Estrutura e florística de dois fragmentos ripários de floresta estacional decidual no noroeste do Rio Grande do Sul, Brasil

Abstract

The riparian forests are located on the river banks and contribute to the quality of water resources and biodiversity conservation. The objective of this research was to evaluate the floristic and structure of two riparian Deciduous Seasonal Forest fragments in northwest of Rio Grande do Sul. The specific objectives were: i) to characterize the floristic and structure of the arboreal component; ii) evaluate the phytosociological aspects of natural regeneration; iii) and determine which environmental variables were correlated to the abundance of the higher density species. The two forest fragments are located at Guarani das Missões county, RS, on the east bank of the river Ijuí. The arboreal component (diameter at breast height (DBH) ≥ 5.0 cm) was measured in 36 plots (18 per fragment). It was systematically distributed in perpendicular tracks to the river bank and distant 40 m from each other. A total of 7200 m2 of surface was sampled. The natural regeneration was sampled in two classes: Seedlings - individuals with height ≥ 0.20 m and DBH < 1 cm; Established Natural Regeneration (RNE) - individuals with 1 cm ≤ DBH < 5 cm. The RNE class was measured in 36 sub-plots of 5 m x 10 m being installed one unit of 10 m x 20 m per plot. The seedlings class was measured by sub-plots of 3.16 m x 3.16 m, each unit had been installed to sub-plot of 5 m x 10 m. A total of 1800 m2 of RNE class and 360 m2 of Seedlings class had been sampled. Considering the two riparian fragments 58 species and 24 botanical families were sampled in the arboreal component. The analysis of the successional category showed predominance of climax light demanding in the two areas, then the greater abundance of climax shade-tolerant was found only in the Pinheiro Machado fragmente (PM), indicating that this fragment has a better condition status. The most important species in PM fragment were Matayba elaeagnoides (10.5%) and Actinostemon concolor (9.8%), in Linha do Mel fragmente (LM) were Matayba elaeagnoides (12.2%) and Gymnanthes klotzschiana (9.5%). Some of the strongest correlations were Gymnanthes klotzschiana (-0.49) and Actinostemon concolor (0.45) with an terrain elevation quota, indicating, respectively, adaptability to sites that accumulate water for a longer time (lower parts of the relief) and well-drained sites (upper parts of the relief). In natural regeneration of riparian forests were observed 71 species, the greatest diversity was observed in the LM fragment compared to the PM fragment. The climax light demanding species showed greater richness in both regeneration classes and in both forests that had been analyzed. The zoochoric dispersion strategy was mainly responsible for increasing the richness of natural regeneration. In both fragments, the majority of species had been found in an aggregated, showing that the regenerating populations are dense established in a few plots. The species Actinostemon concolor (28.1%) and Cupania vernalis (8.4%) had higher natural regeneration relative (RNR) compared to the PM fragment, while Actinostemon concolor (13.7%) and Pavonia sepium (10.1%) stood out the LM fragment. A. concolor, which showed high density in both regeneration classes of the two fragments had also positive correlation with the terrain elevation quota, accumulated litter layer and edge distancy, but it had negative correlation with canopy openness.