Plants of Western Australian granite outcrops



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Weeds

Ornduff (1987) and Ohlemüller (1997) found that

introduced weed species, mainly annuals, comprised

23.7% and 17.0% of the granite herbfield floras sampled

respectively. These figures are high compared with the ca

9% that weeds represent of the WA flora as a whole

(Green 1985; Keighery 1995). They are even more

significant when considered with granite outcrop

herbfield floras elsewhere on earth, which have far fewer

invasive weeds (Porembski et al. 1997; Wyatt 1997).

Weed growth is most pronounced in full sun on

outcrops, especially where soil has been disturbed and

enriched by rabbit dung or agricultural activity. In these

situations, annual grasses such as Briza maxima, Avena



fatua and Ehrharta longifolia often dominate and replace

native annuals throughout much of the south-west.

Weeds are rare only where a dense shrub layer or low

forest of native woody perennials persist. It is clear that a

persistent seed rain of weeds occurs over vast regions of

the south-west, as weeds appear in open areas on

outcrops where little or no disturbance is evident and

native plants dominate.

Hopper (1997) has attributed the high invasibility of

disturbed Western Australian plant communities to the

absence of major glacial soil stripping as an evolutionary

force acting on the flora. Native species are unable to

compete against weeds from habitats where soil

disturbance is a regular pertubation. Further

experimental study of weeds in granite herbfields could

test this hypothesis, and assist attempts at restoration of

invaded outcrops.

Biogeography and Endemism

Western Australian granite outcrops display plant

biogeographic patterns that mirror that of the whole flora

(Hopper 1979, 1992; Hopper et al. 1996); species-richness

and endemism are pronounced in the transitional rainfall

zone (wheatbelt) of the south-west, and attenuate as

rainfall decreases through the pastoral country to the

deserts, Pilbara and Kimberley. For example, the total

number of vascular plant taxa recorded by the senior

author and colleagues on a sample of rocks ranged from

142 (Point Matthew, near Augusta) and 201 (Mt

Frankland) in the highest rainfall forests, 192 (Mt Ney)

and 187 (Yilliminning Rock near Narrogin; Pigott and

Sage 1997) in the transitional rainfall zone, to 85 (Daggar

Hills, near Yalgoo) in the pastoral zone, and 90

(Moolyella Rocks, east of Marble Bar) and 80 (Spear Hill,

west of Marble Bar; Fig 2) in the arid Pilbara.

There are no vascular species shared between

northern (Kimberley, Pilbara) outcrops and those in the

south-west. Moreover, the northern outcrop floras are

virtually identical with that from the matrix of

surrounding terrain, save for outcrop specialists like rock

figs and rock ferns. Walters & Wyatt (1982) similarly

recorded low endemism and little discontinuity between

vascular plants on granite outcrops and adjacent

landforms of the arid Central Mineral Region of Texas.

In contrast, south-western and adjacent pastoral zone

rocks have higher levels of local endemism, especially in

the high rainfall forest region where the outcrops present

the most striking difference in habitat to the surrounding

vegetation matrix (e.g. Wardell Johnson & Williams 1996;

Brooker & Margules 1996).

Biogeographical relationships of outcrop floras across

the south-west are under ongoing study by the senior

author. As a precursor, Hopper & Brown (unpublished)

documented the distribution of 126 orchid taxa on 41

outcrops ranging from the highest rainfall forests

through the transitional zone wheatbelt to the arid zone.

Each rock outcrop was treated as a site in a classification

of the orchid data.

The study highlighted a number of significant trends.

A primary division occurred between the 15 rocks found

in the forested High Rainfall Zone ( >800 mm p.a.) and

the rest ranging from the Transitional Rainfall Zone of

the wheatbelt into the Arid Zone. Subsequent divisions

established as much difference among forest rocks as

among the wheatbelt/arid rocks, even though the forest

rocks were confined to a much smaller area. Moreover,

remarkably, closely adjacent rocks were widely separated

in the classification, indicating significant differences in

their orchids (e.g. 10.3 km Rock and 10.9 km Rock of

Ornduff’s (1987), separated by just 600 m of jarrah forest

on Albany Highway). Conversely, rocks separated

geographically often had similar orchid floras (e.g.

