Climate and Landforms
A Mediterranean climate in an ancient landscape
There are many factors that give this part of the world its very special nature. Two of them are its climate and geological history.
The southwest corner of Australia has a Mediterranean climate, which generally means moist mild winters and warm to hot and mostly dry summers. This basic climatic type is shared with other parts of the world that are also described as Mediterranean, including the countries around the Mediterranean Basin itself, parts of western North America, in southwestern South Africa, parts of central Chile, and part of southeastern Australia. All these countries occur between about 30-50° south or north of the equator.
But the broad classification of Mediterranean, can include considerable variations in temperature, rainfall and of course vegetation types. Within the Gondwana Link area, there is a clear gradient of both rainfall and temperature from the southwestern corner through to the semi-arid interior beyond the Great Western Woodlands, reflected in a transition from the karri and jarrah forests, through drier and warmer woodlands of wandoo, yate and mallees, through to the shrublands bordering the more desert-like environments north east of Kalgoorlie.
Average conditions are useful for classifying climate types and for understanding what vegetation might be expected in a region, but it’s often the variability and reliability, particularly of rainfall, that determines how species and communities evolve and survive in any particular location. Extreme rainfall events, for example, of 200mm or more can fall in a day in just about any part of Gondwana Link – and there can be weeks, months or years in which the rainfall is only a fraction of its long term average.
Some recent studies in the Great Western Woodlands have shown a long term pattern to the rainfall, with 20–30 year below-average periods alternating with 15-year above-average rainfall periods over the last 350 years (Cullen and Grierson 2009), and some similar patterns have been detected for the southwest’s rainfall generally. Recent trends (see map below) show a decrease in rainfall over the southwest – whether or how much of this is due the longer term cycles or is exacerbated by greenhouse-related climate change is unknown, but the trend is clear. We are currently in an extended below average rainfall period across much of the area in which we work, and we don’t know if or when this will turn.
Geology & landforms
The underlying geology is the base on which the landforms we see today have been built, and provide the material from which soils form, so they are a major influence on the resulting vegetation complexes. A very brief summary of the major geological formations within the Gondwana Link area follows, but you can download here further descriptions of a classification into ‘ecodistricts’, based on geology, climatic history, drainage patterns, major soil systems and native vegetation communities that has been applied to part of Gondwana Link and explains the ecological settings in more detail (McQuoid 2009).
South-western Australia can be pictured as a central and dominating plateau of Archaean granites and gneisses, formed somewhere between 2500 and 3700 million years ago. This Yilgran Craton was worn down by glaciers in the great Ice Ages about 250-350 million years ago but other than that, has had no mountain building events or other eroding forces other than the slow wearing away by weather. Through wet and dry cycles, laterisation of the crust occurred and resulted in the iron and aluminium-rich gravels we see particular in the higher rainfall areas, but even this slowed over much of the craton as Australia drifted further north and into an increasingly arid climate. As aridity increased, the great rivers that used to flow to the east and into the ocean where the Nullarbor Plain now exists, became a series of salt lakes, while to the western and southern margins of the craton, rivers continue to flow to the Indian and Southern Oceans.
The western edge of the Yilgarn Craton is marked by the Darling Fault, and within the Gondwana Link area, the area between the Darling Fault and the Dunsborough Fault further to the west is a rift formed by downfaulting of Proterozoic granites when Australia and India separated. The rift valley has since been covered with younger marine sedimentary rocks that are Phanerozoic in origin and formed between 65-250 million years ago. West of this again, and marking the edge of the continent (but extending under the sea) is the Leeuwin Complex, a north-south ridge about 15km wide and 100 km long made up of Proterozoic granites, formed about 1600-2500 million years ago which are partly overlain by Tertiary laterite and sand, and by the sandy limestone of the Tamala Limestone. The Tamala Limestone is derived from coastal dunes that formed between 120 000 years to 1 million years ago (Appleyard, 1989), from calcareous sand that was blown from the west during periods of maximum glaciations when sea levels were low. The sand accumulated on the western side of the laterite covered erosion surfaces of the Leeuwin-Naturaliste ridge, and was lithified by groundwater. Recent erosion of the limestone has led to the formation of numerous cave systems (Myers 1994).
Along the southern and southeastern edges of the Yilgarn Craton, there were other massive events associated with Australia and Antarctica first coming into contact and then later tearing apart. A series of events termed the Albany Fraser Orogen produced complex formations that shape today’s southern coastal and near-coastal landforms, and extend to the north east from the Ravensthorpe area to the Goldfields where they are associated with a lot of that region’s mineral wealth.
Some of these major formations are Proterozoic granites and gneisses, and include the Nornalup Complex where granite outcrops like Mt Lindesay and Mt Frankland occur within the forest mosaic, the Porongurup Range and the coastal granites of the Albany and Bremer regions, such as Mt Manypeaks, the Bremer peninsulas, and many of the islands off shore from Albany and Esperance.
The Stirling Range and the Barren Range in the Fitzgerald River National Park have a different underlying geology, having been formed by upfolding of sandstones and quartz originally laid down under a sea bed. The distinct geology of these peaks and their climatic difference (due to their altitude – albeit, very modest) contribute to their extraordinary concentration of short range endemic plants.
Yet another formation along the southern coast also contains Archaean granites, but with metamorphosed sedimentary and volcanic rocks too, and forms the greenstones of the Ravensthorpe Range. The distinctive mineralogy here also reflects in some of the endemic species.
A younger formation along the south coast is the extensive marine sandstones and valley fill deposits which were laid down as sea bed before being exposed by major sea level changes, and which today are seen as the Pallinup siltstones and the marine plain that forms the Fitzgerald and Esperance sandplains. Known here as spongolite, these rocks are made up of spicules of sea sponges, and often contain fossilised leaves and other organisms. Where they are currently eroding, they form spectacular breakaways.