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1980年代初以来，极端野火事件和燃烧面积的增加被归因于全球变暖和土地使用趋势的结合。 一些土地使用做法，如伐木和放牧，可能会将耐火森林转化为更容易发生火灾的植被类型，从而产生积极的反馈，导致更多的火灾，更多的二氧化碳释放和加剧变暖。 北方半球的许多研究都将野火活动的变化与20世纪世纪的变暖和热带太平洋的厄尔尼诺-南方涛动（ENSO）以及北极涛动等高纬度气候模式的遥相关联系起来。 在南半球，大气变率的主要外大气模式是南半球环形模式（也称为南极涛动），由中纬度和高纬度之间的大气质量重新分布组成。 南部环形模的强度最近的变化与西风带的加强，中纬度风暴路径的变化以及巴塔哥尼亚的温暖干燥条件有关。1950年后南方环形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特征。 来自科罗拉多大学的托马斯·凡勃伦教授将分析树木的年轮，以重建巴塔哥尼亚南部过去400年来的火灾历史，以便将火灾历史的变化与逐年变化、ENSO的20-30年时间变化和大气变化的低频及其相互作用联系起来。这项研究还将确定森林燃烧后，牲畜的影响是如何转换防火南部山毛榉森林更易燃的开放林地和草灌木，从而创造一个积极的反馈，进一步提高火灾的可能性。 这项研究将确定有利于植被类型从耐火森林向易火灌木林转变的物理环境和土地使用条件，方法是根据20世纪后期烧毁的许多地点的卫星图像和历史航空照片绘制燃烧前和燃烧后的植被类型。了解特定地点从耐火森林向易火灌木林转变的潜力，对于减轻和调整土地使用做法以适应野火活动中气候引起的变化至关重要。 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，可能对生态系统的复原力以及向大气排放二氧化碳产生深远的影响。 这项研究将产生的长期（多世纪）火灾记录是必要的，以了解这些最近的火灾活动趋势如何偏离历史变化范围，以及野火活动的变化如何与气候变化以及土地利用趋势相关。 这项研究将阐明畜牧业（具有全球重要性的一种主要土地利用方式）在将耐火森林转变为更容易发生火灾的植被方面的作用，并提供一种预测工具，用于预测与气候变化有关的火灾风险。

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.