Large forest fires have far-reaching impacts on the environment, human health, infrastructure and the economy. Forest fires become large when all forest types across a landscape are dry enough to burn. Mesic forests are the slowest to dry and can act as a barrier to fire growth when they are too wet to burn. Therefore, identifying the factors influencing fire occurrence in mesic forests is important for gauging fire risk across large landscapes. We quantified the key factors influencing the likelihood that an active wildfire would propagate through mesic forest. We analysed 35 large forest fires (> 2500 ha) that occurred in Victoria, Australia where mesic and drier eucalypt forests are interspersed across mountainous terrain. We used a random forest model to evaluate 15 meteorological, topographic and disturbance variables as potential predictors of fire occurrence. These variables were extracted for points within burnt and unburnt patches of mesic forest. The likelihood of an active wildfire spreading through mesic forest increased by 65 % as vapour pressure deficit (VPD, i.e., atmospheric dryness) rose from 2.5 to 7 kPa. Other variables had substantially less influence (< 20 % change in fire occurrence) and their effects were further reduced when VPD was very high (> 6.5 kPa). Mesic forests were less likely to burn in areas with lower aridity, shallower slopes, and more sheltered topographic positions. Mesic forests 13–15 years following stand-replacing disturbance had 6 % higher chance of burning than long undisturbed forests (50 years post-disturbance). Overall, we show that topography and disturbance history cannot substantially counter the effects of high VPD. Therefore, the effectiveness of mesic forest as a barrier to the development of large forest fires is weakening as the climate warms. Our analysis also identifies areas less likely to burn, even under high VPD conditions. These areas could be prioritised as wildfire refugia.

中文翻译：

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大气干燥消除了大型森林火灾发展的障碍

大型森林火灾对环境、人类健康、基础设施和经济产生深远影响。当整个地区的所有森林类型都干燥到足以燃烧时，森林火灾就会变得更大。中湿森林干燥速度最慢，当森林太湿而无法燃烧时，可以充当火势蔓延的屏障。因此，确定影响中生森林火灾发生的因素对于衡量大面积火灾风险非常重要。我们量化了影响活跃野火在湿地森林中传播可能性的关键因素。我们分析了澳大利亚维多利亚州发生的 35 起大型森林火灾（> 2500 公顷），那里的山区分布着湿润和干燥的桉树林。我们使用随机森林模型来评估 15 个气象、地形和干扰变量作为火灾发生的潜在预测因素。这些变量是针对中生森林烧毁和未烧毁斑块内的点提取的。随着蒸气压赤字（VPD，即大气干燥度）从 2.5 kPa 上升至 7 kPa，活跃野火在中生森林中蔓延的可能性增加了 65%。其他变量的影响要小得多（火灾发生的变化 < 20%），并且当 VPD 非常高（> 6.5 kPa）时，它们的影响会进一步降低。在干旱度较低、坡度较浅和地形较为隐蔽的地区，中湿森林不太可能燃烧。与长期未受干扰的森林（干扰后 50 年）相比，在林分更替干扰后 13-15 年的湿润森林发生火灾的可能性高出 6%。总体而言，我们表明地形和扰动历史不能显着抵消高 VPD 的影响。因此，随着气候变暖，中等森林作为大型森林火灾发展屏障的有效性正在减弱。我们的分析还确定了即使在高 VPD 条件下也不易燃烧的区域。这些区域可以优先作为野火避难所。