Fiona Scarff: Effect of plant hydraulic traits on flammability

In this new post Fiona Scarff, a former postdoc at Macquarie University, presents her work on the effect of plant hydraulic traits on flammability, highlights the importance of lab meeting for fostering research and discusses the importance of failure in science.

Bushfire grounds near Bermagui in south-eastern Australia, viewed through a destroyed termite mound. Photos: F. Scarff, J. Lewin.
Bushfire grounds near Bermagui in south-eastern Australia, viewed through a destroyed termite mound. Photos: F. Scarff, J. Lewin.

Our paper is about how readily live plants can burn in wildfires. Plants dry out in the hot, dry, windy weather that accompanies the largest fires. How much they dry out, and how moist they were to start with, is a matter of physiological strategy. We were interested in exploring how plant hydraulic traits, which we might normally think of in connection to drought tolerance, could be influencing the behaviour of live fuels during bushfires. The paper draws connections between plant ecophysiology, fire ecology and wildfire science.

I got the idea for the work in a lab meeting where colleagues were laying out their thinking on regulation of moisture content, as an ecophysiological matter to do with growing and surviving in drier or wetter environments. It was a wonderfully collegial lab and I really enjoyed the imperative of being across everyone else’s work. At the time I was exploring how different plant traits affect flammability, particularly phosphate which correlates with growth rates and weediness, but is also used commercially as a fire retardant.  I thought, there is a lot of sophisticated thinking here around how moist plants are under different circumstances, dating back a century, that hasn’t made it into wildfire science. And my colleagues had all this great data we could bring to this question, which they’d collected thinking about water stress but which we could also apply to questions of flammability. I quickly sketched out the key equations on the back of an envelope and we took it from there.

Co-author Ian Wright, equipped with fly net and compass, measuring internal water pressures in plant samples. The pressure chamber apparatus he is using is colloquially called a pressure bomb, from years of old when ‘bomb’ meant a robust, hollow vessel. It’s a good idea to be careful with the wording when importing them into Australia, as for some reason Customs can be alarmed about having ‘pressure bombs’ loaded onto a plane. Photo: T. Lenz.
Co-author Ian Wright, equipped with fly net and compass, measuring internal water pressures in plant samples. The pressure chamber apparatus he is using is colloquially called a pressure bomb, from years of old when ‘bomb’ meant a robust, hollow vessel. It’s a good idea to be careful with the wording when importing them into Australia, as for some reason Customs can be alarmed about having ‘pressure bombs’ loaded onto a plane. Photo: T. Lenz.

There are a bunch of different traits that determine how dry a plant becomes in the kind of weather associated with big fires: variables like cell solute concentration and the stretchiness of cell walls. I wanted to know whether any of those traits varied enough between species to make a meaningful difference to plant moisture during fire weather – and to how easily those plants could then ignite. Our key finding was that plant moisture during fire weather is expected to vary a lot between species: 3–4-fold in the woodland areas of south-eastern Australia that we studied. For comparison, remote-sensing studies report increases in burned area of 15–25-fold through a shift in live fuel moisture of 2-fold or less. So you’ve got species growing alongside each other within a vegetation formation, that are more different in moisture content than what is observed, on average across the vegetation, in these wildfires of very different sizes. It suggests that this physiological variation is large enough to exert powerful effects on fire activity. Most of that variation is driven by differences in how wet each species runs when it is well-hydrated, before the hot, dry weather sets in and sucks the water out of everything. So if you wanted to represent the way different species manage their moisture contents in models of fire activity, our work shows that the best place to start would be with their moisture content at full saturation. Happily, that is very easy to measure and already widely databased.

When a fire passed through this woodland, all the greenery was combusted or scorched except the cypress pines (bright green shrubs in foreground, Callitris verrucosa). You can see the skeletal remains of the other shrubs and trees. All the other foliage in this scene is newly grown, from after the fire came through. Photo: F. Scarff.
When a fire passed through this woodland, all the greenery was combusted or scorched except the cypress pines (bright green shrubs in foreground, Callitris verrucosa). You can see the skeletal remains of the other shrubs and trees. All the other foliage in this scene is newly grown, from after the fire came through. Photo: F. Scarff.

As I write this I’m watching fine bushfire ash sift down like snow in my backyard. It is a year since massive bushfires consumed the forests of south-eastern Australia. Here in Perth, in the continent’s south-west, a woodland fire is now burning out of control through its 10 000th ha, running before a gusty wind. In the years since that lab meeting a discussion has developed in the fire science community, about whether the moisture content of live plants exerts a strong influence on how wildfires burn. It’s part of a larger deliberation over whether our understanding of fire and its ecology could benefit from drawing distinctions between different plant species as fuel, with all their diversity in physiology, phenology, architecture, fine structure and chemistry. An alternative view is that the influence of these variables is modest, and modelling capacity is better directed at refining how combustible gases mix in the turbulent atmosphere of a burning forest. There’s some sense in which these contrasting views arise from distinct intellectual traditions, that have historically drawn on separate bodies of evidence to address quite unrelated questions at disparate scales. Working in a very collegial and interdisciplinary environment really taught me the merit of looking into other disciplines for pre-made solutions to whatever conundrum one is grappling with. I hope we can see these two intellectual traditions converge to pin down the best way forward in understanding wildfire, whichever way that leads.

Coauthor Tanja Lenz (R) and field assistant Robyn Sinclair (L) at our semi-arid field site. Photo: T. Lenz.
Coauthor Tanja Lenz (R) and field assistant Robyn Sinclair (L) at our semi-arid field site. Photo: T. Lenz.

I was drawn into ecology by an inspiring high school teacher. Looking at my family tree though, I realise I was ripe for recruitment. My grandfather and his brothers were passionate amateur naturalists, and that has percolated down through various branches of the family, whether as an interest in bushwalking, or gathering food in the wild, or a fascination with the cultivation of roses, depending on which aunt or cousin I think of.

In the research lab where we conducted the work for this paper, we were concerned with universal principles of plant ecology – underlying trade-offs and constraints that could inform our understanding of how plants grow and function anywhere in the world. I loved that broad-brush thinking, but now I work at a much more local and applied scale. I’m interested in how local government policy can improve conservation outcomes in urban bushland, whether in relation to fire, introduced predators or management of native vertebrates.

An echidna moves through a blackened fire ground. Photo: J. Lewin.
An echidna moves through a blackened fire ground. Photo: J. Lewin.

Some advice I once received that made a big impression on me: you should train yourself to be delighted whenever you discover you are wrong about something. It’s easy to feel stupid, but in fact you were already wrong, and having made that happy discovery, only now do you have an opportunity to start being right; or perhaps just less egregiously wrong. Or wrong in a more constructive and informative fashion. No matter what happens, I feel certain that I have many happy moments of realising I am wrong ahead of me.

Read the article in full here.

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