Assessing the flammability of New Zealand plants
Sarah Wyse, an Early Career Research Fellow in our Collections Department, describes the first empirical study to measure the flammability of a range New Zealand plant species, recently published in the International Journal of Wildland Fire.
Fire and the New Zealand landscape
New Zealand has an international reputation for pristine primeval landscapes – a reputation no doubt encouraged by The Lord of the Rings films. But, like the rest of the world, New Zealand’s landscapes have been shaped by people, and where people try to tame the wild they invariably resort to fire.
Before the first humans arrived in New Zealand in approximately 1280 AD, New Zealand forests experienced very low fire frequencies. An area of forest might go for centuries or even millennia without facing a fire, and, as a result, New Zealand’s plants exhibit few adaptations to this type of disturbance.
Humans colonised New Zealand in two main waves, both of which were accompanied by extensive forest burning. Polynesian settlement in approximately 1280 AD resulted in an initial burning period that caused rapid forest loss (of likely over 40% of the forest area), while European settlement around 1840 AD saw further burning that converted forest and secondary shrubland to pasture.
Today, wildfires are comparatively frequent, with approximately 3,000 wildfires burning around 6,000 ha of land per year, impacting ecosystems and human infrastructure. Given that climate change scenarios predict increasing summer water deficits and higher temperatures for much of the country, the frequency and impact of wildfires is predicted to increase in many parts of New Zealand in the coming years.
Researching flammability in New Zealand
The flammability of vegetation is a key factor determining the pattern, frequency, and intensity of wildfires. In order for ecologists to understand how fires influence New Zealand’s ecosystems, or for fire managers to identify high-risk landscapes or low flammability species for ‘green firebreaks’, we first need a robust comparison of how well different New Zealand plants burn.
Prior to this research, there were no quantitative assessments comparing the flammability of New Zealand plants; the only study scientists and fire managers had to work from was a qualitative study based on expert opinion by Fogarty in 2001. This research was an important first step, but one that was in need of empirical validation and expansion to include more species, such as commonly planted or invasive exotic species.
In our study, recently published in the International Journal of Wildland Fire, we quantitatively compared the flammability of shoots of common indigenous and exotic New Zealand plants. We also compared our results with those of Fogarty’s (2001) expert-opinion based study, to see how well these two different approaches match up.
The plant barbecue
We collected small branches from 50 indigenous and 10 exotic plant species from around New Zealand. The indigenous species included pioneer forest species, dominant mature-forest trees, shrubs, climbers and tree ferns. The exotic species were either commonly planted in parks, gardens and on farms, or were abundant weeds in native ecosystems or farmland.
The plant samples were then burnt on a custom-built ‘plant barbecue’, where we measured the maximum temperature the sample reached, the length of time it burned for, and then how much of the sample was burnt. The plant barbecue was based on a design used in a study in Argentina, and consisted of an old oil drum fitted with a grill, a gas burner to maintain the grill at around 150 °C, and a blow torch to ignite the sample initially.
How well did the plants burn?
By far the most flammable species we measured was the introduced European gorse (Ulex europaeus). Gorse is one of New Zealand’s most notorious plant invaders, and is a common sight on farmland and in forest edges throughout much of the country. Worryingly, this species is also very quick to regenerate following a fire, where it may outcompete indigenous species. Manna gum (Eucalpytus viminalis), an introduced tree from the fire-prone Australian bush, was the second most flammable species we measured. Also at the top of the list were some New Zealand native trees: silver beech (Lophozonia menziesii), rimu (Dacrydium cupressinum) and the forest pioneer (and producer of fine quality honey) mānuka (Leptospermum scoparium).
At the other end of the spectrum, some of the least flammable species we measured included five-finger (Pseudopanax arboreus), kōtukutuku (Fuchsia excorticata), New Zealand flax (Phormium tenax) and the exotic poplar (Populus nigra): predictably, these were species with high moisture contents in their leaves. With a few exceptions, our results correlated quite well with those from the previous study based on expert opinion. However, species such as silver beech and rimu were found to be more flammable in our study than suggested by Fogarty’s (2001) results, while we found others such as flax and kauri (Agathis australis) to be less flammable than previously thought.
Our results can be used to provide guidelines for individuals and local authorities on suitable species to plant as ‘green firebreaks’: areas of vegetation made up by low flammability species, which act as barriers to help reduce fire spread and help extinguish approaching fire. Unlike the alternatives of lawn or bare ground, green firebreaks made up of indigenous species can better provide ecosystem services and help improve biodiversity.
The results also provide information on some species that are amongst New Zealand’s most flammable plants. In the case of indigenous species these data can be used to indicate species and communities that may face the greatest threats from fire. For exotic species, the data can aid decision-making around planting practices (such as suitable shelterbelt species for farms) or which weed species should be prioritised for control. In particular, our results highlight the importance of ongoing management for widespread, high flammability weed species, such as gorse, for mitigating potential fire risk.
Given that New Zealand has approximately 2,500 indigenous vascular plant species, and a similar number again of naturalised exotic species, our team has quite a bit more work ahead! This project is ongoing, and we are continuing to add more species to our list all the time. We are also working on other projects, including investigating how species burn when mixed together in different combinations. One thing is for sure, we will keep chucking those plants on our barbie!
This research was carried out while Sarah was working at The University of Auckland (UoA), and collaborating with colleagues at Lincoln University (LU). Particular thanks go to the study’s co-authors Professor George Perry (UoA) and Dr Tim Curran (LU), and the rest of the team at Lincoln University. This work was funded by internal funding at both UoA and LU. Sarah is now an Early Career Research Fellow at Kew working on predictive modelling of seed traits.
Wyse*, S. V., Perry, G. L. W., O'Connell, D. M., Holland, P. S., Wright, M. J., Hosted, C. L., Whitelock, S. L., Geary, I. J., Maurin, K. J. L. & Curran, T. J. (2016). A quantitative assessment of shoot flammability for 60 tree and shrub species supports rankings based on expert opinion. International Journal of Wildland Fire 25: 466-477. Available online
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