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.
This research project examined two principal research questions: 1. How is variability in wildfire activity in the Andean-Patagonian region (38 to 52 degrees South Latitude in western South America) related to climate variability and the dominant climate modes of climate variability which are the El Nino-Southern Oscillation (ENSO) and the Southern Annual Mode (SAM)? 2. In Patagonia does the initial burning of tall Nothofagus (southern beech) forests result in a pattern of cover type transition and positive feedback towards increased fire activity, and is this pattern enhanced by livestock? Recent (i.e., post-1950) changes in the magnitude of the SAM are associated with a strengthening of the westerly winds, shifting of storm tracks poleward, and a trend towards warmer and drier conditions in most of Patagonia. We used both documentary fire records (1984-2010) and developed multi-century tree-ring fire history records from more than 600 sampled fire-scarred trees to examine relations between fire and climate variability. The dominant influence on fire is variability in the SAM which reduces seasonal precipitation and favors fire occurrence in the wet western Patagonian forests. Since the latter half of the 20th century there has been an increase in fire in western Patagonian forests linked to the upward trend in the SAM. Given that the positive trend in SAM is likely to continue due to greenhouse gas forcing and forcing by reduced stratospheric ozone, wildfire activity is likely to continue to increase in Patagonia during the 21st century. Fuel properties measured at the level of plant communities as well as flammability traits measured at the level of indiviidual plants indicated that tall Nothofagus forests are less flammable than adjacent tall shrublands. Mapping of 232 fires from satellite images over the period 1984-2010 showed that tall Nothofagus forests are much less likely to burn than the adjacent tall shrublands. Since these tall shrublands originate after the infrequent burning of the relatively fire-resistant forests, initial forest burning results in a more flammable landscape of fire-prone shrublands. Livestock and other introduced animals inhibit tree regeneration after forest burning and also increase plant traits such as lower foliar moisture and vertical ladders of fine fuels that enhance flammability. In addition, the warmer and drier microclimate of shrublands is more conducive to fire spread than the cool moist microclimate of tall forests. Overall, initial burning of tall forests followed by livestock impacts results in positive feedbacks towards more flammable vegetation attributes that increased the likelihood of burning forests even under less extreme drought than was required for initial fire spread in tall forest. Thus, in Patagonian Nothofagus forest landscapes, "fire begets fire" which implies that under continued warming and drying trends the rate of forest burning and conversion to fire-prone shurblands is likely to increase exponentially. The broader impacts of this project included education and training opportunities for 3 graduate students and several undergraduate students. The results of the project were presented at a major symposium on fire and climate change at the 2012 Southern Connection Congress in Dunedin, Chile. This symposium was organized by the senior personnel of this project and consisted of 19 research papers on fire research in 6 countries. This project has yielded research results communicated to the general public through media releases and to resource managers in Chile and Argentina that are informing discussions of the future effects of continued climate warming on fire activity and societal vulnerability to fire in the Andean-Patagonian region.
