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Vegetation Management

For the Sustainable House

The management of vegetation on any site is the very first principal that needs to be considered when building a house. The ultimate vegetation structure and form is what will maintain or enhance the ecological values of any site. The retention of ecological values is the first principal that should be considered by any property owner who wishes to contribute to an ecologically sustainable future. All property owners should consider the range of fauna currently using a site and have a long-term objective of increasing the number of different fauna species that use the allotment.

Vegetation Retention

The retention of existing vegetation is for any new house or major renovation is a key to maintaining wildlife usage of the site. Where an allotment is heavily vegetated it is obviously not possible to retain all existing vegetation. New house designs or renovations should consider mature vegetation as a natural asset to the property. The design should aim to maximize the retention of existing vegetation. Particular attention should be given to any hollow bearing trees on the site. Hollowing bearing trees provide significant habitat values to several groups of wildlife that are dependant on tree hollows for both roosting and nesting. The ultimate long-term survival of some species depends on the availability of tree hollows.

As a general rule of thumb all structures should be located outside the drip line (tree canopy perimeter) of any tree. This is the zone that is generally most critical for the roots of a tree. There are also several species of trees where the critical root zone extends well beyond the drip line of the tree. However, in many circumstances in particular on small sized urban blocks it is not possible to construct a reasonable sized house outside the critical root zone of a tree. In these circumstance alternative construction techniques need to be investigated.

House Design & Maintenance

Traditional slab on ground methods of construction under the drip line of a tree generally results in two outcomes. The first is the death of the tree that then becomes a hazard and is likely to fall on the house. The second outcome is that the tree survives and causes structural damage to the house foundations. Both are circumstances that do little to promote the retention of vegetation in close proximity to houses. The preferred method for construction within a critical root area of a tree is by the use of pier type footings. These are footings that can be placed between major roots and suspend the floor of the house above the root plate of the tree, allowing the natural function of the root system to continue with little or no disturbance to the root plate.

The canopy of a large tree over a house can often cause concern for the household inhabitants. Different species of tree have been labeled from one generation to the next as being widow makers and should be removed where within sight of any residential premises. Whilst it is true that some species are more prone to branch failure than others, the arboricultural knowledge as to which some trees fail has increased significantly to put rest to phenomenon such as summer limb drop. The failure of most trees and limbs can now be linked to particular structural faults that can be identified from an inspection of tree limbs. It is highly recommended that a suitably qualified and experienced arborist inspects and undertakes any maintenance on the tree. This will significantly reduce the risk of limb failure. It is recommended that maintenance inspections be undertaken every two years to manage the risk of limb failure.

Severe storm events are often a concern for house inhabitants that have mature vegetation surrounding their residence. Whilst the chance of limbs failing is significantly reduced by the ongoing inspections maintenance by an arborist, there are design features that can significantly reduce the chance of any property damage during these events. The inclusion of additional structural members in the roof will prevent likelihood of failure on impact. Furthermore, the use of materials such as marine ply to reinforce likely impact locations of the roof have been proved to be both affordable and extremely effective in preventing damage from limb impact.

Construction Impact Mitigation

During the construction process remember that all forms of vegetation provide habitat values for the fauna groups currently using the site. This includes the existing ground cover and shrub layer as well as the taller canopy trees. It is advisable to temporarily fence the areas of vegetation you would like retained on your block. However, you need to ensure you have allowed sufficient area for construction vehicle access and material storage outside the building footprint. It is advisable to speak with your builder to determine the extent area required outside the actual building footprint. Remember that even the compaction of soil in the root zone of a tree from construction vehicles and building activity can be fatal for the tree.

Habitat Enhancement

Once your house has been constructed or renovated you then have the opportunity to enhance the habitat values of your block to make it more attractive to wildlife. The vegetation structure and species selected for your garden will have a significant influence of the diversity of wildlife that decides to share your property.

When planning your garden you need to allocate as much area as possible for habitat rehabilitation. The allocation of the area needs to be balanced with the other needs you may have for your outdoor area such as grassed lawn. The greater the area that can be provided the better your contribution to the biodiversity values of your suburb. Your designated habitat rehabilitation or enhancement areas need try and provide a three tiered vegetation structure this being ground cover, shrubs, and tall canopy trees.

Ground covers such as grasses, sedges, ferns and creeping vines provide great habitat for ground dwelling wildlife. The scattering of fallen branches and rocks also provides a variety of ground habitat types that will certainly increase habitat opportunities. The inclusion of a small water feature at the lowest point of the property will also provide habitat opportunities for additional fauna groups. Any water feature only needs to be shallow and surrounded by very dense sedges or grasses that over hang the water, any logs or rocks protruding from the water also increase it’s use potential. It is recommended that part of the water feature is deep enough to permanently retain some water so it can be stocked with a fish species native to the local area that predate on mosquito larva.

The shrub layer is very important to establish dense thickets of vegetation that provide refuge for small passive bush birds from larger and more aggressive birds. It increases perching and nesting opportunities and invertebrate use of a site. It also provides covered ground foraging opportunities and increases fauna movement opportunities without exposure to potential predators.

Tall trees will provide canopy cover for the site for both specialist canopy foraging species and also provide safe roosting and nesting opportunities for a high diversity of fauna groups. It is possible to achieve substantial canopy cover on your property within only a few years. However, if there were no mature trees on the property, species dependant on tree hollows are unlikely to use your property. This problem can be partly overcome by the use of nest boxes.

Nest boxes come in a variety of shapes and designs for different species. They can be attached to a relatively young tree at elevation to provide an artificial hollow for species that require these features. However, you need to remember that nest boxes are susceptible to invasion by exotic species that will prevent their use by the targeted fauna. All nest boxes need to be monitored and appropriately maintenance action taken if inhabited by undesirable species.

Plant Selection

The selection of plant species for your garden rehabilitation is an extremely important process. Whilst species that are not native to your local catchment are not desirable to have on your allotment, they may be providing some of the only habitat that exists on your property. The removal of exotic species should be prioritized by the ability of the exotic species to spread and displace locally native vegetation. Alternatives to complete removal include poisoning of the exotic vegetation and leaving its dead structure for perching and nesting purposes. It may also provide connectivity between other vegetation for fauna movement purposes.

Lists of plants that are native to your local area are generally available from local Councils. Alternative your local Council may have an Environment Officer who can advise you on species that occur in you suburb. The use of only locally native species will also reduce the requirement for an irrigation system. When selecting plant species for you garden you should endeavour to maximize the diversity of species you plant. Whilst you may be aware that particular species of plant flower well and attract lots of birds, the consequence of planting large numbers of heavily flowering species is a decline in other bird types that will be attracted to your property. The aggressive nature and increased number of the birds attracted to the large volume of flowers in one concentrated location will displace any of the more passive smaller bush birds from your block. Remember the objective is to increase the number of different native species sharing your block not the total number of just a couple of species.

The retention of existing vegetation and subsequent rehabilitation of a diversity of habitats within your allotment will not only maintain but may possibly increase the number of different species using you local area. Maintaining or enhancing the biodiversity values of your allotment is the corner stone to any form of Ecologically Sustainable Development.

Sustainable House Design

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