Flora of disjunct savanna formations in the Brazilian semiarid unveils biogeographical complexities in the Cerrado–Caatinga relationship

Abstract The semiarid landscapes of Northeastern Brazil host disjunct savanna formations, often broadly categorized under the term ‘northeastern cerrados’, whose biogeographical connections with the core Cerrado and Caatinga domains remain debated. While savanna formations on sedimentary basins have been extensively characterized, those established on crystalline substrates remain poorly understood. This study documented the angiosperm flora of a granitic-based savanna on the Serra do Barriga inselberg (Sobral, Ceará, Brazil) and assessed its phytogeographical identity using a comprehensive multivariate suite (UPGMA, NMDS, PCoA, PERMANOVA, PERMADISP, and NODF nestedness), comparing the site with the other 29 areas. We recorded 156 species, with Fabaceae as the most representative family and a predominance of therophytes. Although the area exhibits a typical savanna physiognomy featuring ‘oligarchic’ woody species (e.g., Tabebuia aurea ), statistical analyses consistently positioned the site within the Crystalline Caatinga block. Furthermore, the overall biogeographic results revealed a pronounced species turnover, highlighted by an anti-nestedness system (NODF = 11.56, p < 0.01) among the analyzed areas, and demonstrated that the expression ‘northeastern cerrados’ is an inaccurate generalization, as these patches function as heterogeneous transitional mosaics rather than a cohesive floristic group. For the Serra do Barriga vegetation, the decoupling of structure and composition indicates that the shallow crystalline substrate acts as a primary environmental filter selecting for Caatinga taxa, while local ‘run-off’ mechanisms allow the persistence of savanna elements. Given its distinct identity and role as a refuge for threatened taxa (e.g., Bacopa cochlearia , Amburana cearensis ), the creation of a Natural Monument is recommended to protect this singular ecosystem.