Clusia falcata (Clusiaceae), an endangered species with exceptionally narrow leaves endemic to Chiapas, Mexico

Clusia falcata (Clusiaceae), a new species from Chiapas, Mexico, is described and illustrated. This new taxon is distinctive for its leaves, among the narrowest of all known species, having lanceolate to oblong laminas that are mostly 10 – 15 × 2 – 2.5 cm. The flowers have a quadrangular perianth with succulent petals, and a non-resiniferous androecium and gynoecium. Staminate flowers have a quadrangular androphore bearing 18 – 24 short stamens. Pistillate flowers have 4 pairs of staminodia, with each pair born on an androphore. The species is endemic to montane forests of the Northern and Eastern highlands of Chiapas in Mexico and is currently considered to be endangered.


Introduction
Clusia L. (Clusiaceae) comprises 300 -400 species of hemiepiphytic or terrestrial shrubs and trees distributed from Mexico and the Greater Antilles to Bolivia and southeastern Brazil (Gustafsson et al. 2007). According to molecular evidence the genus is a strongly-supported monophyletic group (Gustafsson & Bittrich 2002;Gustafsson et al. 2007;Luján 2019). One consistent morphological synapomorphy for the genus is the presence of a two-to many-layered hypodermis in the leaves (Vesque 1892;Luján pers. obs.). Other characters that are common across Clusia species and that help distinguish it from related genera are fruits with more than one seed per carpel, seed length ≤ 5 mm and non-vascularised seed aril (Gustafsson et al. 2007).
At least 13 species of Clusia have been reported from Mexico, most of them distributed in montane forests in the southern states of Chiapas, Oaxaca and Veracruz (Tellez et al. 2020). The most widespread species in the country, C. salvinii Donn.Sm. is common in mid-to high-elevation cloud forests and has been collected up to the northwestern state of Sinaloa (Rzedowski & Zamudio 2001). The range of C. salvinii probably represents the latitudinal northern extreme of the natural distribution of the genus in continental America, except for some populations of C. rosea Jacq. in southern Florida, although the range of the latter species is likely influenced by its common cultivation and subsequent naturalisation. Before this study, two species of Clusia were considered endemic to Mexico, namely C. pringlei Lundell, distributed in mid-elevation montane forests of the trans-Mexican volcanic belt and Sierra Madre del Sur, and C. tetratrianthera Maguire, which occurs in low to mid-elevation forests in central Chiapas and Veracruz (Maguire 1979).
As part of an ongoing taxonomic revision of Clusia for Mexico, a set of specimens were identified as representatives of a distinct yet undescribed taxon. Here we provide a description for this new species along with a discussion of its morphological affinities to other taxa in the genus.
RECOGNITION. Clusia falcata can be readily separated from its congeners by its narrow lanceolate to oblong leaves (8 -) 10 -15.5 × (1.4 -) 2 -2.5 cm, with a ratio of leaf width to length of 0.17, which are likely the narrowest leaves observed in the genus thus far. Certain material of C. dukei Maguirea species restricted to lowland wet forests on the Atlantic slopes of Costa Rica and Panamalikewise with very narrow and sometimes falcate leaves, has been indicated as worthy of taxonomic recognition (see Hammel 2010). That material has laminas 9.6 -15 × 1.4 -3 cm (leaf width to length ratio of 0.18), flowers with similarly 4merous perianth and succulent petals. That form of C. dukei differs from C. falcata by its more numerous secondary veins at a wider angle to the primary vein (12 -15 pairs, 2 -3 (-5) mm apart, at 30°-40°a ngle vs 5 -8 pairs, 5 -10 mm apart, at 20°-30°a ngle); and by its androecium with more stamens (32 vs 22 -24 in C. falcata). Clusia dukei overall differs by its clear (vs greenish-white) resin, and by its very different configuration of the resin canals, which are relatively short (interrupted) and widely branched lines that sometimes appear to form a mesh, vs the continuous and usually unbranched lines of C. falcata, more like those of C. flava Jacq. and C. guatemalensis Hemsl. These latter two species have flowers with a similar perianth, androecium and gynoecium, but usually with a higher number of stamens, staminodia, and stigmas than in C. falcata. Most notably, their leaves are never as long and narrow as those of C. falcata.  HABITAT. Clusia falcata grows on forested slopes with Pinus, Quercus and Liquidambar, and on karst areas in tropical rain forests at 950 -1700 m. CONSERVATION STATUS. We estimate an extent of occurrence (EOO) of 1,569.623 km 2 and an area of occupancy (AOO) of 12.0 km 2 for Clusia falcata. The species is known from only three localities, and continuous decline in habitat quality is inferred in at least two of them given their proximity to urban centres (the towns of Jitotol and Yajalón respectively). Only one location is within a protected area, the Reserva de Biosfera Montes Azules, which may ensure the long-term stability of that particular plant population. Given the reduced number of known localities and their likely future instability, we assess the conservation status for C. falcata as endangered (EN) based on the IUCN criteria B1ab(iii)+2ab(iii). PHENOLOGY. Clusia falcata was collected with flowers in September and October, and with fruits in August. ETYMOLOGY. The specific epithet of Clusia falcata derives from the latin word falcatus, which describes the narrow and curved shape of the leaves, a character not previously observed in the genus. NOTES. With the aim of identifying the phylogenetic position of Clusia falcata, sequencing of nrITS was attempted using tissue from the type specimen, although amplification was unsuccessful, probably because the DNA was highly degraded, which is common in tissues sampled from herbarium specimens. Nevertheless, the presence of a non-resiniferous quadrangular androphore bearing multiple free stamens suggests that C. falcata is included in the "Clusia flava group" an informal infrageneric species assembly that includes C. dukei, C. flava, C. guatemalensis, Map 1. Distribution map of Clusia falcata in Chiapas, Mexico.
C. lundellii Standl., C. quadrangula Bartlett, C. torresii Standl. and likely other closely related species from Central America with similar androecial morphology (Hammel 1986). According to molecular evidence, the flava group is a well-supported monophyletic group (Gustafsson et al. 2007;Luján 2019) in which hemiepiphytic habit as well as CAM photosynthesis are commonly present (Vargas-Soto et al. 2009), and includes most, although not all taxa originally proposed by Hammel (1986).
A general pattern observed in evergreen woody plants is that relatively small leaves are more common at high latitudes, high elevations, nutrient poor, and hot arid conditions (Givnish 1987;Wright et al. 2017). Moreover, in some groups (e. g. subtropical bamboos), species with relatively narrower leaves tend to inhabit areas subject to water deficiency (Lin et al. 2020). The exceptionally narrow leaves observed in Clusia falcata suggest that this species may tolerate conditions of limited water availability. Furthermore, one specimen of C. falcata (R. L. Dressler 1689 [MEXU]) was collected on a limestone area, which indicates that the species may tolerate soils with high alkalinity and low nutrient levels. Several environmental and genetic factors are involved in determining leaf size and shape, potentially leading to many equally viable leaf strategies for a given environment (Wright et al. 2017). Further comparative research is needed to better understand the drivers of leaf size and shape variability in Clusia, and to test whether C. falcata is adapted to particularly harsh environmental conditions. ment. Juliet Beentje prepared a wonderful botanical illustration. Two anonymous reviewers provided valuable comments and suggestions which greatly improved this manuscript.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/ licenses/by/4.0/.