By Rowan Reid

Faced with the COVID19 crisis it’s easy to forget the bushfires that devastated so many farms over the summer. But now, as we sit out a winter confined to our properties, many of us will be, optimistically, planting trees. My question is: have the fires changed what species you’ll plant and how you will managed them?

Trees on farms, particularly natives, are seen as increasing the fire risk. But this shouldn’t mean we need to live in an open paddock. Many people who have experienced wildfire first-hand argue that trees can actually help protect homes, animals and people. Bushfire researchers tell us that it is not so much whether there are trees, but what species, their arrangement and how they are managed. In fact, many of the houses that survived the full force of past fires were ‘sheltered’ by garden and farm tree plantings, whereas others out in open paddocks were hit by the full force of the fire front.

This article explores some of the principles of fire behaviour and tree species selection for fire prone areas. Firstly, it is important to point out that every fire is different and the more extreme the conditions the more unpredictable the outcome. Secondly, my family has been lucky: despite living in one of the worst fire regions in the country and losing a house not far away on Ash Wednesday (1983), in the 33 years we’ve been here at Bambra we haven’t experience a wildfire event (so what would I know?).

Let’s start by looking at fire weather and its impact on fire behaviour.

Fire weather and direction

The forest fire danger index (FFDI) integrates the weather (temperature, humidity and wind speed) with measures of drought and the moisture content of fine fuels into a single figure. When the rating was developed in the 1960s the benchmark was Black Friday, 1939. Using records of the weather and fuel conditions the researchers allocated Black Friday a FFDI of 100: the worst they might expect. Over summer, the forest FFDI in Victoria generally ranges from about 10 to 70. If it gets above 30 the authorities might call for a total fire ban. If it gets over 50 the consensus is that, due to fire spotting behaviour, it is nearly impossible to fight the front of a running forest fire. If it approaches 100 then we’re looking at a Black Friday type event.

On February 7, 2009 (Black Saturday) the FFDI was well over 200. No matter how many fire bombers or tankers were on hand, if a fire got going it would run until the fuel ran out, or the weather changed. But, if the fire was in the forest when the cool change hit it could be too late. By then the fire would be generating its own weather – a fire storm – and the only thing that would ‘change’ with the wind would be its direction.

On Black Saturday I was getting data from the Mt Gellibrand weather station to the north of the Otway Ranges. The estimated FFDI peaked at around 350 (calculated from weather data only and excludes drought index or fuel moisture so is only indicative): temperature 44.2 oC, relative humidity 8% and average wind speed 91 km/h at 1pm. The wind gusts exceeded 110 km/hr. When the cool change hit just after 2 pm the FFDI dropped to less than 80 in minutes. With no fire in the area the danger had past – we were safe.

But, to the north of Melbourne the long eastern edge of a fire that had been running for a few hours suddenly became the fire front when the wind changed and there was nothing to stop it running up the face of the mountains and over King Lake and Marysville. The same pattern of events occurred on Ash Wednesday. The FFDI on that day was about 150 before the change, but again, more houses were burnt and lives lost after the south-west wind change than before. Whilst we usually think of a fire coming from the northwest, on days of when the FDI is over 100 it is the south west winds after the change that may present the greatest risk.

Ash Wednesday and Black Saturday were extreme events. It can be argued that what a landholder might have done in terms of clearing firebreaks or planting fire resistant tree species would have had little influence on whether their houses, paddocks or stock survived. But, most fires are more predictable and it is possible to change the odds in your favour. As landholders there is little we can do to change the temperature or humidity. But, we can change the wind speed and the fuel.

Fire and wind

Fire research tells us that if the wind speed doubles, the rate at which a grassfire spreads might increase three-fold or more. A fire’s intensity is dependent on the amount of fuel burnt and the time taken for it to burn. So, an increase in the rate at which the fire spreads directly increases its intensity. The hotter the fire, more it is then able to dry out the fuel in front of it and therefore the faster it burns – and so on.

A well-structured shelterbelt can reduce the wind speed by as much as 50% for a distance of more than 10 times its height. This would reduce the rate of spread of a fire by more than half and the intensity almost four times. Aerial shots of farms burnt by Ash Wednesday show that shelterbelts were able to break up the fire front and deflect the winds which helped save stock and farm buildings.

