Late-glacial and Holocene vegetational history from two sites in the western Cordillera of southwestern Ecuador

Abstract

The deciphering of the late-glacial and Holocene vegetation, glacial-geological, and climatic history of Cajas National Park in southwestern Ecuador is undertaken focusing on close-interval sampling of sediment cores from two high elevations lakes, Lagunas Chorreras (3700 m) and Pallcacocha (4060 m). This study involves extensive dating (both accelerator mass spectrometry and tephra), palynological, and sediment analyses of lakes and bogs. Basal dates for the two cores analyzed for pollen range from about 17 000 to about 15 500 cal. yr BP. Vegetation surveys and surface sample pollen and spore analyses were accomplished for two transects in the western Cordillera. One began east of the main divide near Cuenca, Ecuador and the other began at the crest of the western Cordillera, descending towards the Pacific Ocean. These vegetation surveys coupled with pollen analyses of surface samples were used to establish pollen analogues to help in the interpretation of the fossil pollen records. The results of the pollen analyses from the two lake cores indicate two major climatic periods. (1) The late-glacial pollen record, beginning about 17 000 yr BP and ending near 11 000, is characterized by an herb paramo dominated by pteridophytes (primarily Huperzia spp.) with Asteraceae (assumed to be woody species) and Puya spp. The climate inferred from these pollen records was cooler and moister than today. The sediments for this time period are characterized by low loss-on-ignition percentages, and high magnetic susceptibility values (with secondary peaks between 12 000 and 13 000 yr BP). Low concentrations of charcoal fragments indicate a low incidence of fire, and several pronounced fluctuations in pollen assemblages could be interpreted as changes in the prevailing wind direction and/or climate. (2) The beginning of the Holocene, as represented in the pollen record, is characterized by the disappearance of Puya pollen, vastly diminished Huperzia representation, high charcoal concentrations, and the expansion of moist montane forest pollen. Asteraceae (possibly Gynoxys) are replaced as dominant timberline taxa by Polylepis-dominated timberline forest. The charcoal record suggests that fires were much more prevalent during the early to middle Holocene than during the late-glacial. Changes in Isoëtes concentrations and trends in upland vegetation may be related to fluctuations in lake levels and precipitation.