“Watch this!” he said eagerly. “I’ll show you something you’ve never seen before.” As a newly arrived migrant to Australia there were plenty of things I hadn’t seen before, especially when it came to understanding things about trees. He had my full attention.
He had broken off the end of a branch of a Lemon-scented Gum and was trying to set it on fire with a lighter. It sputtered a little. Smoke came off. But the expected explosion into flame didn’t happen. It was after all a very green and sappy leafy twig, and it was early spring. Never-the-less even though his demonstration turned out to be a fizz I believed him; I was made aware that the living Gum tree has a capacity to carry fire in a manner unknown to one who had lived, hitherto, in a land of Oaks, Beeches, Sycamores and Horse Chestnuts.

 

a very flammable oil

It is a fact known to most people that Gum trees – Eucalypts – produce Eucalyptus oil. And this oil is produced not in their timber, but in their leaf. Pure Eucalyptus oil is in fact a highly flammable liquid. You could run an internal combustion engine on it, that is if you could afford it. (We may have to live a few more years past ‘peak oil’ before bowser prices catch up!) Eucalyptus oil is a well known industrial and domestic solvent. It will shift most stains and dissolve away greases when other solvents give up. It is also a disinfectant. A few drops on a handkerchief will often ease nasal discomfort. It is valued in the pharmaceutical industry and is the source of menthol.
For nearly 20 years many farmers in WA have been showing interest in growing ‘Oil Mallees’ commercially. (Mallees are multi-stemmed trees that grow from a woody lignotuber below ground.) The impetus for the oil mallee initiative was founded upon the commercial production of Eucalyptus oil. Interestingly most of the world production of this valuable oil comes from Malaysia whilst Australia, the home of the Eucalypt, is a very minor producer.

fixing carbon

Apart from oil these trees have a number of advantages in that they are native to our low rainfall wheatbelt. They help stabilise the farm landscape, provide windbreaks, prevent soil erosion, control groundwater and therefore salinity, provide habitat and corridors for wildlife. The lignotuber is also significant. Research by Syd Shea and John Bartle, following the pioneering work of Allan Barton at Murdoch University, has quantified carbon fixation rates* in these woody roots that remain in the ground and grow bigger each year. So now farmers have a basis for calculating carbon credits and entering into carbon trading whenever the time comes; 2015 at the current estimate!
The commercial aspect of growing oil mallees becomes visible when they are harvested on a regular three- or four-year plan. The stems including leaf are
crushed and the oil distilled. The remaining mass can be used to fuel power generation, make activated carbon, and enrich the organic content in our soil.
But there is another aspect to trees that contain a flammable oil in their canopy; the danger to life and property in the event of a bush fire.
There have been enough examples of wildfire raging through suburbs and settlements for us to consider the risks of building our homes among the gum trees. Tiny globules of eucalyptus oil turn into finely atomised aerosols of explosive gas as soon as fire touches leaf on a hot dry day. With strong and gusty winds behind them wildfires progress by leaps and bounds. Leaps of several hundred metres at a time are not uncommon. Our only recourse is to dump thousands of litres of water in the path of the fire, meanwhile at HQ fire control officers look for that hoped for easing of the wind. Its a frightening time.
Now I love gum trees. I thoroughly enjoy living near a native forest. But even I would have reservations about building a new home with such trees around thehouse and garden. In their natural setting our forest gets its ration of rainfall mainly in the winter and early spring. For most of the rest of the year the forest is dry. It flourishes in these time accustomed conditions. But the very act of building in it changes the local soil hydrology. Run off from our roofs and paving favours those trees closest to the homestead. Our nutrient enriched gardens and laundry effluent also feed the trees. Before we know it the tired looking forest that we came to live amidst is burgeoning with green leaf and massive twiggy canopies. All very beautiful. All very lush and shady. But it has become a great sponge-like mass saturated with explosive fuel. And given these conditions who needs a wildfire within ten kilometres on a hot summer day? Most would be fearing the ‘Catastrophic’ classification for their area and listening for that dreaded warning, ‘Pack and leave or stay and defend, Now!’.

fire retardant trees

The answer of course is to place a fire-retardant buffer zone around where we live.Trees we certainly need. But there are many trees which offer only an aqueous nature to the advancing fire. They don’t easily burn. They don’t carry fire. Their watery substance is a natural fire retardant. Such trees as the Oak, Maple, Claret Ash or Jacaranda can still be very beautiful to live among and once established they will thrive on our available rainfall. Better still, you might even grow food on your buffer zone trees. Carob is an eastern Mediterranean tree and its pods will feed domesticated animals. The Macadamia is a native nut tree, easily grown here though its origin is Queensland. Citrus, pome and stone fruit trees all could have a place in our gardens especially if favoured by grey water or rain water run-off.
Having written this I must admit to living among the gum trees myself. But at least we have had the shaggy mass of ultra growth trimmed back. And should
the crunch come we will be counted among those who ‘pack and leave.’
FOOTNOTE
*If you could weigh a tree, taking into account the dry weight of of all that the
tree has ever produced in its lifetime, the trunk, branches, leaves, roots and
bark, it would come to two thirds of the weight of carbon dioxide it has absorbed
over that same period. Thus a one ton tree would have taken up one and a half
tons of CO2. For the 12-year-old forester in the family the chemical formula is
6H2O + 6CO2 = C6H12O6 + 6O2. And it happens in the presence of the
enzyme Chlorophyl. But that’s another story!