Two new endemic tree species from Puerto Rico: Pisonia horneae and Pisonia roqueae (Nyctaginaceae)

Abstract In this paper, we describe two endemic tree species of Pisonia (Caryophyllales: Nyctaginaceae) from Puerto Rico that were erroneously catalogued under the single name Pisonia subcordata var. typica f. gigantophylla, misidentified as P. albida or P. subcordata, and informally named as “P. borinquena” and “P. woodburyana”. The species here named as P. horneae is a rare to locally occasional tree from low elevations in the Northern Karst and the Sierra de Cayey. The other species, here named as P. roqueae, is a rare to locally common tree from mid to high elevations in the Central Mountain Range and the Luquillo Mountains. We provide an account of the taxonomical and nomenclatural history of both species, images, conservation notes, a distribution map, and a key to distinguish the species of Pisonia present in Puerto Rico.


Introduction
The Caribbean Islands are considered one of the hottest hotspots of world biodiversity (Myers et al. 2000) with a richness that has not been fully documented yet, as evidenced by the dozens of new species described from the region every year (Hedges and Conn 2012, Corgosinho and Schizas 2013, Cardoso and Braga 2015, Aguirre-Santoro et al. 2016, Carmenate-Reyes et al. 2017. For example, Puerto Rico -the smallest of the Greater Antilles -is probably the island with the best documented flora of the region with about 2331 species of native plants (Gann et al. 2015(Gann et al. -2017. In spite of the intensive botanical work conducted in the island for more than a century, new species and rediscoveries of plants are still being documented (e.g., in the last decade: Trejo-Torres 2008, 2009, Vega et al. 2010, Caraballo-Ortiz 2013, Bornstein et al. 2014, Trejo-Torres et al. 2014).

An uncertain name used for a Pisonia
Pisonia subcordata Swartz var. typica Heimerl f. gigantophylla Heimerl (Caryophyllales: Nyctaginaceae) was described by Heimerl (1896) using a set of sterile specimens gathered by Paul E. E. Sintenis from at least two localities in central Puerto Rico. Nine years later, Heimerl and Urban (1905) synonymized this name under Pisonia subcordata var. typica Heimerl, as they considered the type collections to be juvenile parts of this species. In 1918, Standley reclassified P. subcordata var. typica f. gigantophylla as a synonym of P. albida (Heimerl) Britton ex Standley (Standley 1918). Coincidently, in 1980 Westra examined and annotated the duplicates of specimen Sintenis 2705 (B, K) and considered them as a mixed collection containing parts of P. albida and P. subcordata Swartz. After examining the type material of the abovementioned taxa, additional specimens, and living plants, we have concluded that the three sets of specimens used to describe P. subcordata var. typica f. gigantophylla (Sintenis 2141, 2705, and4355 [sic. 4335]) comprise two taxa, which however, do not match any of the currently recognized Pisonia species from Puerto Rico or the Caribbean. While the two duplicates of Sintenis 2141 (GH, K) represent the same taxon, at least two of the three duplicates of Sintenis 2705 (B, K, and possibly GH) are mixed and include the same taxon as Sintenis 2141 as well as a second taxon. Although these specimens clearly represent two taxa, their sterile and fragmented nature, as well as the scant protologue, brings uncertainties to assign identities to each of the pieces. Moreover, Sintenis 4355 has not been located yet, and we lack information on its identity or locality.
Our analysis of living and dried specimens shows that the two species of Pisonia recognized by Proctor represent the same two taxa involved in the description of P. subcordata var. typica f. gigantophylla. Therefore, our objective in this paper is to clarify the identities and assign new names for these two species of Pisonia. For each species, we provide descriptions, illustrations, and comments on their abundance, natural history and conservation. In addition, we provide a distribution map and a dichotomous artificial key for the six species of Pisonia reported for Puerto Rico.

Materials and methods
Information on living plants and natural history was obtained from field observations and cultivated plants growing at the nursery of the Conservation Trust of Puerto Rico at San Juan. Species descriptions and measurements were obtained from field notes and dried herbarium specimens. Diagnosis. Pisonia horneae is distinguished from congeners from Puerto Rico by a combination of the following characters: leaves membranaceous and puberulous, twigs puberulous, fruits elliptic-oblanceolate and grayish with 10 rows of viscid glands along their whole length.
