Abstract Subalpine coniferous forests are adapted to cycles of fire and successional development, but increasing fire frequency and severity are altering historical stand structure, composition, and plant diversity. For instance, conifer regeneration has become increasingly variable as a result of prolonged aridity following fire, but the potential cascading effects on understory community development remain virtually unexplored. We used a natural experiment to investigate the relationships among understory plant succession, soil moisture regime, and variable tree canopy development over 30 years following the 1988 fires in the Greater Yellowstone Ecosystem, U.S.A. In 1990, at each of two study areas, plots were established in different site types defined by burn status (burned or unburned) and soil moisture regime (mesic or xeric), determined using biophysical indicators. We asked: (1) Are differences in soil moisture regime associated with differences in understory community composition (species richness, diversity, and functional group representation) during the first three decades after fire? (2) Does the relationship between soil moisture regime and community composition change over time as subcanopy (height >137 cm) tree densities increase? We confirmed our original designations of soil moisture regime using in situ measurements of soil moisture. Over the first decade of succession, species richness and diversity were lower in xeric-burned than in mesic-burned plots. Nearly 30 years after fire at the south-aspect study area, where subcanopy tree densities were low, seral understories diverged by soil moisture regime. There, graminoid cover was 23-fold higher, and forb cover was 3-fold lower, in xeric-burned than in mesic-burned plots. In contrast, in the mixed aspect study area, subcanopy tree densities were markedly higher. There, the understory did not vary by soil moisture regime, appearing to converge successionally and becoming more similar to unburned forest communities. Our results suggest that site-specific soil moisture regimes structure the early trajectory of post-fire understory recovery, but the relationship diminishes as tree canopies develop over time. However, under a warming climate, variable tree canopy development may compound increasing aridity, particularly on xeric sites, resulting in decreases in plant diversity and forb cover, increases in graminoid cover, and the potential for altered successional dynamics.

中文翻译：

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火灾后前 30 年的土壤水分状况和冠层闭合结构亚高山林下发育

摘要 亚高山针叶林适应火灾和演替发展的循环，但火灾频率和严重程度的增加正在改变历史林分结构、组成和植物多样性。例如，由于火灾后长期干旱，针叶树的再生变得越来越多变，但对林下社区发展的潜在连锁影响实际上仍未得到探索。在 1988 年美国大黄石生态系统火灾后的 30 年里，我们使用自然实验来研究林下植物演替、土壤水分状况和可变树冠发育之间的关系。 1990 年，在两个研究区域中的每一个区域都建立了样地在由燃烧状态（燃烧或未燃烧）和土壤水分状况（中等或干旱）定义的不同场地类型中，使用生物物理指标确定。我们问：（1）在火灾发生后的前三年内，土壤水分状况的差异是否与林下群落组成（物种丰富度、多样性和功能群表示）的差异有关？(2) 随着次冠层（高度 >137 cm）树木密度的增加，土壤水分状况和群落组成之间的关系是否会随时间变化？我们使用土壤水分的原位测量确认了我们最初指定的土壤水分状况。在演替的第一个十年中，旱地烧地的物种丰富度和多样性低于中旱烧地。在南向研究区发生火灾近 30 年后，那里的副冠层树密度较低，下层林下层因土壤水分状况而发生分化。在那里，禾本科覆盖率高出 23 倍，和 forb 覆盖率低 3 倍，在干旱燃烧的地块中比在中等燃烧的地块中。相比之下，在混合方面的研究区域，亚冠树密度明显更高。在那里，林下植被不会因土壤湿度状况而异，似乎是连续汇聚并变得更类似于未烧毁的森林群落。我们的研究结果表明，特定地点的土壤水分状况构成了火灾后林下恢复的早期轨迹，但随着树冠的发展，这种关系会随着时间的推移而减弱。然而，在气候变暖的情况下，可变的树冠发育可能会加剧干旱，特别是在干旱地区，导致植物多样性和杂草覆盖率下降，禾本科植物覆盖率增加，并可能改变演替动态。相比之下，在混合方面的研究区域，亚冠树密度明显更高。在那里，林下植被不会因土壤湿度状况而异，似乎是连续汇聚并变得更类似于未烧毁的森林群落。我们的研究结果表明，特定地点的土壤水分状况构成了火灾后林下恢复的早期轨迹，但随着树冠的发展，这种关系会随着时间的推移而减弱。然而，在气候变暖的情况下，可变的树冠发育可能会加剧干旱，特别是在干旱地区，导致植物多样性和杂草覆盖率下降，禾本科植物覆盖率增加，并可能改变演替动态。相比之下，在混合方面的研究区域，亚冠树密度明显更高。在那里，林下植被不会因土壤湿度状况而异，似乎是连续汇聚并变得更类似于未烧毁的森林群落。我们的研究结果表明，特定地点的土壤水分状况构成了火灾后林下恢复的早期轨迹，但随着树冠的发展，这种关系会随着时间的推移而减弱。然而，在气候变暖的情况下，可变的树冠发育可能会加剧干旱，特别是在干旱地区，导致植物多样性和杂草覆盖率下降，禾本科植物覆盖率增加，并可能改变演替动态。似乎连续融合并变得更类似于未烧毁的森林群落。我们的研究结果表明，特定地点的土壤水分状况构成了火灾后林下恢复的早期轨迹，但随着树冠的发展，这种关系会随着时间的推移而减弱。然而，在气候变暖的情况下，可变的树冠发育可能会加剧干旱，特别是在干旱地区，导致植物多样性和杂草覆盖率下降，禾本科植物覆盖率增加，并可能改变演替动态。似乎连续融合并变得更类似于未烧毁的森林群落。我们的研究结果表明，特定地点的土壤水分状况构成了火灾后林下恢复的早期轨迹，但随着树冠的发展，这种关系会随着时间的推移而减弱。然而，在气候变暖的情况下，可变的树冠发育可能会加剧干旱，特别是在干旱地区，导致植物多样性和杂草覆盖率下降，禾本科植物覆盖率增加，并可能改变演替动态。