Abstract Ponderosa pine (Pinus ponderosa) forests occur at their warmer, drier environmental limits in the Mogollon Highlands ecoregion (MHE) of the Southwestern United States, and are commonly found in stringers or discrete stands that form ecotones with interior chaparral. These “rear edge” forests are likely to be highly vulnerable to rapid changes in structure and composition with climate warming, drought, and wildfire. There is increasing interest in understanding historical conditions, ecosystem changes, and restoration needs for MHE forests. However, comprehensive reconstruction analysis of fire regimes and stand structure has not been done for these systems, which differ from many montane ponderosa pine forests by having an abundance of understory shrubs. In this study we used demographic data from field plots, fire scar samples, and dendroecology to reconstruct historical fire regimes and landscape structure at ponderosa pine-dominated sites that spanned a range of environmental conditions on the Prescott and Tonto National Forests. We found strong evidence of historical surface fire regimes with mean fire intervals ranging 1.3–15.6 years across the five MHE sites during the period 1700–1879. We found very little evidence of historical high-severity fire at any study site. Historical forest structure was open with tree densities ranging 84.7–136.4 trees ha−1 and stand basal area (BA) ranging 4.5–8.4 m2 ha−1. Historical composition showed codominance of ponderosa pine, Arizona white oak (Quercus arizonica), Emory oak (Q. emoryi), and Gambel oak (Q. gambelii). Thus, oak species and likely other hardwoods were important historical components of these ecosystems. Contemporary forests are greater in stand density and BA by 359–703% and 285–502%, respectively, compared to historical estimates. In addition, we observed contemporary shifts in species composition. Changes related to disruption of historical fire regimes have increased susceptibility of ponderosa pine forests in the MHE to rapid shifts in structure and composition that may come about with climate change and high-intensity wildfire. Meeting fuels reduction and ecological restoration goals will be challenging for land managers due to vigorous regeneration responses of shrubs to tree thinning, prescribed burning, or other management activities. Managers will be required to balance attention to historical reference conditions, conservation of biological diversity, and needs for fuels management.

中文翻译：

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莫戈隆高地生态区橡松林的火灾状况和结构变化：对生态恢复的影响

摘要 美国西南部莫戈隆高地生态区 (MHE) 的黄松 (Pinusponderosa) 森林在其更温暖、更干燥的环境限制下出现，通常在与内部丛林形成交错带的纵梁或离散林分中发现。这些“后缘”森林很可能极易受到气候变暖、干旱和野火引起的结构和成分快速变化的影响。人们对了解 MHE 森林的历史条件、生态系统变化和恢复需求越来越感兴趣。然而，尚未对这些系统进行火势和林分结构的综合重建分析，这些系统与许多山地黄松森林的不同之处在于拥有丰富的林下灌木。在这项研究中，我们使用了来自田间地块的人口统计数据、火痕样本、和树木生态学，以重建黄松占主导地位的地点的历史火灾状况和景观结构，这些地点跨越了普雷斯科特和通托国家森林的一系列环境条件。我们在 1700 年至 1879 年期间在五个 MHE 地点发现了历史地表火灾状况的有力证据，平均火灾间隔为 1.3-15.6 年。我们在任何研究地点都几乎没有发现历史上严重火灾的证据。历史森林结构是开放的，树木密度范围为 84.7-136.4 棵树 ha-1，林分基础面积 (BA) 范围为 4.5-8.4 m2 ha-1。历史成分显示黄松、亚利桑那白橡木 (Quercus arizonica)、埃默里橡木 (Q. emoryi) 和甘贝尔橡木 (Q. gambelii) 的共性。因此，橡树树种和可能的其他硬木是这些生态系统的重要历史组成部分。与历史估计值相比，当代森林的林分密度和 BA 分别高出 359-703% 和 285-502%。此外，我们观察到物种组成的当代变化。与历史火灾状况中断相关的变化增加了 MHE 中的黄松林对结构和组成的快速变化的敏感性，这可能会随着气候变化和高强度野火而发生。由于灌木对树木间伐、规定的燃烧或其他管理活动的强烈更新反应，实现燃料减少和生态恢复目标对土地管理者来说将是一个挑战。管理人员需要平衡对历史参考条件、生物多样性保护和燃料管理需求的关注。