Why are the Plants and Soils in Foxes Lair so Diverse?
The answer is a bit complicated, but is a combination of rock types, plate tectonics, climate, and plants and microbes.
Geology
This region is a very stable piece of continental rock called the Yilgarn Craton that is mainly composed of light (silica-rich, felsic) rocks that form sandy surfaced soils. In contrast rocks under the ocean floor are dark heavy, and full of heavy (mafic) minerals that form red brown loam to clay soils. Dolerite is a common mafic rock
Heat from the earth’s interior caused mafic rock to well up in mid ocean rifts that pushed huge ‘plates’ of continent and sea floor apart where they collided with other plates to create larger land masses, mountain chains and volcanoes in a process called plate tectonics. These rejuvenate soils by bringing new rock to the surface and steep slopes assist its breakdown.
Although plates collided around it to form the Gondwanaland supercontinent, the Yilgarn Craton stayed largely unaffected above sea level. Soils slowly formed and were eroded and transported into surrounding basins and seas. However stresses in the rock caused cracks in the craton and liquid mafic rock squirted up from depth to fill the cracks that form lines of heavy black rock called mafic dykes. Foxes Lair straddles a the huge Binneringie Dyke that stretches from Quindanning to Coolgardie. Red soils from this dyke are exposed on the southern edge of Foxes Lair.
When the Gondwana supercontinent split and Western Australia separated from adjoining India and Antarctica, land movements caused large changes in climate and river direction.
This region is a very stable piece of continental rock called the Yilgarn Craton that is mainly composed of light (silica-rich, felsic) rocks that form sandy surfaced soils. In contrast rocks under the ocean floor are dark heavy, and full of heavy (mafic) minerals that form red brown loam to clay soils. Dolerite is a common mafic rock
Heat from the earth’s interior caused mafic rock to well up in mid ocean rifts that pushed huge ‘plates’ of continent and sea floor apart where they collided with other plates to create larger land masses, mountain chains and volcanoes in a process called plate tectonics. These rejuvenate soils by bringing new rock to the surface and steep slopes assist its breakdown.
Although plates collided around it to form the Gondwanaland supercontinent, the Yilgarn Craton stayed largely unaffected above sea level. Soils slowly formed and were eroded and transported into surrounding basins and seas. However stresses in the rock caused cracks in the craton and liquid mafic rock squirted up from depth to fill the cracks that form lines of heavy black rock called mafic dykes. Foxes Lair straddles a the huge Binneringie Dyke that stretches from Quindanning to Coolgardie. Red soils from this dyke are exposed on the southern edge of Foxes Lair.
When the Gondwana supercontinent split and Western Australia separated from adjoining India and Antarctica, land movements caused large changes in climate and river direction.
Granitic sandy soil at Arboretum picnic area
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Mafic red brown loam near claypit
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Biology
Vegetation is a good indicator of the soil underneath. For example, marri (Corymbia calophylla) grows on sands and gravels, brown mallet (Eucalyptus astringens) on mottled clays around ironstone ridges, wandoo (Eucalyptus wandoo) on the sand over clay (and other soils), and York gum (Eucalyptus loxophleba) prefers fertile soils that have formed from fresh granite and dolerite bedrock.
Recent research has shown that most soils in the reserve have been created by distinctive native vegetation through root secretions that stimulate microbes and fungi to form clays and laterites. These favour these plants and restrict competing ones.
Two examples are:-
Vegetation is a good indicator of the soil underneath. For example, marri (Corymbia calophylla) grows on sands and gravels, brown mallet (Eucalyptus astringens) on mottled clays around ironstone ridges, wandoo (Eucalyptus wandoo) on the sand over clay (and other soils), and York gum (Eucalyptus loxophleba) prefers fertile soils that have formed from fresh granite and dolerite bedrock.
Recent research has shown that most soils in the reserve have been created by distinctive native vegetation through root secretions that stimulate microbes and fungi to form clays and laterites. These favour these plants and restrict competing ones.
Two examples are:-
Lateritic gravels. Many upland soils in Foxes Lair are lateritic (gravelly) with lots of brown round stones that are high in iron and aluminium. These soils are underlain by bauxite, which is mined in the Darling Range futher west.
