Fuel and restoration treatments seeking to mitigate the likelihood of uncharacteristic high-severity wildfires in forests with historically frequent, low-severity fire regimes are increasingly common, but long-term treatment effects on fuels, aboveground carbon, plant community structure, ecosystem resilience, and other ecosystem attributes are understudied. We present 20-year responses to thinning and prescribed burning treatments commonly used in dry, low-elevation forests of the western United States from a long-term study site in the Northern Rockies that is part of the National Fire and Fire Surrogate Study. We provide a comprehensive synthesis of short-term (<4 years) and mid-term (<14 years) results from previous findings. We then place these results in the context of a mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak that impacted the site 5–10 years post-treatment and describe 20-year responses to assess the longevity of restoration and fuel reduction treatments in light of the MPB outbreak. Thinning treatments had persistently lower forest density and higher tree growth, but effects were more pronounced when thinning was combined with prescribed fire. The thinning+prescribed fire treatment had the additional benefit of maintaining the highest proportion of ponderosa pine (Pinus ponderosa) for overstory and regeneration. No differences in understory native plant cover and richness or exotic species cover remained after 20 years, but exotic species richness, while low relative to native species, was still higher in the thinning+prescribed fire treatment than the control. Aboveground live carbon stocks in thinning treatments recovered to near control and prescribed fire treatment levels by 20 years. The prescribed fire treatment and control had higher fuel loads than thinning treatments due to interactions with the MPB outbreak. The MPB-induced changes to forest structure and fuels increased the fire hazard 20 years post-treatment in the control and prescribed fire treatment. Should a wildfire occur now, the thinning+prescribed fire treatment would likely have the lowest intensity fire and highest tree survival and stable carbon stocks. Our findings show broad support that thinning and prescribed fire increase ponderosa pine forest resilience to both wildfire and bark beetles for up to 20 years, but efficacy is waning and additional fuel treatments are needed to maintain resilience.

中文翻译：

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北落基山脉干燥森林燃料减少和恢复处理的长期效果

燃料和恢复处理旨在减轻历史上频繁发生的低严重性火灾的森林中发生异常高严重性野火的可能性，但长期处理对燃料、地上碳、植物群落结构、生态系统恢复力和其他生态系统属性尚未得到充分研究。我们从北落基山脉的一个长期研究地点（国家火灾和火灾替代研究的一部分）介绍了美国西部干燥、低海拔森林中常用的间伐和规定燃烧处理方法 20 年的反应。我们根据之前的研究结果对短期（<4 年）和中期（<14 年）结果进行了全面综合。然后，我们将这些结果置于处理后 5-10 年影响该地点的山松甲虫（MPB； Dendroctonus ponderosae ）爆发的背景下，并描述 20 年的响应，以根据以下因素评估恢复和燃料减少处理的寿命： MPB爆发。间伐处理持续降低了森林密度，提高了树木生长速度，但当间伐与规定的火烧相结合时，效果更为明显。间伐+规定火烧处理的另一个好处是保持最高比例的黄松（Pinus ponderosa），用于植被覆盖和再生。20年后，林下原生植物覆盖度和丰富度或外来物种覆盖度没有差异，但外来物种丰富度虽然相对于本地物种较低，但在间伐+规定火烧处理中仍然高于对照。疏伐处理中的地上活碳储量在 20 年内恢复到接近控制和规定的火处理水平。由于与 MPB 爆发的相互作用，规定的火灾处理和控制比稀疏处理具有更高的燃料负荷。在控制和规定的火灾处理中，MPB引起的森林结构和燃料的变化在处理后20年增加了火灾危险。如果现在发生野火，间伐+规定的火灾处理可能会产生最低强度的火灾和最高的树木存活率和稳定的碳储量。我们的研究结果表明，间伐和规定火烧可以提高黄松林对野火和树皮甲虫的抵御能力长达 20 年，但功效正在减弱，需要额外的燃料处理来维持抵御能力。