Pollen records of a 520-cm long core from Lake Zempoala (2800 m altitude) and a 884-cm long composite core from Lake Quila (3010 m altitude), both located 65 km SW of Mexico City, show changes in vegetation and climate. The Zempoala record covers the last c. 6320 cal yr BP, while the Quila record spans the last c. 11,610 cal yr BP. These lakes, with submerged aquatics and hydroseral shore vegetation, lie today in the Abies religiosa-dominated forest belt. From c. 6320 to 2530 cal yr BP Zempoala shows a mixed forest with mesophyllous taxa, indicating warmer and more humid conditions than at present. From c. 2530 to 160 cal yr BP Abies forest and Pinus hartwegii forest dominated, indicating a temperate climate. During the first part of this period, from c. 2530 to 1150 cal yr BP abundant Pinus forest is indicative of drier climate conditions. During the last c. 310 cal yr of this period Abies-dominated forest prevailed, with a floral composition resembling the modem Abies forests. This is indicative of temperate and humid climatic conditions. During the last c. 160 cal yr BP the presence of Zea mays and Plantago reflect human impact. From c. 11,610 to 10,650 cal yr BP Quila shows Alnus and Pinus-dominated forest, reflecting humid climatic conditions. From 10,650 to 5000 cal yr BP Pinus and Arceuthobium are indicative of a Pinus hartwegii forest, suggesting slightly lower temperatures than at present. A 25-cm thick volcanic ash horizon at 380-355 cm core depth is the well-known 'yellow ash' marker with a known age of c. 4200-4800 C-14 yr BP. This age is supported by our bracketing dates of 5620 and 4650 C-14 yr BP. From 4630 to 1000 cal yr BP mixed forest, including Pinus, Quercus, Carpinus, Hedyosmum, Juglans and Artemisia, is indicative of mesophytic conditions. From 4630 to 1010 cal yr BP Abies forest was abundant, reflecting colder and more humid conditions. From c. 1010 to 20 cal yr BP Pinus forest was very abundant suggesting, as in site Zempoala, dry climatic conditions. The pollen record from Quila documents four different types of forest: Pinus/Alnus forest (possibly Alnus jorullensis in zone QUI-I), Pinus forest (most likely Pinus hartwegii forest in zone QUI-II), mixed forest in zone QUI-III), and Abies forest (Abies religiosa forest in zone QUI-IV). The last c. 1000 years show a decline of mixed forest and increasing Abies forest, reflecting cooler and more humid conditions. Quila also shows, during the most recent past, human impact by crop cultivation (Zea mays) and vegetation disturbance (Plantago) leading to erosion. Other microfossils, among which coprophilous fungi, were recorded and illustrated. (c) 2005 Elsevier B.V. All rights reserved.
