Clusia chuj (Clusiaceae), a new tree species from the border between Guatemala and Mexico

Clusia chuj (Clusiaceae) is described and illustrated. This new taxon differs from the other species of the genus by being a tree up to 15 m tall with many-flowered inflorescences. The non-resiniferous, quadrangular flowers have thick, pale-yellow petals, and the androecium in the staminate flowers have 20 (– 30) free, short stamens. This new species is mainly found in mountain forests around Tziscao and Yolnabaj lakes near the border between Guatemala and Mexico.


Introduction
Clusia L. (Clusiaceae) includes almost 400 species of Neotropical woody plants that are ecologically important, particularly in montane forests in the Andes and Central America where they are often locally abundant and dominate the canopy of the tree-line between forests and high elevation grasslands (Gustafsson et al. 2007).The taxonomy of the group has proven to be challenging mainly because most species are dioecious and have been described based only on one sexual form.In addition, Clusia species show extensive homoplasy, particularly in their vegetative morphology, likely because they are evolutionarily closely related (Luján 2019).
As part of an ongoing taxonomic revision of the genus Clusia in Mexico and Central America, comparative morphological studies are being conducted using field collected plant material and herbarium collections.A significant number of herbarium specimens used for said taxonomic revision come from expeditions by Dennis E. Breedlove, who collected extensively in the state of Chiapas and other regions in Mexico and Guatemala during the 1960s and 1970s.His valuable plant collections, including over 72,000 plant and fungal specimens, have contributed to the ethnobotanical inventory of plants used by the Maya people (e.g.Breedlove & Laughlin 1993), as well as to many floristic and taxonomic studies (Daniel & Almeda 2012).The new species described in the present work is mainly based on a set of Breedlove specimens collected in the region around lake Tziscao, near the border between Guatemala and Mexico.

