Fire and drought as drivers of early Holocene tree line changes in the Peruvian Andes

Three pollen and charcoal records from three lakes lying at 3400m elevation in southern Peru provided a record of landscape change spanning the last ca.18 000 cal. a BP. The tree line lay close to the site between ca. 16 000 and 12 000 cal. a BP, with Polylepis woodlands growing near the lakes. Progressively drying conditions led to increased fire after 12 000 cal. a BP, coinciding with a decline in Polylepis cover and Andean forest relicts as puna grasslands expanded. A strong decrease in the rate of sediment deposition between ca. 12 000 and ca. 4400 cal. a BP was interpreted to indicate the presence of sedimentary hiatuses. With the return of wet conditions after 4400 cal. a BP, forests did not reassemble around the lakes. Instead, fire-maintained grasslands dominated the landscape. Humans probably induced the intensified fire activity during the late Holocene and thereby deflected local successions. The modern fragmented landscape, with Polylepsis woodlands existing in fire-resistant pockets above the general limit of the Andean tree line, resulted from the intensification of human land use practices during the last 4400 cal. a BP. Copyright (C) 2011 John Wiley & Sons, Ltd.