17,000 years of vegetation, fire and climate change in the eastern foothills of the Andes (lat. 44°S)

Paleoenvironmental records from Patagonia reveal the importance of latitude, longitude and elevation in shaping the response of vegetation to climate change. We examined the vegetation, fire and watershed history from two sites at lat. 44°S, as inferred from pollen, charcoal and lithologic data. These reconstructions were compared with independent paleoenvironmental records to better understand ecosystem dynamics along the southeastern Andes (lat. 41-50°S). Our results show that at lat. 44°S, late-glacial heath-steppe was colonized by trees at ~ 14,200 cal yr BP and forests became more closed at 11,500 cal yr BP. Differences in forest cover between the two sites were likely due to elevation-dependent disparities in humidity and fire. North of lat. 44°S, increasing precipitation favored initial forest development at 16,500 cal yr BP, while dry conditions restricted tree expansion in the south until 7000 cal yr BP. The time-transgressive pattern is attributed to a gradual southward shift in the Southern Westerlies resulting from deglaciation and increasing annual insoltion. Present-day vegetation developed at all latitudes during the middle to late Holocene, when the core of the Southern Westerlies reached its modern position (50°S). Asynchronous forest expansions north and south of lat. 45°S between 5000 and 3000 cal yr BP are ascribed to centennial cycles of contraction and expansion of the Southern Westerlies, which led to seasonal variations in precipitation at the core and northern border of the wind belt. Synchronous oscillations in tree abundance along the eastern Andes prevailed during the last 3000 years. Decreased forest cover at all latitudes between 2500 and 1500 cal yr BP is concurrent with La Nina-like conditions and high solar irradiance. These climate drivers likely decreased effective moisture and favored fires at all latitudes.