Ecological site (ES) concepts describe potential vegetation and ecological mechanisms of plant community change within specific soil, geomorphic and climate conditions.  In this study, a preliminary definition of ES concepts for the caldenal ecoregion was developed by exploring the relationship between landform, soil and vegetation based on 42 sample sites. Woody plant dominance was characterized by plant density (Open, Closed and Fachinal [thicket]), and height (Tall and Short). Herbaceous species composition was grouped in five functional groups: the palatable summer and thin-leaved winter grasses, and the unpalatable winter bunch grasses, forbs, and annuals.  Ward’s cluster analysis was used to group sampling sites based on their soil properties along with vegetation characteristics.  Cluster analysis indicated six distinct plant communities that were easily interpreted.  Open Calden states were found in dunes and sandy plateaus, while thicketized states (Closed and Fachinal) occurred on soils with greater silt-clay content on hillsides, valleys and plains.  A short fachinal state was associated with shallow caliche layer (<0.5 m) and high carbonate levels.  Open and Closed types were dominated by Prosopis caldenia while the Fachinal was dominated by Condalia microphyla.  Palatable grasses were more highly associated with Open sites; however all sites showed a large proportion of unpalatable herbaceous species. The ecological sites derived from this analysis can serve as a basis for recognizing different levels of vulnerability to encroachment and change in grass species composition. This study provides a framework for future data collection to further refine ES concepts in the Caldenal.