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

点击翻译按钮获取中文摘要

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.