Self-thinning forest understoreys reduce wildfire risk, even in a warming climate

Abstract

As climatic changes continue to drive increases in the frequency and severity of forest fires, it is critical to understand all of the factors influencing the risk of forest fire. Using a spatial dataset of areas burnt over a 58-year period in a 528,343-ha study area, we examined three possible drivers of flammability dynamics. These were: that forests became more flammable as fine biomass (fuel) returned following disturbance (H1), that disturbance increased flammability by initiating dense understorey growth that later self-thinned (H2), and that climatic effects were more important than either of these internal dynamics (H3). We found that forests were unlikely to burn for a short ‘young’ period (5-7 years) following fire, very likely to burn as the regrowing understorey became taller and denser (regrowth period), then after a total post-disturbance period of 43-56 years (young + regrowth periods), fire became unlikely and continued to decrease in likelihood (mature period). This trend did not change as the climate warmed, although increases in synoptic variability (mean changes in synoptic systems per season) had a pronounced effect on wildfire likelihood overall. Young forest and regrowth forest became increasingly likely to burn in years of greater synoptic variability and the time taken for forests to mature increased, but in years with the most severe synoptic variability, mature forests were the least likely to burn. Our findings offer an explanation for fire behaviour in numerous long-term studies in diverse forest types globally and indicate that, even in the face of a warming climate, ‘ecologically-cooperative’ approaches may be employed that reinforce rather than disrupt natural ecological controls on forest fire. These range from traditional indigenous fire knowledge, to modern targeting of suppression resources to capitalise on the benefits of self-thinning, and minimise the extent of dense regrowth in the landscape.

Cultural burning, cultural misappropriation, over‐simplification of land management complexity, and ecological illiteracy

“…However, here we argue that more fire in the landscape will not necessarily be a positive step in some ecosystems and in others may exacerbate problems associated with elevated risks of higher fire severity (e.g. see Zylstra et al 2022) as well as have negative impacts on biodiversity (e.g. Dixon et al 2018).…”

“…2021; Zylstra et al . 2022) and how these temporal changes may evolve management practices from those historically applied. For instance, positive feedbacks between vegetation and anthropogenic‐induced climatic changes (increases in temperature associated with carbon‐dioxide levels) have exacerbated the risk of severe wildfires in many terrestrial ecosystems (Alizadeh et al .…”

“…In yet other cases, further fire soon after an ecosystem has commenced recovering after a previous fire may kill germinants, destroy natural regeneration, and greatly impair post-fire recovery (Whelan 1995;Enright et al 2015). Moreover, in some ecosystems, the structure and composition of vegetation after fire may increase flammability, such as when burning promotes new cohorts of dense grasses and shrubs (Zylstra et al 2016(Zylstra et al , 2022Bowd et al 2021).…”

“…Blanket statements like "we need more burning" from sometimes ill-informed members of the public and the media are an oversimplification of the complexity of fire management and may be inconsistent with the ecology and disturbance-tolerance of specific ecosystems. Moreover, it is also important to consider the influence of climatic changes on landscape flammability (McDowell et al 2020;Abram et al 2021;Canadell et al 2021;Zylstra et al 2022) and how these temporal changes may evolve management practices from those historically applied. For instance, positive feedbacks between vegetation and anthropogenic-induced climatic changes (increases in temperature associated with carbon-dioxide levels) have exacerbated the risk of severe wildfires in many terrestrial ecosystems (Alizadeh et al 2021).…”

Considerations in the protection of marsupial gliders and other mature-forest dependent fauna in areas of intensive logging in the tall forests of Victoria, Australia

“…Further, intensive logging makes fires more severe and the resultant regrowth more fire sensitive (Taylor et al 2014;Wilson et al 2018;Lindenmayer et al 2011aLindenmayer et al , 2022bFurlaud et al 2021). This is because logging initially redistributes flammable leaf and branch material from the canopy to ground level, increasing fuel load (see Zylstra et al 2022), makes the physical environment of these areas hotter and drier (Lindenmayer et al 2022b), and within about 10 years changes the forest to be more flammable (see Zylstra et al 2016Zylstra et al , 2021Furlaud et al 2021;Lindenmayer et al , 2022aZylstra et al 2021). Thus, fire severity tends to peak in juvenile regrowth from about age 10-40 years in a range of forest types.…”