A new species of Erythrostemon (Leguminosae, Caesalpinioideae) from the western Río Balsas Depression, Mexico

Abstract A new legume species from a seasonally dry forest of the Western Río Balsas Depression, in the states of Guerrero and Michoacán, Mexico, Erythrostemon guevarafeferii, is herein described and illustrated. The new species shows morphological affinities with Erythrostemon hintonii, from which it is distinguished in having fewer leaflets per pinna, mature leaflets disposed toward the upper half of the pinnae rachises, long inflorescences on curved slender peduncles, abundant red glands on its flowers and inflorescences, and its fruit glabrous with red stipitate glands at maturity. A taxonomic key to the Río Balsas Depression species of Erythrostemon is included.


Introduction
Erythrostemon was re-circumscribed by Gagnon et al. (2016). Th e neotropical genus currently includes a total of 31 species of woody shrubs or small to medium-sized trees. Species distributions follow a bicentric amphitropical distribution pattern in México, Central America and the Caribbean, Brazil, Argentina, Bolivia, Chile and Paraguay (Lewis 1998;Gagnon et al. 2013;Gagnon et al. 2016). Th ey grow in a wide range of habitats including seasonally dry tropical forests, caatinga vegetation, deserts, yungaspuna transition zones, and chaco-transition forests.
Species of Erythrostemon are distinctive by various combinations of several morphological traits (a reference for this cited here). Leafl ets typically are either eglandular or have conspicuous black sessile glands along their margin that renders the margins slightly crenulated. Flowers have sepals that are ovate-lanceolate and their petals bending and either yellow, red, pink or orange, the corolla sometimes laterally compressed. Th e androecium and gynoecium are free from the calyx and the ovary is eglandular or covered in gland-tipped trichomes. Legumes are typically oblong-elliptic pods with papery to slightly woody valves that are chartaceous or slightly woody, glabrous to pubescent, eglandular or with stipitate glands.
During a revision of Caesalpinia sensu lato in Guerrero and Michoacán, several specimens identifi ed in herbaria as Caesalpinia hintonii Sandwith (a species recently transferred to the genus Erythrostemon, Gagnon et al. 2016) proved to be morphologically distinct. Taking into account the above morphological diagnostic characters of Erythrostemon, a detailed study indicated that these specimens belong to that genus, although they do not match the description of any previously described species. Th e overall morphology of the new taxon most closely resembles Erythrostemon hintonii (Sandwith) E. Gagnon & G.P. Lewis, with which it shares a similar distribution area in the Río Balsas Depression. Th e new species is herein described as Erythrostemon guevarafeferii.
Habitat. Seasonally dry tropical forest on rocky slopes, locally common in secondary vegetation along roads, on alluvial soils near seasonal or permanent streams. IUCN Red List category. We recommend that Erythrostemon guevarafeferii be given a conservation assessment of Vulnerable [VU (B1b-iii)], in accordance with IUCN (2012) categories and criteria. Th e extent of occurrence (EOO) of E. guevarafeferii is estimated to be over 2424.18 km 2 , well below the 20,000 km 2 upper limit for Vulnerable status under criterion B1, but its area of occupancy (AOO) is estimated to be less than 10 km 2 (the limit for Endangered status under criterion B2). Th e species is currently known from three discontinuous populations in the states of Michoacán and Guerrero, these separated by diff ering habitat type, human settlement and agricultural land. Erythrostemon guevarafeferii is known in only one protected area, the Reserva de la Biosfera Zicuirán-Infi ernillo in Michoacán. Th e preferred habitat of the species is potentially threatened by future settlement and agricultural activities, as well as by environmental problems associated with drug traffi cking organizations.
Etymology. Th e species epithet is dedicated to Fernando Guevara Fefer who recently passed away. Friend, botany teacher and researcher at the Universidad Michoacana de San Nicolás de Hidalgo, he was interested in the genus Bursera and in fl oristic and vegetation studies within Michoacán, particularly in the Infi ernillo region where the type specimen was collected.

Discussion
Erythrostemon guevarafeferii diff ers from Erythrostemon hintonii in being glabrous at maturity and by sometimes having glands on fl oral structures and fruits. It has smaller leaves with (5-) 7-13 pinnae per leaf and leafl ets (3-) 4-7 (-10) jugate, these on the upper half of the pinna rachis. Flowers are smaller (1.2-1.6 cm long, including the calyx) with light yellow-ochre sepals and salmon-pink petals, its pedicels are articulated at or above their middle. Its legume is glabrous when mature and sometimes has red stipitate glands.
Erythrostemon hintonii diff ers by having leaves with 3-9 pinnae per leaf and leafl ets 4-6 jugate, these distributed along the length of the pinna rachis, fl owers 2 cm long (including the calyx) with red-purple sepals and purple-red petals, pedicels that are articulated near the base of the calyx tube, and a legume that is pubescent and has dark red-brown sessile glands. Some individual plants of E. guevarafeferii have glandular fruits while others in the same locality do not. Th e species is characterized by its arched and pendulous infl orescences, with slender, nearly horizontal pedicels (or these refl exed and sometimes twisted, so that the fl owers are resupinate). Resupinate fl owers on a pendulous infl orescence renders them, once again, in their usual position with the median (standard) petal uppermost.
Field and laboratory observations reveal two kinds of fl ower, with either a short or long pistil, as also observed in Erythrostemon epifanioi (J.L. Contr.) E. Gagnon & G.P. Lewis (as Caesalpinia epifanioi J.L. Contr.; Lewis, 1998) and in another legume genus, Tylosema esculentum (Burch.) A. Schreib., which displays functional heterostyly (Hartley et al. 2002). Flowers with short pistils are functionally male and fl owers with long pistils are hermaphrodite. We hypothesise that the occurrence of both fl ower forms in the same infl orescence ensures that not too many fl owers on each infl orescence set fruit (which would put a strain on maternal resources, as well as mechanically over-loading the peduncle) and that pollinators continue to move pollen throughout or between populations of the species, thus promoting cross-pollination. Andromonoecy, with a labile sex change of fl owers from hermaphrodite to functionally male (the gynoecium development is suppressed in fl owers higher up an infl orescence only if the lower fl owers are successfully pollinated and fruits are set) has been observed in Erythrostemon calycina (Benth.) L.P. Queiroz (as Caesalpinia calycina Benth.) in Brazil (Gibbs et al. 1999).
Taxonomic key to the species of Erythrostemon in Rio Balsas Depression