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The Jarrah Tree
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.
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.
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.
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.
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.
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.