Turraea dargentiana Baider & V. Florens, 2016

Baider, Claudia & Vincent Florens, F. B., 2016, A new and critically endangered species of Turraea (Meliaceae) endemic to the island of Mauritius, Phytotaxa 247 (3), pp. 219-228 : 220-225

Turraea dargentiana Baider & V. Florens , sp. nov. ( Figs. 1 , 2 )

— Quivisia chilosantha Bojer (1837: 58) , nom. nud.

Turraea dargentiana , which resembles T. rigida Ventenat (1808: 48) , is the smallest and the only dioecious species of the genus known in the Mascarenes. Other diagnostic features include the possession of adult leaves that are often irregularly lobed in their distal half.

Type:— MAURITIUS. Mt Camizard , 300 m, 20°20’02.9”S, 57°43’21.34” E, 19 August 2005, female fl., fr., F.B.V. Florens s.n. (holotype MAU 0016259 !) .

Evergreen shrub, dioecious, 0.5–1.2(–1.5) × (0.25–)0.5–1.0(–1.5) m, much-branched, branching as from 10–30(–50) cm high; basal diameter of trunk 1.2–4 cm. Stems cylindrical, tomentulose; bark brown matt, inside bark pale green; young shoots tomentose; leaf scars prominent; lenticels not conspicuous. Branches ascending. Leaves alternate, ± distichous, arranged ± horizontally, mostly near apices; internodes 1–2 cm long at mid-season’s growth, 0.2–0.4 cm long at end of season’s growth. Leaf buds tomentose, dark green when fresh, dark brown when dried. Petioles 4–10 × 1.7–2.4 mm; young leaves terracotta to rusty red; older leaves tomentulose, grey green. Lamina simple, subcoriaceous, obovate to narrowly obovate, some ovate, rarely elliptic, (4–)7–12(–24.5) × (1.5–)3–5(–7.5) cm, green to pale green, older leaves sometimes with yellowish tones, younger leaves sometimes with purplish tones; glabrous, except for few hairs on midrib and, in youngest leaves, sparsely ciliate margin; base cuneate, rarely asymmetrical; margin entire or often with 2–3(–5) lobes, irregularly distributed on each side of the leaf, confined to distal half of leaf; tip acute to obtuse, rarely emarginated; 7–13 pairs of secondary veins, these ± subopposite, prominent adaxially, visible abaxially, linked into arcs to form an intramarginal vein. Inflorescences axillary, 2(–3)-flowered cymes, sometimes ramiflorous; peduncles 1.6–2.5 × 0.8–1 mm, tomentulose; bracteoles 2–3, ± deltoid, 0.5–1 mm long; pedicels 2–5 × 0.6–1.2 mm, tomentose. Flower buds obovate, tomentose, white to pink. Male flower 9–10 mm long; calyx urceolate, 2.2 mm long, lobes 5(–6), 0.3–0.55 mm long, ± deltoid, tomentulose (white hairs), pale purple to green; corolla of (4–)5(–6) petals, these oblong to elliptic, 7.4 × 2.6 mm, slightly pubescent abaxially, glabrous on edges and adaxially; abaxial side white to pale yellowish, adaxial side white, sometimes with pink tinge; stamens 10–12, staminal tube 3.6 mm long, filaments free for 0.6–1 mm; tomentulose on both sides, denser towards tip, base light pink, white at tip, anthers oblong, 1–1.2 mm long including apiculum of 0.1 mm, pubescent on abaxial side, splitting longitudinally; remnant of ovary spherical, 0.8 × 1.1 mm long, pubescent (white hairs), pale yellow with pink tinge; locules 3, ovules 2 per locule, which do not develop; style cylindrical, 4 × 0.5 mm, glabrous except for the pubescence (white hairs) on the lowermost 0.3 mm; stigma capitate, 0.8 × 1.1 mm, smooth except for the strumose uppermost third, white, tip yellow. Female flower 6–7 mm long; nectar shiny, colourless; calyx urceolate, 1.35–1.6 mm long, lobes 5(–6), (0.2–) 0.6–1.0 mm long, ± deltoid, tomentulose (white hairs), pale purple; corolla of (4–)5(–6) petals, these linear–spatulate, elliptic or oblong, 5.3–5.75 × 1.7–2.1 mm, abaxial side slightly pubescent, glabrous on edges and adaxial side; abaxial side pink to pale purple, adaxial side white to pink. Staminodial tube 1.6 mm long, filaments free on last 0.25 mm, adaxial side pubescent, abaxial side glabrous except for the pubescent free parts, pale pink. Staminodia 6, oblong, 0.8–0.9 mm long, including a 0.2 mm apiculum, pubescent, indehiscent; ovary subspherical, 0.85–1 mm long, pubescent with white to pale yellow hairs; locules 3, rarely 4, 2 ovules per locule; style cylindrical, 3.15 × 0.4 mm, glabrous except for the tomentulose (white hairs) lowermost 0.5 mm; stigma capitate, 0.45 × 0.85 mm long, smooth except for the uppermost third, then strumose, white. Fruiting pedicel 3.4–5.4 × 0.6–1.4 mm; capsule ovoid, tip acute, 5.7–6.6 × 6–7 mm, tomentulose, dehiscing along 3 sutures, remaining connected at base; valves outer surface pubescent, inner surface waxy whitish, purplish on fresh, brown-reddish when dry. Seeds 5–6, ovoid, tip falcate, 4.7–5.9 × 2.0–3.5 × 2.1–3.0 mm, aril small, hile very distinct.

