The gorse, grass and pines on the Port Hills above Christchurch are among ideal plants to fuel the wildfires sweeping through the area.
"The Port Hills and Banks Peninsula have large tracts of continuous, highly flammable vegetation," said a senior lecturer in ecology at Lincoln University, Dr Tim Curran.
"This includes scrub dominated by gorse, pine and eucalypt plantations or shelterbelts, pastures and grasslands, and native regrowth dominated by kanuka," Curran told the Science Media Centre.
... repeated fire favours fire-loving exotic plant species
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"Gorse and some pine species retain dead material on the plant, which makes them particularly flammable.
"When combined with hot, dry and windy weather, such vegetation fuels the intense and extensive fires like those we are currently experiencing on the Port Hills."
The key vegetation factors that determine the behaviour of a fire are:
• The biomass (the amount of plant material),
• Its layout (how continuous or connected the vegetation is), and
• The flammability (how well it burns).
Other factors are the weather and the landform: fires spread with the wind and uphill as flames are tilted towards the fresh fuel.
Curran said that in areas with highly-flammable plants and high biomass, particularly dead plant material, and connected-up vegetation in which the fire can spread up into the canopy, "you will get intense fires that burn over a large extent".
Auckland University professor of environmental science George Perry said thin, dead plants such as grasses were highly combustible because of their low moisture content and how fast they dried.
"The vegetation on much of the Port Hills is extremely flammable, especially in those areas where gorse, which is characterised by large amounts of well-connected, dead and fine material, is abundant."
Perry said gorse, pine and other invasive plants were well adapted to regenerate after a fire.
Some New Zealand natives, such as manuka and kanuka, were very flammable, but few were adapted to fire, because of the infrequency of fires in pre-human New Zealand.
"Thus, repeated fire favours fire-loving exotic plant species.
"Fire begets fire."
And we may see more of this in a drier, warmer future.
Perry urges Kiwis to consider how to reduce our risk of fire, for instance by adopting "green fire breaks" - plantings of trees that are of lower flammability.
Curran said his group's experiments had found gorse to be the most flammable plant they tested. Eucalypts, rimu and silver beech were in the second rank. The least flammable group included five-finger and tree fuchsia.