Studies of rock outcrops in tropical South America have increased in recent years, but they have often been restricted to individual countries, single biomes or single disciplines (e.g. through a floristic, functional or genetic lens), limiting their generality. We review the current state of knowledge on the geological and floristic diversity of rock outcrops in tropical South America to identify knowledge gaps and generate testable hypotheses for future research into the biogeography and evolution of their plant communities. We find that the diversity of lowland rock outcrop vegetation is disparately documented and we know less about the evolutionary and biogeographic history of these island-like systems. Based on geological and edaphic factors, we classify South American rock outcrops into four main groups: granite/gneiss, quartzite/metamorphosed sandstone, limestone and ironstone. We hypothesise that these lithologies influence the floristic and evolutionary lineage composition of outcrop floras. However, elevation also plays a role through creating microclimatic conditions and by influencing the degree of insularity from the surrounding vegetation. Our literature review suggests that these lithologies support different floras, but confirming this requires further floristic surveys across the full geological diversity of outcrops. We suggest a research framework to: (i) improve knowledge of outcrop floras and how they relate to floras of surrounding biomes; (ii) investigate the relative roles of niche conservatism and evolution using floristic and phylogenetic approaches; and (iii) assess how outcrop species cope with living in naturally fragmented habitats through analysis of recruitment and gene flow using population genetics. Understanding the biogeography of rock outcrop floras can help provide information for conservation planning and decisions.

Rock outcrops are in urgent need of study because they possess a distinctive and highly specialised flora that is threatened by climate and land-use changes.

There is a need to increase the number of studies in some Latin American countries (e.g. Bolivia, Paraguay, Guyana, French Guiana and Suriname) and lithologies (e.g. limestone).

We suggest that the major determinant of floristic composition on rock outcrops is lithology and outline how this can be tested using community-level floristic data.

Inferring the phylogenetic relationships of the species endemic to rock outcrops and establishing the environments where their closest relatives occur will be a powerful approach to address questions of niche evolution and niche conservatism in the historical assembly of outcrop floras.

Population genetic approaches focusing on plant species that occur both in outcrop habitats and the surrounding vegetation will elucidate their connectivity and, therefore, how insular and vulnerable these environments are.