Competição e dinâmica em floresta tropical seca no semiárido pernambucano

Abstract

Tropical dry forests are the main formations that grow in regions of hot and humid dry climate. In Brazil, one of the most characteristics tropical forests is the Caatinga, which has been mainly modified by the change of soil use for agriculture, livestock and for meeting the energy demand. Besides the anthropic action, the Caatinga is modified by natural phenomena such as drought, which is recurrent in the Brazilian Northeast. Therefore, aiming to monitor the vegetation behavior in the face of acting biotic and abiotic phenomena, this current study was organized in two chapters. The first one aimed to evaluate the vegetal dynamics in periods that overlapped a severe drought in the region; while the second aimed to study the influence of competition and diameter size of individuals on the diametric growth of a dominant species in the forest community, the Cenostigma breacteosum (Tul.) E. Gagnon & G. P. Lewis. This study was conducted in the municipality of Floresta, Pernambuco, with the target area having been monitored for 10 years by permanent plots of 20 m x 20 m each, in which all the shrub-tree individuals with circumference at 1.30 m from the ground (CBH) ≥ 6 cm have been monitored. Shrub-tree community dynamics was evaluated for three periods, from 2012 to 2015, from 2015 to 2018 and from 2008 to 2015. Changes in the phytosociological parameters, mortality rates, recruitment and both diameter and basal area growth of the vegetation were all estimated. Results of the first chapter indicated richness reduction of families and species during all periods, while there was a noticeable decrease in the estimates of horizontal structure parameters. However, Cenostigma bracteosum stood out, with density and dominance close to or over 50% in the periods. The vegetation mean periodic increment, in diameter, was between 0.12 cm year-1 and 0.20 cm year-1. Mortality was higher than recruitment, affecting the net basal area growth of the community, which was negative in all periods. Species with more noticeable responses to the stress were Mimosa ophthalmocentra, Mimosa tenuiflora and Manihot carthaginensis. Species Jatropha molissima and Cnidoscolus quercifolius showed positive results regarding the drought, with the Cenostigma bracteosum, Myracrodruon urundeuva and Aspidosperma pyrifolium species also standing out as well. Long periods of severe drought caused heavy changes in the composition and structure of the shrub-tree vegetation from Caatinga. Regarding the second chapter, data from two areas with different use history were used. All living individuals of the C. bracteosum species in permanent plots were analyzed about their neighbor competition, both in intra and interspecies levels. The competition was quantified through competition indexes, which were evaluated by graphical analysis and linear correlation with the diameter increment. Individuals who were “free” from competition displayed higher mean increments in relation to those who had competitors in their midst. There was a strong positive relation between the diameter size of the individuals and their growth, which may also indicate a secondary effect of the competition influence. The indexes resulted in negative correlations with the increment in diameter; however, obtained correlations were weak, which may be resulting from the random distribution of the shrub-tree individuals in the Caatinga. Size, in diameter, of the object-trees had a higher relation with their diametric growth. Lorimer’s distance-independent index, due to its calculation simplicity, is recommended in order to represent the competition undergone by the C. bracteosum.