Composição florística e estrutura de vegetação arbórea em diferentes condições fisiográficas de um fragmento de floresta estacional semidecidual secundária, na Zona da Mata de Minas Gerais

Abstract

The remnant forests in the Southeastern Brazil are highly fragmented and degraded. To maintain or increase its biodiversity, management plans should be based on the understanding of the floristic and the structure of tree species and its environmental conditions. The present study was carried out in a hilly region, in Paula Cândido county (20°52´2´´ latitude South and 42°58´49´´ longitude West), Minas Gerais state, Brazil. The forest fragment studied is located on the upper third of the slope and on the top of the hills. The point quarter sampling method was used along transects in the North- Top-South and East-Top-West directions. Three different tree species sizes were sampled in each sampling point as follows: level one - plants higher than 3 m with diameter at breast height (DBH) lower than 5 cm; level 2 - plants with DBH between 5 and 10 cm and, level 3 - plants with DBH greater than 10 cm. For each sampling point it was measured the following environmental conditions: photosynthetically active radiation (PAR), leaf area index (LAI), slope, aspect, forest floor and abundance of bamboos in the understorey. The sampling points were grouped on Top and North, South, East and West aspects and the phytosociological analysis was undertaken for each physiographic condition. The PAR varied transmissivity from 7.94% (South) to 16.63 (Top) and LAI varied from 2.43 (Top) to 2.87 (South), indicating that the forest canopy is quite open. It was sampled 2,280 individuals from 120 tree species and 40 families. The Shannon-Weaver diversity index varied from 3.22 to 3.95. Mabea fistulifera Mart. (Euphorbiaceae) dominated in all physiographic conditions. Cecropia hololeuca Miq (Cecropiaceae) presented the second highest Importance Value (IV). Its IV at the level 3 (trees over 10 cm DBH) was 10 times greater than at the level 1, indicating that the environmental condition are not adequate for its natural regeneration. The great majority of tree species were classified as initial secondary, suggesting an initial stage of succession. Species composition and structure varied with the forest fragment physiographic conditions. Nine species were restricted to the Top, two to the North, two to the South, four to the East and five to the West aspect. Some species occurred in several environmental conditions but dominated in some conditions such as Cecropia hololeuca (second highest IV on the Top and South, third on the West, seventh on the East and 13° on the North aspect). In general, the results of this study showed a great variability in species composition and structure due to physiographic conditions, indicating that management plans for the forest fragments should take them into account.