Recreating the Country blog
Deciduous trees act as a fire retardant and are often the reason why some houses survive a bushfire when neighbouring houses are burnt. Why do they provide fire protection and how can they be used in rural areas to make summers safer?
The Weekend Australian Feb 21 2009
A former owner of the Crossways Hotel in Marysville, which survived the fire in 2009, Mr Lawrey is advising residents to plant European trees around their houses rather than eucalypts. "European trees saved my house," he said. "The embers that landed in the trees had time to burn out".
If they land in eucalypts, they burn immediately. "He said all three commercial buildings left standing in Marysville had European trees nearby. They really cooled the fire down when it reached them"
The benefits of deciduous plants.
Deciduous plants are very useful around the home and in public places not only because they add to the ambient beauty but also because they provide cooling shade in the summer and let the sunlight through in the winter.
Evergreen trees are unable to provide this shade contrast. In fact Australian natives often do the reverse and let more sunlight through in the summer by turning the flat surface of their leaves away from the sun to reduce transpiration loss. Some natives like eucalypts let in more light by shedding their leaves during the very hot dry periods.
Deciduous trees cool the air around them through transpiration. The water held in leaves is released from the stomata as water vapour. A Similar evaporative cooling principle is used in air-conditioners. On a hot day it is possible to feel the temperature drop a few degrees as you walk from the shade of native trees under deciduous trees. I experienced this tangible temperature change recently on a visit to the Adelaide Botanic gardens on a 38 degree day walking beneath a very large London Plane tree. It wasn't a coincidence that many more visitors to the gardens that day were seated under this shady tree than could be seen in the rest of the gardens.
Additional benefits of a deciduous trees and vines are fruit & nuts; timber for craft, furniture, and building; firewood, attractive flowers and autumn foliage; shade; privacy screening and as mentioned before they let in the winter sunlight which can make a cold room warmer and dry a boggy track.
How deciduous plants provide protection from bushfires.
Deciduous plants are fire retarding because they have high moisture content in their leaves without the flammable oils. They can provide excellent fire protection in four ways.
1. By shielding buildings from the damaging effects of a fire's radiant heat.
A vegetation screen between a building and an oncoming fire can prevent windows shattering, plastics melting and timbers buckling by reducing the radiant heat emanating from the fire. Radiant heat is short and intense because a bushfire front takes only 30 - 40 seconds to pass through. In extreme fire conditions a house has to withstand temperatures of up to 1200 degrees for about two minutes if it is exposed to the full un-shielded intensity of the radiant heat.
Native trees and shrubs will also shield a house from radiant heat.
2. By catching flying glowing embers before they reach the home.
It's not uncommon to hear stories about people who stayed to save their homes and later celebrated with friends at the pub only to return to a smouldering ruin. The cause of this enormous disappointment and personal tragedy are the airborne embers of burning leaves and bark that cause spot fires ahead of a fire-front and continue to fall after a fire has passed.
Many airborne embers become caught up it the foliage of surrounding trees. If they become caught in deciduous trees they're no longer likely to be a threat.
3. By 'snuffing' out the embers they catch.
The water filled leaves of deciduous trees will cool and extinguish glowing embers in the same way that water puts out a fire. The leaves on the side nearest to the fire-front will be scorched as they take the heat out of the embers but they won't ignite and add to the intensity of the fire.
A eucalypt or a pine could accelerate the fire and burn because they releasing volatile oils which can spontaneously ignite at temperatures above 60 degrees.
4. The leaf litter isn't flammable and breaks down rapidly to humus, improving the soils ability to hold moisture.
Have you ever noticed the smouldering heaps of raked deciduous leaves in the late Autumn? Some over zealous gardeners try to burn them when its much easier to turn them into compost. They are difficult to burn and this feature will stop a fire from spreading. This was observed after the devastating Western Australian fires of 2009 by a firefighter defending a house at Ferndale.
"Every ember that fell on the side of the house that was planted with Blue Gums started a fire in the fallen gum leaves.
Every ember that fell on the other side, amongst the poplar leaves, just petered out.
There was a 100 percent difference.
Where to plant for fire protection.
The key fire sectors are north, north west, west and south west. These are the directions that a fire will come from and also the sides of a property best planted with deciduous trees or vines.
