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

Abstract

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 are attributed 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.

The Last Glacial Maximum and deglaciation in central Patagonia, 44°S–49°S

“…Subsequently, a gradual increase in Nothofagus taxa suggests grass-steppe vegetation with scattered trees and an increase in effective moisture, but still less than modern values (de Porras et al, 2012;Markgraf et al, 2007;de Porras et al, 2014). The pollen record from Lago Los Niños suggests open forest between 14.3-13.2 cal ka BP, with a subsequent trend towards more open landscape and drier conditions until 11.5 cal ka BP (Iglesias et al, 2016). Fluctuations of aquatic taxa in pollen records attest for high millennial environmental variability between ca.…”

“…14 cal ka BP (Markgraf et al, 2007;de Porras et al, 2012;Iglesias et al, 2016). Steppe taxa such as Plantago and Acaena was often present indicating disturbance and unstable postglacial conditions (de Porras et al, 2012;de Porras et al, 2014;Markgraf et al, 2007;Iglesias et al, 2016).…”

“…14 cal ka BP (Markgraf et al, 2007;de Porras et al, 2012;Iglesias et al, 2016). Steppe taxa such as Plantago and Acaena was often present indicating disturbance and unstable postglacial conditions (de Porras et al, 2012;de Porras et al, 2014;Markgraf et al, 2007;Iglesias et al, 2016). Small abundances of hygrophilous arboreal taxa in records from the eastern side of the Andes were interpreted as a long distance transport from forest refugia along the Pacific coast (Markgraf et al, 2007;de Porras, 2014).…”

“…Small abundances of hygrophilous arboreal taxa in records from the eastern side of the Andes were interpreted as a long distance transport from forest refugia along the Pacific coast (Markgraf et al, 2007;de Porras, 2014). Iglesias et al (2016) inferred ~200 mm annual precipitation required to support the reconstructed vegetation community (44° S 71° W). Subsequently, a gradual increase in Nothofagus taxa suggests grass-steppe vegetation with scattered trees and an increase in effective moisture, but still less than modern values (de Porras et al, 2012;Markgraf et al, 2007;de Porras et al, 2014).…”

Mid-late Holocene vegetation response to climatic drivers and biotic disturbances in the Banni grasslands of western India

“…Washed samples were then scanned under a stereomicroscope at a magnification of 15x to identify and count the charcoal particles. Charcoal accumulation rate (CHAR) was then estimated by multiplying the macroscopic charcoal concentrations (count/cm 3 ) by sedimentation rate (cm/yr) giving number of particles cm -2 yr -1 (Zhang et al, 2015;Iglesias et al, 2016) for the samples. We have refrained from the use of CHAR values to statistically estimate the paleofire frequency of the region as our sampling resolution was relatively low Yu et al, 2016).…”