Origins
Eucalypts developed in the warm, moist climate and fertile soils of the ancient southern super-continent, Gondwana.

Australia broke from Gondwana about the tinge Angiosperms, (flowering plants) appeared. Eucalypts may have been among the first of them.

The jarrah tree (Eucalyptus marginata) came from this stock.

It is endemic to the south west coastal region of Western Australia. It has adapted to the wet winters and dry summers, the nutrient poor soils and fire outbreaks of the elevated Darting Scarp and surrounding country. Its adaptations are so perfect it has established tile only tall forest in the world to exist in a truly Mediterranean climate.

Climate
The annual rainfall of the scarp varies from north to south from about 750 mm to 1400 mm. Most of it falls in the winter and soaks through the gravelly ground surface to the clays below.

Jarrah's strategies to cope with dry summers are its extensive root system. its conservative development and slow rate of growth. It has the capacity to transfer nutrients from older to younger parts and to regenerate after fire by shooting from branches and trunk.

Jarrah has two special types of roots:

1. deeply descending sinker roots that pass through cracks in the hard surface to moist clays far below, and

2. extensive fine feeder roots in the surface soil growing on a permanent framework so that they can quickly develop to absorb moisture, even from the occasional summer rains.

Soil
The Darling Scarp has been weathered by rain for many millions of years. Nitrates and phosphates, essential nutrients that are most likely to limit plant growth, have been leached out and are in short supply. Jarrah seedlings frequently germinate and grow after the occasional fire, on the ash-bed where the nitrates and phosphates are more readily available and competing understorey plants have been killed.

If the seedlings arc sheltered from sunlight by the forest canopy, they may remain as compact shrubs for tip to fifty years.

During this time they put down sinker roots and also establish a ligno-tuber; that is a root swollen with stored food and containing buds ready to grow up rapidly when a gap appears in the canopy.

The ligno-tuber is a storehouse for the minerals, such as phosphates, needed for tree growth.

Typically. jarrah has a leafy crown forming a shady canopy, which inhibits the growth of seedlings below. In this way the big, old trees control the density of the forest , keeping it in balance with the supply of water and minerals.

Wildfire
The wide spacing of trees resulting from canopy control of understorey and seedling growth. meant that it was difficult for fire to take hold in the natural, old growth forest. This is confirmed by the examination of growth rings near the ground in large, old stumps. Also. as litter accumulated on the shaded forest floor, the ground was only dry for relatively short periods.

There is no evidence of widespread fires in the undisturbed forest of the past. though there must have been some small fires that assisted seedling germination and growth. The forest controlled the burning patterns to be most benificial to its existence.

Disturbance
All these evolutionary features give jarrah a high degree of resilience. where resilience is defined as the process of restoration of initial structure and function after a disturbance or period of stress.

Unfortunately, humanity's interference through clearing, logging. burning and fragmenting with roads, mining operations and towns has significantly reduced the forest's ability to restore itself.

Selective logging and clearfelling have reduced the canopy. When trees are cut down they can no longer take tip groundwater, transpiring much of it into the atmosphere. Then water tables rise and waterlogging occurs in winter. This, in turn, can release some of tile salt normally stored deep below the surface and may encourage the spread of tile deadly dieback fungus. With the removal of the canopy, too many seedlings are now released to grow and further disturb the control of ground moisture.

Litter is now regarded as fuel instead of food. When litter is burnt nutrients and energy are lost to the atmosphere and nutrient and energy cycles are upset.

Disturbance to the interaction between soil, water and temperature has allowed weeds to invade and colonize disturbed areas.

Fragmenting the forest with networks of roads, logged and burnt areas has allowed invasion by feral animals, particularly predatory foxes and cats. which have had a serious effect on wildlife, particularly small mammals like woylies.

Constant, regular burning regimes have favoured some members of the associated understorey plants but reduced the distribution of others.

Canopy and litter destruction have had enormous effects on populations of insects and other invertebrates; we now see damaging outbreaks of leaf-eating insects that can regularly defoliate and weaken young seedlings and mature trees. Many effects are still unknown.

Precautionary Principle
Where there is a lack of knowledge of how an ecosystem works and there are threats of serious or irreversible consequences, precautions should be taken to prevent damage that cannot be anticipated.

Tile jarrah forest is a very ancient and complex ecosystem and far too little is known about how it works.

What is a closed canopy forest?

A forest where the leafy canopy excludes sunlight from the forest floor

 

Did the jarrah forest have a closed canopy before it was subject to human disturbance?

Evidence indicates the answer is yes. The leafy canopy prevented the understorey from becoming too thick and maintained the litter layer. So the fire hazard was controlled naturally. Logging and burning decreases the canopy and so increases the fire hazard.