Additional specimens examined (paratypes): — MAURITIUS. Mt Camizard, 300 m, 20°20’02.9” S, 57°43’21.34” E, 06 November 2005, female fl., F.B.V. Florens & C. Baider s.n. ( MAU 0016262 ) ; Mt Camizard , 14 March 2005, female fl. bud, open fr., F.B.V. Florens & C. Baider s.n. ( MAU 0016261 ) ; Mt Camizard , 300 m, 23 February 2013, fl. bud, male fl., C. Baider 2610 & F.B.V. Florens ( MAU 0016266 ) ; Mt Lion , 413 m, 20°21’41.04”S 57°43’31.26”E, 28 February 2011, fl. bud, F.B.V. Florens s.n. ( MAU 0016264 ) ; Mt Lion , 421 m, 20°21’41.67”S, 57°43’32.11”E, 28 October 2014, female fl. bud, F.B.V. Florens s.n. ( MAU 0016265 ) ; Ferney, 320 m, 200 20’01”S, 57041 ’28”E, 17 March 2012, male fl. bud, C. Baider 2580 & F.B.V. Florens ( MAU 0016263 ) ; Le Pouce, 745 m, 20°11’53.25”S, 57°31’45.82”E, 14 October 2011, C. Baider 2543 & F.B.V. Florens ( MAU 0016267 ) ; Le Pouce, 743 m, 20°11’52.90”S, 57°31’45.45”E, 28 October 2011, fr., C. Baider 2523 & F.B.V. Florens ( MAU 0016268 ) ; Le Pouce, 13 February 1975, fl. bud., M.J.E.Coode & J.M.E. Guého 4844 ( MAU 0016260 ) ; without locality, no date, fl. bud, L.M.A.A. Petit-Thouars s.n. ( P Pret n. 113 13/73 ), L.M.A.A. Petit-Thouars s.n. ( P Pret n. 111 10/73 ), L.M.A.A. Petit-Thouars s.n. ( P Pret n. 111 10/74 ) .

Etymology: —The epithet is in recognition of the considerable botanical contribution and exceptional dedication to the conservation of the local flora of Mr. Gabriel D’Argent (born 1925), retired forester from the Forestry Services ( Mauritius).

