Islands of vegetation on quartzite-sandstone outcrops, Pai Inácio Mountain, Chapada Diamantina, Bahia, Brazil

Islands of vegetation on rocky surfaces were studied on two plateaus at Pai Inácio Mountain (41°28'; 12°27'S) in the Chapada Diamantina. Both plateaus have quartzite-sandstone outcrops interspersed with sandy, acidic soils at the summit between 1,100 and 1,170 meters above sea level, with a well-defined dry season. Islands are defined as clumps of one or more species of vascular plants completely surrounded by a rocky surface devoid of vascular plants. The study included 39 vegetation islands of different sizes on each plateau, with 63 herb and shrub species, of which 22 are common to both plateaus. Liliopsida species had the highest abundance, frequency and dominance, with Velloziaceae, Cyperaceae, Orchidaceae, Bromeliaceae, and Guttiferae families predominating on both plateaus. The chamaephyte life-form was also predominant. Similar species richness was detected on both plateaus, with most of the islands composed of up to five species. Four species groups were revealed using UPGMA and Jaccard´s similarity index, two groups with species typical of sunlit islands, one group with species from shadier sites, and a fourth composed of more generalist species. The Vellozia hemisphaerica - Trilepis lhotzkiana association was typical of rocky outcrops at Pai Inácio Mountain. Orchidaceae and Cyperaceae were the only families present in the two smallest island-size classes, while Guttiferae, Rubiaceae and Bromeliaceae were mostly present in the largest class. Variation in species composition and abundance between the two plateaus suggest environmental and isolation differences affecting the species spatial distribution in the vegetation islands on each plateau.

campo rupestre; phytosociology; rocky outcrop; inselberg; vegetation island