Abstract Nearly a century of fire suppression in most forested land of the United States has limited researchers’ ability to construct and rigorously test conceptual models of forest structural development in mixed-conifer ecosystems. As a result, land managers must rely on conceptual models of forest development that may overemphasize idealized stand structures and developmental pathways, which ultimately hampers management of many forest systems for resilience to future climate change impacts. We sought to determine the relative importance of fire history (frequency, severity, and time since fire) and biophysical variables on forest structural diversity and development. Importantly, we conducted our study in an ecosystem with a contemporary active fire regime where wildland fire has been managed as an ecosystem process for four decades. Using data from unburned (≥80 years since fire), once-burned and twice-burned mixed-conifer forests in the Bob Marshall Wilderness of Northwest Montana, we conducted a hierarchical clustering analysis to identify forest stand structure classes. We then used a Classification and Regression Tree analysis, combined with other post-hoc analyses, to elucidate the biophysical and disturbance history drivers that lead to each structure class. The cluster analysis revealed six forest structure classes. The CART analysis indicated that time since fire plays a large role in determining forest structure, but at intermediate time scales structure is further shaped by repeat fires and interactions with biophysical variables. The CART and post-hoc analyses did not, however, indicate a singular fire history or biophysical pathway to any one structure class. We synthesize our results in a conceptual model of forest structural development under an active fire regime. This model supports existing theory that succession following severe fire plays a large role in shaping forest structure. It also recognizes the role of fire at variable severities and frequencies, the physical environment, and tree community composition in influencing forest structural development. The complexity of forest structure and development generated by an active fire regime points to the need to incorporate a process-based view of wildfire if the goal is to manage for improved resiliency and adaptive capacity to future climate change impacts.

中文翻译：

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活跃火势混交针叶林的结构多样性和发展

摘要 美国大部分林地近一个世纪的灭火工作限制了研究人员构建和严格测试混合针叶林生态系统中森林结构发展的概念模型的能力。因此，土地管理者必须依赖森林发展的概念模型，这些模型可能过分强调理想化的林分结构和发展路径，这最终会阻碍许多森林系统的管理，以适应未来气候变化的影响。我们试图确定火灾历史（频率、严重程度和火灾发生后的时间）和生物物理变量对森林结构多样性和发展的相对重要性。重要的是，我们在一个具有当代活跃火灾制度的生态系统中进行了研究，在这个生态系统中，荒地火灾已经作为一个生态系统过程进行了四年的管理。使用来自蒙大拿州西北部 Bob Marshall Wilderness 未燃烧（火灾发生后 80 年以上）、一次燃烧和两次燃烧的混合针叶林的数据，我们进行了层次聚类分析，以确定林分结构类别。然后，我们使用分类和回归树分析，结合其他事后分析，阐明导致每个结构类别的生物物理和干扰历史驱动因素。聚类分析揭示了六个森林结构类别。CART 分析表明，自火灾以来的时间在决定森林结构方面发挥着重要作用，但在中间时间尺度上，结构会因重复火灾和与生物物理变量的相互作用而进一步形成。然而，CART 和事后分析并未表明任何一种结构类别的单一火灾历史或生物物理途径。我们将我们的结果综合在活跃火灾状况下森林结构发展的概念模型中。该模型支持现有理论，即严重火灾后的演替对森林结构的形成起着重要作用。它还认识到不同严重程度和频率的火灾、物理环境和树木群落组成在影响森林结构发展方面的作用。如果目标是管理提高对未来气候变化影响的复原力和适应能力，则由活跃的火灾制度产生的森林结构和发展的复杂性表明，需要纳入基于过程的野火观点。该模型支持现有理论，即严重火灾后的演替对森林结构的形成起着重要作用。它还认识到不同严重程度和频率的火灾、物理环境和树木群落组成在影响森林结构发展方面的作用。如果目标是管理提高对未来气候变化影响的复原力和适应能力，则由活跃的火灾制度产生的森林结构和发展的复杂性表明，需要纳入基于过程的野火观点。该模型支持现有理论，即严重火灾后的演替对森林结构的形成起着重要作用。它还认识到不同严重程度和频率的火灾、物理环境和树木群落组成在影响森林结构发展方面的作用。如果目标是管理提高对未来气候变化影响的复原力和适应能力，则由活跃的火灾制度产生的森林结构和发展的复杂性表明，需要纳入基于过程的野火观点。