Wildfire Regime Shifts in Southern South America from Tree-Ring Reconstructed Fire History Networks: Climatic Controls, Land Use, and Ecological Feedbacks

在世界许多地方，自20世纪80年代初以来，极端野火事件和被烧毁面积的增加被归因于全球变暖和土地利用趋势的共同作用。一些土地利用做法，如伐木和放牧，可能会将耐火森林转变为更容易发生火灾的植被类型，从而对更多的火灾、更多的二氧化碳释放和加剧的变暖产生积极的反馈。在北半球进行的大量研究已经将野火活动的变化与20世纪的变暖以及与热带太平洋的厄尔Niño-Southern涛动（ENSO）和高纬度气候模式（如北极涛动）的遥相关联系起来。在南半球，大气变率的主要温带模态是南环模态（也称为南极涛动），它由大气质量在中纬度和高纬度之间的再分布组成。最近南环模强度的变化与西风带的增强、中纬度风暴路径的变化以及巴塔哥尼亚更温暖、更干燥的条件有关。1950年后南环模的正趋势被解释为温室气体增加和/或平流层臭氧减少的人为强迫的标志。科罗拉多大学的Thomas Veblen教授将通过分析树木年轮来重建巴塔哥尼亚南部过去400年的火灾历史，以便将火灾历史的变化与年变化、ENSO的20-30年时间变化和大气低频变化及其相互作用联系起来。这项研究还将确定森林燃烧之后牲畜的影响如何将耐火的南方山毛榉林转变为更易燃的开阔林地和草灌丛地，从而产生正反馈，进一步增强火灾潜力。该研究将通过绘制卫星图像和历史航空照片中的燃烧前和燃烧后植被类型，确定有利于植被类型从防火森林向易火灌木转变的物理环境和土地利用条件。这些植被类型来自于20世纪后半叶许多被烧毁的地点。了解从防火森林向火灾易发灌木地转变的具体地点潜力，对于减轻和调整土地利用做法以适应气候引起的野火活动变化至关重要。在最近的气候趋势下，极端天气（包括引发火灾的干旱）的规模和频率都在增加，并可能对生态系统的恢复能力以及大气中的二氧化碳排放产生深远影响。这项研究将产生的长期（多世纪）火灾记录对于理解火灾活动的这些近期趋势如何偏离历史变异性范围，以及野火活动的变异性如何与气候变化和土地利用趋势相关联是必要的。这项研究将阐明牲畜饲养（具有全球重要性的主要土地利用）在将防火森林转变为更容易发生火灾的植被方面的作用，并提供预测与气候变化有关的火灾风险的预测工具。

In many parts of the world, extreme wildfire events and increases in area burned since the early 1980s have been attributed to a combination of global warming and land-use trends. Some land-use practices, such as logging and grazing, may convert fire-resistant forests to more fire-prone vegetation types, thus creating positive feedbacks towards more fires, more CO2 release, and enhanced warming. Numerous studies in the Northern Hemisphere have linked variability in wildfire activity to both 20th century warming and teleconnections with El Niño-Southern Oscillation (ENSO) in the tropical Pacific and high-latitude climate modes such as the Arctic Oscillation. In the Southern Hemisphere the leading extratropical mode of atmospheric variability is the Southern Annular Mode (also known as the Antarctic Oscillation) consisting of a redistribution of atmospheric mass between middle and high latitudes. Recent changes in the magnitude of the Southern Annular Mode are associated with a strengthening of the westerlies, variations in mid-latitude storm tracks, and warmer-drier conditions in Patagonia. The post-1950 positive trend in the Southern Annular Mode is interpreted as a signature of anthropogenic forcing by increased greenhouse gases and/or reduced stratospheric ozone. Professor Thomas Veblen from the University of Colorado will analyze tree rings to reconstruct fire history in southern Patagonia over the past 400 years in order to relate variability in fire history to year-to-year variability, as well as the 20-30 year temporal variability of ENSO and the low-frequency of atmospheric variability, and their interactions. This study will also determine how forest burning followed by livestock impacts is converting fire-resistant southern beech forests to more flammable open woodlands and grass-shrublands, thus creating a positive feedback further enhancing fire potential. The study will determine the physical setting and land-use conditions that favor shifts in vegetation types from fire-resistant forests to fire-prone shrublands by mapping pre-burn and post-burn vegetation types from satellite imagery and historical air photos at numerous sites burned in the latter part of the 20th century. Understanding the site-specific potential for shifts from fire-resistant forests to fire-prone shrublands is essential for mitigating and adapting land-use practices to climate-induced changes in wildfire activity. Under recent climate trends, weather extremes (including fire-promoting droughts) are increasing in magnitude and frequency, and can have far-reaching implications for ecosystems resilience as well as CO2 emissions to the atmosphere. The long-term (multi-century) fire records that this study will produce are necessary to understand how these recent trends in fire activity depart from the historical range of variability, and how variability in wildfire activity relates to climate variation as well as land-use trends. The study will elucidate the role of livestock raising (a major land use of global importance) in converting fire-resistant forests to more fire-prone vegetation, and provide a forecasting tool for predicting fire risk in relation to climate variability.