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 climatic change over the past 13,000 yr. The oldest pollen assemblage consists of a mixture of paramo elements (tropical alpine vegetation), including high percentages of Poaceae (40%) that decline upward, moist montane 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 characteristic of paramo, and decreasing values of Plantago tubulosa and the wet montane forest elements Hedyosmum and Podocarpaceae. Charcoal percentages are at a maximum during this: period, magnetic susceptibility and sand percentages are high, and percentages of organic matter are low. Several explanations for these changes are possible, including a reduction in temperature and moisture, more frequent periods of aridity with increased fires, or natural succession. The Holocene record begins with pronounced increases in organic carbon and pollen of wet montane forest, primarily Hedyosmum, Podocarpaceae, and Urticales. High values of Podocarpaceae pollen (>35%) and a decline in charcoal suggest temperature and moisture levels above modern-day values. Wet montane forest pollen remain high and charcoal values are low from about 10,000 to 6000 yr B.P., suggesting that warm and moist conditions prevailed for about 4000 yr. Subsequently Podocarpaceae and Urticales decline, and for a brief time Alnus is prominent in the pollen record. Following the Alnus maximum at about 5000 yr B.P., Poaceae, Ambrosia and Chenopodiaceae/Amaranthaceae become frequent, Increased paramo and disturbance indicator pollen suggest increased anthropogenic activities in this region from the middle Holocene to the present. (C) 1995 University of Washington.