Croton crossolepis P.E.Berry & Kainul., 2017

Berry, Paul E. & Kainulainen, Kent, 2017, Croton crossolepis (Euphorbiaceae), a new species with an unusual trichome type from southwestern Madagascar, Phytotaxa 307 (1), pp. 95-100 : 95-99

Croton crossolepis P.E.Berry & Kainul. , sp. nov.

Type:— MADAGASCAR. Toliara, Atsimo-Andrefana: along Route Nationale 7 on S side of road, ca. 30 km E of Toliara, 23°19’54”S 43°55’15”E, 190 m, 8 February 2009, B.W. van Ee, P. E. Berry, B. Dorsey & H. Razanatsoa 823 (holotype MICH-1210791 !; isotypes G!, MO!, P!, TAN). Figs. 1–3 .

= Croton mavoravina var. thysanolepis Radcliffe-Smith (2016: 32) . Type:— MADAGASCAR. Toliara, Atsimo-Andrefana: environs de Tuléar , près de la colline de la Table, 10–12 January 1947, H. Humbert 19816 (holotype K!; isotype P-00433491 !).

Diagnosis: Croton crossolepis is similar to C. mavoravina Leandri but differs in having fringed lepidote trichomes on the young stems and petioles, as well as on the veins of the undersides of the leaves over an underlayer of stellate trichomes (vs. stellate trichomes with several porrect central rays throughout in C. mavoravina ), as well as proportionately narrower and longer bracts subtending the flowers (4–8.5 × 0.3–0.5 mm vs. 1–3 × 0.4–1.2 mm in C. mavoravina ).

Well-branched shrubs to small trees 1–5 m tall ( Fig. 2A, B, D ). Young stems stout, somewhat gnarled, loosely dichotomously-branched, with numerous rusty-colored lepidote trichomes with a fringed, silvery margin ( Fig. 3C ), showing a bright green phelloderm layer when decorticated ( Fig. 2C ). Older stems gray, with numerous vertical, lenticellate striations. Leaves alternate, subcoriaceous, clustered at the branch tips; petioles 1.5–5(–8) cm long, lepidote, with two laterally divergent glands at the junction with the blade, each very shortly stipitate, the stipe lepidote, the apex concave, lustrous, and 0.5–0.8 mm diam.; blades oval-elliptic, (2–)3–6(–10) × 2–4(–6) cm, base shallowly cordate, apex rounded to acute, margins entire, with 10–13 secondary veins on either side of the midvein, adaxially green with multiradiate trichomes, abaxially much paler, appearing brown-dotted on a silvery-white background, with mostly multiradiate trichomes between the veins but with irregularly shaped, rusty-tinged lepidote trichomes along the midvein and the secondary and tertiary veins, these with stiffly fimbriate, horizontal rays ( Fig. 3B ). Inflorescence pseudoterminal, thyrsoid-spicate, 1–6 cm long, flowers densely arranged along the axis, only seen in bud or post-anthesis, bracts lanceolate-acicular, 4–8.5 × 0.3–0.5 mm, exceeding the buds, densely lepidote and brown-dotted, bracteoles 2, flanking each flower, acicular, 1–2 mm long. Staminate flowers: buds globose, densely lepidote, pedicels 1–2 mm long; sepals 5, foliaceous, ca. 3.3 × 2.0 mm, adaxially glabrous, abaxially densely lepidote; petals 5, ellipticspatulate, ca. 3.5 × 1.2 mm, adaxially glabrous, abaxially papillate and sparsely lepidote, margins ciliate; stamens 16–18, filaments 3–5 mm long, glabrous to the base, anthers elliptic, 1.2–1.5 × 0.8–1.2 mm; disc glands 5, opposite the sepals, sessile, ca. 0.5 × 1.0 mm; receptacle pilose. Pistillate flowers and fruits not seen.

Additional specimens examined (paratypes):— MADAGASCAR. Toliara: Tuléar , 27 February 1943, R. Decary 18841 (G, K, MO, P-00433492 ) ; La Table , ca. 20 km N of Toliara, slope and along ridge, 23°25’26”S 43°46’03”E, 50–120 m, 19 May 2004, Z. Rogers & F. Rakotonasolo 486 (K, MICH, MO, P-05572382 ) ; Ankororoka, préfecture Tuléar , 18 January 1971, J. P. Schmitt 169-S ( P-00154330 ) ; 28 km E of Tuléar by road, 12 November 1985, T. Willing 32 ( MO-2990275 ) .

Distribution: —This species appears to be restricted to a narrow swath of deciduous or spiny scrub forest in southwestern Toliara Province (Atsimo-Andrefana Region), mainly along Route Nationale 7 starting from La Table, about 20 road km east of Toliara, and extending another 25 km or so (east of Ankororoka) to about a third of the distance from Tuléar to Andranovory (the junction with Route Nationale 10 to Fort Dauphin; see Fig. 1 ).

Habitat and Phenology: —Deciduous bush on limestone substrates, elevation 10– 190 m. One individual was seen with post-anthesis flowers in late November, and all others collections examined were in bud in January, February, and May. The species likely flowers for short periods of time following the infrequent periods of heavy rain that mostly occur from October to April in the region.

Etymology: —The specific epithet refers to the stiffly fringed-scaled trichomes on the stems and scattered along the veins of the leaf undersides. In the field we referred to this species as the “Velcro” Croton because of its tendency to cling to clothing with its fimbriate scales, akin to the VELCRO® brand self-engaging hook and loop fasteners.

