Megafires in dry conifer forests of the Southwest US are driving transitions to alternative vegetative states, including extensive shrubfields dominated by Gambel oak (Quercus gambelii). Recent tree-ring research on oak shrubfields that predate the 20th century suggests that these are not a seral stage of conifer succession but are enduring stable states that can persist for centuries. Here we combine soil charcoal radiocarbon dating with tree-ring evidence to refine the fire origin dates for three oak shrubfields (<300 ha) in the Jemez Mountains of northern New Mexico and test three hypotheses that shrubfields were established by tree-killing fires caused by (1) megadrought; (2) forest infilling associated with decadal-scale climate influences on fire spread; or (3) anthropogenic interruptions of fire spread. Integrated tree-ring and radiocarbon evidence indicate that one shrubfield established in 1664 CE, another in 1522 CE, and the third long predated the oldest tree-ring evidence, establishing sometime prior to 1500 CE. Although megadrought alone was insufficient to drive the transitions to shrub-dominated states, a combination of drought and anthropogenic impacts on fire spread may account for the origins of all three shrub patches. Our study shows that these shrubfields can persist >500 years, meaning modern forest-shrub conversion of patches as large as >10,000 ha will likely persist for centuries.

美国西南部干燥针叶林中的大火正推动向其他植物生长州的过渡，包括由甘贝尔橡木（Quercus gambelii）主导的广阔灌木丛。）。最近在20世纪之前对橡树灌木丛进行的树木年轮研究表明，这些不是针叶树演替的串行阶段，而是持久稳定的状态，可以持续几个世纪。在这里，我们将土壤木炭放射性碳测年与树木年轮证据相结合，以完善新墨西哥州北部杰米兹山的三个橡树灌木丛（<300公顷）的火源日期，并检验三个假说灌木丛是由杀树火造成的（1）特大干旱；（2）与年代际气候对火势蔓延的影响相关的森林充填；或（3）人为中断火势蔓延。综合的树木年轮和放射性碳证据表明，一个灌木丛建立于公元1664年，另一个灌木丛建立于1522年，而第三个灌木丛早于最早的树木年轮证据，建立于公元1500年之前。尽管仅是大干旱不足以推动向灌木为主的州过渡，但干旱和人为因素对火势蔓延的综合影响可能解释了所有三个灌木丛的起源。我们的研究表明，这些灌木丛可以持续> 500年，这意味着面积超过10,000公顷的斑块的现代森林灌木转换可能会持续几个世纪。