Increasing forest fuel aridity with climate change may be expanding mid‐to‐high‐elevation forests' vulnerability to large, severe, and frequent wildfire. Long‐lasting changes in forests' structure and composition may occur if dominant tree species are poorly adapted to shifting wildfire patterns. We hypothesized that altered fire activity may lower existing forest resilience and disrupt the recovery of upper‐montane and subalpine conifer forest types. We empirically tested this hypothesis by quantifying post‐fire forest structure and conifer tree regeneration after spatially large, severe, and rapidly repeated wildfires (<12‐yr interval) in the Central Cascade Range in the U.S. Pacific Northwest. Post‐fire conifer regeneration was generally very poor among plots that experienced either a single high‐severity fire or rapid reburn, driven primarily by lack of proximate seed source. Pre‐fire dominant, shade‐tolerant species' abundance was highly negatively correlated with increasing seed source distances and dry, exposed post‐fire environmental conditions. In rapidly reburned plots, the order of burn severity was critical and promoted establishment of all conifer species, if low‐then‐high severity, or primarily fire‐adapted pines, if high‐then‐low severity. Our findings suggest that these forests, affected by expansive high‐severity and/or short‐interval wildfire, may transition into a patchy, low‐density, pine‐dominated forest state under future warming trends. These emerging, early seral ecosystems will incorporate more fire‐adapted tree species, lower tree densities, and more non‐forest patches than prior forests, likely expanding their resilience to anticipated increases in fire frequency. If future larger, more severe, and more frequent wildfire patterns manifest as expected in the Cascade Range, previously denser, moist mid‐to‐high‐elevation forests may begin resembling their drier, lower‐elevation mixed‐conifer counterparts in structure and composition.

中文翻译：

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高强度和短间隔的野火限制了中级联山脉的森林恢复

随着气候变化，森林燃料干旱的增加可能会使中高海拔森林的脆弱性扩大，使其更容易遭受大规模，严重和频繁的野火。如果优势树种不善于适应不断变化的野火格局，则森林的结构和组成可能会发生长期变化。我们假设火灾活动的改变可能会降低现有森林的复原力，并破坏上山区和亚高山针叶林类型的恢复。我们通过量化美国太平洋西北部中部级联山脉的空间大，严重和快速重复的野火（<12年间隔）后的后火后森林结构和针叶树再生来对这一假设进行经验检验。在经历过一次高烈度火灾或快速重燃的地块中，火后针叶树的再生通常很差，主要是由于缺乏邻近的种子来源。火灾前占优势的耐荫树种的丰度与种子源距离的增加以及干燥，暴露于火灾后的环境条件高度相关。在快速重烧的样地中，烧伤严重程度的顺序至关重要，如果严重程度从低到高，则促进所有针叶树种的建立；如果严重程度从高到低，则主要促进火适应的松树的建立。我们的发现表明，这些森林在未来的变暖趋势下，可能会受到高强度高强度和/或短间隔野火的影响，转变为片状，低密度，松树为主的森林状态。与新兴森林相比，这些新兴的，早期的串行生态系统将包含更多的适应火灾的树种，更低的树密度和更多的非森林斑块，可能将其弹性扩展到预期的发火频率上。如果将来在喀斯喀特山脉表现出预期的更大，更严重，更频繁的野火模式，那么以前茂密，潮湿的中高海拔森林可能在结构和组成上开始类似于它们的干燥，低海拔混合针叶林。