A LATE-GLACIAL HOLOCENE POLLEN RECORD FROM THE EASTERN ANDES OF NORTHERN PERU

A pollen and sediment record of a core 4.2-m-long from Laguna Baja (7 degrees 42' S, 77 degrees 32' W, 3575 m) in the Cordillera Oriental of northern Peru suggests several episodes of major vegetational and cli matic change over the past 13,000 yr. The oldest pollen assemblage con sists of a mixture of paramo elements (tropical alpine vegetation), in cluding high percentages of Poaceae (40%) that decline upward, moist m ontane forest (Compositae and Polylepis), and wet montane forest (e.g. , Hedyosmum and Podocarpaceae). Organic carbon content range from <2% to 8%. About 11,600 yr B,P. this mixed pollen assemblage was replaced by Poaceae (>60%), with high percentages of Jamesonia, a fern characte ristic of paramo, and decreasing values of Plantago tubulosa and the w et montane forest elements Hedyosmum and Podocarpaceae. Charcoal perce ntages are at a maximum during this: period, magnetic susceptibility a nd sand percentages are high, and percentages of organic matter are lo w. Several explanations for these changes are possible, including a re duction in temperature and moisture, more frequent periods of aridity with increased fires, or natural succession. The Holocene record begin s with pronounced increases in organic carbon and pollen of wet montan e forest, primarily Hedyosmum, Podocarpaceae, and Urticales. High valu es of Podocarpaceae pollen (>35%) and a decline in charcoal suggest te mperature and moisture levels above modern-day values. Wet montane for est pollen remain high and charcoal values are low from about 10,000 t o 6000 yr B.P., suggesting that warm and moist conditions prevailed fo r about 4000 yr. Subsequently Podocarpaceae and Urticales decline, and for a brief time Alnus is prominent in the pollen record. Following t he Alnus maximum at about 5000 yr B.P., Poaceae, Ambrosia and Chenopod iaceae/Amaranthaceae become frequent, Increased paramo and disturbance indicator pollen suggest increased anthropogenic activities in this r egion from the middle Holocene to the present. (C) 1995 University of Washington.