Pacific Northwest Research Station, USDA Forest Service, 11175 Auke Lake Way, Juneau, Alaska, 99801, USA.
WA Franke College of Forestry and Conservation, University of Montana, 32 Campus Drive, Missoula, Montana, 59812, USA.
Ecol Appl. 2020 Mar;30(2):e02023. doi: 10.1002/eap.2023. Epub 2019 Nov 11.
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.
在美国西部，为了恢复历史上频繁发生的低严重度火灾的森林，通常会进行减少燃料措施，以重新建立开阔的早期演替阶段条件，同时减少燃料连续性和负荷。仅在 2001 年至 2016 年期间，美国就对超过 2600 万公顷的联邦土地实施了减少燃料措施（例如疏伐、计划火烧等），反映了减轻高严重度野火风险的紧迫性。然而，在 2001 年至 2012 年期间，美国近 2000 万公顷的土地受到了山松甲虫（Dendroctonus ponderosae）的影响，这使得受野火危害处理的林分的恢复效果更加复杂。需要了解处理后自然干扰对长期森林结构和群落的影响，特别是考虑到燃料处理的实施越来越多，预计气候变暖将加剧干扰的频率和严重程度。我们使用一个多变量实验设计（对照、疏伐、火烧、疏伐+火烧），测试了旨在降低受山松甲虫爆发挑战的林分中高严重度野火危险的处理措施与自然干扰之间的相互作用效应，该设计是在一个以黄松（Pinus ponderosa）为主的森林中进行的。林分于 2002 年进行处理，然后于 2005 年至 2012 年期间受到山松甲虫爆发的影响。我们评估了林分上层和下层森林群落结构、组成和多样性随时间的变化。存在明显的疏伐、火烧和年份效应。疏伐立即降低了林分上层密度；由于山松甲虫的影响，未疏伐的林分中松树密度比疏伐林分降低了 4.5 倍。火烧立即减少了禾本科植物、灌木和总下层植被的盖度，减少幅度高达 52%，与未火烧的处理相比，物种均匀度更高，但到 2016 年，由于生长和山松甲虫爆发，差异消失了。同样，多变量分析表明，处理后森林群落有明显差异，但随着时间的推移变得更加相似，尽管关键的下层和上层属性仍然区分对照和疏伐+火烧。这项研究表明，长期的森林实验对于评估处理的持久性以及处理和自然干扰的复合效应具有重要意义。我们证明了处理引起的生长与山松甲虫引起的树木死亡之间的同质化效应，这对只处理林分上层（疏伐）或下层（火烧）的管理策略提出了挑战，表明只有综合处理才能提供预期的恢复的独特结构和组成结果。
