Identifying and managing disturbance‐stimulated flammability in woody ecosystems

Abstract

Many forest types globally have been subject to an increase in the frequency of, and area burnt by, high‐severity wildfire. Here we explore the role that previous disturbance has played in increasing the extent and severity of subsequent forest fires. We summarise evidence documenting and explaining the mechanisms underpinning a pulse of flammability that may follow disturbances such as fire, logging, clearing or windthrow (a process we term disturbance‐stimulated flammability). Disturbance sometimes initiates a short initial period of low flammability, but then drives an extended period of increased flammability as vegetation regrows. Our analysis initially focuses on well‐documented cases in Australia, but we also discuss where these pattens may apply elsewhere, including in the Northern Hemisphere. We outline the mechanisms by which disturbance drives flammability through disrupting the ecological controls that limit it in undisturbed forests. We then develop and test a conceptual model to aid prediction of woody vegetation communities where such patterns of disturbance‐stimulated flammability may occur. We discuss the interaction of ecological controls with climate change, which is driving larger and more severe fires. We also explore the current state of knowledge around the point where disturbed, fire‐prone stands are sufficiently widespread in landscapes that they may promote spatial contagion of high‐severity wildfire that overwhelms any reduction in fire spread offered by less‐flammable stands.We discuss how land managers might deal with the major challenges that changes in landscape cover and altered fire regimes may have created. This is especially pertinent in landscapes now dominated by extensive areas of young forest regenerating after logging, regrowing following broadscale fire including prescribed burning, or regenerating following agricultural land abandonment.Where disturbance is found to stimulate flammability, then key management actions should consider the long‐term benefits of: (i) limiting disturbance‐based management like logging or burning that creates young forests and triggers understorey development; (ii) protecting young forests from disturbances and assisting them to transition to an older, less‐flammable state; and (iii) reinforcing the fire‐inhibitory properties of older, less‐flammable stands through methods for rapid fire detection and suppression.

What did it used to look like? A case study from tall, wet mainland Mountain Ash forests prior to British invasion

“…Evidence of multi-aged forest present at the time of wildfire in 1939, likely generated by low-severity fire or partial stand replacing disturbance within old-growth stands, is consistent with work on ecological control theory in tall, wet eastern Australian forests (Lindenmayer & Zylstra, 2024) and south-west Australian forests (Zylstra et al, 2023). Ecological control theory posits that plant communities exert ecological controls on fire through the processes of growth and succession over time (Zylstra et al, 2023).…”

“…This is especially important as fire in young stands of forest has the potential to burn into (and destroy) adjacent old forest (Figure 10). Of course, new technologies may not lead to the rapid suppression of all wildfires, but efforts to increase the amount of old-growth Mountain Ash forests may help limit spatial contagion in high-severity fires at a landscape scale , thereby enhancing larger scale forest restoration and protection (Lindenmayer & Zylstra, 2024;Lindenmayer, Zylstra, Kooyman, Taylor, et al, 2022).…”

“…A key part of ecological control theory is that forest flammability is mediated by ecological processes that result in the transition of plants in the lower strata that are available as fuel into taller plants that are less likely to ignite and therefore more likely to act as "overstorey shelter", slowing the wind speed beneath them (Lindenmayer & Zylstra, 2024). The distribution of foliage biomass between lower-strata plants acting as fuel and taller plants acting as overstorey shelter is often related to the stage of forest maturity, where disturbance that removes overstorey shelter promotes regrowth as fuel until the forest once again matures (Specht & Morgan, 1981).…”

“…Where an abundance of these species occur (as in early-successional forests), flammability dynamics may be altered, which could elevate the risk of severe bushfire Furlaud et al, 2021;Lindenmayer & Zylstra, 2024;Taylor et al, 2014Taylor et al, , 2020. Notably, forestry manuals have repeatedly highlighted the fire risks associated with thinning operations in a range of forest types (Forestry Tasmania, 2001;Sebire & Fagg, 1997).…”