Measuring the burn!

Fire. Even in the 21st century fire remains an important part of our lives. This week I spent several hours stacking firewood ahead of the coming winter. On some of the recent less windy days we have been surrounded by plumes of smoke as farmers burn off their cereal stubble.  I have been awoken by the fire siren echoing around Lincoln as it has called the firemen to attend a house fire caused by a malfunctioning food smoker.

Ecoblitz glade
Native forest, not as resistant to fire as we thought.

Looking from my 5th floor window at the University I see reminders of the past effects of fire. Much of New Zealand was forest, or at least scrub, prior to the arrival of humans. Humans used fire as the main tool to clear away large areas of these unwanted trees (as we have done all over the world). The Canterbury Plains retain almost none of this old forest or scrub. Looking to the future, with warmer temperatures and a dryer east coast, it is likely that fire will continue to play an important role in the area. Every summer we have wildfire burns that require huge efforts to get under control. One came a couple of kilometers from Lincoln a few years back (in fact, as I write this, I see that there is an out of control fire just a little to the west of Lincoln).

Cabbage tree (Cordyline australis) has a moderate flammability.

So how do we make our landscapes safer? There are a couple of things that we could do. We could reduce the proportion of plants that are really flammable from the landscapes to make conditions for fires less common. For example, the Canterbury Plains have many shelterbelts of trees to protect stock from the prevailing winds. If your shelterbelt contains a highly flammable species it then provides a route for wildfires to spread.

We could also plant greenbreaks with plants of low flammability to stop fires spreading. Of course to do this we would need to know just how flammable different species are. The only source of information about flammability of New Zealand species is information compiled by experts who ranked species in 2001. While this has been useful the information in not quantitative and has not been tested. That’s where the plant barbeque comes in.

Tim Curran (Lincoln University) and colleagues Sarah Wyse (Kew Gardens) and George Perry (University of Auckland), as well as a host of Lincoln University students, including a well-known all black, have designed a barbeque on which they can place plant samples. The grill maintains its temperature around 150C and the sample is ignited with a blowtorch. They recorded the time that the sample burned for, maximum temperature reached and, once burning was complete, how much of the sample had been burnt.

The group collected small  branches from 60 species from around the country. Ten species were common introduced species, like gorse, poplar and manna gum. Fifty native species were also tested ranging from forest trees, like silver beech (Lophozonia menziesii) and totara (Podocarpus totara), to understory scrub, like supplejack (Ripogonum scangens) and flax (Phormium tenax). Each had their turn on the barbie. The results have just been published in the International Journal of Wildland Fire.

plant BBQ measuring max temps
Measuring the burn.

The most flammable species was gorse (Ulex europaeus), an introduced species that was used a lot along fences and is now a major pest. The eucalypt species was next (Eucalyptus viminalis), unsurprisingly give it is from fire-prone Australia. Surprisingly, given that historically fire hasn’t been that common in New Zealand, common native forest and scrub trees rimu (Dacrydium cupressinum), silver beech and manuka (Leptospermum scoparium) were also very flammable. At the other end of the list were species suitable for greenbreaks, like five-finger (Pseudopanax arboreus), poplar (Populaus nigra), and flax.

The expert opinion proved to be fairly accurate with 80% of species matching with the burn data. However, some species like rimu and silver beech were very underestimated in their flammability and flax was significantly overestimated. Finding these discrepancies shows the value of taking a measurement approach. The plant barbie is an approach that can be used productively around the world.

Poplars (Populus nigra) have low flammability.

So what about the future? This work is a good start on making the landscape more resilient to wildfires. For example, we should remove shelterbelts of gorse and eucalypts and replace with species like poplars or flax. This would stop fires spreading so quickly. We need to think carefully about putting greenbelts of low flammable species around bush remnants of natives, like silver beech. Otherwise, our conservation and restoration efforts may just go up in smoke. With this kind of information we can make sure that a dry future doesn’t have to be a fire future.

Leave a Reply