Efeitos da silvicultura do eucalipto na dinâmica da vegetação em área de pecuária no Rio Grande do Sul, Brasil

Abstract

The overall objective of this study was to investigate the influence of the forestry (eucalyptus plantations) in Campos Sulinos biome vegetation. It were installed permanent and temporary plots in different grassland types of vegetation of (dry and wet grassland field), field with livestock, riparian forests, and within the eucalyptus plantations, to be assessed over time. The plots were evaluated about two years. The study areas were Tapera and Passarinho Farms, located in Pinheiro Machado, Brazil. In one farm, there was the eucalyptus introduction for wood pulp production. These settlements were introduced by mosaic form, maintaining the Permanent Preservation Areas (APP) limits. With the eucalyptus introduction in the Tapera farm area, the cattle industry was abandoned, resulting in grassland, forest and riparian woods vegetation changes. In the second farm, the main activity is cattle raising. In this area there s a predominance of herbaceous vegetation, grasses and other plants with the main purpose of grazing. The results for riparian forest it were high homogeneity with low species diversity. In the upper stratum, the most representative families were Anacardiaceae (3 species), Myrtaceae (2 species) and Lauraceae (2 species). The species with the greatest individuals number were Lithraea brasiliensis, Scutia buxifolia, Allophyllus edulis, Blepharocalyx salicifolius Schinus polygamus, Myrrhinium atropurpureum and Ocotea acutifolia. The individuals diameter classes distribution followed the pattern of native forests (inverted "J"), focusing on smaller classes. Regarding the field of pasture, the most representative botanical families were Poaceae, Rubiaceae and Apiaceae. In the field (dry and wet) with the absence of grazing were Poaceae, Asteraceae and Apiaceae. The predominant species in field with grazing were Paspalum notatum, Axonopus affinis and Piptochaetium montevidense and in ungrazed field, Baccharis trimera, Paspalum pauciciliatum, Cortaderia sp., Eryngium horridum, Axonopus compressus and Calamagrostis viridiflavescens. In the study field with eucalyptus sp. and without eucalyptus, the most representative families were Poaceae, Apiaceae and Asteraceae on both occasions, differing only in their biomass percentage, which in eucalyptus, was lower. The most characteristic species in eucalyptus field in year 1 were Eryngium horridum, Conyza sp. and Baccharis trimera, and in year 2, Baccharis trimera, and Eryngium horridum, Aspilia montevidensis, and in field without eucalyptus in year 1 were Baccharis trimera, Paspalum pauciciliatum, Cortaderia sp. and Eryngium horridum. So the eucalyptus plantations have been contributed positively to the studied fragment in relation to coexistence of natural formation in riparian vegetation, due to cattle removal and later fencing the area, allowing development of natural regeneration. The species diversity is greater in livestock activity field than in fields without this activity, but the increases in vegetation biomass without cattle. The cattle removal from the fields, the forestry activities, may cause biomass and botanical composition changes, reducing species diversity and increasing dominance of tall grasses and shrubs and other species. Species diversity and biomass are greater in field without eucalyptus plantation than in fields with this activity. In wet and dry fields analysis without livestock, with eucalyptus plantations with no livestock and only with livestock, the average diversity and biomass of dry field, wet grassland and eucalyptus field didn t differ statistically. The livestock field, dry and wet grassland showed no significant difference. The livestock field had the highest average for that variable and the field with eucalyptus had the lowest average for the same variable. Although the results suggest that eucalyptus field shows lower species richness, the livestock removal from the fields may cause changes in biomass, with consequent reduction in species diversity.