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<b>Aim:</b> Identify the vegetation patterns that make up the boundaries of the Caldenal - Pampean Austral ecotone, and determine the main variables that drive biogeographic changes in areas with a high rate of ecosystem degradation.<b>Location:</b> The southwestern region of Buenos Aires province (Argentina), a broad ecotone between the phytogeographic districts of Caldenal and Pampean Austral.<b>Methods:</b> Three vegetation censuses were carried out at 11 sites by establishing plots and subplots, recording species richness, cover, and shrub characteristics. Environmental data included 10 bioclimatic variables and the human influence index (HII). Generalised Dissimilarity Model (GDM) was used to analyse compositional changes, accounting for natural and anthropogenic effects, resulting in calibration analysis, focusing on explained deviance and predictor significance on beta diversity patterns, and detailed maps.<b>Results: </b>A total of 179 species were identified in the thirty-three vegetation censuses. Sites were classified into five groups based on their vegetation composition. The GDM explained 68.48% of the variation in species compositional turnover. The variables that contributed the most to determine these changes were potential evapotranspiration, HII, and maximum temperature. Three main different vegetation patterns were recognised: the Caldenal district, the Pampean Austral district, and in the Sierra de la Ventana region, which were also divided into subregions.<b>Conclusions:</b> Our results suggest the presence of at least five groups of plant communities, which coexist in an environmental gradient, in this region. The Pampean Austral district has an herbaceous layer that transitions to a woody layer in the Caldenal district as potential evapotranspiration increases. Also, human activities like agriculture and livestock grazing have a greater impact on the eastern border of the Caldenal district. The maps presented in this work define the boundary between these districts, and highlight the large compositional differences found in the Ventania system, so it should be considered separately from the Pampean Austral district.