Data from: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.美国西部亚高山森林中，以林分更替型火制度（stand-replacing fire regimes）为特征的野火愈发频发，正持续扩大早期演替森林（early-seral forests）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后林分的可燃物载荷（fuel loads）及其空间变异性仍知之甚少。我们以1988年黄石公园大火后自然更新的24年生美扭松（Pinus contorta var. latifolia）林分（n=82）为研究对象，采集可燃物剖面数据，旨在解答三个核心问题：（1）年轻美扭松林分内部及不同林分间，冠层可燃物载荷与地表可燃物载荷呈现出怎样的变化规律？（2）冠层与地表可燃物载荷如何随美扭松林分火烧前、火烧后的林分结构及环境条件发生变化？（3）火烧后8年至24年期间，地表可燃物载荷发生了怎样的动态变化？尽管所有研究林分均由同一场大火更新而来，但整个研究区域内的可燃物组合仍存在极大的空间异质性。有效冠层可燃物载荷与冠层可燃物体积密度（canopy bulk density）的平均值分别为8.5 兆克每公顷（范围：0.0~46.6）与0.24 千克每立方米（范围：0.0~2.3），达到甚至超过了成熟美扭松林的对应水平。地表可燃物总载荷的平均值为123 兆克每公顷（范围：43~207），其中88%属于1000小时级可燃物（1000-hr fuel class）。枯落物、1小时级可燃物（1-hr fuel class）及10小时级可燃物（10-hr fuel class）的载荷均低于成熟美扭松林的已有报道值，而1000小时级可燃物（1000-hr fuel class）载荷则与之持平甚至更高。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。火烧后美扭松的种群密度是冠层可燃物与细级地表可燃物载荷最强的正向预测因子。火烧前林分的演替阶段是火烧后林分中100小时级可燃物（100-hr fuel class）与1000小时级可燃物（1000-hr fuel class）载荷的最佳预测因子，同时也显著影响健康原木与腐朽原木的规模及占比：火烧前为晚期演替的林分，其健康原木占比更高；火烧前为早期演替的林分，其腐朽原木占比更高。我们的研究数据显示，76%的年轻火烧后美扭松林的1000小时级可燃物（1000-hr fuel class）载荷超过了与高强度地表火灾潜在可能性相关的阈值，另有63%的林分超过了与主动冠层火灾潜在可能性相关的阈值。黄石国家公园的火轮回期（fire rotations）预计将缩短至数十年，且目前尚无证据表明缺乏可支撑重复火灾发生的可燃物可推翻这一预测。