Vegetational changes during the Holocene in Extra-Andean Patagonia, Santa Cruz Province, Argentina
Abstract Modern pollen-vegetation relationship was used in the interpretation of stratigraphical sequences from two archeological sites, La Martita cave (48°24′S, 69°15′W) and Alero Cardenas (47°18′S, 70°26′W) located in Extra-Andean Patagonia in the Santa Cruz Province. The modern pollen assemblages are related to vegetation units that extend from the Andes toward the Atlantic coast, reflecting the increase of aridity from west to east. The more humid areas in the west are represented by grass steppe with Festuca pallescens; in those areas of intermediate humidity shrub steppe can be found with a conspicuous stratum of Stipa (S. speciosa, S. humilis and S. chrysophylla) and shrubs (Nardophyllum obtusifolium, Berberis heterophylla, Senecio filaginoides, Mulinum spinosum) and in the most arid areas scrub steppe expands with Nassauvia glomerulosa and cushion plants such as Azorella caespitosa, Accantholippia seriphiodes, Chuquiraga aurea, Brachyclados caespitosus. A vegetational and paleoclimatic history is presented based on comparison with modern analogs and with other palynological records from southern Patagonia. Prior to 8000 yr B.P. a grass steppe extended indicating relatively high precipitation under cold conditions. Between 8000 and 6000 yr B.P. an increase of shrub-steppe taxa, dominated by Asteraceae tubuliflorae would relate to an increase in temperature, while precipitation remained in the previous range of about 200 mm. The mid-Holocene (5000 yr B.P.) is reflected in the La Martita sequence by a short-term return of steppe taxa, primarily grasses. This steppe could indicate cold conditions similar to those of the early Holocene, or an increase in precipitation. After 4500 yr B.P. the vegetation is represented by grass-shrub steppe. After 3500 yr B.P. the Alero Cardenas record shows a shrub steppe dominated by Asteraceae tubuliflorae. In records near the steppe-forest ecotone, at 47 and 50°S, the more extensive late Holocene forest suggests greater effective moisture, probably related to cooler temperatures. In Alero Cardenas grasses increase and shrubs are irregularly present. In Altiplanicie Central steppe vegetation is analogous to the present vegetation. In the present century taxa indicative of disturbance are present (Rumex acetosella and Brassicaceae).