Environmental factors and community dynamics at the southernmost part of the North American Graminetum

We have delineated the spatio-temporal plant assemblages prevailing under contrasting environmental conditions at the southernmost part of the North American Graminetum in central México and discuss the relative importance of these factors in determining plant community structure and composition. 353 line-transect samples were collected during 11 years from exclosures and adjacent grazed areas within three Bouteloua-dominated grasslands and one Acacia schaffneri shrubland and analyzed using TWINSPAN. In addition, eight edaphic variables were utilized to evaluate similarities in soil properties among sites, using unweighted-pair groups method. Results from TWINSPAN were translated into mosaic patterns to show the distribution of communities as a function of environmental factors over time. Under no or moderate grazing, summer precipitation promotes an initial differentiation of vegetation into high, low or average rainfall communities. This situation is altered in grasslands degraded by intensive grazing pressure, where rainfall patterns play a subordinate role to that of livestock herbivory. Soil influences are secondary to those of precipitation and grazing in affecting grassland structure and composition. In shrubland, community stability is related to high floristic and edaphic spatial heterogeneity in the face of climatic variability.[PUBLICATION ABSTRACT]