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Pollen analysis on a 9.54-m sediment core from lake Chignahuapan in the upper Lerma basin, the highest intermontane basin in Central Mexico (2570 m asl), documents vegetation and limnological changes over the past ∽23,000 14C yr. The core was drilled near the archaeological site of Santa Cruz Atizapán, a site with a long history of human occupation, abandoned at the end of the Epiclassic period (ca. 900 AD). Six radiocarbon AMS dates and two well-dated volcanic events, the Upper Toluca Pumice with an age of 11,600 14C yr B.P. and the Tres Cruces Tephra of 8500 14C yr B.P., provide the chronological framework for the lacustrine sequence. From ca. 23,000 14C yr B.P. to ca. 11,600 14C yr B.P. the plant communities were woodlands and grasslands based on the pollen data. The glacial advances MII-1 and MII-2 correlate with abundant non-arboreal pollen, mainly grasses, from ca. 21,000 to 16,000 14C yr B.P., and at ca. 12,600 14C yr B.P. During the late Pleistocene, lake Chignahuapan was a shallow freshwater lake with a phase of lower level between 19,000 and 16,000 14C yr B.P. After 10,000 14C yr B.P., tree cover in the area increased, and a more variable lake level is documented. Late Holocene (ca. 3100 14C yr B.P.) deforestation was concurrent with human population expansion at the beginning of the Formative period (1500 B.C.). Agriculture and manipulation of the lacustrine environment by human lakeshore populations appear at 1200 14C yr B.P. (550 A.D.) with the appearance of Zea mays pollen and abundant charcoal particles.

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L as investigaciones palinológicas permiten reconocer la evolución, la adaptación, la migración y la extinción de las especies vegetales, así como su biodiversidad actual y pasada, eventos estrechamente relacionados con los cambios climáticos (Van der Hammen, 1995). En México, este tipo de estudios se han realizado fundamentalmente en el Eje Volcánico Transmexicano (EVT) y en el sureste de México; éste último, junto con Centroamérica, conforman la región Caribe.Los estudios paleoambientales en el EVT se inician con Resumen: En los sedimentos de la laguna de Chignahuapan, Estado de México, México, se identificaron 96 palinomorfos. Se encontraron 8 taxa correspondientes a algas, un género y una familia tanto para Equisetopsida como para Lycopodiopsida, y 6 familias con 10 géneros para Polypodiopsida. Las Pinophyta estuvieron representadas por tres familias con tres géneros, las Liliopsida por 10 familias y 13 géneros, y las Magnoliopsida por 44 familias con 50 géneros. En los bosques templados cercanos a la Laguna de Chignahuapan sobresalen los siguientes taxa: Abies, Alnus, Pinus, Quercus y Salix. Entre los elementos de la vegetación acuática y subacuática se presentan: Cyperus, Luzula, Myriophyllum, Typha y Schoenoplectus. La actividad del hombre se infiere por la presencia de Zea mays y Cucurbitaceae. Palabras clave: paleoecología, Holoceno, laguna, cuenca del río Lerma, México. Abstract: In sediments from Chignahuapan Lake, State of Mexico, Mexico, 96 palynomorphs were identified. Eight taxa were found for algae, one family with one genus both for Equisetopsida and Lycopodiopsida, and 6 families with 10 genera for Polypodiopsida. Pinophyta were represented by 3 families and 3 genera, Liliopsida by 10 families and 13 genera, and Magnoliopsida by 44 families and 50 genera. The temperate forest surrounding Chignahuapan lake included components such as Abies, Alnus, Pinus, Quercus and Salix; aquatic and subaquatic vegetation components such as Cyperus, Luzula, Myriophyllum, Typha and Schoenoplectus were also present. Human activity was inferred from the occurrence of Zea mays and Cucurbitaceae.

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