Influences of humans and ENSO on fire history of Austrocedrus chilensis woodlands in northern Patagonia, Argentina

Abstract

:In northern Patagonia, Argentina, five areas near the ecotone of xeric woodlands and the steppe were sampled for fire history to assess spatial and temporal variations in fire regimes. A total of 214 fire-scar samples from the xeric conifer Austrocedrus chilensis (D. Don) Florin & Bout, yielded 430 cross-dated fire dates from AD 1439 to 1989. A regional trend of increasing fire frequency during the latter half of the 19th century coincides with increased native American occupation of the area. There is a marked decline in fire frequency following the demise of the native American population in the late 1800s and in association with increasingly effective fire suppression during the 20th century. Inter-site variations in the frequency of small fires appear to reflect intra-regional variations in human activities. In contrast, regional synchroneity of years of more widespread fires implies greater climatic control of major burning events. El Niño-Southern Oscillation (ENSO) events are a major influence on the subtropical anticyclone of the southeast Pacific that affects weather in northern Patagonia, and, at a time scale of fifty years, periods of widespread fire closely track increased ENSO events as determined from historical sources and tree-ring reconstructions. However, the multi-decadal relationship of increased frequency of years of widespread fire and increased ENSO activity could only be tentatively established due to the relatively small number of pre-1800 fire dates and the potentially confounding influence of variations in human activities. This tentative association of increased fire occurrence with greater climatic variability at a fifty-year time scale complements earlier research that relates more widespread fire to droughts preceded by years of above-average moisture availability at time scales of 1 to 4 years.