Estudio fitosociológico en campo natural y su relación con variables edáficas e implicancias en la respuesta a la historia de manejo del campo

The present work was carried out at the Mario A. Cassinoni Experimental Station of the Faculty of Agronomy - University of the Republic, located on National Route No. 3 Gral. José Gervasio Artigas, kilometer 363, in the department of Paysandú, Uruguay. This study was developed on two paddocks: paddock 13, which covers 38 hectares of re-established natural field, and is subjected to an experiment with continuous and rotational grazing methods with different levels of forage supply; and paddock 18, which covers 36 hectares of virgin natural field and is under grazing exclusion for a period of 6 months. The objective was to study the association between the plant communities of a natural field and the different environmental factors, such as the types of soil present, the history of field management, and the implication of the different grazing management used. To achieve this, a survey of the botanical composition was carried out, using the surface method with systematic sampling by means of a 1 m² square, where the cover of the identified species, bare soil, mulch, dry debris and feces was subjectively estimated in terms of absolute abundance. A total of 301 samples were taken between both paddocks. The soil types present are Brunosols, Planosols, Lithosols, Gleysols and Solonetz, which are found in similar proportions in both paddocks. For the analysis of the results, spreadsheets with information matrices were used to perform a statistical analysis. InfoStat statistical software was used, applying multivariate and univariate descriptive techniques (ANAVA) and Tukey mean comparison tests with significance levels of 10%, 5% and 1%. A total of 207 species belonging to 39 families and 13 tribes were recorded for the Gramineae family. The results show different trends for the variables analyzed according to the factors evaluated.  The botanical composition at different hierarchical levels and the components of the total cover observed are determined and conditioned by the history of field management and edaphic characteristics. At the same time, there are trends in behavior according to the factors analyzed at the level of species, families, tribes and the different groupings of plant communities. Plant diversity was evaluated through biological diversity indexes, measured in terms of specific richness, Shannon-Wiener index, Simpson index and Pielou's equity index. The results indicate that the virgin field presents greater richness, diversity and equity, with less dominance, compared to the re-established field. This suggests that, after a long period of tapestry intervention in a natural field, the situation of a virgin field is not completely recovered. In terms of soil types, the Solonetz, despite not having the greatest richness, shows the greatest diversity, the least species dominance and therefore the greatest equity. The plant diversity of the natural field is characterized by a great functional complexity that is strongly related to environmental factors and their variations.