[Variations of structure and of flora of the "carrasco" vegetation of the Ibiapaba plateau, state of Cearsá, Brazil].

Abstract

"Carrasco" is a closed, tall-shrubby, xerophilous vegetation on Quartz Sand soils between altitudes of 700 and 900 m on Ibiapaba and Araripe plateaus in the Brazilian semi-arid domain. As it is still very poorly known, this paper aims to describe the structure and composition of the woody community and its spatial variation in relation to the soil. One-hundred 10 x 10 m plots were located by pairs of random numbers in a coordinate system at the locality of Jaburuna (3 degrees 54'34"S, 40 degrees 59'24"W, about 830 m altitude), municipality of Ubajara, north Ibiapaba. All woody plants with a minimum stem diameter of 3 cm at ground level had their vertical height (not for climbers) and stem perimeter recorded. A soil extract from 0-50 cm depth was taken at the centre of each plot. Published surveys of other carrasco areas on the south Ibiapaba were considered for comparison. The community structure showed great dominance concentration, the most abundant species (Acacia langsdorffii, Piptadenia moniliformis, Thiloa glaucocarpa) varied between surveys. At Jaburuna 81% of all sampled plants were shrubs, 14% trees (the most with less than 10 cm of trunk diameter and 6 m height), and 5% climbers. Canberra distances and Jaccard's Indices were calculated from a primary matrix of 87 species (with 5 or more individuals) and 175 plots. Several methods of cluster analysis were employed, all showed great floristic variation from place to place. A matrix of soil physical and chemical variates per plot was constructed, and the canonical correspondence analysis was applied to both primary and soil matrices. Lower pH and higher sum of bases (e. g. Brunfelsia cuneifolia, Neojeobertia candoleana), higher content of gross sand (e. g. Acacia glomerosa, Aspidosperma subincanumn) and higher content of fine sand (e. g. Aspidosperma discolor, Hymenaea velutina) were the main variates separating species. The floristic richness at Jaburuna was of 74 species, and the surveys showed no difference in Shannon's diversity, which varied from 2.87 to 3.16 nats/individual.