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Chapter 1
Introduction to plant diversity of the Pilbara
Todd E. Erickson and David J. Merritt
Regional characteristics of the ‘The Pilbara’
Landscape and climate
In the northwestern corner of Australia’s arid zone lies the Pilbara biogeographical region (‘the Pilbara’). This region represents a unique, ancient landscape that consists of distinct geology, climate, topography, and vegetation (Figure 1.1)1. Situated largely in the summer rainfall zone of the Australian tropics, the majority of the rainfall occurs when temperatures are at their peak between December and March (Figure 1.2)2. Maximum temperatures consistently exceed 40–45°C in parts of the Pilbara during summer. In addition to these extreme temperatures, boom and bust cycles of moisture availability can be highly variable and dependent on summer thunderstorms, tropical cyclones and sporadic winter rains. Therefore, the flora of this arid region is highly attuned to survive harsh seasonal fluctuations in temperature and moisture availability and this is strongly reflected in the biology of the seeds found throughout the Pilbara.
Within the Pilbara, four floristic subregions are recognised: the Hamersley, Eortescue, Chichester, and Roebourne subregions (Figure 1.1)1. The elevated ranges, ridges, and mesa outcrops of the Hamersley Ranges located in the south (Figure 1.3) are the landscape features most characteristically associated with the Pilbara. Some of the world’s oldest rocks, at least 3.5 billion years old, can be found in these landscapes. Feeding out from, and winding through these hills, are deep gullies, drainage lines, and creeks that flow into the many rivers, drainage flats, and floodplain networks of the Eortescue Marsh to the north and the Gascoyne region to the south.
The Fortescue subregion bisects the Pilbara in an east-west direction and forms a distinct topographical barrier between the southern mountains of the Hamersley Range, and the northern regions that contain more isolated rocky ranges (e.g. Chichester Range), granitic outcrops, extensive plains, coarse river systems (e.g. the De Grey, Fortescue, and Yule Rivers), and the fringing coastal zone. The extensive area of Fortescue Marsh provides a stong biogeographic boundary or barrier for many Pilbara species, both plants and animals. Ecological studies of geckos have highlighted that this extensive topographical divide has created a long-standing genetic divergence between the north and south3. Yet, outside of these studies, very little is known about the unique evolutionary history of the Pilbara’s flora and fauna and further studies should be undertaken across a wide range of organisms to shed light on this history.
Figure 1.1 Much of inland Australia is considered arid to semi-arid with an average annual rainfall of <350 mm (yellow shading in A). The Pilbara region is situated in northwestern Australia and is known for its unique geology, topography, and vegetation associations (black outline in A). Floristically, the Pilbara is divided into four subregions: the Hamersley, Fortescue, Chichester, and Roebourne (B). Regional towns are scattered throughout the Pilbara and support various mining, pastoral, and tourism industries associated with major national parks.
Figure 1.2 Long-term climate data for Newman Airport 1971–20142. Data represents average monthly rainfall and average monthly maximum and minimum temperatures. Peak rainfall and hot summer temperatures in the Pilbara occur between December and March, with rainfall being highly variable from year to year.
Figure 1.3 Escarpments covered in Triodia hummock grasslands are characteristic of the Hamersley subregion. Plant diversity and community structure changes across escarpment ridges (A), mesa cliff faces (B), rocky mid-slopes (C), and drainage flats usually dominated by Acacia shrublands (D).
Regional plant communities and species diversity
Across the whole Pilbara, total plant diversity is approaching 1,800 species, with approximately 15% of these species endemic to the region4. Floristically, hummock grasslands (Triodia spp.) dominate the mountain chains and the more skeletal soils of the undulating plains (Figure 1.4). The region is also known as a secondary centre of species richness and endemism for Acacia species in Australia5. Criss-crossing and bisecting the expansive hummock grasslands are shrublands situated on the deeper soils and drainage flats made up predominately of wattles and grevilleas (Acacia and Grevillea spp.), scattered mallees and trees (Eucalyptus and Corymbia spp.), and tussock grasslands (Aristida and Eragrostis spp.). More striking are annual species that abound after good rains as well as fire ephemerals and herbaceous elements that grow in higher densities immediately after fire and in the buffered corners of the landscapes (e.g. rocky mesa outcrops and permanent waterholes).
Figure 1.4 Hummock grasslands of the Pilbara dominate most landscapes. Currently, 22 different spinifex grasses (Triodia species) are recognised in the region6 (Photo: Alexandra Rouillard).
Species selection for restoration and conservation programs
When considering the differences in plant species and community assemblages, both through space and time, it becomes evident that there is a wide array of vegetation types present in Pilbara ecosystems. If land restoration and conservation projects are to be successful, a wide selection of species from these vegetation types needs to be available. The four most common families in the Pilbara, in terms of species richness, are the Fabaceae, Poaceae, Malvaceae, and Asteraceae (Table 1.1)4. The core elements of the vegetation comprise long-lived perennial grasses, shrubs, and trees, and include species of Triodia, Eragrostis, Acacia, Senna, Tephrosia and Eucalyptus. Targeting plants from these framework groups and obtaining sufficient quantities of seeds will ensure that the majority of species are available for restoration and conservation programs.
Table 1.1 The top 10 families and genera by number of species in the Pilbara.
References
- 1. McKenzie N., van Leeuwen S., Pinder A. (2009) Introduction to the Pilbara biodiversity survey, 2002–2007. Records of the Western Australian Museum, Supplement 78, 3–89.
- 2. BoM (2015) Bureau of Meteorology, Commonwealth of Australia, URL: http://www.bom.gov.au
- 3. Pepper M., Doughty P., Keogh J. S. (2013) Geodiversity and endemism in the iconic Australian Pilbara region: a review of landscape evolution and biotic response in an ancient refugium. Journal of Biogeography 40, 1225–1239.
- 4. DPaW (2007–) Nature Map: Mapping Western Australia’s Biodiversity, Department of Parks and Wildlife, URL: http://naturemap.dpaw.wa.gov.au/
- 5. González-Qrozco C. E., Laffan S. W, Miller J. T. (2011) Spatial distribution of species richness and endemism of the genus Acacia in Australia. Australian Journal of Botany 59, 600–608.
- 6. Western Australian Herbarium (1998–) FloraBase – the Western Australian Flora, Department of Parks and Wildlife, URL: http://florabase.dpaw.wa.gov.au/
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Chapter 2
Seed collection, cleaning, and storage procedures
Todd E. Erickson and David J. Merritt
Seed Collection
For many plant species in the Pilbara, outside of those that are dominant and easily collected, knowledge of flowering phenology and seed maturation timing is limited. The phenological information reported in this section is compiled from personal observations of the authors (Chapter 5), the flowering period reported on databases such as FloraBase1, and, if available, records from commercial seed collections made within the last five years. As further knowledge is gathered, guidelines as to the optimal time for collecting will become more readily available. Nevertheless, it is clear that timing of seed collection in these arid landscapes is heavily influenced by localised rainfall. Seed collection programs need to be carefully planned and communicated to on-ground practitioners well in advance of the collection season to ensure the timely collection of priority species. Importantly, collection activities may need to be organised at short notice to capitalise on sporadic rainfall events that are a char...
