Are Australian native rainforest trees the answer to our climate emergency?

The geographic break up of Gondwanaland that began 180 million years ago saw the separation of Africa, South America, India and Antarctica. The Australian continent moved north from Antarctica, with northern Australia later separating from South East Asia, Indonesia and New Guinea, with a final split occurring about 10,000 years ago.

The Daintree in Far North Queensland contains the world’s oldest tropical rainforest, which is part of an incredibly diverse and important ecosystem that contains over 50% of the world’s biodiversity. Preservation of these ecosystems are vital to the human habitability of our planet.

With the drying of the Australian continent and the invasion of dry hard-leaved sclerophyll evergreens (such as eucalypts) into this space, our rainforests today occupy less than 1% of our continent, stretching in pockets along the east coast from the subtropics to temperate Tasmania.

Defining a rainforest

It is the rapid recycling and fungal decomposition of dead leaves and branches that have helped Australia’s rainforests build high-carbon organic soils that are incredibly fertile, supported by high rainfall, high humidity and extremely low light conditions.

Trees can reach 30–45m tall, with Gothic cathedral-like buttresses and wide canopies providing extremely dense, dark shade. Monocotyledons, such as the truly ancient pines we now know as Bunya, Kauri and Hoop Pines, were previously more prominent but have now been partially replaced by flowering rainforest trees (dicotyledons).

A middle layer of palms and vines break out when gaps in the dark canopy appear and light breaks through. Because light on the ground is very low, ferns, mosses and fungi thrive. Deep loamy fertile soils hold moisture and decomposing leaves retain growth, without need for phosphorus as in most Australian soils. Rainfall can reach from 1.3–4m annually, which creates high humidity and dense shade, preventing dry sclerophyll plants like wattles, eucalypts and casuarinas from entering.

Directors of Australia’s botanic gardens (that stretch from the subtropics of Brisbane and Sydney to the Mediterranean climates of Melbourne and Adelaide) trialled broad-leaved evergreen trees and prehistoric pines over 175 years ago. Adaption to lower rainfall areas from well-watered fertile soils with a minimum of just 600mm rainfall has proven successful, with almost equal tree heights in Adelaide and Melbourne despite their dry summers and autumns.

Rainforest summary

Low light, high rainfall (700mm–2m), high organic soil content.

The threat posed by eucalypts

Our rainforests are threatened by a warming climate that suits eucalypts. The long term threat from dry sclerophyll plants, such as eucalypts and wattles, which are adapted to fire, now threaten our precious rainforest evergreens at their edges, posing an even greater threat to their survival than climate warming.

Fire actually benefits eucalyptus trees. Now that eucalypts have been planted around the world, it is not only Australia that is reappraising their suitability in a warming climate. Eucalypts grow very quickly and their oil rich leaves and peeling, flammable bark have many foresters claiming that eucalypts are the worst trees in the world for wildfires.

“Looking at the eucalyptus forest outside my window in Tasmania, I see a gigantic fire hazard.” David Bowman, forest ecologist, University of Tasmania.

Dry sclerophyll plants

Native dry sclerophyll plants have adapted to dryness with usually parallel hard tissue, short internodes and stiff leathery leaves. These plants thrive in the Mediterranean climates of California, South Africa, Chile and Australia in full sun. In addition to eucalypts and acacias, Australian plants including banksias, casuarinas, callistemons and melaleucas also thrive in these conditions. Typical dry sclerophyll plants from the Mediterranean include rosemary, lavender, Stone Pine and Italian Cypress.

The major threat to survival of our precious rainforests comes from adjoining sclerophyll vegetation (basically eucalypts, which are assisted by fire). As our weather system moves across Western Australia from west to east, it passes over our hot arid desert, from which weather systems dry out the edges of our evergreen rainforests. Rainforest trees are slow to burn but do not recover from fires like eucalypts.

Eighty percent of Australia’s forest trees are eucalypts. Their hard evergreen leaves hang obliquely to minimise moisture loss but, in the process, encourage soils to dry out. This creates the dangerous conditions that appear to encourage the flammable eucalypt trees to suicide.

But in destroying competing plants, eucalypts rapidly replace themselves by spreading seeds and rapidly resprouting from undamaged lignotubers.

Dry sclerophyll summary

Dry warm summers and wet wild winters (e.g. Mediterranean, California, Chile, South Africa, Victoria’s Mallee), low rainfall (200–500mm), low moisture and carbon soil content.

Note: Wet sclerophyll forests contain gigantic flammable eucalypts that almost equal the world’s tallest trees, that being Californian redwoods. These massive Mountain ash trees create open forests with tree ferns underneath but have far too much light to establish true native, closed canopy rainforests and they are just as flammable as dry sclerophyll forests.

Written by Clive Blazey, co-founder of The Diggers Club and The Diggers Foundation.