On Hydrangea peruviana, an endangered species from Ecuador, and Hydrangea oerstedii, very common in Costa Rica and Panama, and seven threatened Central and South American Hydrangeas, which have been confounded with these

Abstract

Hydrangea section Cornidia, currently consisting of 19 accepted taxa, occurs from Mexico to Chile and Argentina, with one species in southeast Asia. Its representatives are root-climbing lianas which may grow up to 60 m high in the tree canopy of temperate to (sub)tropical forests. Our extensive field work throughout its distribution area, study of herbarium specimens and ongoing molecular studies have resulted in the discovery of species new to science, as well as new insights into the circumscription of many taxa. We here present amended descriptions for seven Hydrangea species of Central and South America and discuss the taxonomical situation of two Colombian Hydrangeas, including an identification key, illustrations, and distribution maps. Field work was carried out in Costa Rica, Panama, Ecuador and Peru, including exploration in areas where the genus had not been collected before. These specimens and observations were complemented with the study of specimens of 41 herbaria of North, Central and South America, as well as Europe. Detailed morphological studies of all species were carried out, based on living plants in their natural habitat, as well as on dried specimens from our own collections and all available herbarium material. Type material was studied in detail for all species concerned. Based on an extensive number of morphological characters, combined with distribution patterns, phenological differences and ecological preferences, including molecular data in most cases, Hydrangea peruviana and H. oerstedii are clearly distinct taxa, as well as the other seven species mentioned here, which had been synonymized with either of these two species. The present study results in the recognition of 26 species in section Cornidia and exemplifies the urgent need for profound taxonomic studies in plants, as in many families we do not dispose of well-circumscribed units for conservation to mitigate the already occurring unprecedented loss of biodiversity.

Molecular Phylogeny, Character Evolution, and Biogeography of Hydrangea Section Cornidia, Hydrangeaceae

“…Subsection Monosegia is characterized by a single pseudoumbellate cyme and its series by the presence or absence of "sterile flowers, " respectively. However, it should be noted that the latter flowers are not sterile but have enlarged sepals, and hence, we prefer the term enlarged "marginal flowers, " following Samain et al (2014Samain et al ( , 2021. Subsection Polysegia, according to Briquet (1919), is recognized by the thyrsoid inflorescence, consisting of several pseudo-umbellate cymes, and its series, among others, by short stamens and pubescent leaves or long stamens and glabrous leaves.…”

“…The focal group was represented by 89 individuals corresponding to 21 currently recognized species (Samain et al, 2014(Samain et al, , 2019(Samain et al, , 2021, 11 species that will be recognized (pending publication of ongoing taxonomical studies), and six or seven undescribed taxa, representing about 60-70% of the expected total species diversity in this clade (Supplementary Material 1). Our sampling within Cornidia includes the most comprehensive taxonomical, morphological, and geographical representation of this group to date.…”

“…As a result of our extensive field and herbarium studies, several new Cornidia species were discovered or existing species circumscriptions were amended (Samain et al, 2014(Samain et al, , 2019(Samain et al, , 2021. Our molecular phylogenetic hypothesis allows us to test the monophyly of many morphologically defined taxonomic entities for which more than one accession could be included (Figure 2).…”

“…Most of the species grow at higher elevations (1,000-3,000 m) at tropical and subtropical latitudes, favoring temperate climates, generally occurring in the cloud forest or in the ecotone between cloud forest and tropical rainforest. This section consists of 26 currently accepted species, as well as a yet undefined number of potentially new species and taxa that have been erroneously synonymized (Samain et al, 2014(Samain et al, , 2019(Samain et al, , 2021Samain and Martínez Salas, 2015). All taxa of Hydrangea section Cornidia (hereafter shortened as Cornidia) are evergreen root climbers growing up to 60 m high in the canopy of mostly primary forests, or rarely on boulders and rock walls (Granados Mendoza et al, 2014), generally functionally dioecious, rarely monoecious, with opposite coriaceous leaves and hortensia-like whitish, greenish, yellowish, reddish, or purplish inflorescences, with or without enlarged marginal flowers (Samain et al, 2019(Samain et al, , 2021.…”

“…Given that the majority of the species of Cornidia are threatened by extinction due to habitat destruction and climate change (Samain et al, 2014(Samain et al, , 2019(Samain et al, , 2021, the delimitation of units for conservation and identification of their specific threats are urgent. We have recently described seven new species for Mexico (Samain et al, 2014(Samain et al, , 2019, and the ongoing study of Hydrangeas of Central and South America includes both the description of species new to science and the preparation of amended descriptions of taxa, which do fit within earlier described species that had been erroneously synonymized (Samain et al, 2021).…”