Neotropical xerophytic forest ecosystems evolved with fires that shaped their resilience to disturbance events. However, it is unknown whether forest resilience to fires persists under a new fire regime influenced by anthropogenic disturbance and climate change. We asked whether there was evidence for a fire severity threshold causing an abrupt transition from a forest to an alternative shrub thicket state in the presence of typical postfire management. We studied a heterogeneous wildfire event to assess medium-term effects (11 years) of varying fire severity in a xerophytic Caldén forest in central Argentina. We conducted vegetation surveys in patches that were exposed to low (LFS), medium (MFS), and high (HFS) fire severities but had similar prefire woody canopy cover. Satellite images were used to quantify fire severity using a delta Normalized Burning Ratio (dNBR) and to map prefire canopy cover. Postfire total woody canopy cover was higher in low and medium than high severity patches, but the understory woody component was highest in HFS patches. The density of woody plants was over three times higher under HFS than MFS and LFS due to the contribution of small woody plants to the total density. Unlike LFS and MFS patches, the small plants in HFS patches were persistent, multistem shrubs that resulted from the resprouting of top-killed Prosopis caldenia trees and, more importantly, from young shrubs that probably established after the wildfire. Our results suggest that the Caldén forest is resilient to fires of low to moderate severities but not to high-severity fires. Fire severities with dNBR values > ~600 triggered an abrupt transition to a shrub thicket state. Postfire grazing and controlled-fire treatments likely contributed to shrub dominance after high-severity wildfire. Forest to shrub thicket transitions enable recurring high-severity fire events. We propose that repeated fires combined with grazing can trap the system in a shrub thicket state. Further studies are needed to determine whether the relationships between fire and vegetation structure examined in this case study represent general mechanisms of irreversible state changes across the Caldenal forest region and whether analogous threshold relationships exist in other fire-prone woodland ecosystems.

中文翻译：

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新热带干燥森林中应对火灾严重程度的阈值和替代状态

新热带旱生森林生态系统随着火灾的发展而演变，从而形成了它们对干扰事件的抵御能力。然而，在受人为干扰和气候变化影响的新火灾制度下，森林对火灾的抵抗力是否持续存在尚不清楚。我们询问是否有证据表明，在典型的火后管理存在的情况下，火灾严重程度阈值会导致从森林突然转变为替代灌木丛状态。我们研究了一次异质野火事件，以评估阿根廷中部卡尔登旱生森林中不同火灾严重程度的中期影响（11 年）。我们对暴露于低（LFS）、中（MFS）和高（HFS）火灾严重程度但具有相似火灾前木质树冠覆盖的斑块进行了植被调查。卫星图像用于使用 delta 标准化燃烧比 (dNBR) 量化火灾严重程度，并绘制火前树冠覆盖图。火后低度和中度斑块的总木本冠层覆盖度高于高度斑块，但林下木质成分在高强度斑块中最高。由于小型木本植物对总密度的贡献，HFS 下的木本植物密度比 MFS 和 LFS 高出三倍以上。与 LFS 和 MFS 斑块不同，HFS 斑块中的小植物是持久的多茎灌木，它们是由顶部被杀死的Prosopis caldenia树重新发芽形成的，更重要的是，来自可能在野火后生长的年轻灌木。我们的研究结果表明，卡尔登森林能够抵御低至中等严重程度的火灾，但不能抵抗高严重程度的火灾。dNBR 值 > ~600 的火灾严重程度会引发向灌木丛状态的突然转变。火后放牧和控制火处理可能导致高强度野火后灌木丛的优势。森林到灌木丛的过渡导致高严重程度的火灾事件反复发生。我们认为，反复的火灾与放牧相结合可以使系统陷入灌木丛状态。需要进一步的研究来确定本案例研究中检查的火灾与植被结构之间的关系是否代表了卡尔德纳尔森林地区不可逆状态变化的一般机制，以及其他易发生火灾的林地生态系统中是否存在类似的阈值关系。