From wind tunnel research and field studies we know a lot about the pattern of wind flow over and around shelterbelts. As shown in the diagram below, if you want to use a shelterbelt to protect your house and sheds then it is best to locate them at least 3 or 4 tree heights away from the shelterbelt. If the trees in your shelterbelt are 20 metres tall this is 60-80 metres away, which should mean that if the belt itself catches alight you’ll be ‘as safe as houses’.

Topography – slope and aspect

The next factor to consider is topography. Ground slope directly affects the rate at which a fire spreads. Fires travel much faster uphill than downhill: every 10 degree increase in slope doubles the speed at which a fire spreads. Aspect is also important. If you have the opportunity to site your house then don’t place it at the top of a north or west facing slope. If the house is already there then you need to think carefully about the vegetation on the slope below.

Fuel load, type, arrangement and connectivity

Fuel is any material that can be burnt but not all fuels are the same. The size, moisture content and arrangement of the fuels will influence the way the fire burns and the risk of spot fires. Fine fuels, including grass, pine needles, leaves and bark, ignite and burn quickly, carrying the fire forward. Coarser fuels, such as branches and logs, burn for longer but only after the fire front has passed.

The arrangement of fine fuels is therefore critical. Pastures trampled by stock will burn more slowly than a standing pasture. The more continuous the fuel the easier it is for the fire to run, pre-drying any fine fuels as it goes. Reducing, compacting or separating the fine fuel components can dramatically reduce a fire’s rate of spread and its intensity. We graze sheep under most of our trees, particularly to the north and west of the house.

Good and bad tree species for fire

The foliage of some tree species contain oils or other compounds that increase their flammability. Eucalypt leaves contain oils that vaporise as they are heated and burst into flame when ignited. Other tree and shrub species have a high moisture content in their leaves during the fire season and are less likely to burn. When selecting tree and shrub species farmers should also consider the following:

1. Bark type. Choose species with a tight clean bark. Stringybark eucalypts and those that shed long ribbons of gum bark can act as a ladder carrying the fire up into the canopy. Once alight, bark can fly through the air causing spot fires hundreds of metres ahead. Of the eucalypts Spotted Gum, which sheds its bark in small pieces, is much better than Manna Gum (which produces long ribbons) or Messmate (Stringybark).
2. Leaf shape, moisture content and oil content. The flammability of tree foliage can be tested by holding a green branch over a hot fire. Let the leaves heat up before ignition. Watch to see if it is the leaf itself that ignites or the volatile oils released when heated. Watch the embers fly through the air and note how long they stay alight. You’ll soon identify the better species to grow near your house and the ones that should be cleared. Wattles are better than any of the eucalypts or tea-trees but then there are many other options, including exotic species.
3. Propensity of trees to retain dry leaves or branches during the dry season. We all know that the deciduous trees lose their leaves in autumn, well after the fire season. However, most of the evergreen species lose a proportion of their leaf area as moisture becomes more limiting in the depth of summer. If these leaves and the small dead twigs persist in the canopy they present a real fire risk.
4. Effect on the vigour and density of understorey plants and groundcovers. If a tree has the right bark and leaf characteristics but can also suppress the ground vegetation – without dropping a lot of fine litter – they can create a very effective fire break. This might be a result of heavy shading or intense competition. The only risk is that the bare ground will be prone to soil erosion during summer storms so this is really only a strategy for flat land. Species that allow the grass to persist under the canopy are a better option on sloping ground particularly if the grass can be grazed down before summer. Pasture growth and survival can be improved by pruning up the tree canopy and spacing the trees more widely.

As important as species selection, is the arrangement and management of the trees. An open ‘parkland’ of trees is safer than a ‘plantation’. This suits those of us growing sawlogs: we thin to promote diameter growth which reduces the connectivity of the canopy. By pruning the lower branches and keeping grass down it is possible to reduce the risk of a fire jumping up into the canopy. I use grazing combined with pruning to create a fire shield that will deflect the wind and trap the embers so as to protect my farm buildings.

Be willing to manage – don’t leave it to nature

Too often landholder seem resigned to the trees they have and are reluctant to correct past mistakes or manage their existing fuel loads. I’ve certainly made mistakes. I planted tiny eucalyptus seedlings to shelter our house from the winter winds thinking that pruning and spacing would make them safe from fire. But the species I chose (Shining Gum and Blue Gum) are now producing long ribbons of bark and lots of ground fuel that can carry a fire. Although I burn under the wide-spaced eucalypts to reduce the ground fuels I’ve started harvesting the eucalypts (for timber) and are planting Oaks instead. I appreciate that in some states, laws aimed at protecting native vegetation (even those trees planted by the landholder themselves) don’t allow you to manage the trees (that’s a topic for another blog).