Taxonomic Notes: —Examination of all Turraea samples from the Mascarenes deposited at P and MAU revealed previous collections of T. dargentiana . The species was collected by du Petit Thouars between 1793 and 1795, but his three existing samples at P had previously been determined as T. rigida by A.J. Scott in 1974 (in sched.). In fact, Bojer (1837) mentioned one small shrubby Turraea species on the east side of the Le Pouce Mt that flowered from May to June ( Quivisia chilosantha Bojer 1837: 58 , nom. nud.). This species was later synonymised under T. rigida ( Bosser et al. 1976 -onwards). The flowering season, small shrubby nature of the plant and also the precise locality where it was said to occur ( Bojer 1837) correspond exactly to a population of T. dargentiana . No other shrubby Turraea species was found in that precise locality despite intensive surveys. Furthermore, a sample of T. dargentiana was collected more recently at Le Pouce Mt in 1975 (Coode & Guého 4844 [MAU 0016260]), but was, at that time, identified only to genus level.

Comparison with other Mauritian species: — Turraea dargentiana resembles Turraea rigida and the two species are sympatric in SE Mauritius. However, T. rigida is a shrub or treelet that can reach up to 3 m high, with dark green and more coriaceous leaves, and larger flowers which are said to be white ( Bosser et al. 1976), but which vary from light pink to purple. Turraea dargentiana is a smaller shrub (none of the 200 individuals observed exceeded 1.5 m, a few grew to over 1 m high and many adults not even exceeding 50 cm). Furthermore, flowers of T. dargentiana have smaller petals (5.3 – 7.4 mm long) compared to T. rigida (10 – 19 mm long). Turraea dargentiana is also the only recorded Turraea from the Mascarene Islands to be dioecious, although at first sight, flowers appear to be bisexual. Close examination shows that pollen sacs would split (and release pollen) only in male flowers, and would remain intact in female flowers. This situation is known as cryptic dioecy (retention of opposite sex structures within the flowers of each sex) ( Humeau et al. 1999). Monitoring of a few individuals at Mt Camizard for some years has revealed no sex variation through time within a given individual, with seedlings found only around female plants. There is also no apparent signs of leaky dioecy (presence of fruits in low numbers on male plants) ( Baker & Cox 1984, Humeau et al. 1999), nor of dichogamy (male and female parts ripening at different times). Cryptic and leaky dioecy have been reported for a number of species in the Mascarenes ( Humeau et al. 1999, Venkatasamy et al. 2007) and other islands ( Percy and Cronk 1997, Anderson et al. 2015), with higher percentages of dioecious species found on oceanic tropical islands of the Pacific Ocean ( Baker & Cox 1984), and the Mascarenes ( Humeau et al. 1999).

Phenology: — Turraea dargentiana was found to start having flower buds as from October, flowering starts towards mid-October but spanning mainly from February to May, extending to August, and fruiting is from March to November.

