Ecological responses to mid- and late-Holocene climate variability in the Patagonian Andes (lat 34°-55°S)

IGLESIAS, V.; WHITLOCK, C.; BIANCHI, M.M.; MARKGRAF, V; NAVARRO, D.; BARTLEIN, P.J.

San Francisco

Simposio; American Geophysical Union (AGU) Fall Meeting; 2009

American Geophysical Union

Along the eastern Andes, a prominent ecotone separates North Patagonian forest, dominated by Nothofagus spp. and at middle latitudes by Nothofagus spp. and Austrocedrus, from steppe. The elevational position of the ecotone is determined by effective moisture, which in turn is governed by the strength and position of the Southern Westerlies. As a result, changes in forest composition and fire activity provide an opportunity to examine past climate variations in this region. The vegetation and fire history is inferred from a longitudinal transect of 8 lake and bog sites. Pollen data indicate that, on millennial time scales, late-glacial steppe was replaced by open forest in the early Holocene and then by closed forest in the middle and late Holocene (south of lat 36S). Charcoal data suggest that fire activity south of lat 41S was highest between 11 and 8-6 ka and significantly decreased between 8-6 and 3 ka. The opposite pattern occurred north of lat 41S, where fire activity was highest between 6 and 3 ka. Between lat 40 and 43S, fires were nearly as frequent between 3 and 1 ka as in the early Holocene, and this period was followed by a decline in fires to the 20th century. The long-term vegetation and fire patterns are partly explained as a response to variations in seasonal and annual insolation and their effect on summer (fire-season) conditions. In the last 5000 years, changes in precipitation and ENSO variability, inferred from independent data, matched closely with trade-offs in the abundance of Nothofagus, Austrocedrus, and Poaceae pollen and variations in charcoal influx. Four climate scenarios (dry conditions/low variability, dry conditions/high variability, wet conditions/high variability, wet conditions/low variability) registered distinctive ecotonal responses. Pollen and charcoal data suggest that wet conditions led to the expansion of forest into steppe, and high moisture variability, mainly through its effects on fire occurrence, shaped the composition of the ecotone. Wet/low variability stable conditions at 4-3.5 ka favored the expansion of Austrocedrus into steppe. An invasion of Nothofagus into Austrocedrus forest resulted from increased fire activity associated to wet, variable conditions at 3.5-2.8 ka, 1.4-1.2 ka, and 0.5-0.2 ka. Dry conditions allowed the expansion of the steppe at the expense of forest species (i.e., 4.8-4 ka, 2.5-1.7 ka). Our findings suggest that precipitation and ENSO variability explain much of the fine-scale ecotonal features of the late Holocene but that these relations must be understood in the context of millennial-scale climate variations that shape broad patterns of vegetation and fire in the region.