Landscape variation in tree regeneration and snag fall drive fuel loads in 25-yr old post-fire lodgepole pine forests

Escalating wildfire in subalpine forests with stand-replacing fire regimes is increasing the extent of early-seral forests throughout the western US. Post-fire succession generates the fuel for future fires, but little is known about fuel loads and their variability in young post-fire stands. We sampled fuel profiles in 24-year-old post-fire lodgepole pine (Pinus contorta var. latifolia) stands (n=82) that regenerated from the 1988 Yellowstone Fires to answer three questions. (1) How do canopy and surface fuel loads vary within and among young lodgepole pine stands? (2) How do canopy and surface fuels vary with pre- and post-fire lodgepole pine stand structure and environmental conditions? (3) How have surface fuels changed between 8 and 24 years post-fire? Fuel complexes varied tremendously across the landscape despite having regenerated from the same fires. Available canopy fuel loads and canopy bulk density averaged 8.5 Mg ha-1 [range 0.0-46.6] and 0.24 kg m3 [range: 0.0-2.3], respectively, meeting or exceeding levels in mature lodgepole pine forests. Total surface-fuel loads averaged 123 Mg ha-1 [range: 43 - 207], and 88% was in the 1000-hr fuel class. Litter, 1-hr, and 10-hr surface fuel loads were lower than reported for mature lodgepole pine forests, and 1000-hr fuel loads were similar or greater. Among-plot variation was greater in canopy fuels than surface fuels, and within-plot variation was greater than among-plot variation for nearly all fuels. Post-fire lodgepole pine density was the strongest positive predictor of canopy and fine surface fuel loads. Pre-fire successional stage was the best predictor of 100-hr and 1000-hr fuel loads in the post-fire stands and strongly influenced the size and proportion of sound logs (greater when late successional stands had burned) and rotten logs (greater when early successional stands had burned). Our data suggest that 76% of the young post-fire lodgepole pine forests have 1000-hr fuel loads that exceed levels associated with high-severity surface fire potential, and 63% exceed levels associated with active crown fire potential. Fire rotations in Yellowstone National Park are predicted to shorten to a few decades and this prediction cannot be ruled out by a lack of fuels to carry repeated fires.

美国西部各地，林分更替型火灾制度下的亚高山森林野火持续蔓延，正不断扩大早期演替森林的分布范围。火烧后演替过程会为后续火灾提供可燃物，但目前学界对年轻火烧迹地林分的可燃物载量及其变异特征仍知之甚少。本研究针对1988年黄石公园火灾后自然更新的82个24年生黑松（Pinus contorta var. latifolia）林分开展可燃物剖面采样，旨在解答三个核心研究问题：（1）年轻黑松林分内部及不同林分间，冠层与地表可燃物载量存在怎样的变异规律？（2）冠层与地表可燃物载荷如何随火前、火后黑松林分结构及环境条件发生变化？（3）火灾后8年至24年期间，地表可燃物发生了哪些动态变化？尽管所有林分均源自同一场火灾，但景观尺度下的可燃物复合体仍存在显著差异。可利用冠层可燃物载量与冠层体积密度的平均值分别为8.5 Mg·ha⁻¹[取值范围0.0~46.6]与0.24 kg·m⁻³[取值范围0.0~2.3]，达到或超过成熟黑松森林的对应水平。地表可燃物总载量平均值为123 Mg·ha⁻¹[取值范围43~207]，其中88%属于1000小时时滞可燃物类别。枯落物、1小时时滞及10小时时滞地表可燃物载量均低于成熟黑松森林的已有报道值，而1000小时时滞可燃物载量则与之相当甚至更高。冠层可燃物的样间变异大于地表可燃物，而几乎所有可燃物类型的样内变异均大于样间变异。火后黑松种群密度是冠层与细地表可燃物载量最强的正向预测因子。火前演替阶段是火烧迹地林分中100小时时滞与1000小时时滞可燃物载量的最佳预测因子，同时显著影响健康原木与腐朽原木的规模及占比：火前为晚期演替的林分火烧后健康原木占比更高，火前为早期演替的林分火烧后腐朽原木占比更高。本研究数据显示，76%的年轻火烧迹地黑松森林的1000小时时滞可燃物载量超过了与高强度地表火灾潜势相关的阈值，63%的林分则超过了与主动冠层火灾潜势相关的阈值。黄石国家公园内的火灾轮期预计将缩短至数十年，且无法因缺乏可承载重复火灾的可燃物而排除该预测的合理性。