Habitat and ecology. Pisonia horneae is found in moist limestone forests at low elevations in the Northern Karst of Puerto Rico (10-150 m; Fig. 3), frequently on slopes and forest edges with relatively undisturbed vegetation. In eastern Puerto Rico, the species have been recorded from volcanic moist substrates from approximately 10 to 300 m (Fig. 3).
Although P. horneae is a species from low elevations and can grow near the coast, it has not been observed as part of the coastal vegetation. In contrast, the congener P. subcordata is distributed on coastal zones directly exposed to sea spray on elevations typically less than 10 m.
Phenology. Pisonia horneae sheds leaves during March and April. After shedding, the plant usually produces flowers by April, though flowers have been also recorded in January. Fruits have been recorded in May and June.
Vernacular names. As with other tree species of Pisonia from Puerto Rico, P. horneae is locally known as corcho.
Etymology. We are honored to dedicate this species to Frances Elvira Worth Horne , an American illustrator who lived in Puerto Rico and, for 45 years, painted its plants and birds. Ms. Horne composed 750 watercolors of Puerto Rican plants to complement Nathaniel Lord Britton's Flora Borinqueña (Jackson 1997), a popular book on common plants of Puerto Rico which was never published. Only 44 of her plant watercolors were published in the botanical journal Addisonia from 1922 to 1932 (Jackson, 1997), while 48 were included in the book Common Trees of Puerto Rico and the Virgin Islands (Little et al. 1977). She donated her collection of plant watercolors to The New York Botanical Garden in 1963 (Jackson 1997), which can be consulted at http://plants.jstor.org [accessed 02.08.2017]).
Abundance and conservation. Pisonia horneae can be considered a rare to locally occasional species. Despite its broad distributional range along the Northern Karst of Puerto Rico, trees of P. horneae are usually found isolated or forming small groups. The only localities where we have observed the species as locally common include a few karstic ravines in the municipality of Quebradillas and some limestone hills with relatively undisturbed forest in Ciales (Bo. Hato Viejo) and Dorado (Bo. Rio Lajas). Pisonia horneae seems to be extremely rare outside the Northern Karst, as the species is only known from three scattered localities in eastern Puerto Rico in the municipalities of Rio Grande, San Lorenzo, and Yabucoa. In Rio Grande, P. horneae seems to be uncommon and is only known from a single specimen (described below) and an unvouchered observation of a solitary sapling made in 2006 by biologist J. A. Sustache from the SJ Herbarium (pers. comm.). The authors visited the historical locality of Cerro Gregorio in San Lorenzo in December 2016, but no Pisonia were located. The locality in Yabucoa contained a single individual; however, it was fruiting and multiple seedlings were observed around the tree, suggesting the presence of additional individuals at the site (see Discussion section). Pisonia horneae is currently designated as a critical element of the flora of Puerto Rico by the Department of Natural and Environmental Resources of The Commonwealth of Puerto Rico (listed as P. woodburyana) (DNER 2008). It is also preliminary considered as an "Imperiled" (sensu NatureServe) or "Vulnerable" (sensu IUCN Red List) species by Gann et al. (2015Gann et al. ( -2017.    Diagnosis. Pisonia roqueae is distinguished from congeners from Puerto Rico by a combination of the following characters: staminate inflorescences with globose crowns, twigs glabrescent, leaves coriaceous and glabrescent, and fruits clavate and reddishblack with five rows of viscid glands on their distal half. Description. Trees dioecious up to 25 m high with trunks up to 1 m in diameter. Bark finely and vertically striated, grayish with lenticels about 3 mm in diameter. Twigs ancipitous, ferruginous-brownish, and pubescent when young; terete, glabrescent, and grayish when old. Leaves clustered towards the ends of branches, opposite, subopposite, or sub-verticillate, decussate; petioles up to 3.5 cm long, yellowish green; blades elliptic, obovate, or roundish, 10-17 (-25) cm × 7-12 (-16) cm, apex acute to rounded, sometimes cuspidate, base acute to cuneate, margin entire or slightly wavy, adaxial and abaxial sides glabrous, coriaceous, drying coriaceous, dark green above, slightly paler below; veins pinnate, reticulate, up to eleven pairs, opposite or alternate, blackish. Inflorescences terminal or axillary, dendroid, to 7 cm long; crowns compact; staminate inflorescences with a globose crown; pistillate inflorescences with a flabellate crown. Flowers fragrant; pistillate flowers with perianth cylindrical-campanulate-oblong at anthesis, 2 mm long, puberulent; staminate flowers perianth campanulate at anthesis, green, about 3 mm long, puberulent. Infructescences dendroid, 4-7 cm long, drying reddish or brownish; peduncle angled, 2-3 cm long; branches irregularly forked, with minute bracteoles around the base of anthocarps. Fruits anthocarps (achenes), clavate, 10-15 mm x 2 mm, longitudinally striate, tip cuspidate; husk glabrous, with five lines of glands over discrete ribs along the distal third or half; glands capitate, about 0.5 mm long, viscid.
