In the western United States, restoration of forests with historically frequent, low-severity fire regimes often includes fuel reduction that reestablish open, early-seral conditions while reducing fuel continuity and loading. Between 2001 and 2016, fuel reduction (e.g., thinning, prescribed burning, etc.) was implemented on over 26 million hectares of federal lands alone in the United States, reflecting the urgency to mitigate risk from high-severity wildfire. However, between 2001 and 2012, nearly 20 million hectares in the United States were impacted by mountain pine beetle (MPB; Dendroctonus ponderosae), compounding restoration effects in wildfire-hazard-treated stands. Knowledge of the effects of treatments followed by natural disturbance on long-term forest structure and communities is needed, especially considering that fuel treatments are increasingly being implemented and warming climate is predicted to exacerbate disturbance frequency and severity. We tested the interacting effects of treatments designed to reduce high-severity wildfire hazard in stands subsequently challenged by MPB outbreak on vegetation dynamics using a factorial experimental design (control, thin only, burn only, thin + burn) in a ponderosa pine (Pinus ponderosa)-dominated forest. Stands were treated by 2002, then impacted by MPB outbreak from 2005 to 2012. We assessed change in overstory and understory forest community structure, composition, and diversity over time. There were distinct thinning, burning, and year effects. Thinning immediately reduced overstory density; pine density then declined 4.5 times more in unthinned than thinned treatments due to MPB. Burning immediately reduced graminoid, shrub, and total understory cover by as much as 52%, resulting in greater species evenness than unburned treatments, but differences disappeared by 2016 due to growth and MPB outbreak. Similarly, multivariate analyses indicated forest communities were starkly different after treatment but became more similar over time, though key understory and overstory attributes still distinguish control and thin + burn. This study shows the value of long-term silvicultural experiments to evaluate treatment longevity and the compounded effects of treatment and natural disturbance. We demonstrate the homogenizing effects of treatment-induced growth coupled with MPB-caused tree mortality on management strategies that just treat the overstory (thinning) or understory (burning), showing that only combined treatments can provide the unique structural and compositional outcomes expected of restoration.

中文翻译：

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在干燥的松树林中，复合扰动后的植被动态：燃料处理，然后是树皮甲虫暴发。

在美国西部，历史上频繁发生，严重程度较低的火灾情况下的森林恢复通常包括减少燃料，以恢复开放的早期条件，同时减少燃料的连续性和负荷。在2001年至2016年之间，仅在美国就对超过2600万公顷的联邦土地实施了燃油削减（例如稀疏，规定的燃烧等）措施，这反映出迫切需要减轻高强度野火的风险。但是，在2001年至2012年之间，美国近2000万公顷的土地受到了山松甲虫（MPB; Dendroctonus Poterosae）的影响，在经过野火危害处理的林分中，恢复作用更为严重。需要了解自然干扰后的处理对长期森林结构和社区的影响，特别是考虑到越来越多地实施燃料处理，并且预计气候变暖会加剧干扰的频率和严重性。我们使用了分解实验设计（对照，仅稀薄，仅燃烧，稀薄+燃烧）在美国黄松（Pinus tankerosa）中进行了因子分解实验设计，测试了旨在减少随后受到MPB爆发挑战的林分中高强度野火危害的处理措施的相互作用效果。 ）为主的森林。到2002年对林分进行了处理，然后在2005年至2012年受到MPB爆发的影响。我们评估了林上层和林下森林群落结构，组成和多样性随时间的变化。有明显的变薄，燃烧和年度影响。变薄立即降低了楼层的密度；然后，由于MPB，未稀疏处理的松密度比稀疏处理降低了4.5倍。燃烧会立即使类固醇，灌木和林下总覆盖物减少多达52％，从而比未燃烧处理产生更高的物种均匀度，但由于增长和MPB爆发，到2016年差异消失了。同样，多变量分析表明，经过处理后，森林群落完全不同，但是随着时间的推移，森林群落变得越来越相似，尽管关键的林下和林下的属性仍然可以区分控制和稀疏+烧伤。这项研究显示了长期的造林实验对评估治疗寿命以及治疗与自然干扰的复合作用的价值。我们证明了治疗诱导的生长同MPB引起的树木死亡率同质化对仅治疗树皮过薄（稀疏）或树下枯燥（燃烧）的管理策略的影响，