In historically open-canopied and fire-dependent upland oak (Quercus spp.) forests of the central and eastern United States, fire exclusion is contributing to an increase in competing non-oak tree species that are often shade-tolerant and fire-sensitive. As these non-oak species encroach and oak abundance declines, forests are becoming denser and will likely become cooler, moister, and less flammable through a hypothesized feedback loop termed mesophication. To better understand how this gradual shift in forest composition could affect flammability of leaf litter, the primary fuel in these systems, we measured leaf litter traits and moisture dynamics of two pyrophytic oaks and three non-oaks, and implemented experimental burns on plots (~3-m²) with fuel beds comprised of single- and mixed-species leaf litter. Non-oaks produced thinner, smaller leaves with a greater specific leaf area compared to oaks, and had a higher initial fuel moisture content, traits associated with low flammability. Our experimental burns confirmed that non-oak leaf litter flammability was lower than that of oaks and that flammability decreased linearly with increasing non-oak leaf litter contribution to the fuel bed. These species-driven changes in fuelbed flammability may provide a mechanism whereby encroaching non-oak tree species create self-promoting conditions that are less favorable for regeneration of fire-dependent upland oaks. Thus, without other management interventions, our ability to reintroduce fire into these systems as a management tool to prevent further compositional shifts and improve oak regeneration will likely decline as non-oak species’ contribution to the fine fuel bed increases.

在美国中部和东部历史悠久的开放式棚顶且依赖火的高地栎树（Quercus spp。）森林中，防火排斥加剧了竞争性非栎树种的增长，这些树种通常耐荫且对火很敏感。随着这些非橡木树种的侵占和橡木树的丰度下降，森林正在变得茂密，并有可能通过所谓的“介词化”的反馈回路变得凉爽，潮湿和不易燃。为了更好地了解森林组成的这种逐渐变化如何影响这些系统中主要燃料枯叶落叶的可燃性，我们测量了两种自生栎和三种非栎的枯叶凋落物性状和水分动态，并在田间进行了实验性烧伤（〜 3米²），其燃料床由单种和混合种类的凋落物组成。与橡树相比，非橡树产生的叶片更薄，更小，比叶面积更大，并且初始燃料的水分含量更高，具有易燃性低的特点。我们的实验燃烧证实，非橡树叶垫料的可燃性低于橡木，并且可燃性随着非橡树叶垫料对燃料床的贡献的增加而线性降低。这些物种驱动的燃料床可燃性变化可能提供了一种机制，在这种机制下，非橡木树种的入侵会产生不利于依赖火的陆地栎树再生的自我促进条件。因此，无需其他管理干预，