There 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.

中文翻译：

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变薄和火灾严重程度之间有什么关联？

全球范围内一直关注野火对生命、财产和生物多样性的影响。已提出机械减薄作为降低火灾严重性的一种方式。然而，其有效性似乎因地区和生态系统而异。在这里，我们试图回答这个问题：在管理的桉树林中，间伐是否会降低野火的严重程度？为此，我们完成了对影响两个级别火灾严重程度的因素的实证研究——2019-2020 年澳大利亚东吉普斯兰的低地和潮湿/潮湿森林中的树冠烧伤和树冠烧伤/树冠焦烧。我们发现，在树冠烧伤和树冠烧伤/树冠焦烧的最佳拟合模型中，间伐、森林类型和自上次主要干扰以来的时间之间存在复杂的相互作用。在较年轻的稀疏林中，树冠烧伤的概率高于未稀疏林。间伐林中树冠烧伤风险的特点是与自上次干扰以来的时间增加呈负相关；在未砍伐的森林中没有这种影响。我们发现，自上次干扰以来，无论是在变薄还是未变薄的低地森林中，树冠烧伤/树冠焦化的概率都随着时间的推移而增加。在潮湿/潮湿的森林中，冠部烧伤/冠部焦化的可能性也随着时间的推移而增加，但未间伐和间伐林分的模式不同。与未稀疏的森林相比，年轻的稀疏森林中树冠烧伤/树冠焦烧的风险较低，但这种模式随着自上次干扰以来的时间增加而逆转。我们的分析表明，间伐的效果是可变的，这取决于火灾的严重程度、管理目标的森林类型和该森林的年龄（反映自上次重大干扰以来的时间）。因此，在不考虑这些因素的情况下，不应实施减薄干预以减轻火灾严重性，并且在许多情况下不太可能成为可行的管理选择。这是因为，在某些情况下，变薄会导致火灾严重性升高（例如