Fire controls tree cover in many savannas by suppressing saplings through repeated topkill and resprouting, causing a demographic bottleneck. Tree cover can increase dramatically if even a small fraction of saplings escape this fire trap, so modelling and management of savanna vegetation should account for occasional individuals that escape the fire trap because they are "better" (i.e. they grow faster than average) or because they are "lucky" (they experience an occasional longer-than-average interval without fire or a below-average fire severity). We quantified variation in growth rates and topkill probability in Quercus laevis (turkey oak) in longleaf pine savanna to estimate the percentage of stems expected to escape the fire trap due to variability in 1) growth rate, 2) fire severity, and 3) fire interval. For trees growing at the mean rate and exposed to the mean fire severity and the mean fire interval, no saplings are expected to become adults under typical fire frequencies. Introducing variability in any of these factors, however, allows some individuals to escape the fire trap. A variable fire interval had the greatest influence, allowing 8% of stems to become adults within a century. In contrast, introducing variation in fire severity and growth rate should allow 2.8% and 0.3% of stems to become adults, respectively. Thus, most trees that escape the fire trap do so because of luck. By chance, they experience long fire-free intervals and/or a low-severity fire when they are not yet large enough to resist an average fire. Fewer stems escape the fire trap by being unusually fast-growing individuals. It is important to quantify these sources of variation and their consequences to improve understanding, prediction, and management of vegetation dynamics of fire-maintained savannas. Here we also present a new approach to quantifying variation in fire severity utilizing a latent-variable model of logistic regression.

中文翻译：

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好运胜于好：稀树草原树木如何在一个多变的世界中逃脱火灾陷阱

火通过反复顶杀和重新发芽抑制树苗来控制许多热带稀树草原的树木覆盖，造成人口瓶颈。如果即使一小部分树苗逃脱了这个火陷阱，树木覆盖率也会显着增加，因此稀树草原植被的建模和管理应该考虑偶尔逃脱火陷阱的个体，因为它们“更好”（即它们比平均生长速度更快）或因为他们是“幸运的”（他们偶尔会经历比平均时间更长的时间间隔没有火灾或火灾严重程度低于平均水平）。我们量化了长叶松稀树草原中 Quercus laevis（火鸡橡树）的生长速率和最高杀灭概率的变化，以估计由于 1) 生长速率、2) 火灾严重程度和 3) 火灾的可变性而预计逃离火灾陷阱的茎的百分比间隔。对于以平均速度生长并暴露于平均火灾严重程度和平均火灾间隔的树木，在典型的火灾频率下，预计没有树苗会长大。然而，在这些因素中的任何一个中引入可变性，都可以让一些人逃离火灾陷阱。可变的火灾间隔影响最大，使 8% 的茎在一个世纪内成为成年人。相比之下，引入火灾严重程度和增长率的变化应该分别允许 2.8% 和 0.3% 的茎成为成年人。因此，大多数逃出火坑的树木都是靠运气。偶然地，当它们还不足以抵抗普通火灾时，它们会经历长时间的无火间隔和/或低严重性火灾。由于生长异常迅速，很少有茎能逃过火坑。量化这些变化的来源及其后果以提高对火灾维持的稀树草原植被动态的理解、预测和管理非常重要。在这里，我们还提出了一种使用逻辑回归的潜在变量模型来量化火灾严重程度变化的新方法。