Fire behaviour can seem unpredictable but there is a lot of good information available that can help farmers make sensible decisions about where they might grow trees, which species are best and how they should be managed in order to reduce the risks. Of course, we all hope we never experience a wildfire but with climate change pushing up temperature and the increasing fuel loads in our public forests, it almost seems inevitable. Having a tree-less farm might make you feel safer, but in the event of a wildfire it might only improve the view of the flames as they come hurtling towards you.

Rowan Reid is a Forest Scientist, Senior Fellow of the University of Melbourne, originator of the Australian Master TreeGrower Program and the author of “Heartwood – the art and science of growing trees for conservation and profit” More: http://www.agroforestry.net.au

Rowan Reid

I graduated with a degree in Forest Science in early 1980’s. At the time, our public forests were being managed for a mix of conservation, water, recreation and timber production. But because of a lack of trust, the management paradigm was shifting away from complementary multiple use to distinct value optimisation. The forests were divided into single-purpose zones; one for conservation the other for timber production. This simple act drew the battlelines for the forestry wars. 

We now have a winner, but I suspect that the real loser will be the very environment they were fighting over.

The fight for control of the native forests was always the wrong battle. Both sides missed the point and their arguments have distracted our attention from the real issue. Locking up our forests, was never the answer. Now it risks becoming the new problem. 

As the attention of both sides shifts to the farmland I worry we are about to make the very same mistakes. The belief that conservation and production are mutually exclusive is already dividing our rural landscape. Rather than solving the problem, nature conservation policies and incentives are undermining property rights and encouraging landholders to neglect their remnant native vegetation and only plant exotic trees. On the other hand, the demand for ‘eco-friendly’ sources of wood is supporting large-scale industrial, monoculture forests of exotic species. Whilst it’s fair to argue that these plantations, like wheat, are ‘just another crop’, to do so ignores the very things that makes trees, and forests, so special. 

It’s time to let go of this artificial dichotomy; there is no divide in nature between production and conservation. In fact, one drives the other. As we approach the environmental, social and productivity limits of our conventional systems, it’s time for a fresh approach. It’s time to bury the hatchet and pick up the spade, and the chainsaw. It’s time to support a suite of emerging approaches to growing new forests that are not primarily commercial but make good business sense. They don’t look like conventional conservation plantings, yet improve biodiversity and reduce land degradation. 

These forests are not compromises. They are not negotiated points along some tug-of-war line between two opposing ideologies. They are elegant solutions, appropriate for each owner, their place and the time, which provide a balance of conservation, aesthetics and profit. Together they can deliver much more than can be achieved by simply dividing up the landscape between competing interests. 

Back on our own family farm in the Otway Ranges I will continue to selectively harvest sawlogs from the mixed species native forest we planted 32 years ago along a degraded creek. We mill and dry the timber on farm, some of which has made it to the finest furniture showrooms in Melbourne. We planted the forest for soil and water conservation but we managed the trees for profit. They provide shade and shelter for our stock and habitat for endangered wildlife.

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We commonly find Sugar Gliders hiding under the bark of the trees we plan to harvest. Does that mean that our forest should be locked up? Should we be prevented from cutting down trees because the forest we grew also supports wildlife? In fact, we’re now seeing evidence of how our harvesting is actually improving biodiversity, carbon sequestration and water quality by creating dead stumps, leaky weirs and encouraging understorey.

We share a landscape that has been actively managed by humans for food, fuel, water, fire control, shelter, building materials, and art and cultural heritage for more than 40,000 years. Having a stake in the future of our natural resources is not about picking sides. It is about celebrating a story that argues that the growing and felling of a tree for timber is not necessarily a step back; it can be a leap forward, a revelation, an act of conservation. 

So, if you think conservation is about saving every tree, please step back, let us show you that there may be another pathway through the undergrowth. You might well be surprised: when it comes to forests, we can have our cake and eat it too.

Rowan Reid is a founding member of the Otway Agroforestry Network, Senior Fellow of the University of Melbourne and the author of “Heartwood – the art and science of growing trees for conservation and profit”

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