A new species of Aetanthus (Loranthaceae) from Colombia with notes on A. engelsii

Abstract Murillo-Serna, J.S., F.J. Roldán-Palacio, I. Carmona-Gallego & F. Alzate (2021). A new species of Aetanthus (Loranthaceae) from Colombia with notes on A. engelsii. Candollea 76: 71–76. In English, English and Spanish abstracts. Aetanthus alternifolius F.J. Roldán, Carmona, Alzate & J.S. Murillo (Loranthaceae) is newly described and illustrated from the department of Santander in northeastern Colombia. This new mistletoe species, known only from the type locality, is discriminate by its indeterminate shoots, alternate or subopposite, obovate leaves, cymose inflorescences bearing three dyads, flower buds c. 9 cm long, with a finely toothed and sometimes fissured calyculus. Its morphological affinities are discussed especially in relation to A. colombianus A.C. Smith and A. coriaceus Patsch. The rediscovery of original material in P of the poorly known Phyllostephanus engelsii Tiegh. (≡ Aetanthus engelsii (Tiegh.) Engl.) is discussed. An emended description is provided and a lectotype is designated. An identification key to the ten species of Aetanthus (Eichler) Engl. occurring in Colombia is also presented.

Rediscovery of original material of Phyllostephanus engelsii Tiegh. ( Aetanthus engelsii (Tiegh.) Engl.) in P, allowed us to provide an emended description of this poorly known species not included in Kuijt (2014)'s monograph of the genus Aetanthus. Its taxonomic status is discussed and a lectotype is designated. A breakdown of diagnostic characters of the genus is also discussed and an identification key to the species known from Colombia is provided.
Etymology. -The epithet alternifolius refers to the phyllotaxy in mature shoots.
Distribution and habitat. -Aetanthus alternifolius is known only from a single specimen collected in the municipality of California, in montane oak (Quercus humboldtii Bonpl.) forests belonging to the surrounding area of the Páramo de Santurbán (Santander, Colombia) close to 2800 m. Host is unknown for this species.
Phenology. -This species was collected with several inflorescences in anthesis and flower buds at the end of January. Fruits remain unknown.
Conservation status. -Aetanthus alternifolius occurs in montane oak forests that are severely threatened by mining and agricultural activities (Avella & Rangel, 2014). Nevertheless, additional studies are required for this mistletoe, especially regarding its host range, distribution and ecology. We suggest considering this species as "Data Deficient" [DD], according to the IUCN Red List Categories and Criteria (IUCN, 2012), awaiting the results of new studies.
Notes. -Species of the genus Aetanthus display a series of diagnostic characters that are useful to determine taxa at species level. Some of those are: ramification pattern (percurrent or indeterminate vs. aborted terminally or determinate); phyllotaxy (opposite-decussate or whorled), number of dyads per inflorescence and flower length. Some features are unique among Aetanthus species, as a strikingly dentate calyculus in A. mutisii, sessile leaves in A. sessilifolius (Kuijt, 2014) and the alternate phyllotaxy observed in A. alternifolius.
Aetanthus alternifolius has morphological affinities with A. colombianus and A. coriaceus, displaying similarities in growth habit and flower length. However, these three species can be distinguished by their phyllotaxy, inflorescence structure and stem shape. Most relevant characters to discriminate the three species are presented in Table 1.
Aetanthus alternifolius occurs sympatrically in the Central Cordillera, along with three additional species of the genus, which can be differentiated by the following morphological characters: A. megaphyllus has dichotomous branching and greater sized leaves with stout leaf blades and petioles; A. validus exhibits a dichotomous ramification with 3-whorled leaves and A. mutisii has a percurrent habit, whorled phyllotaxy and flowers ranging 15-22 cm long with a strikingly dentate calyculus. Kuijt (2009Kuijt ( , 2014 reports that it is common that Loranthaceae taxa are known only from a few localities and usually, taxonomic novelties are published based on a single gathering. This may be due to habit-imposed difficulties and collectors' taxonomic interests (Cavallin et al., 2016). The precise geographic location of the type gathering is uncertain only refering to Colombia. The protologue was also very succinct just mentioning A. engelsii has 4-whorled leaves and tetratomic branching (Van Tieghem, 1895).