Mechanisms by which growth and succession limit the impact of fire—A comment on Zylstra et al.'s model

Abstract

Zylstra et al. (2023) hypothesise that ecological controls contribute to a decline in the potential behaviour and severity of fires burning in tall open eucalypt forests unburnt for multi‐decadal periods (>52 years). They model potential flame height in forests dominated by Eucalyptus jacksonii in southwestern Australia and argue that self‐thinning of understorey shrubs leads to a decline in suspended litter (woody necromass), which in turn leads to reduced flame heights in older fuels. Forest floor and suspended litter were not sampled as part of their field survey, and instead they describe the dynamics of these fuel components by fitting equations to data reported from previous studies. The form of equation they use to describe the accumulation of suspended litter does not account for the contributions of woody necromass other than from understorey shrubs. In particular, twig fall from the forest overstorey is likely to make an important contribution to suspended litter and is much slower to decompose than leaves and necromass from understorey shrubs. Their assumption of a decline in suspended litter is inconsistent with published research on fuel dynamics in tall open forests in southwestern Australia, which demonstrates that dead fine fuel loadings do not decline when the forest remains unburnt. In the absence of field data to support their assumption of decline in suspended litter, the broader conclusions of Zylstra et al. (2023) regarding changes in potential behaviour and severity of fires in E. jacksonii dominant forest unburnt for >52 years remain open to question. Read the free Plain Language Summary for this article on the Journal blog.