From a distance the Island still rises green above the glistening blue. The tree canopy screens most of the houses above the waterfront. These trees make the Island more beautiful to look at and much more attractive and pleasant to live on. They also give food and shelter to the many other creatures for whom the Island is home. They are a vital part of the Island's charm with an appeal which translates into real estate dollars.
The Island's distinctive spotted gum forests are a special feature of Pittwater, as it is one of the only places where the spotted gum naturally occurs in the Sydney region. At a larger scale, our spotted gums have nature conservation significance in New South Wales, as only small amounts of this forest type are conserved in National Parks.
Our NativeTrees - Life In a Web of Relationships!
It's easy to take trees for granted. Trees look tough. They seem to be an enduring part of the landscape.
What is not so obvious is the hidden web of relationships within which a tree lives. Links with the physical environment - light, air, water, soils and nutrients. Partnerships with other plants and many sorts of animals - soil micro-organisms, insects, birds, pertiles and marsupials all form the tree's community.
Trees grow best in a plant community which includes shrubs and ground cover plants, as found in healthy bushland. Other small plants help to keep the soil moist and recycled nutrients from these plants can be used by the trees. They are also food for insects, birds and other animals.
Changes in the living or non-living enivironment may affect the life of a tree - sometimes for better, but often for worse. Death often takes years as the tree goes into a slow decline.
Our Native Trees - Under Threat?
All is not well with the Island's native trees. The Dieback study concluded there is a moderate problem with the dieback of native trees in the Island's urbanised areas and that natural regeneration is not occuring adequately to ensure the long-term survival of a healthy tree canopy
In the gullies of the south side many dead trees are obvious. There are other patches where the trees are still alive but their foliage is thinning, the ends of their branches are bare and skeletal. In many other places individual trees have died or are stressed. Dead trees are usually removed from people's blocks - their loss unnoticed (soon forgotten?) by passers-by. At the same time we are changing the environment in ways which prevent seedlings from growing up as replacements. It is death by a thousand cuts for the forest canopy as a whole.
What We Need To Do
1. Value our native trees highly and understand what they need.
2. Act to preserve existing trees.
3. Plant young ones to provide a new generation for the future. Choose species indigenous to your part of the Island.
Roots - What Matters?
Changes affecting roots are a common cause of sick trees.
The tree's large roots support the tree in the ground, bracing it against winds and the great weight of its own trunk and branches. Larger roots may be deep or shallow depending on the soil. Smaller feeder roots are also vital, supplying the tree with nutrients. These fine roots extend out to the dripline under the tree's outermost foliage. Nutrients are concentrated in the area beneath the tree's branches, just 30-60-cm below the soil surface. As organic matter breaks down on the soil surface, fine feeder roots recycle the nutrients back to the tree.
Friendly Fungi
A surprising fact is that healthy trees (and many other plants) depend on the help of various species of fungus growing on their roots. These are called mycorrhizal fungi. We often think of fungi as being undesirable, but certain fungi have a mutually beneficial relationship with their tree host.
In the soil these fungi take the form of a huge web of tiny threads called hyphae, vastly increasing the tree's root length . The tree provides the fungi with sugars, and the fungi help the roots to collect essential nutrients and moisture, usually in short supply in local soils. Occasionally the fungi may produce fruiting bodies such as toadstools.
Micro-organism Mates
In many trees and shrubs of plants in the Proteaceae family, such as Banksia, Hakea and Grevillea, another special kind of root develops. A particular microorganism in the surface root area promotes the growth of a dense mat of fine roots, particularly in soils rich in decayed organic matter. These fine proteoid roots efficiently collect phosphorus, usually in short supply in undisturbed Sydney soils.
What can go wrong?
Any change affecting the health of roots and the fungi and microorganisms which help them to function will threaten the life of the tree. Soil normally contains some air, vital to the healthy growth of roots, fungi and many beneficial soil microorganisms.
Excess Water and Nutrients - More Is Not Better
On Scotland Island excess water and excess nutrients have been identified as the major cause of tree death.
Excess Water
Problems occur due to:
· waterlogging -excess water displaces vital oxygen from the soil which becomes too low in oxygen. The constant wetness encourages harmful root-rotting fungi. .
