Saxicolous vascular flora of karst outcrops: An overlooked component of Brazilian biodiversity

Abstract

Rock outcrops are islands in the landscape that usually harbor a xeric saxicolous flora with endemic and endangered species. They are abundant in the Neotropical region, but floristic and ecological studies are centered on quartzite and granite-gneissic lithologies, leaving a knowledge gap regarding those of calcareous origin. It is expected that outcrops of carbonate rocks, like other rock outcrops, harbor a peculiar saxicolous flora tolerant to drought, and share communities of species associated with the distinct microhabitats. To test these assumptions, we carried out a floristic survey of the saxicolous vascular vegetation of two calcareous outcrops at Lagoa Santa region, Minas Gerais, Brazil, the historical site studied in the XIX Century by Eugen Warming – one of the fathers of Ecology. We also analysed the floristic similarity between the outcrops, among the five recognized microhabitats, crevices, exposed rock, fissures, soil islands, and epiphytic, as well as the life form spectra. Our results registered 45 families, 92 genera and 124 species, standing out Orchidaceae (14 species), Bromeliaceae (11 species), Piperaceae (10 species) and Asteraceae (8 species). There was a low similarity between the outcrops (Ij = 0.45), but the cluster analysis revealed three floristic groups inhabiting the microhabitats (exposed rock, crevices-soil islands, and fissure-epiphytic), each one sharing a similar subset of species. The distribution of the life forms into the microhabitats was homogeneous in the two outcrops, meaning that there is a correspondence between them. Three saxicolous species are threatened, but most of the inventoried species (73%) were never assessed and may be under extinction risk. Our results highlight that the saxicolous flora on outcrops of carbonate rocks deserve more attention on conservation once they have high biological importance, share communities of saxicolous plants inhabiting particular microhabitats, and face strong pressures of anthropic activities, especially mining.