The temperate rainforest within The Otways mountain range has remained intact for more than 40 million years.
Approximately 380 to 500 million years ago all the earths’ land was in a supercontinent called Pangea. About 180 to 200 million years ago the southern part on Pangea started to separate and form a southern land mass. This was named Gondwana land. This included modern day southern Africa, India, Australia, Antarctica and South America.
In the eocene paleogenic period about 38-55 million years ago Gondwana had almost split apart but Australia, Antarctica and South America remained joined. The Australian part of Gondwana land was then near the South Pole.
The Eocene world was experiencing increased atmospheric CO2 and a warm wet climate. Antarctica had no ice and the Pangean conifer forest of a previous colder period was replaced by temperate rainforest. These forests contain 3 to 15 tree species with slender even trunks that grow to a great height.
The Australian southern temperate rainforest is characterised by Antarctic beech myrtle. Trees in these forests can live a long time and some Tasmanian Huon pine are thought to be more than 2000 years old.
Australia was joined to South America by a forest corridor and similar rainforest trees are found in Australia and South America. About 40 million years ago sea floor spreading accelerated and a wide sunken rift valley developed between Southern Australia and Antarctica as they began to tear apart. Australia was now joined to Antarctica via Tasmania. With continued sea floor spreading and Australia moving north Tasmania was separated from Antarctica.
The sub polar currents that had been forced north now circulated around the Antarctic landmass that became frozen. Australia having moved north was now warmer and drier.
Rainforests require a wet climate so they are now present in coastal ranges where there is adequate rain. The Otway forest inhabits the southern slopes of the Otway range west of Melbourne. This area has the highest rainfall in Victoria and provides the habitat for the most western temperate rainforest on mainland Australia. This forest has the closest Australian mainland floristic resemblance to the cool temperate rainforests of Tasmania.
In the Otways there remains only 11.8 square kilometers of pure beech myrtle rainforest on public land. The rainforest exists between Lavers Hill and the headwaters of the Cumberland River with the greatest concentration in the Aire Valley.
The trees of a rainforest form a canopy that excludes more than 70% of sky. This results in little sun reaching the forest floor that is not densely populated by grasses but is a suitable habitat for ferns and lichens which like shade. Rainforests are populated by angiosperms that have coated seeds in contrast to the conifers that have naked seeds.
Angiosperms principally propagate by seeds that are fire sensitive and only 40% of rainforest trees regrow after fire. In the Otways the original native rainforest microclimate was naturally fire-retardant. Fires were usually canopy fires. The low fuel load of the forest floor and the cool dense undergrowth resulted in low intensity fires that were extinguished. These fires did not kill the rainforest trees and they were infrequent enough to allow regrowth by seeds.
Native rainforest species like Myrtle Beech once covered all of Victoria and most of South-eastern Australia.
In Australia Eucalypts were also developing a presence and their canopy allowed more sun to the forest floor. This resulted in grasses replacing the ferns. The forest now had a high fuel load of grass and the high oil containing eucalypts. Fires in this forest were high intensity fires. Fires were more frequent and intense enough to kill the rainforest trees and frequent enough to prevent seedling growth.
Because Eucalypts regrow after fire and some of their seeds require fire to aid germination eucalypts had a selective advantage. As a result the majestic temperate rainforest were decimated by thousands of years of Aboriginal fire stick farming, and finally reduced to remnants by centuries of European Australian grazing, mining, and logging.
The great forests were replaced by fire-fuelling eucalypts with a grass fodder understory, which has delivered the perfect conditions for super-fires.
Restoring areas of rainforest would assist in hydrating and stabilising the landscape, shading and rebuilding topsoil, and increasing rainfall to our water catchments. Re-vegetating fire-ravaged zones with rainforest corridors would create firebreaks and connect fragmented animal and plant species populations, encouraging greater genetic diversity.Tweet
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