Ready to fire: flammability traits may allow manuka to dominate

Marley Hill, Port Hills, ablaze CC-By-NC Jon Sullivan

The Port Hills fire after a day of burning. Image from Adrian Paterson.

Some of my strongest memories are associated with fire. There is the satisfaction of splitting and stockpiling wood for the winter. The excitement of giant bonfires on Guy Fawkes Night and at parties growing up (especially the massive one we had on our farm for 6th form (Year 12) UE). The pleasure of getting the mix of flame right for a good barbeque heat. The security of sitting around the fireplace in the lounge as a snowy gale lashes the house. The excitement of keeping a fire under control as you burn off a paddock full of barley stubble. The sense of dread as a wildfire bears down on your town.

Looking from the top of the Port Hills at a burned gulley. Image from Adrian Paterson.

A month ago there was a massive wild fire on the Port Hills around Christchurch not far from Lincoln University. Flames up to 30 m high ravaged a good part of the hills for five days. I had to evacuate the in-laws from their house in a valley near the fire-front as ash rained down around me. Luckily a wind change took the fire away from them but this didn’t help others who lost their homes. In the dry landscape of Canterbury, fire is always a potential hazard waiting around the corner. Understanding more about how plants burn is a useful thing to know that might allow us to manage future fires.

One of my colleagues, Dr Tim Curran, is a specialist in understanding how plants burn. He has helped to develop a plant barbeque that can measure the flammability of different species. Tim uses this information to inform people  on which plant species they can plant to minimize fire threats. Tim is interested in more than just burning plants however (although he IS extremely passionate about burning plants). Tim is also interested in how plants come to have different levels of flammability, what traits they have to survive fires and the evolution of these traits.

Tim has joined a number of colleagues from the University of Auckland and Landcare Research to look at the evolution of one of New Zealand’s few fire-adapted plants, manuka or tea-tree (Leptospermum scoparium). A key fire-related trait to help a plant population to survive is called serotiny. Serotiny is when seeds are held in the canopy of the tree and are released after a fire. The seeds dominate what is left after the fire and the species produces many seedlings relative to species without these canopy seed-banks therefore allowing the species to dominate the local habitat. Of course, manuka is itself highly flammable and if it dominates local habitats it will make these areas more prone to fires.

Manuka on Onawe Peninsula. Image from Adrian Paterson.

In a paper in the Journal of Biogeography, Tim as his colleagues explain how they tested their ideas on manuka fire adaptations. They compared manuka samples from 137 populations from  across New Zealand. For many of the sites there were charcoal records for the last few thousand years that recorded the frequency of fires in the past. The proportion of serotinous capsules on branches was recorded from the samples as well as their flammability. Environmental conditions, like slope and elevation, rainfall, soil water and so on were also measured for these sites.

Lattitude strongly determined serotiny, with higher levels of serotiny in the north of New Zealand and virtually none in the south. Serotiny was also high associated with lower rainfall and higher temperatures. Fire history did not strongly determine serotiny levels and how well samples burned from each population was not linked to this fire adaptation. Manuka is highly flammable but with the relatively low frequency fire regime in New Zealand serotiny is not necessary for manuka to survive. Serotiny, especially in dry eastern conditions may make manuka more dominant, however, especially when a site is recovering from a fire.

Tim Curran showing Lincoln Bio-Blitz participants how his plant BBQ works. Image from Adrian Paterson.

Cooler and wet southern sites, with little history of fire, are inhabitated by populations with virtually no serotiny. Tim and his co-authors suggest that as humans transform the local habitats that conditions for fire in these landscapes will become more favorable and that serotiny will likely spread through these manuka populations. As fires become more frequent they will favour manuka with serotiny which, in turn, will increase the dominance of this flammable species.

With the forecast for dryer and warmer summers, particularly in the eastern parts of New Zealand, we need to understand how our plants will respond to the inevitable fires that will come. Understanding plant fire traits will allow us to better live with these species in our landscapes while planning for a safer future. You’ll continue to find Tim burning plants on his barbie for a long time yet.

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