The spatial overlap of multiple ecological disturbances in close succession has the capacity to alter trajectories of ecosystem recovery. Widespread bark beetle outbreaks and wildfire have affected many forests in western North America in the past two decades in areas of important habitat for native ungulates. Bark beetle outbreaks prior to fire may deplete seed supply of the host species, and differences in fire‐related regeneration strategies among species may shift the species composition and structure of the initial forest trajectory. Subsequent browsing of postfire tree regeneration by large ungulates, such as elk (Cervus canadensis), may limit the capacity for regeneration to grow above the browse zone to form the next forest canopy. Five stand‐replacing wildfires burned ~60,000 ha of subalpine forest that had previously been affected by severe (>90% mortality) outbreaks of spruce beetle (SB, Dendroctonus rufipennis) in Engelmann spruce (Picea engelmannii) in 2012–2013 in southwestern Colorado. Here we examine the drivers of variability in abundance of newly established conifer tree seedlings [spruce and subalpine fir (Abies lasiocarpa)] and resprouts of quaking aspen (Populus tremuloides) following the short‐interval sequence of SB outbreaks and wildfire (2–8 yr between SB outbreak and fire) at sites where we previously reconstructed severities of SB and fire. We then examine the implications of ungulate browsing for forest recovery. We found that abundances of postfire spruce seedling establishment decreased substantially in areas of severe SB outbreak. Prolific aspen resprouting in stands with live aspen prior to fire will favor an initial postfire forest trajectory dominated by aspen. However, preferential browsing of postfire aspen resprouts by ungulates will likely slow the rate of canopy recovery but browsing is unlikely to alter the species composition of the future forest canopy. Collectively, our results show that SB outbreak prior to fire increases the vulnerability of spruce–fir forests to shifts in forest type (conifer to aspen) and physiognomic community type (conifer forest to non‐forest). By identifying where compounded disturbance interactions are likely to limit recovery of forests or tree species, our findings are useful for developing adaptive management strategies in the context of warming climate and shifting disturbance regimes.

中文翻译：

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在短时间间隔，复合扰动相互作用之后，预计将通过地震消除白杨树的未来优势

多个连续的生态扰动在空间上的重叠具有改变生态系统恢复轨迹的能力。在过去的二十年中，广泛的树皮甲虫暴发和野火影响了北美西部有蹄类动物重要栖息地的许多森林。火灾前的树皮甲虫暴发可能耗尽宿主物种的种子供应，并且物种之间与火灾相关的再生策略的差异可能会改变原始森林轨迹的物种组成和结构。随后浏览大有蹄类动物（如麋鹿（加拿大鹿）的火后树再生），可能会限制再生能力在浏览区域上方生长以形成下一个森林冠层。2012年至2013年，科罗拉多州西南部地区发生了五次替换林场的大火，烧毁了约60,000公顷的亚高山森林，此前曾受到恩格尔曼云杉（Picea engelmannii）的云杉甲虫（SB，Dendroctonus rufipennis）严重暴发（死亡率超过90％）的影响。在这里，我们审查变异的司机纷纷新成立的针叶树树苗[云杉和冷杉亚高山（的金莲冷杉）和发抖白杨的resprouts（颤杨），在我们之前重建了SB和火的严重程度的地点，按照SB爆发和野火的短时间间隔序列（SB爆发和火之间的2-8年）。然后，我们研究有蹄类动物浏览对森林恢复的影响。我们发现严重的SB暴发地区火后云杉幼苗的形成大量减少。在火灾前用活白杨在林分中繁殖的高产白杨将有利于以白杨为主导的最初的篝火森林轨迹。但是，有蹄类动物对火后白杨新芽的优先浏览可能会减慢冠层恢复的速度，但是浏览不太可能改变未来森林冠层的物种组成。总的来说，我们的结果表明，火灾前SB爆发增加了云杉杉木林易受森林类型（针叶树到白杨）和地貌群落类型（针叶树森林到非森林）转变的脆弱性。通过确定复合干扰相互作用可能限制森林或树木物种恢复的地方，我们的发现对于在气候变暖和干扰机制变化的背景下制定适应性管理策略很有用。