What are the associations between thinning and fire severity?

Abstract

AbstractThere has been concern globally about the impacts of wildfires on lives, property and biodiversity. Mechanical thinning has been proposed as a way to reduce fire severity. However, its effectiveness appears to vary between regions and ecosystems. Here, we sought to answer the question: Does thinning reduce the severity of wildfire in managed eucalypt forests? We did this by completing an empirical study of the factors affecting two levels of fire severity – crown burn and crown burn/crown scorch in lowland and wet/damp forest burned in 2019‐2020 in East Gippsland, Australia. We found complex interactions between thinning, forest type and time since the last major disturbance in the best‐fitting models for crown burn and crown burn/crown scorch. The probability of a crown burn was higher in younger stands of thinned forest than in unthinned forest. Crown burn risk in thinned forest was characterised by an inverse relationship with increasing time since the last disturbance; there were no such effects in unthinned forest. We found that the probability of crown burn/crown scorch increased with time since last disturbance in both thinned and unthinned lowland forest. The probability of crown burn/crown scorch also increased over time in wet/damp forest, but the patterns were different between unthinned and thinned stands. Risk of crown burn/crown scorch was lower in young thinned forest relative to unthinned forest, but this pattern reversed with increasing time since the last disturbance. Our analyses showed the efficacy of thinning was variable, depending on fire severity, type of forest targeted for management and the age of that forest (as reflected by the time since the last major disturbance). Therefore, thinning interventions to mitigate fire severity should not be implemented without consideration of these factors and are unlikely to be a viable management option in many circumstances. This is because, in some cases, thinning can lead to elevated fire severity (e.g. soon after thinning for crown burn and in older forests for crown burn/crown scorch) and hence have opposite effects to those intended from such activities.