Distribution and habitat: —Endemic to Mauritius; known from Le Pouce and the Bambou Mountains ( Fig. 3 ). Turraea dargentiana is currently known to grow in wet native vegetation remnants of 240–745 m elevation, on ferralitic soil or lithosols formed from volcanic rocks ( Willaime 1984). The sites are in the humid or super-humid zones of Mauritius ( Halais & Davy 1969, Parish & Feilllafé 1965), with annual rainfall varying from 1,400 mm (Le Pouce) to 2,500 mm (Bambou Mt Range) ( Willaime 1984) and mean annual temperatures of about 20 ºC ( Halais & Davy 1969). In the Bambou Mountains, the species was found at Mt Camizard, Mt Lion and at Ferney Valley (about 1.25 km east north-east of Bestel). Elsewhere, it was found only at Le Pouce. At Mt Camizard the species grows rather sparsely in the understory of wet native lowland forest of 8 – 12 m tall centred around 20° 20’ 00” S and 57° 42’ 55” E at about 240 m elevation. This habitat is dominated in the canopy by Diospyros tessellaria Poiret (1804: 430) ( Ebenaceae ) and Labourdonnaisia Bojer (1841: 295) spp. ( Sapotaceae ) with the most common native species in the understorey being Warneckea trinervis ( Candolle 1828: 3, 5) Jacques-Félix (1978: 235) ( Melastomataceae ) and different species of Eugenia Linnaeus (1753: 450) ( Myrtaceae ). The forest is however invaded by alien species that sum up to 84 % of all woody stems above 1.3 m diameter, of which Cinnamomum verum J. Presl (in Berchtold & Presl 1825: 37) ( Lauraceae ) and Psidium cattleianum Sabine (1821: 317) ( Myrtaceae ) are the most dominant ( Florens et al. 2012). At Mt Lion, T. dargentiana occurs more densely in the understorey of shorter vegetation (4 – 8 m tall) but in a more localised distribution very close to the summit. The population is centred on 20° 21’ 29” S and 57° 43’ 06” E at about 470 m elevation and grows in a vegetation where the main species are Labourdonnasia callophylloides Bojer (1841: 295) ( Sapotaceae ), Warneckea trinervis , and Gaertnera psychotrioides Candolle (1830: 531) Baker (1877: 231) ( Rubiaceae ). Turraea dargentiana occurs more rarely at Ferney Valley close to the ridge top in similar forest type as in Mt Camizard. The population at Ferney Valley occurs around 20° 20’ 01” S and 57° 41’ 28” E at an elevation of about 320 m. Other than in the Bambou ranges, the species has been recorded only at Le Pouce Mt near the summit at 780 m elevation. The species occurs there as a few small clumps centred on 20° 11’ 43” S and 57° 31’ 19” E in very dense stunted vegetation 2–3 m tall which is at places nearly impenetrable owing to dense branching of the stunted trees close to the ground. The dominant species is Erythrospermum monticolum Petit-Thouars (1806: 67) ( Achariaceae ) and other common species include Syzygium commersonii Guého & A.J. Scott (in Scott 1980: 491) ( Myrtaceae ) and Geniostoma pedunculatum Bojer (1837: 215 , nom. nud.) ex Candolle (1845: 28) ( Loganiaceae ).

Ecology and Conservation status: —Plants are distributed on two mountain ranges which are some 23 km apart. The three localities in the Bambou Mountains in the south east of the island occur within a maximum of 5 km of each other. In each locality, plants tend to occur typically within clumps, with a few solitary interspersed individuals between the clumps. At Le Pouce Mt, the same pattern of spatial distribution was noted. Therefore, T. dargentiana is known in the wild from 4 localities, with about 200 individuals, of which about 185 are adults. The species typically grows in good quality native forest where> 50 % of the canopy cover is composed of native species. However, invasion by alien plants is happening at all sites. The main invasive plant species are C. verum , P. cattleianum and Syzygium jambos ( Linnaeus 1753: 470) Alston (1931: 115) ( Myrtaceae ) in Mt Camizard and Ferney, P. cattleianum and Ligustrum robustum subsp. walkeri (Decaisne 1879: 27) Green (1985: 130) ( Oleaceae ) on Mt Lion and Ossaea marginata ( Desrousseaux 1797: 32) Triana (1871: 147) ( Melastomataceae ) and P. cattleianum on Le Pouce Mt. It has been shown that invasion by such alien plant species have deleterious impacts on native woody plants in Mauritius, including elevated mortality and reduced growth rates ( Florens 2008), as well as, reduced production of flowers and fruits ( Baider & Florens 2006, Monty et al. 2013), that can result in very low or halted natural regeneration ( Baider & Florens 2011). Invasion by alien plants may even interfere with insect pollination since it was shown that the species richness and abundance of native butterflies—a group often used to indicate change in other insect group ( Thomas 2005)—are much lower in forests invaded by alien weeds compared to adjacent areas where the weeds have been removed ( Florens et al. 2010). Fortunately, there does not seem to be any predation of fruits or seeds by invasive animals like rats or monkeys as are often seen with several other native plants in Mauritius ( Baider & Florens 2006, 2013). It is worthy to note that the four localities where the species occurs are all still relatively well preserved. The species’ absence from more alien invaded forests suggests that it may be particularly vulnerable to alien plant invasion presumably caused by its very small size, which results in it being over-topped and shaded by even small alien plants. Previous studies in Mauritian forests showed that ground flora or understorey species appear to be more at risk to alien plants than larger species ( Baider & Florens 2011). Throughout its whole known range, the species occurs only in mountain reserves and therefore receives generally little protection. The only site where some conservation management is being done in the exact area where T. dargentiana occurs is at Mt Camizard where the private landowner is controlling invasive alien plants to restore the native forest. However, it appears that the weeding of all alien plants in one go may be changing the forest’s microclimate too suddenly (with higher light exposure, and dryer conditions) for the species since several seedlings and adults have died shortly after the weeding. A more gradual or stepwise alien plants weeding is advisable to avoid or minimise the apparent short-term deleterious impacts of weeding mentioned above. Despite these possible short-term negative impact it is expected that, in the medium to longer run, alien weeding will still bring strong benefits by increasing fitness and reproductive output as shown for other species ( Baider & Florens 2006, Monty et al. 2013).