Habitat and ecology. Pisonia roqueae is mainly distributed from middle to high elevations. In the Central Mountain Range and Sierra de Luquillo, the species have been found on wet and moist serpentine or volcanic forests from 480 to 950 m. The only known locality from the Northern Karst is on a wet limestone forest at ca. 330 m.
Unlike other species of Pisonia from Puerto Rico, adult trees of P. roqueae can reach considerable heights (> 15 m). Some large trees can be found at the Maricao State Forest and at the Luquillo Mountains in EL Verde Field Station, including the Luquillo Forest Dynamics Plot (http://luq.lternet.edu), where the species (treated as either "P. borinquena" or P. subcordata) has been included in long-term studies (e.g., Kress et al. 2010).
Phenology. Pisonia roqueae has been recorded flowering in April, June, and July, and fruiting from January to April.
Vernacular names. As with other tree species of Pisonia from Puerto Rico, P. roqueae is locally known as corcho or corcho blanco.
Etymology. It is our honor to name Pisonia roqueae after Dr. Ana Cristina Roqué Geigel de Duprey , an amateur ethnobotanist from Puerto Rico who dedicated over three decades of her life to prepare the bilingual manuscript "Botánica Antillana: Introduction to the study of the picturesque flora of Porto Rico and West Indies", aimed to make botany accessible to the general public. Her manuscript (Roqué de Duprey 1925) was never published and remained in oblivion to the botanical community until recently when JCTT and collaborators divulged its existence (Martínez 2015). Roqué de Duprey is mostly known for being an educator, writer, suffragist, and one of the founders of the University of Puerto Rico-Mayagüez, among other educational institutions.
Abundance and conservation. Pisonia roqueae has been observed as a locally common tree in two localities in the eastern and western Central Mountain Range (Monte La Torrecilla in Barranquitas and Maricao State Forest, respectively), occasional at the El Yunque   (Proctor et al. 48064,US [00707324]). Note the dendroid staminate inflorescences with globose crowns and the leafless branches during the flowering period. Photo courtesy of The United States National Herbarium, Smithsonian Institution. National Forest (Luquillo Mountains), and rare elsewhere. It has been also recorded from Cayey (including an unvouchered locality at Bo. Jájome, 300-350 m, 18 Feb 2015, by O. Monzón), Guayama at Sierra de Cayey, and a single record from the Northern Karst distributed in the moist limestone forests of the Northern Karst and the eastern Central Mountain Range; P. roqueae is a species restricted to wet forests with elevations above 300 m along the Central Mountain Range and the Luquillo Mountains; P. subcordata, a tree distributed throughout the West Indies, is mainly found on coastal forests and thickets along the northern and eastern coast; and P. taina Trejo, an endemic and rare tree from Puerto Rico, has a scattered distribution across the central and western parts of the island (Trejo-Torres 2005). The distributional ranges of P. albida, P. horneae, P. roqueae, and P. subcordata seems to be consistent with specific substrate types and/or environmental conditions. However, we still lack scientific studies exploring possible biotic or abiotic factors shaping these distributions. A possible explanation is the existence of species-specific interactions with ectomycorrhizal fungi, which can help species survive in particular abiotic conditions (Van der Putten et al. 2010, Pickles et al. 2015 and have been found to interact with Pisonia and other members of the Pisonieae clade for at least 14 Myr (Hayward and Horton 2014). We also lack molecular studies to infer the phylogenetic relationships and species divergence times among Caribbean Pisonia.
