Fire history in northern Patagonia: The roles of humans and climatic variation

The effects of humans and climatic variation on fire history in northern Pa tagonia, Argentina, were examined by dating fire scars on 458 trees at 21 s ites in rain forests of Fitzroya cupressoides and xeric woodlands of Austro cedrus chilensis from 39 degrees to 43 degrees S latitude. Climatic variati on associated with fires was analyzed on the basis of 20th-century observat ional records and tree ring proxy records of climatic variation since appro ximately AD 1500, In the Austrocedrus woodlands, fire frequency increases a fter about 1850, coincident with greater use of the area by Native American hunters. Increased burning, particularly in the zone of more mesic forests , is also strongly associated with forest clearing by European settlers fro m about 1880 to the early 1900s. The marked decline in fire frequency durin g the 20th century coincides with both the demise of Native American hunter s in the 1890s and increasingly effective fire exclusion. Strong synchroneity in the years of widespread fire at sample sites dispers ed over a north-south distance of similar to 400 km indicates a strong clim atic influence on fire occurrence at an annual scale. Tree ring reconstruct ions of regional precipitation and temperature show a steeply declining inf luence of climatic variability on fire occurrence from annual to multidecad al scales. It is the interannual variability in climate, rather than variat ions in average climatic conditions over longer periods, that strongly infl uences fire regimes in northern Patagonia. Although climatic variability ov errides human influences on fire regimes at an interannual scale, human act ivity is an equally important determinant of fire frequency at multidecadal scales. Climatic conditions conducive to widespread fire in both xeric Austrocedrus woodlands and Fitzroya rain forests are typical of the late stages of La N ina (cold phase of the Southern Oscillation) events, as indicated by trends in the Southern Oscillation Index and eastern tropical Pacific sea surface temperatures during the 1-2 years before and after fire event years, Years of extreme fire occurrence are associated both with dry winter-springs of La Nina events and with the warm summers following El Nino events. Years in which the southeast Pacific subtropical anticyclone is intense and located farther south than normal are years of enhanced drought and fire. Similarl y, years of widespread fire in northern Patagonia are associated with varia tions in mean sea level atmospheric pressure at about 50 degrees-60 degrees S latitude in the South American-Antarctic Peninsula sector of the Souther n Ocean, as reconstructed from tree rings for AD 1746-1984. Precipitation a nd, hence, fire regimes in northern Patagonia are significantly influenced by high-latitude blocking events, which drive westerly cyclonic storms nort hward, Variations at decadal to centennial time scales in major circulation features, such as ENSO activity and the meridionality of regional air flow at high latitudes, as well as changes in the degree of coupling of these f eatures, influence climate and fire regimes of northern Patagonia.