Forests that keep fires small, and how they do it

Philip Zylstra, Grant Wardell-Johnson, Daniel Falster, Melissa Howe, Nathan McQuoid, Simon Neville

This is a plain language summary of a Functional Ecology research article which can be found here.

South-western Australia has long prided itself on its record of prescribed burning to reduce fuels, but a recent analysis of their records gave some surprising findings. Burning kept fire risk down for a few years, but, compared to old forests that had not been burned, ‘fuel reduced’ areas became much more fire prone for decades afterward. Once those treated areas were compared to old forests with no record of fire for decades, it was clear that the burning programs had actually created the main fire risk.

To find out why this would be, we surveyed one of the forests in the area across a wide range of ages – the iconic Red Tingle. Measuring details that ranged from species composition to the size, crown shape, and spacing of each plant, we found trends in the ways that these parameters changed, then used them to regrow a virtual forest for 100 years. Most fire behaviour models don’t consider the effect of these changes, but we used a state-of-the-art model called FRaME, which looks at the way everything from leaf size to plant spacing affects fire behaviour.

Ecological controls on fire. Thefigure shows the stages of tingle growth and succession after fire, as the forest ages from left to right. Traditional fire management has focused on burning the forest to create the short-term period of open understorey and low flammability we’ve called young forest. Such management, however, also produces the following much longer regrowth age of forest, promoting dense understorey regrowth that burns with large, difficult to control flames. These two stages of recovery together constitute the Disturbed period. In contrast, pre-European forests were dominated by older forests in the Post-disturbance period of mature forest. This occurs because short-lived shrubs that were stimulated by burning eventually self-thin or are outcompeted by taller plants, which self- prune their lower, shaded branches. Due to these “ecological controls” on fire, plants in mature forest are too tall to burn as fuel a lot of the time, and instead slow fires by slowing the wind beneath them (credit: the author)

What we found was striking: mature forests were modelled to burn with much smaller flames because forest understoreys thin themselves over time (Fig. 1), and plants prune their own lower branches when they become too shaded and unproductive. Whereas the traditional view sees forests as becoming increasingly dangerous as plants grow back, we’ve shown that the opposite is true. The plants in forests like Red Tingle start out as fuel when they are small and close to the ground, but when a gap forms between the ground and the bulk of the foliage, those plants are usually out of reach of the flames and instead act to slow the wind beneath them and thereby calm the fire. Left alone, these forests burn with much smaller flames, store far more carbon, and provide safe habitat for the many species now threatened by frequent fire. Once we know such natural controls on fire, it is possible to work with and reinforce them; an approach called ecological cooperation.

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