Journal of the Royal Society of Western Australia, 87(2), June 2004 
hereafter referred to as plant communities. The 
dendrogram was also cut at the two group level to 
distinguish broad vegetation types (i.e., vegetation type 'S' 
and vegetation type 'M') and at the eight group level to 
distinguish vegetation subtypes . Although the 
determinations of cut levels and group unions were 
ultimately subjective, choices were guided by 
homogeneity analysis (i.e where average group 
homogeneity begins to plateau as more groups are 
defined by the classification; see Pressey & Bedward 
1991; Bedward et at. 1992), the shape of the dendrogram 
(e.g. cuts occurred at distinct gaps in the hierarchy), and 
the desire to ensure that sites grouped together had a 
reasonable consistency in terms of dominant and 
common species. 
The names and descriptions of the 2 vegetation types, 
8 subtypes and 16 plant communities so derived follow. 
Dominant and indicator species for each community are 
shown in Tables 1 & 2, respectively. 
Vegetation type 'S': hummock grassland with emergent 
trees & shrubs 
This vegetation type dominates the upland ridges, 
hills and mountains as well as the pediment slopes. It is 
dominated by various species of hummock grasses of the 
genus Triodia, colloquially known as "spinifex" (Jacobs 
1992). The genus is endemic to Australia and dominates 
large parts of the arid northern Australia. Hummock 
grasslands are rarely 'pure' grasslands as scattered trees 
and/or tall shrubs are usually present. Under some 
classification schemes (e.g. Specht 1981), these formations 
would be described as woodland or open woodland, but 
the term hummock grassland is preferred given the low 
cover of trees. In the study area, these trees and shrubs 
typically cover 5-10% and are chiefly of the genera 
Eucalyptus , Acacia and Senna (syn. Cassia). Hummock 
grasses themselves typically cover 20-25% of the soil 
surface. 
Subtype 1: Hummock grassland with emergent Eucalyptus 
leucophloia on ridge, mountain and hill slopes 
Community 1A: Triodia wiseana - T. basedowii hummock 
grassland with emergent Eucalyptus leucophloia and 
tall Acacia shrubs (on Marra mamba ridges and 
slopes; 9 sites). 
T. wiseana dominates this community, but is often 
mixed with T. basedowii. E. leucophloia (snappy gum) is 
the dominant tree at around 5% cover. Tall Acacia 
shrubs and small trees such as A. marramamba 
(indicator species and seemingly restricted to this 
geological substrate), A. atkinsiana (also an indicator 
species), A. tnaitlandii, A. pruinocarpa, A. hamersleyensis, 
A. pyrifolia and, in places, A. aneura s.l. are scattered 
throughout and sometimes form distinctive groves 
along the contour on less steep slopes. Shrubs, 
including Senna glutinosa subsp. pruinosa, S. glutinosa 
subsp. glutinosa and Keraudrenia velutina subsp. 
'elliptica' ms, form a very sparse, low-shrub layer. 
Grasses such as Eriachne mucronata, Eragrostis eriopoda 
and Paraneurachne muelleri are occasionally found 
between the hummocks, but herbs and subshrubs are 
rarely found (Tables 1 & 3) except in the first 1-3 years 
following fire (pers. obs.). 
Community IB: Triodia basedowii - T. wiseana hummock 
grassland with emergent Eucalyptus leucophloia , 
Corymbia hamersleyana and low Acacia shrubs, (on 
Brockman ridges and slopes;13 sites). 
This community is similar to community 1A in terms 
of structure and dominant species (although T. basedowii 
is often the most common species in the hummock grass 
mix). The major difference is the suite of shrubs present. 
Here small shrubs, such as A. adoxa and A. hilliana, 
dominate a sparse, low shrub layer (3-4% cover), with a 
tall shrub layer virtually absent (although A. maitlandii 
can be locally common). Another distinction from 1A is 
the bloodwood tree C. hamersleyana being a sparse but 
regular feature of the upper stratum. Subshrubs (small, 
predominantly non-woody shrubs such as Sida spp., 
Stemodia grossa , Goodenia spp.) are again uncommon in 
this community (Tables 1 & 3), yet are generally distinct 
from those of community 1A, with only one of twenty 
species common to both communities (namely Abutilon 
otocarpum). 
Community 1C: Triodia brizoides hummock grassland 
with emergent Eucalyptus leucophloia (on shallow, 
skeletal soils on slopes and ridges; 7 sites) 
Although also structurally similar to 1A, this 
community is dominated by T. brizoides (20-25% cover), a 
species which seems able to tolerate the shallow, skeletal 
soils found around outcrops of bedrock and on steeper 
slopes. Other perennial grasses such as E. mucronata , 
Paspalidium clementii , Triodia bitextura and Amphipogon 
caricinus are also present, particularly in the interstices of 
rocks and outcrops. Shrubs are less common than in 
communities 1A and IB, the most common being S. 
glutinosa subsp. glutinosa, A. maitlandii and Dodonaea 
coriacea (the latter is an indicator species; Table 2). 
Subtype 2: Hummock grassland of Triodia basedowii on 
pediment slopes. 
Community 2A: Triodia basedowii hummock grassland 
with emergent Eucalyptus gamophylla mallee (on lower 
to middle pediment slopes; 17 sites). 
The gentle pediment slopes (of around 1-3°) have 
shallow soils (20-30 cm deep) and are covered with small 
pebbles forming an ironstone pavement. Here the 
hummock grassland is dominated by T. basedowii and the 
mallee £. gamophylla is the dominant and characteristic 
emergent. No trees are found with the exception of Hakea 
lorea and C. deserticola at low densities. There is little 
development of a shrub layer (only ~2% cover) with the 
main species being the hybrid Senna artemisioides subsp. 
oligophylla x subsp. helmsii, Bonamia rosea and Indigofera 
monophylla. Acacia shrubs (such as A. stowardii and A. 
ancistrocarpa) occur but are relatively sparse. Semi¬ 
perennial grasses are fairly common between hummocks 
at some sites. 
Community 2B: Triodia basedowii hummock grassland 
with emergent Eucalyptus leucophloia and mixed Acacia 
and Senna shrubs (on upper pediment slopes; 12 
sites). 
Again T. basedowii is the dominant hummock grass 
and species present. A very low density of E. leucophloia 
occurs mixed with leguminous shrubs and small trees 
68 
