Diversity and plant trait-soil relationships among rock outcrops in the Brazilian Atlantic rainforest

We investigated how outcrops of different geological origins enhance the plant megadiversity of the Atlantic rainforest hotspot. We collected vegetation, topographic, and soil fertility data from 50 2 m(2) plots in each of nine rock outcrops (three ironstones -or cangas, three quartzites and three granitoids) in the Iron Quadrangle, SE Brazil. We examined the response of community diversity and structure patterns to edaphic and topographic gradients by means of diversity profiles, clustering and ordination analyses. Species were organized into nine functional groups. We inventoried 17,690 individuals belonging to 352 species. Functional groups with largest cover were sclerophytic shrubs (in cangas), graminoid and poikilohydric herbs (in both granitoids and quartzites). Granitoid plant communities were the least diverse, on account of fewer substrate types leading to more xeric conditions. The multivariate analyses sorted the outcrops by geological origin, although within-lithotype similarity was low. There was stronger similarity between cangas and quartzites, separated from granitoids. Soil was nutrient-poor, and variables most influencing this pattern were number of substrates, topographic heterogeneity, soil depth, and aluminum saturation. Saxicolous plant communities responded more strongly to microtopographic than soil fertility parameters. Each lithotype contributes differently to the high alpha- and especially beta-diversity within the Atlantic Rainforest matrix.