Plants for Firebreaks and Shelterbelts

July 2015

A study identifying the best plant options for fire resistant shelterbelts

Using a “plant barbecue”, a team of researchers has determined the flammability of both indigenous and exotic tree species with the aim of helping farmers and homeowners to create low combustibility shelter belts and landscapes and so reduce the risk of fires igniting and/or spreading rapidly.

Fires were uncommon in NZ prior to humans arriving. When they did occur the most likely cause would have been lightning strike but that was most frequent on the West Coast where wet conditions would have prevented or limited burning.

Maori probably used fire to clear some of the landscape although large conflagrations may have been accidental. However, the arrival of Europeans led to the widespread use of fire to clear the land for settlement and farming.

There has been a belief that NZ indigenous vegetation has low flammability and thus constitutes a low fire risk, whereas exotic species pose a higher risk. In 2002 Liam Fogerty of Scion surveyed a large number of fire managers around the country asking them to rank the flammability of species from their experience. The result was a list that has been used as a guide by councils and landowners to make safer plantings and to develop “green firebreaks” – wide strips of plants that have low flammability and so resist fire and help to reduce its spread.

However, the Fogerty list is subjective and assessments of the flammability of one species may have been biased by the presence of other flammable species around it – when a fire is hot enough any vegetation will burn. So a collaborative study involving Auckland and Lincoln Universities has been carried out to test the rankings that the Fogerty study came up with and to provide some objective measurements of flammability.

Dr Tim Curran, senior lecturer in ecology at Lincoln University and a member of the research team, says that sorting out test equipment and procedures for green plants was the first step.

“A lot of work has been done to test the flammability of building materials and strict measurement standards have been worked out, but these are time-consuming and expensive and not appropriate for plant materials,” he says.

“Fortunately a device for measuring plant flammability on a lot of samples very quickly, essentially a plant barbecue, had been devised in Argentina and we were able to build one.”

The device was completed in October 2012 and the first experiments measured how the proportion of dead material in gorse changed its flammability. Subsequently a wide range of species was studied and data collected.

“We now have results on more than 60 species including 27 of the 42 in the Fogerty survey. By and large our tests gave similar rankings to those of the experts, so there is considerable overlap between the two different approaches and that adds some robustness to the findings,” says Tim.

“There were also some surprising results. Two native species were among the most flammable that we tested – rimu and silver beech. Rimu trees occasionally retain dead material and have some very closely packed needles on their stems, and that would partly explain why they burn so well, but silver beech was the real surprise. Both were similar in flammability to a eucalypt that we tested.”

“The interesting thing is that both those species often occur in quite wet environments like the West Coast and the wetter parts of the mountains etc. In those environments they are not going to burn very readily but our findings suggest that in extended dry conditions in those areas these plants would prove quite flammable.”

These results imply that a reasonable degree of flammability is inherent in the New Zealand flora, and that it was the lack of ignition rather than low flammability that was the reason for very few fires prior to human arrival.

A few species were found to be less flammable than the Fogerty list suggested – ake ake, flax and kauri, but there was agreement that gorse was the most flammable.

In extreme climatic conditions any plant will burn, so there is no such thing as a fireproof hedge, according to Tim. Low flammability plants will help up to a point, but after that the fire takes on a life of its own, driven by the weather. But he gives some clear recommendations:

  • Clear any dead material out of shelterbelts regularly. Dead material drives fires.
  • Don’t use gorse for a hedge. If you do have gorse, keep it trimmed back so that old dead material does not accumulate.
  • For green hedgerows choose low flammability native species such as five-finger or tree fuchsia
  • For green shelterbelts use low flammability species such as poplar.

“Landowners need to decide what benefits they want their shelterbelts to provide and match those up with the prevailing conditions,” says Tim. “Plant flammability should be a consideration, not just on farms but in peri-urban locations as well. In areas like Wellington, the Hutt Valley, Porirua and around parts of the Marlborough Sounds, some houses are very close to patches of bush that may contain a lot of gorse, and that is why the fire authorities in the Wellington region have been very keen for people to plant green firebreaks.”

“Often houses are on ridges, and on the slope below is a mix of gorse and other bush plants. Flames go quickest uphill, so if a fire starts down at the bottom it could rapidly be a serious problem for the people at the top. A green firebreak should help check the spread of such fires.”

The flammability work so far has involved researchers at the University of Auckland, the Fire Engineering Department of the University of Canterbury and a fire scientist from Scion. Amongst other things they have been working on an improved “plant barbecue” with a number of additional features that allow more precise measurements of flammability – slower and more expensive but providing a check on results from the original device. A comparison of the results from the two devices should indicate whether future flammability testing should be done in the more sophisticated device or whether the much more portable device is good enough.

The team also wants to start testing larger plant samples.

“One of the issues in plant flammability research is that sometimes measurements are made of quite small plant components, like leaves or bunches of leaves, and that result is often scaled up to estimate how whole plant might respond in terms of flammability. We have some concerns about the accuracy of that,” says Tim.

“We hope to work with the University of Canterbury’s furniture calorimeter, which is essentially a room in which we could burn whole shrubs or tree branches, and see how the results correlate with the small-scale tests and also with the Fogerty survey results. The key is to have people with a wide range of expertise collaborate to use a variety of different measures. We hope in that way to get a better indication of the flammability of species mixtures in the field.”

When New Zealanders think about bush fires they probably think of Australia and elsewhere, but climate change predictions are that many parts of New Zealand will have hotter, drier and less humid weather with higher winds, which will all increase fire risk.

“What is happening now is probably not going to be the situation in 30 years time and it is predicted that we will see considerable changes in fire weather patterns throughout the country. So ignition sources and fuel are probably the biggest elements that we can manipulate and have some effect on,” says Tim.

“The best precautions anyone can take are to determine what the fuel is around the site and how it is distributed. This project is trying to determine where the risky fuels are around the landscape so that we can work to minimise those.”

“It will also indicate how a fire might burn in a given situation and what resources we would need to combat it. Alternatively we can start to establish green fire breaks of low flammability plants.”

“This work is giving us the extra knowledge we require, and hopefully policymakers and others will see the need to be planning for a more fire prone environment in the future and that there will be a much wider uptake of the information around the community.”

For further information on shelterbelt guidelines: