Scrub Fire Dangers

More than half of Great Barrier Island is covered in Manuka/Kanuka scrub, which contributes fuels to create varying degrees of fire risk. Typically, the Barrier spends about a third of the year in extreme Scrub Fire Danger. The island's topography, where steep slopes are common, leads to more rapid fire spread and thus higher fire intensities. A mild and moderately windy climate doesn't help, but relatively high humidity levels offer some protection.

In recent years, an intensive research program has been going on to come up with a NZ Fire Danger Rating System to help support fire management decision-making. In particular, the Forest and Rural Fire Research programme aims to close a gap in our knowledge of scrub fuels, a dominant vegetation type in NZ's landscape. Much more is known of forest and grassland fires even though elevated scrub fuels such as manuka and kanuka dry more quickly and such fires are generally more responsive to changes in wind speed and fuel moisture content.

Most residents are aware to a greater or lesser extent of our history of fire, and there are valuable insights to be gained from listening to locals who have experienced seeing them or been involved in fighting them.

Manuka/kanuka scrub encompasses a whole range of fuel sizes from fine to coarse, but typically has extremely high fine fuel loadings, including needle-like leaves. Fuel loads could be described as moderate to heavy, with 10-50 tonnes per hectare of available fuel (or between 1 and 5 tonnes per quarter acre).

Observations of scrub fire behaviour from several wildfires and dozens of experimental burning trials

have resulted in a scrubland fire behaviour model. Still under trial, it predicts the rate and spread of fire in manuka and other scrub fuels. Over half the experimental fires were carried out using manuka/kanuka fuels in the northern half of the North island. In addition, samples have been taken, (including from this island), of fuel loadings in varying heights of scrub to make assessing the fire hazard easier.

Manuka and kanuka typically contain about half the moisture of other common plant species, while harbouring high levels of flammable volatile oils. The arrangement of fuels within manuka/kanuka stands can lead to intense fire behaviour; fuels being aerated and exposed to the drying effects of the surrounding atmosphere. Flaky bark, a high proportion of fine dead branch material, and dense undergrowth provide ladder fuels allowing the spread of fire from the surface into crowns, where fires can spread rapidly.

When wood is heated, such as in proximity to a fire, water is driven off and, as the temperature increases, flammable gasses are produced, especially above 250C. At that temperature, if not lit by an adjacent flame, the gasses self-ignite in the presence of the oxygen in the air. Very quickly, at about 270C, the fire becomes self-supporting as long as there is sufficient fuel, heat and oxygen. Along with convected heat acting in the immediate area of the fire, infra-red radiation is produced that dries and pre-heats other fuels more distant. At extreme temperatures, this radiation can quickly disable and/or kill humans in direct line of sight.

Predicting fire behaviour and the difficulties of suppression is a science. The more we know, the more the negative impacts of fires can be prevented or minimised.