Inter-hemispheric comparison of fire history: The Colorado Front Range,U.S.A., and the Northern Patagonian Andes,Argentina

Fire history was compared between the Colorado Front Range (U.S.A.) and northern Patagonia (Argentina) by dating fire-scars on 525 Pinus ponderosa and 418 Austrocedrus chilensis, respectively, and determining fire weather on the basis of instrumental and tree-ring proxy records of climatic variation. Years of above average moisture availability preceding fire years, rather than drought alone, is conducive to years of widespread fire in the Colorado Front Range and the northern Patagonian study areas. Above-average precipitation promotes fire by enhancing the growth of herbaceous plants which increases the quantity of fine fuels during the fire season a few years later. The short-term variability in moisture availability that is conducive to widespread burning is strongly related to El Niño Southern Oscillation (ENSO) activity. The warm (El Niño) phase of ENSO is associated with greater moisture availability during the spring in both regions which leads to peaks in fire occurrence several years after El Niño events. The warmer and drier springs associated with la Niña events exacerbate the drying of fuels so that fire years commonly coincide with La Niña events. In both regions, there was a dramatic decline in fire occurrence after the early 1900s due to a decline in intentionally set fires by Native Americans and European settlers, fuel reduction by livestock grazing, and increasingly effective organized fire suppression activities after the 1920s. In both regions there was a marked increase in fire frequency during the mid-and late-19th centuries which coincides with increased ignitions by Native Americans and/or European settlers. However, year-to-year variability in ring widths of Pinus ponderosa and Austrocedrus chilensis also increased from relatively low values in the late 1700s and early 1800s to peaks in the 1850s and 1860s. This implies frequent alternation of years of above and below average moisture availability during the mid-19th century when the frequencies of major fire years rise. The high correlation of tree-growth variability betweem the two regions implies a strong inter-hemispheric variation in climatic variability at a centennial time scale which closely parallels a variety of proxy records of ENSO activity. Based on the relationship of fire and ENSO events documented in the current study, this long-term trend in ENSO activity probably contributed to the mid- and late-19th century increase in fire spread in both regions. These similar trends in fire occurrence have contributed to similar patterns of forest structures, forest health, and current hazard of catastrophic wildfire in the Colorado Front Range and northern Patagonia.