This study provides a preliminary phylogeny of the Celtis species from the neotropical region with emphasis on the South American species. We analyzed 19 taxa using the plastid psbA-trnH and nuclear ITS 4-5, and FA16180b markers. The sequence data were analyzed using maximum parsimony, maximum likelihood, and Bayesian inference. The South American species were highly supported as monophyletic, while the North American species were recovered as paraphyletic. The endocarp morphology corroborated the lineages within the molecular phylogeny and helps to differentiate the species, to recognize a new species Celtis serratissima, and to reestablish two previously known species, Celtis clausseniana (Wedd.) Miq. and Celtis spinosissima (Wedd.) Miq. These three species emerged among one of the three lineages of the tropical South American species. Celtis serratissima is thus described, illustrated, and compared to its most closely related species.

The genus Celtis L. (Cannabaceae) comprises around 73 species distributed in temperate and tropical regions of both hemispheres. Most South American species belong to the subgenus Mertensia Planch., and they are characterized by showing high morphological plasticity. A recent taxonomic revision of this group was carried out with an excessively reductionist criterion regarding the number of accepted species and that does not clearly reflect the morphological differences among them. In order to contribute to the correct delimitation of the specific entities, a morpho-descriptive approach is presented for ten taxa of the genus Celtis (C. brasiliensis, C. chichape, C. clausseniana, C. fluminensis, C. pallida var. pallida, C pallida var. discolor, C. serratissima, C. spinosa, C. spinosissima and C. tala) growing in South American Southern Cone. The morphology was analyzed through the use of a stereomicroscope and/or the tpsDig program for digital images of specimens. As a result, a key is provided for the determination of the aforementioned species and a detailed morphological description of the vegetative (habit, bark, branches, spines, brachyblasts, indumentum and leaves) and reproductive structures (inflorescences, flowers, fruits and endocarp -pyrene-). In addition, a specimen collection guide is proposed with a list of the main morphological characters that should be observed in the field, to facilitate future identifications.

El género Celtis L. (Celtidaceae) comprende unos 70 taxones de árboles o arbustos de regiones templadas y tropicales. En la Argentina habitan cuatro especies, todas pertenecientes al subgénero Mertensia Planch., cuya similitud y variabilidad inter e intraespecíficas han generado confusiones taxonómicas dentro del grupo. Un caso emblemático es el de C. pallida Torr. y C. tala Gillies ex Planch., considerados sinónimos de C. ehrenbergiana (Klotzsch) Liebm. Como estas entidades son claramente diferenciables a campo, se realizó la revisión taxonómica de dichos taxones. Tras un extenso muestreo, un estudio morfológico detallado y a través de un Análisis de Componentes Principales y de Conglomerados, se concluyó que lo considerado en Argentina como C. ehrenbergiana, abarca tres entidades: C. tala, C. pallida var. pallida y C. pallida var. discolor Hunz. & Dottori, y se propone la rehabilitación de dichos taxones. Asimismo, los resultados de los Análisis de la Varianza y de Similitud corroboran diferencias significativas (p-valor < 0.05) entre grupos. Se establecieron como caracteres de mayor importancia taxonómica: porte de los individuos, textura de corteza, pilosidad de láminas, nervaduras y pecíolos, espinas estipulares/ramas espinosas, abundancia y aspecto de domacios. Además, algunos especímenes de C. pallida son morfológicamente intermedios entre ambas variedades, por lo cual su identidad será esclarecida por medio del uso de herramientas moleculares.

In preparation for a new Neotropical taxonomic review and a global phylogeny focusing on the Neotropical lineages of Celtis, some nomenclatural changes are needed. Lectotypes are designated for Celtis alnifolia, C. asperula, C. brasiliensis, C. clausseniana, C. diffusa, C. morifolia, C. fluminensis, C. glycycarpa, C. hilariana, C. membranacea, C. orthacanthos, C. selloviana and Momisia integrifolia, and an epitype is designated for Celtis asperula. Celtis flavovenarum was proposed as a new name to replace the homonym C. morifolia. For C. spinosa we provide the locality of its type.