Bad fire days are always associated with strong northerly or north westerly winds. North winds are warm in summer as they blow across the northern deserts before they reach the southern states. These warmed gusty winds can strip the moisture from leaves and cause less hardy plants to wilt.
Wind directions can change. A cool change is always very welcome when it brings relief from extreme heat, but it can turn a fire to the east. If a fire is coming from the north with a narrow front and is turned toward the east by a westerly change, the long eastern edge of the fire becomes the new and much longer fire front.
To protect your home against all these possibilities imagine a semicircular zone from the south-west extending to the north and plant this zone with deciduous plants to guard against the unpredictable nature of fire. Choose trees appropriate to the size of the space available because large deciduous trees can dry out garden beds and cause structural damage if planted too close to a building.
What deciduous plants are suitable for your country property.
When choosing suitable plants you will have more success if you factor in your annual rainfall, aspect (south & east facing positions and slopes are cooler than north & west facing), drainage and soil type. This knowledge is critical and can make the difference between success and failure whether you are planting hardy natives or exotic deciduous trees. This knowledge is also very useful when you visit your local nursery.
Generally in dry areas drought tolerant plants will cope with a long dry spell and hold on to their leaf moisture in the dry months. This is a very important feature if they are to function as a fire shield.
I have added some potential uses and cautionary notes in brackets below as an alert to help your choice and avoid future regrets. Always check the eventual size of the trees you're planting and keep them at least 1.5 times their mature height from any structures with foundations. Very large trees like the Beeches, some larger maple species, Weeping Willow and London Plane can cause structural damage and their roots can lift paved areas. On the brighter side, an avenue of London Plane trees planted in the fire sector will proved a very effective fire shield from 40 meters away from a homestead.
The following genera of deciduous trees prefer moist soils.
They are unsuitable as fire shields in dry climates unless they are in a moist gully or well irrigated:
Willows (an environmental weed on waterways, one species used traditionally to make cricket bats).
Poplars (buy non suckering cultivars, valuable timber).
Most oaks (valuable timber, acorns are edible if the bitter tannins are removed by repeated boiling).
Maples (valuable timber).
Birches (valuable timber).
Beech (valuable timber, edible nuts).
The following genera of deciduous trees are hardy and tolerate very dry conditions:
Ash (Desert Ash is very hardy but potentially a weed, grafted cultivars like the Claret Ash are not weedy, valuable timber).
Plane (valuable timber, the roots are deep, invasive and potentially damaging).
Hawthorn (spread by birds and can become an environmental weed),
Black Locust (boat building timber, firewood, will sucker).
Honey Locust (valuable hardwood)
The following common fruit and nut varieties will cope well with long dry periods when established:
For a good crop in dryer locations drip irrigation is recommended.
Quince (will sucker), pomegranate, plum, persimmon, pecan, pear, mulberry, fig (will sucker), apricot and almond.
Apple, peach, nectarine are less drought tolerant but will still survive without irrigation if 'the dry' is not too prolonged.
Walnut (Black Walnut provides valuable timber), hazelnut, chestnut and cherry definitely need moist soil with good drainage and prefer cooler summers
Ornamental pears and ornamental prunus are a very useful medium sized trees that will cope with drought and poor drainage. The prunus species which include almond, apricot and plum are tough and adaptable to most soils particularly if they are grafted onto plum rootstock.
I have included this link to a comprehensive guide to fruit ornamental trees written by Flemmings. It should answer any questions you may have about the suitability of different trees. www.flemings.com.au/tree_guide.asp
Flemmings are a well known nursery dynasty that have researched and published this excellent guide to trees that they sell through retail outlets. There are cheaper wholesalers of deciduous and fruit trees, so it is definitely worth shopping around.
Ornamental and fruiting grape vines are also very hardy and can be used as a trellised fire shield.
Deciduous trees provide many benefits around human settlements but reducing the fire risk is arguably a most critical asset that should be considered for every rural home.
Planting back the indigenous and native vegetation in rural and urban areas is essential to protect our indigenous flora and fauna and to restore the resilience of the natural environment to change, but it would be foolish to ignore the advantages that deciduous plants offer to human safety in the advent of a serious fire.
Stephen Murphy is qualified in Geology and Environmental Management and has been a nurseryman and a designer of natural landscapes for over 30 years. He loves the bush, supports Landcare and is a volunteer helping to conserve local reserves.