Fire exclusion over the last two centuries has driven a significant fire deficit in the forests of western North America, leading to widespread changes in the composition and structure of these historically fire‐adapted ecosystems. Fuel treatments have been increasingly applied over the last few decades to mitigate fire hazard, yet it is unclear whether these fuel‐focused treatments restore the fire‐adapted conditions and species that will allow forests to persist into the future. A vital prerequisite of restoring fire‐adaptedness is ongoing establishment of fire‐tolerant tree species, and both the type and reoccurrence of fuel treatments are likely to strongly influence stand trajectories. Here, we leveraged a long‐term study of repeated fuel treatments in a Sierra Nevada mixed‐conifer forest to examine the regeneration response of six native tree species to the repeated application of common fuel treatments: prescribed fire, mechanical, mechanical plus fire, and untreated controls. Our objectives were to (1) quantify differences in forest structure and composition following the repeated application of alternative fuel treatments that may influence the establishment environment and then (2) identify the stand structure and climate conditions influencing seedling dynamics. We found that both treatment type and intensity are highly influential in shifting forests toward more fire‐adapted conditions and determining species‐specific regeneration dynamics. Specifically, the conifer species tracked here increased in either colonization or persistence potential following repeated applications of fire, indicating fire may be most effective for restoring regeneration conditions broadly across species. Fire alone, however, was not enough to promote fire‐adapted composition, with concurrent mechanical treatments creating more favorable conditions for promoting colonization and increasing abundances of fire‐tolerant ponderosa pine. Yet, even with repeated fuel treatment application, establishment of fire‐intolerant species far exceeded that of fire‐tolerant species over this 20‐year study period. Moreover, increasing growing season water stress negatively impacted seedling dynamics across all species regardless of treatment type and intensity, an important consideration for ongoing management under heightened climatic stress. While repeated treatments are waypoints in restoring fire‐adapted conditions, more intense treatments via gap‐creation or hotter prescribed fires targeting removal of fire‐intolerant species will be necessary to sustain recruitment of fire‐tolerant species.

中文翻译：

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在美国内华达山脉针叶混交林中，重复的燃料处理达不到适应火灾的再生目标


过去两个世纪的防火措施导致北美西部森林出现严重的火灾赤字，导致这些历史上适应火灾的生态系统的组成和结构发生广泛变化。在过去的几十年里，燃料处理越来越多地用于减轻火灾隐患，但目前尚不清楚这些以燃料为重点的处理是否能恢复适应火灾的条件和物种，从而使森林能够持续存在。恢复耐火性的一个重要先决条件是持续建立耐火树种，而燃料处理的类型和重复发生都可能强烈影响林分的轨迹。在这里，我们利用对内华达山脉针叶树混合林中重复燃料处理的长期研究，检查了六种本地树种对重复应用常见燃料处理的再生反应：计划火、机械、机械加火和未经处理的控制。我们的目标是 （1） 量化重复应用可能影响种植环境的替代燃料处理后森林结构和组成的差异，然后 （2） 确定影响幼苗动态的林分结构和气候条件。我们发现，处理类型和强度对将森林转变为更适应火灾的条件和确定特定物种的再生动力学具有很大影响。具体来说，这里追踪的针叶树物种在反复施火后定植或持久性潜力增加，这表明火可能最有效地恢复物种之间的再生条件。 然而，仅靠火并不足以促进适应火的成分，同时进行的机械处理为促进定殖和增加耐火黄松的丰度创造了更有利的条件。然而，即使在这 20 年的研究期间，不耐火物种的建立也远远超过耐火物种的建立。此外，无论处理类型和强度如何，生长季节水分胁迫的增加都会对所有物种的幼苗动态产生负面影响，这是在气候胁迫加剧的情况下进行持续管理的重要考虑因素。虽然重复处理是恢复耐火条件的航路点，但为了维持耐火物种的招募，需要通过创造间隙或更热的计划用火来进行更密集的处理，以清除不耐火的物种。