Dinâmica da vegetação do sub-bosque sob influência de clareiras causadas pela exploração em uma floresta de terra firme no município de Moju – Pará, Brasil.

Abstract

The dynamics of the floristic, structure and diversity of an understorey vegetation influence by gaps caused by reduced impact logging was analysed during 12 years in a terra firme rain forest in the Eastern Amazonian, in the municipality of Moju, in Pará state, Brazil. Angiosperm individuals from 10 cm height to 5 cm DBH was surveyed in a 468 m² sample area in six occasions, and they were analyzed considering life-forms (trees, shrub, lianas, herbs and palms) and size classes (CT1 = 10cm ≤ HT < 50cm; CT2 = 50cm ≤ HT ≤ 130cm; CT3 = 130cm < HT and DAP < 2cm; and CT4 = 2cm ≤ DAP < 5 cm) of the individuals. Data was analyzed considering their life-forms (trees, shrub, lianas, herbs and palms) and size categories (CT1 = 10cm ≤ HT < 50 cm; CT2 = 50cm ≤ HT ≤ 130cm; CT3 = 130cm < HT e DAP < 2 cm e; CT4 = 2cm ≤ DAP < 5cm), considering abundance, frequency, relative natural regeneration, The Shannon Index, Evenness and Sorensen Similarity Index. There were found 17,264 plants from 73 families, 180 genera and 256 species. An increase in the number of species and individuals was observed until 3 years after logging, but decreasing in the following years. Three was the most representative life-form followed by lianas and herbs. CT1 was the most representative size class. There were great changes in the composition of less abundant species during the study period. Size class and relative natural regeneration depended on abundance of species and they had many changes in the studied period. Burseraceae, Marantaceae, Violaceae and Lecythidaceae were the most important families in the study area, while the most representative species were Monotagma sp1., Monotagma sp2., Rinorea guianensis, Lecythis idatimon e Protium pilosum. The most frequent life forms were trees, herbs and lianas; shrub became less important with time, while palms had their importance increased. Small changes occurred in the community diversity during the study period and no significant differences were found between assessments. Initially diversity was higher in the low size classes, decreasing in the following years, but increasing in the higher size classes. Evenness was higher in the bigger size classes because of the low density of individuals and species in these classes. Diversity was higher for trees and lower for palms. Similarity decreased with time, being higher between close years. Similarity was higher between CT1 and CT2 over the study period. The dynamics of life forms depended on light and consequently on the forest succession phase decreasing according to the process of canopy closing, which allows the establishment of the individuals that will compose the forest community. Changes with time suggested that the vegetation tends to stability. Gaps caused significant changes on the understorey diversity, boosting the process of maintaining diversity in the study forest.