LATE QUATERNARY VEGETATION HISTORY OF THE UPPER LERMA BASIN, CENTRAL MEXICO

Recent paleoenvironmental research in continuous and semicontinuous lacustrine sequences of central Mexico provides a picture of the climate change during the last 25,000 yr. Most of the sites has good resolution for the full/interglacial cycle, but only a few display enough resolution in the Holocene. In the Upper Lerma Basin (ULB), the record for the last 10,000 yr is less disturbed and has better resolution. The (ULB) is the highest intermontane basin in Central Mexico (2570 m) and it is the origin of the longest river in the country. There are three water bodies connected by the Lerma river. Most of the paleoecological research in the area is related to the limnological evolution of the lakes and pollen data are scarce. A core of 9.54 m was drilled in lake Chignahuapan near the archeological site of Santa Cruz Atizapan. Evidence of history of plant communities in the surrounding area during the last 22,000 yr is presented. The chronology is based on six AMS radiocarbon dates and two tephra layers. Pollen data indicate that during the late Pleistocene, from ca. 22,000 yr BP. to 11,600 yr BP., open forests of pine, oak and alder with abundant herbaceous vegetation were growing in the area. Glacial advances correlate with increases in grass pollen at ca. 16,000 yr. BP. and ca.14,000 yr BP. Magnetic data indicate high sediment input to the lake, in agreement with the pollen stratigraphy indicating open communities and a lower tree-line. During the Holocene, plant communities changed notoriously with the expansion of pine and oak forests, the reduction of the grass assemblage and higher pollen concentration values. After the fall of the TCT tephra, with an age of 8500 yr BP, the fir forest, indicative of mesic conditions in the highlands, is documented. The late Holocene record shows the presence of Teosinte and Zea mays pollen, associated with high levels of charcoal particles related with agricultural activities.