Conservation assessment:— Croton crossolepis is currently known from just six collections in scrub forest east of Toliara, between La Table and Ankororoka, along Route Nationale 7. This is an area that has been severely threatened by deforestation and fires set to clear the land, and only La Table now belongs to a protected area, the Tsinjoriake Reserve. We visited La Table twice, once in the wet season and once in the dry season, and did not find any individuals of this species, so we assume that it is not common there. Since the overall area of occupancy is less than 2,000 km 2, only six locations are known, and the quality of habitat in the area is in evident decline, we consider that it should be assigned a conservation status of Vulnerable (VU B2abiii) following the IUCN Red List Categories and Criteria (IUCN 2012).

Notes:— Although we still lack fruits and pistillate flowers of this species, it is distinctive enough in other features from the remaining members of the genus in the arid southeast of Madagascar that we are confident in describing it here as a new species. Most of the numerous other species of Croton that occur in the area around Toliara are much smaller-leaved, including those that co-occur with it on La Table, such as C. inops Baillon (1890: 864) and C. chauvetiae Leandri (1970: 310) . At the easternmost (type) location of C. crossolepis , we observed considerable variation in the size of the plants, with some quite small and compact on open ground of limestone outcrops ( Fig. 2D ), while others nearby in more protected, forested spots grew as small trees to 5 m tall ( Fig. 2A , left side).

One of the main features that differentiates this species from its congeners is the unusual type of trichomes that are found on the stems and undersides of the leaves. These are irregularly lepidote scales that have a markedly brown central disc but fringed rays that are silvery-translucent; when coming into contact with a shirt sleeve, the stiff, partially separated rays of these trichomes often cling to the fabric almost like hook and loop fasteners. These larger scales are superimposed upon a denser layer of uniformly silvery, multiradiate trichomes underneath ( Fig. 2E, F , 3B ). Another particular feature of this species are the acicular bracts that subtend the flowers on the inflorescence ( Fig. 2F ). Radcliffe-Smith (2016) treated this species as a variety of C. mavoravina , but that species, which occurs farther east in Toliara Province, is characterized by more membranaceous, wavy-margined leaves with uniformly stellate pubescence on both sides of the leaves, as well as on the stems and petioles. It also has smaller bracts and more slender, straighter shoots than C. crossolepis . In the molecular phylogeny of Malagasy species of Croton by Haber et al. (2017), Croton crossolepis is treated as “ Croton sp. nov. A,” and it is a member of the Western Indian Ocean Region (WIOR) Clade 1, which also includes C. mavoravina . Within that clade, C. crossolepis is sister to a subclade of 12 species that in turn is divided into two well supported groups. The first of these two groups includes C. mavoravina along with several others smaller-leaved species from the south of Madagascar, including C. aubrevilecta Leandri (1970: 310), C. miarensis Leandri (1969: 504) , and C. kimosorum Leandri (1939: 29) . Such a pattern suggests that C. crossolepis may be related to C. mavoravina , but is certainly not conspecific with it. We chose not to simply make a new combination with the Radcliffe-Smith (2016) varietal name “ thysanolepis ” because that is not required by the Code, but more importantly we chose to use our own collection as the type because it is well documented with a georeferenced locality, photographs are available in the Tropicos® database, and molecular sequence data from it are published in Haber et al. (2017).

In view of our current lack of knowledge about the pistillate flowers and fruits of this species, we would strongly encourage botanists visiting southern Madagascar to search for this species in an effort to characterize it more substantially and to improve the threat assessment for the species. Mature inflorescences, particularly ones with female flowers and fruits would be particularly helpful. Likewise, since we do not yet understand the significance of the green phelloderm layer underneath the young stem cortex, determining the extent of this feature on older wood would also be desirable.

Figures

FIGURE 1. Distribution map of Croton crossolepis from known, geolocatable specimens in southwestern Toliara Province, Madagascar, all along Route Nationale 7 (in yellow). Inset shows location of the satellite photo within Madagascar. Google Earth Image © 2017 DigitalGlobe, reproduced per attribution guidelines.

FIGURE 2. Croton crossolepis, photos of living plants. A. Large individual ca. 5 m tall (on left). Shorter tree on right with trilobed leaves is Jatropha mahafalensis Jumelle & Perrier de la Bâthie (1910: 180), and the lanceolate-leaved shrubs in the understory are Euphorbia antso Denis (1921: 45). B. Closeup of branches showing stout stems and drooping leaves. C. Bicolored leaves with rusty-colored lepidote trichomes on stems. Note green cambial layer of the stem where outer bark was peeled off. D. A small individual on bare limestone just 10 m away from individual shown in A. E. Detail of stem apex showing the silvery underside of a leaf with overlaying brown lepidote scales concentrated along the midvein and secondary veins. F. Branch apex showing the subterminal inflorescence in bud. Note sessile buds and elongate, linear bracts. (From van Ee et al. 823 except for D, photographed nearby; photos: P.E. Berry and B.W. van Ee).

FIGURE 3. Croton crossolepis, micrographs of dried material. A. View of the base of a leaf with the junction of the petiole, showing scattered stellate trichomes on the adaxial surface and one of the shortly stipulate acropetiolar glands. B. Closeup of the center of a leaf blade on the abaxial surface, showing a background of pale-silvery, stellate trichomes and fewer larger, brown-centered, fimbriate-rayed lepidote trichomes concentrated on the midvein and secondary veins. C. Closeup of a young stem showing densely overlapping browncentered lepidote trichomes with sharp, fringed ray tips (these exemplify the specific epithet “crossolepis”). D. A rehydrated staminate flower (anthers became detached during drying, but one of the anthers is shown in upper right), showing the venose sepals and petals adaxially and ca. 18 filaments. (A–C from van Ee et al. 823, D from Willing 32; photos: K. Kainulainen).