Croton calcareus Riina & Mateo-Ram., 2020

Mateo-Ramírez, Leticia & Riina, Ricarda, 2020, Croton calcareus: a new species of dragon’s blood (Euphorbiaceae) from dry forest in the state of Chiapas, Mexico, European Journal of Taxonomy 657 (657), pp. 1-14 : 4-9

Croton calcareus Riina & Mateo-Ram. sp. nov.

urn:lsid:ipni.org:names:77209678-1

Figs 2–4

Diagnosis

Croton calcareus Riina & Mateo-Ram. sp. nov. is most closely related to C. redolens Pittier and can be distinguished from the latter mainly by its larger fruits (12–13 × 12–15 mm in C. calcareus sp. nov. vs 5–6 × 7–8 mm in C. redolens ) and an indumentum of sessile, stellate, multiradiate, porrect trichomes on the adaxial side of leaves (vs stipitate, stellate, porrect trichomes in C. redolens ). The new species has been confused with C. draco Schltdl. , another dragon’s blood from Mexico, but it differs from this species in having shorter inflorescences (8–10 cm vs 15–43 cm in C. draco ) and larger fruits (only 4–7 × 4–7 mm in C. draco ).

Etymology

The specific epithet refers to the calcareous substrates where the species occurs.

Type material

MEXICO • Chiapas, Mpio, La Trinitaria, 4 km E of La Trinitaria along Mex 190 ; 16°8′ N, 92°2′ W; alt. 1480 m; 8 Jul. 1990; R.J. Hampshire, A. Reyes García and L. Hernández 1161; holotype: MEXU [ 858873 ] ; isotypes: BM , MA [ 540108 ] , MO [ 4323513 ] .

Paratypes

MEXICO – Chiapas • Mpio, Ocozocoautla, steep-walled canyon at the head of the Río de la Venta at the Chorreadero near Derna ; 16 Dec. 1972; D.E. Breedlove, R.F. Thorne 30301; MEXU • El Aguacero, 13 km al NO de Ocozocoautla; 29 Mar. 1985; E. Cabrera and H. de Cabrera 7870; MEXU • El Aguacero, canyon of the Río La Venta ; alt. 540–660 m; 19 Nov. 1984; G. Davidse, M. Sousa, O. Tellez, E. Martinez and J. Davidse 30083; MA, MO • 13 km E of Ocozocoautla on Rte 190 , then N on road to Aguacero; alt. 630–700 m; 2 Oct. 1984; M.J. Huft, E. Cabrera and R. Torres 2253; MEXU, MO • “Cascada El Aguacero en el Río La Venta” [ El Aguacero waterfall on the river La Venta ]; 16°45′17″ N, 93°31′20″ W; alt. 600 m; 12 Dec. 1987; E. Martinez and A. Reyes 22018; MEXU, MICH • El Aguacero, “a 33 km al E de Cintalapa” [33 km E of Cintalapa ]; alt. 540–660 m; 19 Nov. 1984; O. Tellez, M. Sousa, E. Martinez, G. Davidse and J. Davidse 8120; MO, MEXU • 7 mi SW of Tuxtla Gutierrez ; alt. 3000 ft.; 11 Aug. 1962; G.L. Webster, K. Miller and L. Miller 12966; DAV, MO .

Description

Small monoecious trees, up to 6 m high; young branches with a dense indumentum of rosulate (sometimes also stipitate) trichomes; latex from the main trunk and branches reddish to dark red (observation from herbarium specimens). Stipules short-triangular when young (ca 1.5 mm) to filiform 8–13.5 mm long, covered with a dense indumentum of rosulate and multiradiate trichomes, colleters (ovoid glands) at base, margin and apex. Leaf blade ovate to broadly ovate, 5–13 × 3.5–11 cm; base rounded to cordate; apex acute to obtuse; margin entire or slightly dentate; colleters (ovoid glands) along the margin; venation 3–5-plinerved, brochidodromous; primary and secondary veins raised on both surfaces; petiolar nectary glands 2–6, patelliform, sessile, subsessile or stipitate, attached to the petiole on the adaxial side, but also sometimes visible from the abaxial side; petiole 1.5–8 cm long, with a more or less dense indumentum of stellate adpressed trichomes; adaxial leaf indumentum very dense on young leaves to nearly glabrous on old leaves, sessile, stellate-porrect and stellate-multiradiate trichomes; abaxial leaf surface very densely pubescent on young leaves to sparser with age, with sessile or stipitate stellate or multiradiate trichomes. Inflorescences terminal, erect thyrses 8–10 cm long, axis costate, with a dense indumentum of stellatemultiradiate trichomes; bracts triangular, ca 1 × 1 mm; cymules usually bisexual, regularly spaced at the base and more congested toward the apex. Staminate flowers shortly pedicellate, pedicels 1–2 mm long; sepals valvate, triangular-ovate, 2–2.8 × 1.1–2 mm, adaxial surface with scattered simple trichomes, abaxial surface with a dense indumentum of stellate-multiradiate trichomes; petals narrow-elliptical, 1.7–1.8 × 0.7–0.8 mm, with scattered simple or stellate hairs on both surfaces, margin lanate; receptacle densely pilose; stamens 16; filaments glabrous, 2–3 mm long, anthers 0.6–0.9 × 0.4–0.5 mm. Pistillate flowers with pedicels 1.5–2 mm long, with stellate-multiradiate trichomes; sepals valvate, triangularovate, 2–2.2 × 1.2–1.3 mm, adaxial and abaxial surfaces with dense covering of stellate-multiradiate trichomes; petals absent or reduced to a filament with a colleter at the tip; ovary subglobose, densely covered with stellate-multiradiate trichomes; styles bifid, with six terminal tips, the lower half of the styles with stellate-multiradiate trichomes. Capsules depressed-subglobose, slightly trilobate, 12–13 × 12–15 mm; columella 9–10 mm long; seeds ovoid, 7–8 × 5.5–6 mm, brown, surface smooth, shiny, caruncle trapezoid, 0.6 × 1.2 mm.

