Compositional patterns of vegetation and their relationship to temporal and spatial environmental variation, with emphasis on climatic factors, were investigated in plant communities located in the southernmost portion of the North American Graminetum, in central México. Data from 353 samples, obtained in four ecologically contrasting plant communities during 11 years, were analyzed by partial canonical correspondence analysis. Eight climatic variables and eighteen covariables (seventeen edaphic and one resource management) were included in the ordination. A relationship between floristic change and weather variation, once covariables effects were fitted, was examined. Despite a strong contrast in ecological conditions among study sites, a set of four climatic variables was finally found in which each variable contributed independently and statistically (P < 0.01) to the total variance in the vegetation data. Thus, environmental variables other than climatic could not conceal the important role of weather as a mediator of floristic change through time. Summer precipitation and summer maximum temperature showed the highest correlations with the first two species axes, 0.77 and –0.39, respectively. Contribution of these two climatic variables to variance in the vegetation data explained by environmental variables was approximately 81%. Annual species were abundant during rainy years, while abundance of perennial grasses and shrubs showed no clear relationship to weather variation. This study explicitly probes the important role of rain patterns in shaping structure and composition of semiarid communities and considers how other environmental factors can affect plant communities at the southernmost part of the North American Graminetum.