiphophylla and the 
other dominated by A. victoriae and chenopods; and, 3) 
the two coolabah-mulga woodland sites of the drainage 
basins (community 8) were clumped with mulga 
72 