· excessive drainage through the soil leaching out nutrients and causing erosion or soil compaction.
These problems occur in the course of residential development, when septic tanks are installed, construction works alter drainage lines, and stormwater runoff occurs.
Excess Nutrients
Native vegetation is mostly adapted to soils with very low levels of nutrients. Too much extra becomes a toxic overdose for the tree and the beneficial fungi and soil organisms.
Changes in Soil Level - More Soil Can Be Fatal
An extra 10cm or more of soil within the dripline can starve roots and mycorrhizal fungi of essential soil oxygen. The greater the depth of fill, the less the chance of survival of plants, particularly of older trees.
Less Soil Can Be Fatal Too
Removing surface soil and excavation within the dripline will harm the tree by cutting off its feeder roots. Death may be swift. Removal of larger roots by cutting trenchs for drains and foundations also harm a tree. Another major cause of tree death on Scotland Island has been excavation for road building. Trees have died, fallen and been removed so the evidence is historical rather than before our eyes.
Hard Surfaces Make Life Hard
Placing hard surfaces around trees such as concrete or pavers reduce air in the soil and the soil becomes too acidic for beneficial soil organisms.
Soil Compaction
Soil beneath a tree may become compacted due to constant walking or vehicle use. The soil organisms suffer yet again and so does the tree.
Clearing Shrubs and Ground-Covers
When the native shrubs and ground covers around a tree are removed, part of the support community of other organisms is lost and the soil moisture and temperature changes. Physical damage to the bark from lawnmowing and whippersnippering will provide a way into the tree for harmful fungi. A ring of such injuries around the trunk can ring-bark the tree.
What About Insects?
Insect attack is often a symptom rather than a cause of tree death. A healthy eucalypt or angophora produces kino, a sticky sap which engulfs and kills insects attacking the wood of the tree. Borers can ringbark a tree unable to make kino because its mycorrhizal fungi have been killed. Wood rotting fungus then move into the dying wood.
Eucalypts and other native trees are hosts to many leaf eating and sap-sucking insects. On small trees some of these could be removed if the tree if being defoliated. Larger trees can cope with such attacks. Insects are controlled by a host of predators such as birds, other insects, funguses, and other pathogens.
Is This Tree Sick?
Here's a guide so you can give your trees a quick medical check-up.
A healthy tree should have a fairly dense canopy of foliage on most of the smaller twigs in its crown.
The leaves will usually have a few holes and chewed bits, but should be mostly green.
A tree suffering stress will have sparse foliage, with bare and dying smaller branches twigs. Watch for signs of small branches having sprouted and then died. This may happen repeatedly as the tree attempts to balance its canopy with functioning roots. As roots die off, so does the canopy. Eventually a few sad tufts of foliage on otherwise bare branches signal a tree at the end of its life. A tree that has died quickly from extreme damage, or from being poisoned, does not have a chance to grow these extra tufts of twigs and foliage.
Tree Dieback - What Can You Do?
Prevention is better than cure.
Plan development on your block with your understanding of the needs of trees in mind. Ask a few questions and imagine the trees are giving the answers.
Where should the wastewater go? Is this the right place for a BBQ area with all the compaction it will cause? Should I pave this area? Will a lawn area here change soil levels? Where should the tracks and footpaths go? If I build a terrace here, what will happen to the tree roots? Do I really need a rockery for exotic plants (and their fertilisers)? How close can I whippersnipper? What about allowing the native vegetation here to do its own thing?
When a Tree Looks Sick
Check through the factors listed here to see whether there has been some change in the tree's environment. If so, can you reverse or modify this change? Call a tree surgeon with horticultural training for further diagnosis and advice.
Life After Death
With development, some tree loss or damage may be inevitable. Since natural regeneration of trees on your block may no longer occur, planting young trees is essential. Wherever possible plant local natives - ones indigenous to the Island rather than somewhere else in Australia. If conditions in a part of your block have changed e.g. due to wastewater disposal you will need to choose species adapted to these changed conditions .
As you notice more about the trees around you and help them to thrive, you will feel that you too are a member of the community of trees.