Assessing the flammability of New Zealand plants

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. 

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.