Across much of the eastern United States, oak forests are undergoing mesophication as shade-tolerant competitors become more abundant and suppress oak regeneration. Given the historical role of anthropogenic surface fires in promoting oak dominance, prescribed fire has become important in efforts to reverse mesophication and sustain oaks. In 2000 we established the Ohio Hills Fire and Fire Surrogate (FFS) study to examine whether repeated prescribed fire (Fire), mechanical partial harvest (Mech), and their combined application (Mech + Fire) reduced the dominance of subcanopy mesophytic competitors, increased the abundance of large oak–hickory advance regeneration, created a more diverse and productive ground-layer flora, and produced fuel beds more conducive to prescribed fire, reducing the risk of high-severity wildfire. Here we report on the ~20-year effects of treatments on vegetation and fuels and examine the support for interactive effects across a topographic-moisture and energy gradient. In general, we found that Fire and Mech + Fire treatments tended to reverse mesophication while the Mech-only treatment did not. The moderate and occasionally high-intensity fires resulted in effects that were ultimately very similar between the two fire treatments but were modulated by topography with increasing fire severity on drier sites. In particular, we found support for an interaction effect between treatment and topography on forest structure and tree regeneration responses. Fire generally reduced mesophytic tree density in the midstory and sapling strata across all site conditions, while leading to substantial gains in the abundance of large oak–hickory advance regeneration on dry and intermediate landscape positions. Fire also promoted ground-layer diversity and created compositionally distinct communities across all site conditions, primarily through the increased richness of native perennial herbs. However, the fire had limited effects on fine surface fuel loading and increased the loading of large woody fuels, potentially increasing the risk of high-severity wildfire during drought conditions. We conclude that two decades of repeated fires, with and without mechanical density reduction, significantly shifted the trajectory of mesophication across most of the landscape, particularly on dry and intermediate sites, highlighting the capacity of a periodic fire regime to sustain eastern oak forests and promote plant diversity but modulated by topography.

中文翻译：

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维持东部橡树林：火灾和地形对植被和燃料的协同效应

在美国东部的大部分地区，随着耐荫竞争者变得更加丰富并抑制橡树再生，橡树林正在经历中温化。鉴于人为地表火灾在促进橡树优势方面的历史作用，规定的火灾在扭转中温化和维持橡树的努力中变得非常重要。 2000 年，我们建立了俄亥俄山火和火替代 (FFS) 研究，以检验重复规定的火 (Fire)、机械部分收获 (Mech) 以及它们的组合应用 (Mech + Fire) 是否会降低冠层下中生竞争者的优势，增加丰富的大型橡树和山核桃木促进了再生，创造了更加多样化和生产力更高的地面植物群，并产生了更有利于规定火灾的燃料床，降低了严重野火的风险。在这里，我们报告了大约 20 年的处理对植被和燃料的影响，并研究了对跨地形水分和能量梯度的相互作用效应的支持。总的来说，我们发现火和机甲+火治疗倾向于逆转中化，而仅机甲治疗则不然。中度和偶尔的高强度火灾所产生的影响最终在两种火灾处理之间非常相似，但受到地形的调节，在干燥地区火灾严重程度不断增加。特别是，我们发现了处理和地形之间的相互作用对森林结构和树木再生反应的影响。在所有场地条件下，火灾通常会降低中层和幼树层的中生树木密度，同时导致干燥和中间景观位置上大量大型橡树-山核桃提前再生。火灾还促进了地面层的多样性，并在所有场地条件下创建了组成独特的群落，这主要是通过增加当地多年生草本植物的丰富度来实现的。然而，火灾对精细表面燃料装载量的影响有限，并增加了大型木质燃料的装载量，可能增加干旱条件下发生高强度野火的风险。我们的结论是，二十年的反复火灾，无论机械密度是否降低，都显着改变了大部分景观的中温化轨迹，特别是在干燥和中间的地点，突显了周期性火灾制度维持东部橡树林和促进中温化的能力。植物多样性但受地形调节。