Phylogenetic placement and relationships

The ITS alignment consisted of 686 aligned positions. The resulting tree topology ( Fig. 1 ) was congruent with previous analyses of Croton using the ITS region for the clades involved ( Riina et al. 2009; van Ee et al. 2011). This phylogeny indicates that Croton calcareus sp. nov. is indeed a member of Croton sect. Cyclostigma and that it is most closely related to C. redolens and C. gossypiifolius Vahl ( Fig. 1 ). The two sequences of C. calcareus sp. nov. are identical and share two unique positions with respect to the rest of sequences in the matrix. The accessions of Mexican species with which C. calcareus sp. nov. has been confused in the past (i.e., C. draco , C. xalapensis Kunth and C. verapazensis Donn.Sm. ) are indicated with an asterisk in the phylogeny ( Fig. 1 ). Of these, C. draco is the most closely related to the new species, coming out in the same clade (sect. Cyclostigma ), the other two species belong to two different sections/clades of Croton ( Fig. 1 ).

Distribution, ecology and phenology

Croton calcareus sp. nov. appears to be restricted to the state of Chiapas ( Mexico) where it grows on limestone outcrops, in low deciduous forests on rocky slopes, steep river canyons and river floodplains,

between 550–1500 m elevation ( Fig. 4 ). Flowering between August and February and probably fruiting between March and July.

Conservation status

The species could be categorized as Vulnerable (VU D2; IUCN 2012). The extent of occurrence of Croton calcareus sp. nov. is only 2718 km 2 and it is only known from four different localities in two notadjacent municipalities. The species appears to be under low human pressure since three of the known localities (Municipality of Ocozocoautla) are within the buffer zone of the Selva el Ocote Biosphere Reserve ( Ochoa-Gaona 1996; UNESCO 2012). Most of the known localities are of limited access due to their vicinity to limestone rocky formations along rivers.

Figures

Fig. 2. Croton calcareus Riina & Mateo-Ram. sp. nov. A. Image of an isotype (MA). B. Apical portion of a young branch showing the short young stipule (right arrow) and fully developed stipule (left arrow); the arrows point to the base of each stipule. C. Petiolar nectary glands visible from the adaxial side on a leaf with a rounded base. D. Petiolar nectary glands visible from the abaxial side on a leaf with a cordate base. E. Branch showing a young inflorescence. F. Section of inflorescence with detail of male flowers. G. Mature fruit. Scale bars = 1 mm. Vouchers: A, C, E, G: Hampshire et al. 1161 (MA); B, D, F: Davidse et al. 30083 (MA).

Fig. 3. Image of one of the paratypes of Croton calcareus Riina & Mateo-Ram. sp. nov. (Cabrera and Cabrera 7870, MEXU), previously indentified as C. xalapensis Kunth, to show the variation in leaf shape and indumentum density. It also shows immature fruits (younger than those in the type specimens).

Fig. 4. Distribution map showing the localities (black squares) of Croton calcareus Riina & Mateo- Ram. sp. nov., all known from southern Mexico.

Fig. 1. Bayesian majority rule consensus phylogram of nuclear ITS data from a selection of Croton L. taxa including two accessions of Croton calcareus Riina & Mateo-Ram. sp. nov. Species marked with an asterisk correspond to those with which C. calcareus sp. nov. has been previously confused. The relevant clades are labelled on the left side. Numbers next to nodes indicate clade support (Bayesian posterior probabilities).