Resumo. This study aims to identify structural similarities between the remnants of seasonal deciduous forest (SDF) in Santa Catarina state and check the influence of environmental factors that leads the formation of these groups. Associated with this primary objective, it was also sought to identify the areas of contact between this forest type and Araucaria forest (mixed ombrophilous forest – MOF). We analyzed data from 78 sample units (AU) installed in the remnants of SDF between 2008 and 2009 by professional team of Floristic and Forest Inventory of Santa Catarina project (IFFSC). The data were processed through the Detrended Correspondence Analysis (DCA) and Canonical Correspondence Analysis (CCA), using a matrix of species density associated with a environmental variables matrix, that consisted of altitude and climate parameters. The proportion of variance explained by the first three axes of DCA was low, accounting for approximately 21% of the information contained in the original data set. This result provides low power of interpretation of the results. However, when DCA was applied to the data of mean density of species in each river basin, the results pointed to better explanatory eigenvectors (three first eigenvectors explain 58.1% of total variance). The first ordination axis of DCA grouped the eastern watersheds SUs at one end, and the western watersheds SUs at the other end of this axis. At the right end of the first axis are arranged the watersheds of the river Canoas, Pelotas and Peixe, which are characterized by the presence of species such as Ocotea pulchella, Zanthoxylum fagara, Lithrea brasiliensis, Matayba elaeagnoides, Cinnamodendron dinisii commonly associated with Mixed Ombrophilous Forest (Araucaria forest). At the left end, the grouped SUs are mainly those of western watersheds, a group that includes the basins of the Jacutinga, Irani, Chapecó, Antas and Peperi-guaçu rivers. Among the species that most strongly influence the results in the analysis, we highlight Apuleia leiocarpa, Rauvolfia sellowii, Bastardiopsis densiflora, Chrysophyllum gonocarpum, Cordia trichotoma, Holocalyx balansae, Myrocarpus frondosus and Pisonia zapallo, which are common in seasonal forests of the hinterlands of southern Brazil. The analysis through the ACC pointed to similar results and joint interpretation of environmental data indicated that the basins located in the east are associated with colder climates where frosts are more frequent as a result of higher altitudes. As the altitudes tend to decline from east to west and portions, the authors propose the existence of a transition zone between the two phytoecological regions (MOF and SDF) in the range of 600 m asl, where occurs the interdigitation of elements from seasonal flora and those of Araucaria forest, resulting in the delineation of a core area of the SDF below this altitudinal level.