The most reliable characteristics to set apart the six species of Pisonia from Puerto Rico are the morphology of inflorescences, flowers, and fruits along with leaf shape and size. However, Pisonia trees have a fugacious reproductive season and herbarium specimens are often found either sterile or fertile but lacking well-developed leaves (Fig. 6). As a result, many specimens are incomplete representations of the species, which has partially contributed to their misidentification (Trejo-Torres 2005). Therefore, when collecting Pisonia in the field we recommend to include, whenever possible, multiple vouchers from a single plant or population throughout seasons to properly document the range of phenological stages of plants, including fertile and sterile material. Pisonia fruits usually persist on branches or on the ground below trees during several months. Hence, we included a comparison of fruit morphology for the six species of Pisonia from Puerto Rico to assist with their identification (Fig. 4). Observational information may also assist with identification of the two-new species here described and we have noted that, unlike other Pisonia from Puerto Rico, fresh leaves of P. horneae are usually light green and young plants often have a purplish midvein. Likewise, leaves of P. roqueae are typically dark green and shiny.

Notes on reproduction and conservation
Most of the observed plants of P. horneae and P. roqueae were either isolated or forming small groups. Although both species have broad distributions within their respective habitats, most trees are restricted to ravine banks, cliffs, or rocky areas, especially for P. horneae. These observations might suggest that either trees tend to colonize these particular habitats due to physiological requirements, or that they represent relicts of a former, more continuous population that was severely fragmented during the intense deforestation period experienced in the island for the past centuries.
As with other Pisonia species, both P. horneae and P. roqueae are dioecious and their flowers might require pollen to set fruits (Douglas andManos 2007, Nores et al. 2015).
However, at least two species of Pisonia from Puerto Rico (P. taina (Trejo-Torres 2005) and P. subcordata (M.A. Vives-Heyliger, pers. comm.)) have been reported as rarely subandroecious, a condition that can help mitigate the impact of reproductive isolation by inbreeding. We currently lack information on reproductive biology for the vast majority of Pisonia species (Douglas and Manos 2007), but field observations made by one of the authors suggest that P. horneae and P. subcordata can achieve high fruit sets in sites where trees are locally abundant (MACO, pers. obs.). This suggests that Pisonia, as other dioecious tropical trees, can be particularly susceptible to reproductive failure from isolation and low population sizes (House 1992). Reproductive success of dioecious plants will depend on multiple factors such as the spatial distribution of trees, the male-female ratio within populations, and the foraging range of their effective pollinators (Lin et al. 2015). Thus, conservation efforts, besides protecting extant trees and natural areas where the species are present, should take into consideration increasing neighborhood densities by planting additional individuals near isolated trees, even though these actions would not warrant maintenance of genetic diversity and the long-term survival of the species.
To our knowledge, there are no reports of seed dispersal for any Neotropical Pisonia, which contrast with the Indo-Pacific region where seed dispersal has been well-documented, especially for P. grandis R. Br. and P. umbellifera (J.R. Forst. & G. Forst.) Seem. (e.g., Ridley 1930, Cleland 1952, Walker et al. 1991, Murphy and Legge 2003, Burger 2005. In 2005 after an intense fruiting season at one of the densest known populations of P. horneae at the municipality of Quebradillas in northwestern Puerto Rico, infructescences detached from branches when fruits were ripe, but they remained under their maternal trees. Subsequently, seedlings grew densely clustered and the vast majority of them died within the next few months ( Fig. 1D; MACO, pers. obs.). These observations, along with the low recruitment detected beyond parental trees, suggest that at least some populations of P. horneae might be experiencing low seed dispersal rates.
Key to the species of Pisonia from Puerto Rico