Major altitudinal shifts in Andean vegetation on the Amazonian flank show temporary loss of biota in the Holocene

The Lateglacial and Holocene vegetation history from the Amazonian flank of the south Colombian Andes has been studied in a 12 m long sediment core from Lake La Cocha (2780 m altitude). 18 AMS 14C ages and 550 pollen samples yielded a history of a ~25 yr resolution. Montane forest extended up to 2200 m during the Lateglacial and shifted during a 10,000 yr period with millennial-scale and centennial-scale variability superimposed, to its present-day limit at 3550 m. We hypothesise that Intertropical Convergence Zone (ITCZ)-modulated trans-Amazonian moisture flows are an important driver of forest dynamics but influence of ENSO (El Niño Southern Oscillation) variability after 7000 yr BP may also be reflected in the record. Quasi-forest stability lasted for maximally 200 to 600 years. Upslope forest shifts up to 300 m/100 yr, reflecting a 1.5°C temperature increase were common during the Holocene. During several intervals of fastest forest migration the subpáramo disappeared for short intervals of time, suggesting that upslope forest migration exceeded the migration capacity of the subpáramo biome. From Lateglacial time onwards a suite of arboreal taxa show successive upslope expansion events, suggesting internal forest dynamics during the shift of residence areas from their glacial to Holocene altitudinal intervals. Logging, preferentially Podocarpus, frequent fires, forest disturbance and changes of the diatom flora in the lake suggest strong human impact after 1405 cal. yr BP.