| Shrnutí: | La Michilía biosphere reserve is located in the southeastern part of Durango state, in a zone of semidry temperate climate, embracing an area of about 70,000 ha, 7000 of which belong to Cerro Blanco, the nucleus zone. Its vegetation is representative of that of millions of hectares of the Mexican Sierra Madre Occidental, especially of its eastern slopes at altitudes between 2000 and 3000 m. The plant communities were delimited and maped on scale 1:50,000. The altitudinal distribution of several associations is sketched on profiles of vegetation. The following major vegetation units were recognized: Quercus forest, Quercus - Pinus forest, Pinus forest, Pinus - Quercus forest, Pseudotsuga forest, Cupressus forest, Juniperus communities, grassland, Arctostaphylos pungens scrub, Quercus microphylla scrub, Acacia schaffneri scrub and aquatic and semiaquatic vegetation. Oak and oak-pine forests form the bulk of the vegetation, and are represented by diverse associations. The structure and floristic composition of the major associations are described. From a comparison of diversity indices and structural parameters for two size classes of woody elements an interpretation of the successional trends of these communities is attempted. Relationships between plant cover and the most important abiotic factors, i.e. climate, geology, soils and geomorphology, are commented. The altitudinal levels of vegetation are scanty uniform due to geomorphology but, in general terms, it is observed that pines are predominant above 2700 m of altitude. Between 2500 and 2700 m oak-pine forests are common, and between 2200 and 2600 m oak forests and grasslands prevail. This distributional pattern is modified by relief, and reversions of vegetational levels in ravines and mountain valleys are common. Further studies about the dynamics and regeneration of these forests and the relationships between vegetation, wild fauna and livestock are recommended, since in a great part of its distributional area this ecosystem is simultaneously exploited for cattle raising and forestry.
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