Boyagin Rock and Pingaring Rock on the western and

eastern sides of the south-central wheatbelt respectively).

These patterns suggest significant barriers to orchid


Journal of the Royal Society of Western Australia, 80(3), September 1997

147


Table 1

Lists of Western Australian orchid taxa that occur on granite outcrops.



Caladenia voigtii ms

Corybas recurvus

Cyanicula amplexans ms

Cyanicula ashbyae ms

Cyanicula caerulea subsp apertala ms

Cyanicula deformis ms

Cyanicula fragrans ms

Cyanicula gemmata ms

Cyanicula sericea ms

Cyrtostylis huegelii

Cyrtostylis robusta

Diuris aff longifolia

Diuris brumalis

Diuris conspicillata

Diuris laevis

Diuris laxiflora

Diuris longifolia

Diuris maculata

Diuris picta

Diuris pulchella

Diuris recurva

Diuris setacea

Drakonorchis barbarossa ms

Drakonorchis drakeoides ms

Drakonorchis mesocera ms

Elythranthera brunonis

Elythranthera emarginata

Eriochilus dilatatus subsp dilatatus ms

Eriochilus dilatatus subsp multiflorus ms

Eriochilus dilatatus subsp undulatus ms

Eriochilus helonomos ms

Eriochilus pulchellus ms

Eriochilus scaber

Genoplesium nigricans

Leptoceras menziesii

Lyperanthus serratus

Microtis aff parviflora

Microtis atrata

Microtis brownii

Microtis eremaea

Microtis graniticola

Microtis media subsp eremicola

Microtis media subsp media

Monadenia bracteata

Paracaleana nigrita

Paracaleana triens ms

Prasophyllum aff parvifolium

Prasophyllum brownii

Prasophyllum cucullatum

Prasophyllum elatum

Prasophyllum fimbria

Prasophyllum gibbosum

Prasophyllum gracile

Prasophyllum parvifolium

Prasophyllum ringens

Pterostylis aff nana

Pterostylis aff rufa

Pterostylis allantoidea

Pterostylis aspera

Pterostylis barbata

Pterostylis elegantisima

Pterostylis hamiltonii

Pterostylis mutica

Pterostylis recurva

Pterostylis roensis

Pterostylis sanguinea

Pterostylis sargentii

Pterostylis scabra

Pterostylis vittata

Pyrorchis nigricans

Spiculaea ciliata

Thelymitra aff holmsii

Thelymitra aff longifolia

Thelymitra aff nuda

Thelymitra aff pauciflora

Thelymitra antennifera

Thelymitra benthamiana

Thelymitra crinita

Thelymitra cucullata

Thelymitra aff dedmaniarum

Thelymitra flexuosa

Thelymitra macrophylla

Thelymitra spiralis

Taxa that predominantly occur on granite

outcrops

Caladenia caesarea subsp maritima ms

Caladenia exstans ms

Caladenia granitora ms

Caladenia hoffmanii subsp graniticola ms

Caladenia integra

Caladenia longicauda subsp clivicola ms

Caladenia longicauda subsp rigidula ms

Caladenia multiclavia

Caladenia nivalis

Caladenia remota subsp remota ms

Cyanicula ashbyae ms

Cyanicula fragrans ms

Diuris conspicillata

Diuris picta

Diuris pulchella

Eriochilus pulchellus ms

Microtis eremaea

Microtis graniticola

Microtis media subsp eremicola

Spiculaea ciliata

Thelymitra aff nuda

Thelymitra aff dedmaniarum

Declared Rare orchid taxa of granite

outcrops

Caladenia caesarea subsp maritima ms

Caladenia exstans ms

Caladenia hoffmanii subsp graniticola ms

Caladenia voigtii ms

Thelymitra aff dedmaniarum

All taxa

Caladenia attingens subsp attingens ms

Caladenia attingens subsp gracillima ms

Caladenia brevisura ms

Caladenia brownii ms

Caladenia caesarea subsp maritima ms

Caladenia caesarea subsp transiens ms

Caladenia citrina ms

Caladenia denticulata

Caladenia dimidia ms

Caladenia discoidea

Caladenia doutchiae

Caladenia exstans ms

Caladenia falcata

Caladenia filifera

Caladenia flaccida subsp flaccida ms

Caladenia flaccida subsp pulchra ms

