Pollen data from 3860 m in the Agua El Marrano valley on the northwest side of Iztaccihuatl volcano (southeastern part of the Basin of Mexico, 19N), combined with data on the glacial chronology and tephrochronology, document Holocene climatic and vegetation changes near the present timberline. Glacial advances occurred between 12 000 and-10 500 cal. yr BP (Milpulco-l) and between 8300 and 7300 cal. yr BP (Milpulco-2). A 450-cm core was drilled in sediments accumulated in a Milpulco-I moraine depression. The sequence covers the last 11 000 cal. yr BP, according to three 14C dates and the presence of a distinctive pumice deposit dated elsewhere at 5700 cal. yr BP. The pollen record of Agua El Marrano shows barren conditions after glacier retreat, followed by a gradual colonization by elements of the alpine grasslands around 10900 cal. yr BP This plant community remained until c. 7200 cal. yr BP. Between c. 7200 cal. yr BP and 6500 cal. yr BP coniferous forest grew close to or at the site, but alpine grasslands were re-established between 6500 cal. yr BP and c. 5000 cal. yr BP, apparently in connection with mid-Holocene dry conditions recorded in central Mexico. The upper timberline attained its modern altitude (- 4000 m) in the last 3000 years. During this period no significant changes are observed in the pollen spectra. Comparison with other palaeoecological records from central Mexico suggests that the timberline fluctuated in response to both temperature and precipitation changes during the Holocene.
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