Materials and Methods
Herbarium specimens were studied at CAS, MEXU, MO and XAL (acronyms follow Thiers, continuously updated) using dissecting stereoscopes.The unified species concept from de Queiroz (2007), in which species are interpreted as separate evolving lineages based on their defining properties, was utilised.In this case, phenotypic diagnosability and geographic distribution were used as lines of evidence for species recognition.Morphological characters, including flower colours, were gathered from herbarium specimens and data registered on their labels.Terminology from Beentje (2016) was used to describe morphological characters.Species conservation status was assessed following the IUCN Red List categories and criteria (IUCN 2019).The GeoCat tool (Bachman et al. 2011), with a cell width of 2 km, was used to estimate extent of occurrence (EOO) and area of occupancy (AOO).A distribution map for the species was generated based on specimen locality information and using QGIS software (QGIS Development Team 2023).
RECOGNITION.Clusia chuj is similar to C. guatemalensis Hemsl.but it differs in usually having smaller lamina (5 -7.5 vs 8 -11.5 cm long) with obtuse to acute apex (vs acute to acuminate), resin lines in a more acute angle to the midvein than the secondary veins (vs in a similar angle than the secondary veins), and fewer stamens (20 [-30] vs >50).Clusia chuj resembles taxa from Costa Rica and Panama including C. dukei Maguire from which it can be separated by its free-standing tree habit (vs mostly hemiepiphytic), lamina with obtuse to acute apex (vs strongly acuminate), secondary veins 1 -2 mm apart (vs 2 -5 mm apart), and C. torresii Standl.from which it differs by having mostly continuous resin lines (vs reticulate), stamens 20 [-30] (vs 65 -80), and smaller fruits and seeds.Clusia chuj is also similar to C. gracilis Standl., it differs in having a free-standing tree habit (vs hemiepiphytic), leaves with obtuse to acute apex (vs acuminate), evident secondary veins (vs inconspicuous), relatively more floriferous inflorescences with shorter pedicels and light-yellow petals (vs brownish-red).
In general, Clusia chuj can be separated from most allied Central American taxa by having inflorescences with relatively more flowers, staminate flowers with typically 20 stamens (vs >30 in most taxa), and ovoid fruits (vs ellipsoid).The species is reported as a (3) 6 -15 m tree, whereas almost all similar taxa are usually hemiepiphytic shrubs or smaller trees.Future study of morphological characters from the pistillate flowers that are missing in this description may contribute to delimiting these taxa more clearly.DISTRIBUTION.Most collections come from the area around Lake Tziscao (Chiapas, Mexico) and Lake Yolnabaj ([Yolnajab], Huehuetenango, Guatemala).Outside this area, Clusia chuj has been collected in Cerro de La Santa Cruz, in San Cristóbal de Las Casas, in the Sierra Madre de Chiapas and near Laguna El Suspiro in Southern Mexico close to the border with Guatemala (Map 1).HABITAT.Clusia chuj mostly occurs in montane forests associated with Liquidambar, Magnolia, Pinus, Quercus and Vochysia, between 1000 -2400 m in the Sierra Madre de Chiapas and South of the Chiapas Central Plateau.One collection comes from primary lowland wet forest at 860 m.This species is apparently common near bodies of water as most specimens come from collections made around lakes in Guatemala and Mexico.CONSERVATION STATUS.For Clusia chuj the estimated extent of occurrence (EOO) is 11,633.920km 2 and the area of occupancy (AOO) is 28,000 km 2 .Most of the specimens were collected in Lagunas de Montebello which is a UNESCO Biosphere Reserve, and near Laguna El Suspiro in Montes Azules Biosphere Reserve.These plant populations may remain stable in the future as they are within protected areas.The additional three known localities for the species, which include a forested hill in the town of San Cristóbal de las Casas, a ridge above the town of Siltepec in Mexico, and around Yolnajab lake in Guatemala, are all near urban centres and touristic developments, therefore highly vulnerable to habitat transformation.Given the reduced number of known localities and their likely decline in quality of habitat, we assess the conservation status for C. chuj as vulnerable (VU) based on the IUCN criteria B1ab(iii).ETYMOLOGY.This species is named after the Chuj people, an indigenous ethnic group primarily located in the highlands across the border between Guatemala and Mexico.The Chuj people have a long history that predates the arrival of the Spanish colonisers in the Americas, however they have faced various challenges, including limited access to education and healthcare, poverty, and discrimination Clusia chuj is described from collections mostly made by Breedlove in the 1970s which have been stored in herbaria for over 50 years.This clearly illustrates the critical importance of natural history collections as repositories of untapped biodiversity information.In addition, study of plant collections in most local herbaria in Guatemala, Mexico as well as in international institutions, demonstrateed that C. chuj has not been re-collected in decades.Furthermore, no recent plant specimens from the lake Tziscao (Chiapas, Mexico) or lake Yolnabaj (Huehuetenango, Guatemala) regions have been located in any of the visited herbaria or queried databases, which suggests that fieldwork campaigns and plant collecting efforts have declined in these localities.As suggested by Wilson (2017), botanical field exploration should be promoted and supported so that we can make progress towards completing the plant species inventory in these highly diverse tropical regions and inform conservation efforts that are so urgently needed in the current context of global biodiversity loss.
Map 1. Distribution map of Clusia chuj from all known herbarium specimens.

Fig. 1 .
Fig. 1.Clusia chuj.A petiole base; B habit of flowering staminate branch; C -D details of the androecium of a staminate flower bud; E immature fruit; F mature fruit; G sepal side view and adaxial side; H petal side view and adaxial side; J flower bud; K inflorescence; L detail of lamina showing secondary veins and resin lines.From D. E. Breedlove & A. R. Smith 32247 (CAs [330947]), except e -F which are from D. E. Breedlove 14972 (♀) (CAs [472310]).drawn by juliet beentje.
. The Chuj people continue to maintain their cultural identity, and work towards the preservation and promotion of their ancestral traditions (Limón Aguirre 2008).NOTES.The presence of a non-resiniferous quadrangular androphore bearing multiple free stamens indicates that Clusia chuj is part of the "Flava group" an informal assemblage of Central American species that includes C. dukei, C. flava Jacq., C. guatemalensis, C. gracilis, C. lundellii Standl., C. quadrangula Bartlett, C. torresii (Hammel 1986), and likely other undescribed species.