Caladenia flava subsp flava ms

Caladenia flava subsp maculata ms

Caladenia flava subsp sylvestris ms

Caladenia footeana ms

Caladenia granitora ms

Caladenia heberleana ms

Caladenia hirta subsp rosea ms

Caladenia hoffmanii subsp graniticola ms

Caladenia incensa ms

Caladenia incrassata ms

Caladenia infundibularis

Caladenia integra

Caladenia latifolia

Caladenia lobata

Caladenia longicauda subsp clivicola ms

Caladenia longicauda subsp eminens ms

Caladenia longicauda subsp longicauda ms

Caladenia longicauda subsp rigidula ms

Caladenia longiclavata

Caladenia macrostylis

Caladenia marginata

Caladenia microchila ms

Caladenia multiclavia

Caladenia nivalis ms

Caladenia pachychila ms

Caladenia pholcoidea ms

Caladenia polychroma ms

Caladenia radialis

Caladenia reptans subsp impensa ms

Caladenia reptans subsp reptans ms

Caladenia rhomboidiformis

Caladenia roei

Caladenia saccharata

Caladenia serotina ms

Caladenia sigmoidea

Caladenia splendens ms

Caladenia hiemalis ms

Caladenia horistes ms

Caladenia pendens subsp pendens ms

Caladenia remota subsp remota ms

Caladenia pendens subsp talbotii ms

dispersal, particularly between forest rocks, and high

levels of local extinction and stochastic events underlying

the presence of orchids on individual rocks in the south-

west and adjacent arid zone. There have been dynamic

climatic fluctuations across the south-west for several

million years as Australia drifted northwards and arid

conditions overtook much of central Australia (Hopper

1979; Hopper et al. 1996). The diversity of microhabitats

on granite outcrops provided refuge for plants adapted

to both dry or wet conditions as the surrounding matrix

waxed and waned climatically (Marchant 1973; Main

1997). Survivorship in small populations on granite

refuges undoubtedly was a matter of chance in the face

of such repeated climatic turmoil.


148

Journal of the Royal Society of Western Australia, 80(3), September 1997

Interestingly, the above conditions of small disjunct

outcrop populations undergoing recurrent stresses is

predicted as ideal for genetic divergence and speciation

(Grant 1981). Is this prediction borne out by studies of

Western Australian outcrop plants? Table 1 provides a

recently updated list of 141 orchid taxa recorded from

Western Australian granite outcrops. Of these, 22 (16%)

are more or less endemic. The endemics have

geographical ranges from widespread on outcrops

throughout the south-west (e.g. Spiculaea ciliata; Fig 5) to

highly restricted to a few adjacent outcrops less than 10

km apart (e.g. Caladenia caesarea subsp maritima,



Thelymitra aff dedmaniarum).

In terms of evolutionary origins, these endemics

display at least three patterns (Hopper and Brown,

unpublished);

• relictual, with no obvious close relatives and

therefore likely to have been on granites for a long

period of time (e.g. Spiculaea ciliata, a monotypic

genus);


• derived by speciation from allopatric congeners of

habitats other than granite (e.g. C. granitora, from

coastal granites east of Albany, sister species to C.

infundibularis of western high rainfall forests and

coastal heaths; C. hoffmanii subsp graniticola, of east

central wheatbelt outcrops, sister to C. hoffmanii

subsp hoffmanii of lateritic loams well to the north-

west in the Northampton region); and

• derived by speciation from allopatric congeners of

other granite outcrops (e.g. C. exstans, from

outcrops east of Esperance, sister to C. integra of

western wheatbelt outcrops).