Laterites have diverse and colourful wildflowers, particularly the Proteaceae (banksias, hakeas, grevilleas etc) and Casuarinaceae (tammas, sheoaks) families. These plants are dominant here because they can extract phosphorus (a scarce nutrient) that is unavailable to most plants on these infertile soils. They do these with special cluster roots that release an organic acid in to the soil in winter. The acid releases phosphorus from soil particles for the plants to use. Iron that causes the red and brown colours in soil, and aluminium are also released. Soil bacteria then use the remaining acid for food, and in doing so, cause the iron and aluminium to become solid again (precipitate), on gravel stones in the topsoil or down plant root channels. If you look inside gravel stones, you will often see the deposited iron layers.
Laterites have diverse and colourful wildflowers, particularly the Proteaceae (banksias, hakeas, grevilleas etc) and Casuarinaceae (tammas, sheoaks) families. These plants are dominant here because they can extract phosphorus (a scarce nutrient) that is unavailable to most plants on these infertile soils. They do these with special cluster roots that release an organic acid in to the soil in winter. The acid releases phosphorus from soil particles for the plants to use. Iron that causes the red and brown colours in soil, and aluminium are also released. Soil bacteria then use the remaining acid for food, and in doing so, cause the iron and aluminium to become solid again (precipitate), on gravel stones in the topsoil or down plant root channels. If you look inside gravel stones, you will often see the deposited iron layers.
closeup of a cluster root
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iron layers in gravel
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Iron deposited in root channels
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Over thousands of years, a deep laterite profile forms. This is often a gravelly topsoil underlain by ironstone, a mottled iron rich clay (mottled zone), a white clay (pallid zone), and finally the crystalline rock (light coloured granite or dark dolerite).
Laterites vary greatly. They range from those with dark heavy gravel stones and red-brown soil to others with pale light sandy gravels, and even ironstone pipes.
Laterites vary greatly. They range from those with dark heavy gravel stones and red-brown soil to others with pale light sandy gravels, and even ironstone pipes.
- Mesas are lateritic gravel flat-topped hills with steep sided slopes on one or all sides called breakaways.
Exposed domed clay subsoil near the claypit
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 Reddish clay nodules formed around roots
Clay subsoils Many eucalypts secrete organic chemicals from their roots that stimulate microbes and fungi to produce clay subsoils. In the image below left from Chillnup, sandy topsoil has been blown away to show nodules of reddish clay that has formed around the mallee roots. Most eucalypt tree and mallee soils are duplex, with a sand or loam topsoil overlying dense clay subsoil. |
Climate and geological uplift
About 100 million years ago and less the final separation of Western Australia from India then Antarctica, caused the gradual uplift of the Darling Range, other movements that blocked some west flowing rivers and reduced inland rainfall. This led to our present broad valleys in the east, and more active valleys to the west where rivers have forced their way through the Darling Range. Slope changes and water erosion has cut into the previously gently sloping landscape around Narrogin.
These events have coincided with a change in the world's alternating warm wet and cold dry climate cycles..
Wet cycles favour gentle landscapes and laterites. Dry cycles coincide with cold weather. barer soils, infrequent flash floods and very strong winds that cause salt lakes and adjoining dunes. The steep valleys and lateritic mesas and breakaways that can bee seen on the Breakway Walk formed then.
We are currently in a wetter geological period (excluding man made climate change).
About 100 million years ago and less the final separation of Western Australia from India then Antarctica, caused the gradual uplift of the Darling Range, other movements that blocked some west flowing rivers and reduced inland rainfall. This led to our present broad valleys in the east, and more active valleys to the west where rivers have forced their way through the Darling Range. Slope changes and water erosion has cut into the previously gently sloping landscape around Narrogin.
These events have coincided with a change in the world's alternating warm wet and cold dry climate cycles..
Wet cycles favour gentle landscapes and laterites. Dry cycles coincide with cold weather. barer soils, infrequent flash floods and very strong winds that cause salt lakes and adjoining dunes. The steep valleys and lateritic mesas and breakaways that can bee seen on the Breakway Walk formed then.
We are currently in a wetter geological period (excluding man made climate change).
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