Late quaternary vegetation history and paleoecology of Laguna Pedro Palo (subandean forest belt, Eastern Cordillera, Colombia)

Abstract

Laguna Pedro Palo is situated on a saddle at ca. 2000 m altitude in the subandean forest belt on the western slope of the Eastern Cordillera of Colombia. Three pollen records from the site show the vegetation history and paleoclimatological sequence since ca. 13, 000 BP. The pollen records were correlated on the basis of radiocarbon-dated horizons (11,950 ± 100, 11,380 ± 130, 10,380 ± 90, and 10,280 ± 90 yr BP) and palynostratigraphy. Almost no palynological data are available from the subandean forest belt during glacial conditions. Most important is the direct evidence that the upper Andean forest line shifted to below 2000 m alt. during glacial conditions, providing evidence of a glacial forest line depression of ca. 1400 m and aninferred temperature depression of ca. 8°C. During the early Late Glacial (or Pleniglacial-Late Glacial transition) under dry climatic conditions, (sub)andean (semi-)open dry vegetation was locally in contact with dry paramo vegetation and a proper Andean forest belt was apparently absent. Open shrubparamo and grassparamo vegetation surrounded the lake at 2000 m alt. Subsequently patches of Andean forest reached closer to the area, indicating gradually less dry conditions shortly before 12,000 yr BP. During the Guantiva interstadial (ca. 12,000–11,000 yr BP) a well-developed Andean forest belt occurred from ca. 1800 to 2800 m alt. consisting of mainly Quercus forest and Weinmmannia forest. Climatic conditions were cool temperate and more humid than before the Guantiva interstadial. Alnus carr vegetation occurred around the lake. During the next El Abra stadial (ca. 11,000–10,150 yr BP) a cooler climate (−3°C) is indicated by alowering of the upper Andean forest line by ca. 500 m from its position during the Guantiva interstadial. Carr vegetation of Alnus diminished and disappeared at the end of the El Abra stadial, an approximate time-equivalent of the Younger Dryas event. Subsequently the upper Andean and subandean forest lines reached Holocene elevations, ca. 3300 and 2400 m, respectively. First a massive increase of Cecropia, a pioneer tree, and shortly later Acalypha and Alchornea dominated the pollen spectra of the subandean forest belt. In the Andean forest belt Quercus and Weinmannia forest types dominated with important contributions of Hedyosmum, Ilex, Juglans, Miconia, Urticaceae and Brunellia. A Late Holocene pollen record shows a phase showing human disturbance of the forest belt. Agricultural activity is indicated by relative high percentages of Gramineae and Chenopodiaceae followed by a possible forest succession phase in which Cecropia played a major part. Finally a “post-conquest” phase (after ca. 1500 AD) is registered, shown by relative high percentage of Gramineae and Rumex acetosella.