Thus, the orchid data do indeed support the hypothesis

that conditions on south-west granite outcrops have

facilitated genetic divergence and speciation. Genetic

studies of a few other granite taxa lend further support,

e.g. the herb Isotoma petraea (Bussell & James 1997), and

eucalypts endemic to granite (Hopper & Burgman 1983;

Sampson  et al. 1988). Clearly, more work along these

lines is needed.

There is a large number of granite endemics in south-

western Australia, especially among the perennials that

dominate the woody vegetation and herbfields. We have

already shown that 16% of orchids on outcrops are

endemic. For eucalypts, around 24% are endemic

(Hopper, unpublished). The level of endemism for the

whole granite outcrop flora is difficult to determine

without more penetrating research, but there is no doubt

that south-western Australia has higher levels than any

other system documented (e.g. Walters & Wyatt 1982;

Porembski et al. 1995, 1997).

The refugial opportunities offered by south-west

granite outcrops are also evident in species that have

highly disjunct outliers well removed from the main

geographical distribution. These include populations

on wet outcrop sites in much lower rainfall areas than

the main species’ stand (e.g. the Jilakin Rock stand of

jarrah Eucalyptus marginata, the Twine Rock stand of E.



wandoo, and the Kuendar stand of E. rudis).

Conversely, arid-adapted species penetrate high

rainfall areas on dry north-facing slopes of granites

(e.g. populations of Eucalyptus drummondii west of

Margaret River).

Conservation

Granite outcrops occupy a very small proportion of

most Western Australian landscapes in which they occur.

Especially in the south-west, the outcrop plant

communities are, therefore, by definition, rare, and likely

to contain rare species.

Hopper  et al. (1990) found that endangered granite

outcrop plants numbered 29 (12.2%) of the 238 plants

declared in 1989 as Rare Flora under the Wildlife

Conservation Act. These endangered plants ranged from

large mallees (e.g. Eucalyptus crucis subsp crucis) and

small trees (Acacia denticulosa, Banksia verticillata), through

compact shrubs (e.g. Drummondita hasselii var longifolia,

Verticordia staminosa) and climbers (Kennedia beckxiana, K.

macrophylla) to diminutive herbs (Tribonanthes purpurea)

and annual aquatics (Myriophyllum petraeum). While some

endangered taxa have several populations spread across

a number of disjunct outcrops, some are confined to very

few localities (e.g. Myriophyllum lapidicola, known from

just two gnammas). Accidental destruction of such

populations could be catastrophic. Conservation in the

wild in such cases needs to be backed up by off-site

activites such as germplasm storage and artificial

propagation (underway at Kings Park and Botanic

Garden for M. lapidicola and other critically endangered

granite endemics; Dixon 1994).

Apart from endangered species, the diverse

communities on south-western granite outcrops are

noteworthy in the rapidity with which they change

within and between rocks. They are complex, ever-

changing, and rare in their own right.

While many granite outcrops have been spared direct

clearing due to their unsuitability for agriculture, and

some are included within conservation reserves, most

face threatening processes that need management if the

native biota is to persist. Such processes include

replacement or damage by invasive weeds, feral animals,

grazing, inappropriate fire regimes, clearing, loss of

shrub layer, salinity and dieback disease. We have briefly

addressed the issue of weeds above, and highlighted the

importance of maintaining undisturbed soil and dense

shrub layers to control weeds effectively. Such

restoration activities require an ongoing presence and

commitment to a given outcrop. Local communities are

vital in this context.

Western Australians are fortunate in being custodians

of a unique and diverse suite of granite outcrop plants.

We hope that this brief review will stimulate others to

study and conserve what is a remarkable heritage.

Acknowledgements: 

To all those colleagues who helped with field work,

discussion and ideas, our sincere thanks. L Sage assisted with the analysis

of granite orchid biogeography. R Wyatt and R Ornduff hosted SH on

sabbatical in the USA in 1990, and greatly facilitated development of an

international perspective on granite outcrop plants. S Porembski has

recently broadened this international perspective. We are grateful for

their interest and ideas.



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