Australia: The Land Where Time Began

A biography of the Australian continent 

Nutrient status (especially phosphorus) and location of rainforests

Research has been carried out on pockets of monsoon forest surrounded by savanna that studied 144 rainforest boundaries in northwest Australia. It was found that there was no connection between tree species diversity in the savanna and the level of phosphorus in the soil. Soils in the northern savannas had less than 500 PPM of phosphorus, as expected for impoverished soils of the region. It was found that some of the most diverse monsoon forests had soils with less than 200 PPM phosphorus. Such soils would be expected to have sclerophyll cover, according to current beliefs. It has been believed that sclerophylly evolved as a reaction to the depleted nature of the soil, preadapting sclerophyll vegetation to the dry conditions that replaced the moist conditions as the climate dried. A number of recent studies have found rainforest growing on many different soil types with greatly varying phosphorus concentrations. Rainforests are closed ecosystems, being maintained by the recycling of nutrients, the rate of circulation of nutrients being the determining factor of the biological fertility. They are another example of an ecosystem where their smallest biological components, the microbes and small invertebrates, control the health of the entire system by carrying out the processes that make the recycling possibly, mineralising biological detritus such as dead leaves. It is the biological fertility of rainforests that allow them to grow on nutrient deficient soils.

An example of rainforest on nutrient deficient soils is that on Fraser Island, off the Queensland coast.

Parent material and soil type as a determining factor of rainforest distribution

The suggestion that rainforests are only found on soils with a particular texture. The particular species of rainforest trees can restricted to soils with particular drainage characteristics. On Fraser Island the subtropical rainforest occurs in areas of high fertility, which were formed by the nutrient recycling, not because of nutrients in the parent material, while eucalypt occur on soils of lower fertility. The eucalypt forests don't recycle nutrients in the manner of rainforests.

The suggestion that rainforests occur along rainfall gradients

It has been believed that rainforests grow in areas with rainfall above 600 mm/year, eucalypt savanna replacing the rainforest in areas where rainfall drops below 600 mm/year. There is not a strong correlation between areas with rainfall and rainforest. Nothofagus is an example of rainforest tree in which some species, considered to require perhumid conditions, can tolerate some degree of water stress. In northern Australia rainforest commonly occurs on infertile soils in areas with low rainfall. This goes against the suggestion, based on an Edpahic compensation mechanism, that rainforests require fertile soils if they are to grow in areas of low rainfall.

Sources & Further reading

• Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
• Mary E. White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
• Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
• Dawn W. Frith & Clifford B. Frith, Cape York Peninsula: A Natural History, Reed, 1995