Populations of Carex sect. Rhynchocystis (Cyperaceae) from the Macaronesian archipelagos (Azores and Madeira) have traditionally been treated either as a variety of the widely distributed Western Palearctic C. pendula, or directly synonymized under it. However, recent phylogenetic studies have shown that Azorean populations of C. pendula display a certain degree of differentiation from mainland plants, while the phylogenetic relationships of Madeiran populations remain unclear. Here we perform an integrated systematic study focused on the Macaronesian populations of Carex sect. Rhynchocystis to elucidate their phylogenetic relationships and taxonomic status. We reconstructed a molecular phylogeny based on five DNA regions and conducted a multivariate morphological analysis. Divergence time estimates show that the Macaronesian populations can be traced back to a Plio-Pleistocene origin. Our results suggest that these island populations of C. pendula are better treated as two distinct species within Carex sect. Rhynchocystis (i.e., C. leviosa from the Azores and C. sequeirae from Madeira). We provide morphological characters to differentiate the new species from C. pendula s.str., detailed descriptions of the three taxa, a revised key for the entire section, as well as detailed analytical drawings of the two newly described species. We also perform a critical evaluation of the taxonomic diversity of Carex in the Azores and Madeira. Finally, we assessed the conservation status of the new species at a global scale under IUCN categories and criteria, resulting in the proposal of the categories “Least Concern” for C. leviosa and “Critically Endangered” for C. sequeirae

Phylogenetic Analyses—Samples of all species of Carex. sect. Rhynchocystis were used to representatively cover its range. We sampled the following populations (1 specimen per population): (1) six of C. agastachys; (2) five of C. bequaertii subsp. bequaertii and three of C. bequaertii subsp. mossii; (3) three of C. microcarpa; (4) eight of C. pendula; (5) one of C. penduliformis; (6) five of C. pendula from the Azores, treated by us as C. leviosa; (7) five of C. pendula from Madeira, treated by us as C. sequeirae; and (8) two species from each of the closely related Carex sect. Ceratocystis Dumort., Carex sect. Phacocystis Dumort., Carex sect. Sylvaticae Rouy, and Carex sect. Spirostachyae (Drejer) L.H.Bailey, as outgroups (Míguez et al. 2017; Appendix 1). We sequenced and analyzed three cDNA (i.e., atpIH, matK, and rpl32-trnL^UAG) and two nDNA (i.e., ETS and ITS) regions.

Macromorphological Study—The morphological study included a total of 75 herbarium specimens from 13 herbaria (E, BM, K, LISU, M, MADJ, MADM, MHA, P, SEV, TUM, UPOS, UPS; Appendix 2; abbreviations follow Thiers 2020). Our sampling was designed to explore the morphological variation within the monophyletic group formed by C. agastachys, C. leviosa, C. pendula and C. sequeirae.