Another important factor concerning the species’ rarity may be related to its unique sexual reproductive system within the Mascarenes Turraea . In the genus Dombeya Cavanilles (1787: 121) in Réunion, cryptic dioecy occurs in species found mostly in mid- to high elevation, whereas leaky dioecy is more common in the lowlands ( Humeau et al. 1999). However, the strictly dioecious species typically have large populations, suggesting that this might have been the case for T. dargentiana . Moreover, dioecious species have higher chances of local extinction caused by stochastic events, especially if their population becomes fragmented.

The species should be considered Critically Endangered (B1a, b ii,iii,iv,v; C2aii) according to the IUCN Red List criteria ( IUCN 2001). This assessment is based on the species’ limited geographical distribution—with an Extent of Occurrence <34 km 2, and an Area of Occupancy of 16 km 2; calculations following GeoCAT version β, ( Bachman et al. 2011)—, its severely fragmented populations, the high probability of losing one of the localities in a near future (at Ferney, with its two aged known plant) and the fact that the majority of individuals are found in habitats currently being degraded by invasive alien plants, which is leading to declining and ageing populations.

A species recovery programme appears necessary given the paucity of juveniles that indicate low natural regeneration hence declining populations. We suggest as immediate action to gradually remove invasive alien weeds from the immediate vicinity of all the known plants to increase their survival and natural regeneration, and to secure ripe seeds for temporary ex-situ propagation in view of augmentation programmes in its current habitat where invasive alien plants would have been controlled.

Figures

FIGURE 1. Turraea dargentiana. A. Habit; B. Dehisced fruit, showing three valves; C. Fruit; D. Female flower showing remnants of staminodial tube. Drawn by Danielle Florens, after F.B.V. Florens s.n. (MAU 0016259); scale bar = 2 cm (A), 1 cm (B–D).

FIGURE 2. A.Adult plant in its forest understorey habitat; B. Abaxial surface of a young leaf showing purplish tinge; C. Abaxial surface of a mature leaf; D. Flower buds (male) at various stages of development; E. Male flowers; F. Female flowers; G. Unripe fruits with dry petals still attached. Photos: F.B. Vincent Florens.

FIGURE 3. Distribution of known current localities of Turraea dargentiana on Mauritius. Each dot is 1 km in diameter. 1: Mt Camizard; 2: Mt Lion; 3: Ridge top at ‘Ferney Valley’ (1.25 km E–NE of Bestel); 4: Le Pouce Mt.