Fire on the mountain: habitat flammability changes over decades

“Close to, the fire on the mountain was very much more alarming than it had seemed from a distance. They could smell it now, and hear it; smell the smoke more bitter than a farm bonfire; hear the soft, dreadful sound of flames consuming the bracken, like paper crumpled in the hand, and the sudden crackling roar as a bush or a patch of gorse went up. And they could see the flames, leaping high, bright red and yellow at the edges of the fire but ferocious and near-invisible at its heart.” Susan Cooper, The Grey King

A major fire on a mountainside plays a vital role in Susan Cooper’s “The Grey King”, part of the Dark is Rising sequence. Hasn’t made me freak out about wildfires in the hills since my childhood at all. No siree. Nuh uh. Image by Adrian Paterson.

One of my favourite and formative series of books was the Dark is Rising sequence written by Susan Cooper. This is series of five interlinking books that follows young Will Stanton as he becomes an Old One of the Light. It is mostly set in present day (well the late 70s I guess) but interweaves English and Celtic folklore, time travel and strangeness, as well as simply growing up, in a complex but fascinating way. The writing is great and evocative, as well as economical. All five books make up a page count of one Harry Potter.

The second book, The Dark is Rising, is set in the cold midwinter and there are few books that capture the starkness of this season so well. I first read the series when I was about 11 or 12 and could identify with Will and the other characters. I have revisited this series many times over the years, including reading them to my sons.

My favourite book of the sequence is The Grey King. Set in the mountains of Wales, where Will goes to recuperate on a relative’s farm following an illness, this is mainly the story of Bran Davies. Bran lives in the valley, on a farm, in an unhappy and lonely relationship with his father. Throughout the series there is reference to a poem, a cryptic set of lines that ultimately drives the shape of the story. One line runs “Fire on the mountain shall find the harp of gold“. During the course of The Grey King this line plays out in a literal sense where Will and Bran are placed in peril as a wildfire threatens to trap and burn them on a mountainside. This image of being caught by fire in the mountains has stayed with me ever since.

“But Will … felt that nothing could halt or check the inferno before them, snarling high over their heads now as it reached a tangle of blackberry bushes. It was like a huge beast raging over the mountain, gobbling up everything in its path with irresistible greed.” Susan Cooper, The Grey King

As a boy growing up in South Otago where the hills are lush, green and well-watered, I never had much to do with wildfires. Burning off straw stubble was about as close as we got. As an undergrad at University of Otago I had ecology lectures by the infamous Alan Marks. A topic that saw him get extremely heated about was fires in the high country of the Southern Alps and the changes that they made to the local ecosystems. Visualising these fires always took me back to the images from Cooper’s books.

“There was not much smoke, for so much fire. In a line along the lower slope of the mountain, which they could only just see above the hedge from where they stood, flames were blazing in the bracken. It was like a long wound, a gash in the peaceful brown slope, quivering with deadly, ominous life. Yet there was little colour in it, and they were too far away to hear any sound.” Susan Cooper, The Grey King

Smoke from the 2017 Port Hills fire. This wildfire burned for several days. Image from Adrian Paterson.

Fires in the high country, usually on or around sheep and cattle stations, generally reduces native diversity in favour of introduced species. Although historically fires were not unknown in the dry rain shadow areas of the Southern Alps, the frequency of fires, either purposely set for farming/hunting purposes or accidentally started, have increased since the arrival of humans.

Much of the research into fires in the mountains of New Zealand have focused on these affects on biodiversity and the impacts on local ecosystems. For example, Jagoba Malumbres-Olarte found that burned tussock areas were more likely to be colonised by introduced spider species than locals after burning.

One crucial piece of the fire puzzle concerns long term changes that may occur with flammability of high country habitats with changing diversity. While fires are devastating in their own right, the aftermath and recovery of habitats is often just as important. Burnt habitats are right for takeover by pest species which means fires can potentially change these ecosystems in the longer term.

“Though the fire blazed no longer on the mountain, a strong, dead smell of burning hung in the air. … All the near slope of the mountain, Bird Rock and the valley edge were blackened and charred, and here and there wisps of smoke still rose.” Susan Cooper, The Grey King

A spark is rising… Image by Adrian Paterson

Tim Curran, Lincoln University, has spent the last few years burning plants. By doing this he has collected a lot of information about how many of the plant species of New Zealand, both native and exotic, burn, how easy they are to ignite, how much fuel they have, how hot they burn and so on. Knowing how flammable each species is all very interesting. Knowing how a bunch of species in your local plant community species is even more important.

The particular mix of species alters the overall flammability of an area, how likely it is  for fires to ignite and spread. This is vital information to know for fire-prone areas like the dry high country. Likewise, does the ongoing change in species mix with more invasive species establishing in these mountain habitats change the flammability of the area?

Curran and other colleagues from Lincoln University and Auckland University of Technology measured the flammability of 51 species common to high country areas. They made the most of 103 sites that have been surveyed for over 25 years from throughout the South Island high country. By measuring the flammability of each species and which species were found in a particular site (and the density of each species at these sites) they were able to calculate an overall flammability score for each site. They could also follow the score for each site over 25 years.

In a paper published in the Journal of Ecology the team showed the effects of an overall increase in exotic species into the sites over the 25 years. The mountain habitats had moved from sites dominated by native, and highly flammable, species like Chionochloa rigida, Dracophyllum acerosum, Aciphylla aurea and Festuca novaezelandiae, to increasingly common introduced, and much less flammable, species like Crepis capillaris,
Pilosella officinarum, Pilosella piloselloides
and Hypochaeris radicata.

What does this mean for future fires on the mountainsides? Areas with invasive plant species will decrease flammability. In addition, areas that have had fires that have removed native species and been colonised by exotic species that it is likely that fires will be smaller, not as hot and will not burn in an area for as long. If this is the case then fires will not clear areas of vegetation as much as they did in the past. Fires that don’t burn as hot or for as long will leave plenty of vegetation behind. In turn this will select for the kinds of plant species that do better in more dense conditions and create a positive feedback that selects for plants with these traits. Mountain habitats will become less flammable overall. It will be difficult for mountain habitats in these dry areas to return to their historical state.

The future for fires in the mountains, even with warming temperatures and potential increase in droughts, is likely to be a trend towards fewer fires, perhaps that are easier to control. But still dangerous. Still unpredictable. Still shaping ecosystems. Still scary.

But up there on the slope. the fire was gaining a greater hold, as the gusting wind caught it in patches, and gradually they were forced downwards, into the trees edging the road. In triumphant thunder the fire roared after them.” Susan Cooper, The Grey King


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