This study of the zacatonal alpino zone of the volcanoes Popocatepetl (5452 in) and Nevado de Toluca (4690 m) in the central region of the Transmexican Volcanic Belt, is a follow-up of the study of ALMEIDA et al. (1994). This tropical alpine zacatonal represents the potential vegetation of the altitudinal belt between 3950 and 4500 in. These altitudinal limits constitute the uppermost limit of Pinus hartwegii forests and the lower limit of the periglacial areas that are almost without vegetation. On both volcanoes a total of 117 vegetation phytosociological relevees were made along the altitudinal gradient. They were clustered according to a classification analysis. The syntaxonomic scheme of the different units is presented and discussed. Special emphasis is given to the association level, aspects such as floristic composition, altitudinal zone, distribution and relation with environmental factors. Syntaxonomically new is the class Drabo jorullensis-Calama-grostietea tolucensis, which includes herbaceous formations dominated by tussock grasses, ground rosettes and cushions growing on Andosols and Lithosols. This class includes the orders Festucetalia lividae (ord. nov.) and Calamagrostietalia tolucensis. The first is located above 4150 m, and colonizes umbric Lithosols
the latter type of grassland, dense zacatonal formations, is present between about 3950 and 4150 m, and preferably on Andosols. The first order includes the alliance Plantagino tolucensis-Festucion lividae (all. nov.). The vegetation of this alliance is distributed on gentle slopes and almost level grounds with intense cryoturbation. This alliance comprises two associations, one of which is the new association Drabo nivicolae-Plantaginetum tolucensis. The pioneer vegetation consists of small tussocks and rosettes, and occurs in zones of colluvial accumulation and wind swept passes. It is restricted to Nevado de Toluca. The second order includes the new alliance Lupinomontani-calamagrostion tolucensis, which comprises the zacatonal associations above the pine transition zones of Pinus hartwegii. One out of four, the newly described association Lupino mexicani-Calamagrostietum tolucensis, is developed on slopes above the upper limit of these pine forests, only at Nevado de Toluca. A total of 22 families, 42 genera and 61 species of vascular plants are presented. The more representative families are Caryophyllaceae, Asteraceae, Brassicaceae y Poaceae
among the genera are: Arenaria, Calamagrostis, Draba, Festuca, Senecio y Trisetum. Among the species with a wide range are: Arenaria bryoides, Calamagrostis tolucensis, Draba jorullensis, Senecio procumbens and Trisetum spicatum. More genera and species are documented from Nevado de Toluca than from Popocatepetl, due to historic-geological, edaphic, topographic and perturbation differences. The physiognomically dominant growth forms are the tussocks. The xerophytic aspect of the alpine vegetation on these volcanoes is determined by low temperatures during the largest part of the year, and by dryness which is reflected by the sheath of dry leaves of the tussocks. There is no arbustive fringe of transition among the sand paths limiting the high monotonous coniferous Pinus hartwegii forest and the "zacatonal alpino".
