Understanding how severe disturbances and their interactions affect forests is key to projecting ecological change under a warming climate. Substantial increases in some biotic disturbances, such as bark beetle outbreaks, in temperate forest ecosystems may compromise recovery to a forest vegetation type (i.e., physiognomic recovery or resilience), especially if subsequent biotic disturbances (e.g. herbivory) alter recovery mechanisms. From 2005 to 2017, severe outbreaks (> 90% mortality) of spruce bark beetles (SB, Dendroctonus rufipennis) affected Engelmann spruce (Picea engelmannii) across 325,000 ha of spruce and subalpine fir (Abies lasiocarpa) forest in the southern Rocky Mountains, USA. Concurrently, an outbreak of western balsam bark beetle (WBBB, Dryocoetes confuses) infested subalpine fir across at least 47,000 of these hectares. We explored the capacity of 105 stands affected by one or two bark beetle outbreaks and browsing of juvenile trees by ungulates to return to a forest vegetation type in the context of pre-outbreak forest conditions and topography. Nine initial forest trajectories (i.e. at least several decades) were identified from four pre-outbreak forest types affected by three biotic disturbances that occurred at different spatial scales and severities. Most stands (86%) contained surviving non-host adult trees in the main canopy [fir and aspen (Populus tremuloides)] and many surviving juveniles of all species, implying that they are currently on a trajectory for physiognomic recovery. Stands composed exclusively of large-diameter spruce were affected by a severe SB outbreak and were most vulnerable to a transition to a low-density forest, below regional stocking levels (<370 trees per hectare). Greater pre-outbreak stand structural complexity and species diversity were key traits of stands with a higher potential for physiognomic recovery. However, all multi-species stands shifted in relative composition of the main canopy to non-host species, suggesting low potential for compositional recovery over the next several decades. Most post-outbreak stands (86%) exceeded regional stocking levels with trees taller than the browse zone (<2 m). As such, ungulate browsing on over half of all juveniles will primarily affect the rate of infilling of the forest canopy and preferential browsing of more palatable species will influence the composition of the future forest canopy.

中文翻译：

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云杉和西香脂树皮甲虫同步爆发后的森林恢复因有蹄类动物的浏览而减慢

了解严重干扰及其相互作用如何影响森林是预测气候变暖下生态变化的关键。温带森林生态系统中某些生物干扰（例如树皮甲虫爆发）的大幅增加可能会影响森林植被类型的恢复（即地貌恢复或恢复力），特别是如果随后的生物干扰（例如食草）改变了恢复机制。从 2005 年到 2017 年，云杉树皮甲虫（SB，Dendroctonus rufipennis）的严重爆发（> 90% 死亡率）影响了恩格曼云杉（Picea engelmannii），分布在美国南部落石山的 325,000 公顷云杉和亚高山冷杉（Abies lasiocarpay）森林. 与此同时，西部香脂树皮甲虫（WBBB， Dryocoetes 混淆）的爆发在这些至少 47,000 公顷的亚高山冷杉中肆虐。我们探索了 105 个受一两次树皮甲虫爆发和有蹄类动物掠食幼树影响的林分的能力，以便在爆发前的森林条件和地形的背景下恢复到森林植被类型。从受三种生物干扰影响的四种爆发前森林类型中确定了九个初始森林轨迹（即至少几十年），这些干扰发生在不同的空间尺度和严重程度。大多数林分 (86%) 在主树冠 [冷杉和白杨 (Populus tremuloides)] 中包含幸存的非寄主成年树和所有物种的许多幸存的幼树，这意味着它们目前正处于恢复地貌的轨道上。完全由大直径云杉组成的林分受到严重 SB 爆发的影响，最容易向低密度森林过渡，低于区域放养水平（<370 棵树/公顷）。更大的爆发前林分结构复杂性和物种多样性是具有更高地貌恢复潜力的林分的关键特征。然而，所有多物种林分的主冠层相对组成向非寄主物种转变，表明未来几十年组成恢复的潜力很低。大多数爆发后的林分 (86%) 超过了区域放养水平，树木高于浏览区 (<2 m)。因此，超过一半的幼年有蹄类动物捕食将主要影响森林冠层的填充率，优先捕食更可口的物种将影响未来森林冠层的组成。更大的爆发前林分结构复杂性和物种多样性是具有更高地貌恢复潜力的林分的关键特征。然而，所有多物种林分的主冠层相对组成向非寄主物种转变，表明未来几十年组成恢复的潜力很低。大多数爆发后的林分 (86%) 超过了区域放养水平，树木高于浏览区 (<2 m)。因此，超过一半的幼年有蹄类动物捕食将主要影响森林冠层的填充率，优先捕食更可口的物种将影响未来森林冠层的组成。更大的爆发前林分结构复杂性和物种多样性是具有更高地貌恢复潜力的林分的关键特征。然而，所有多物种林分的主冠层相对组成向非寄主物种转变，表明未来几十年组成恢复的潜力很低。大多数爆发后的林分 (86%) 超过了区域放养水平，树木高于浏览区 (<2 m)。因此，超过一半的幼年有蹄类动物捕食将主要影响森林冠层的填充率，优先捕食更可口的物种将影响未来森林冠层的组成。所有多物种在主冠层的相对组成上向非寄主物种转变，表明未来几十年组成恢复的潜力很低。大多数爆发后的林分 (86%) 超过了区域放养水平，树木高于浏览区 (<2 m)。因此，超过一半的幼年有蹄类动物捕食将主要影响森林冠层的填充率，优先捕食更可口的物种将影响未来森林冠层的组成。所有多物种在主冠层的相对组成上向非寄主物种转变，表明未来几十年组成恢复的潜力很低。大多数爆发后的林分 (86%) 超过了区域放养水平，树木高于浏览区 (<2 m)。因此，超过一半的幼年有蹄类动物捕食将主要影响森林冠层的填充率，优先捕食更可口的物种将影响未来森林冠层的组成。