Grindelia rheophila (Asteraceae, Astereae): A new species and a river-borne exception in a genus shaped by Deserts

Grindelia rheophila (Asteraceae, Astereae), the first known rheophytic gumweed species is proposed, described, and illustrated here. Unlike most species of the genus, which are adapted to almost permanently dry environments, this new species thrives in dynamic riverine habitats with fast-flowing water. Grindelia rheophila is characterized by the erect to decumbent growth form, long linear-elliptic leaves, and a pappus with hemipaleaceous and bristle elements, which differentiate it from the morphologically similar G. buphthalmoides, G. pulchella and G. scorzonerifolia. The new species is known only at the northern headwaters of the Ibicuí river basin along the edge of the South Brazilian plateau in the state of Rio Grande do Sul, Brazil, where it grows along rocky flood-prone margins of the watercourse, as rapids and waterfalls are influenced by seasonal hydrological fluctuations. We highlight the vulnerability of rheophile ecosystems to anthropogenic threats, particularly dam construction and climate-driven extreme weather events, justifying the recognition of the new species as Critically Endangered (CR A2ac+3c+4ac; B1a,b(i,ii,iii,iv),c(iii)). A watercolor illustration, habitat photographs, a distribution map, and an identification key for Brazilian Grindelia are also provided.