Angraecum jeannineanum Fournel & Micheneau, 2015

Fournel, Jacques, Micheneau, Claire & Baider, Claudia, 2015, A new critically endangered species of Angraecum (Orchidaceae) endemic to the island of Mauritius, Indian Ocean, Phytotaxa 222 (3), pp. 211-220 : 213-217

Angraecum jeannineanum Fournel & Micheneau , sp. nov. ( Fig. 2 )

Diagnosis: Plant epiphytic, of the section Hadrangis Schltr. , close to Angraecum cadetii Bosser , with a bag-shaped labellum but differing by its smaller and thicker dark green leaves, with both thinner and smaller inflorescences and flowers, the latter being clearly non-fleshy, and more greenish in colour.

Type: — MAURITIUS. Plaine Champagne, 11 March 1965, fl., Guého s.n. (holotype MAU 11690!).

Description

Epiphytic; stem erect, 1–5 cm long, ± 5 mm in diameter. Roots smooth, greyish, ± 2 mm in diameter; distributed at the base of the plant. Leaves 3–6, distichous, fan-shaped; with a short sheath, 7–8 mm long; blade slightly coriaceous; margin entire, dark green above, lighter colour below; venation 4–6, visible on the abaxial surface; ligule, 0.8–1.0 × 3–8 cm, slightly contracted and folded along the midrib towards the base, otherwise flat, apex unequally bilobed. Inflorescence invaginated slightly downward, 3–7 cm long, ± 3 mm in diameter; peduncle 1–2 cm long; 3–5 sheaths, infundibuliform, 4–10 mm long, imbricate, covering most of the base of the peduncle; sheaths above more distant, not covering the peduncle tip; axis 2–5 cm long, internodes thick; 1–5 flowered, 0.6–1.0 cm apart; bracteoles mostly oval, rounded tip, 6–9 mm long, sheathing the ovary almost completely. Flower pale green, margin white-cream. Perianth thin. Dorsal sepal ovate, 7–10 × 4.5–5.5 mm, keeled at the back, apex acute, curved. Lateral sepals resembling the dorsal sepal, slightly more oblique, 6.5–9.5 × 4.5–5.5 mm, tip curved downwards. Petals ovate to narrowly ovate, 6–9 × 3–4 mm, keeled at the back, tip acute, curved. Labellum bag-shaped, base rounded, 6–8 mm long × 4.5–5.5 mm deep, side edges rounded, lamina slightly marked, forming a curved tip. Column 2.5 mm high; lateral lobes slightly developed, rounded; rostellum medium tooth short, acicular. Anther semi-circular, 3 mm large, indented forward, with a small rounded keel at the back. Pollinia yellow, nearly quadrate, 1 mm long; stipe formed as a membranous piece, tubular, 0.9 mm large; viscidium under the stipe, of similar shape, as a unique piece. Ovary pale green, cylindrical, 6–7 mm long, ± 3 mm in diameter, 6-ribbed. Capsule ovoid, 1.2 × 1 cm.

Taxonomical Notes: —The four species of Angraecum section Hadrangis have thicker leaves and flowers compared to the other Angraecum species in the Mascarenes. Angraecum jeannineanum is similar to A. cadetii but it is, in general, a smaller plant ( Fig. 3 ). Angraecum bracteosum is a much larger plant, with more inflorescences and flowers per inflorescence than all the other species of the section. Angraecum striatum has distinct striate leaves and is the species that occurs at the highest elevations of all (between 800 and 2,000 m).

Species of section Hadrangis can be readily separated by the aperture pattern of the flower, or in other words, by the area available for an animal to access the nectar. This was determined by measuring the distance between the labellum to the base of the pollinia (called ‘height’ in Fig. 1A ), and the distance between the edges of the labellum (called ‘width’ in Fig. 1A ). These two-dimensional measurements show a unique flower aperture pattern for each species ( Fig. 4 ). Angraecum cadetii has the largest aperture ratio (for both ‘height’ and ‘width’) of the four species, allowing small birds ( Zosterops spp. ) to reach its nectar but without removing the pollinia. Flowers of the other three species have smaller aperture ratios ( Fig. 4 ). This smaller aperture of the flower forces Zosterops spp. to be in contact with the orchid column when probing for nectar, resulting in the bird effecting pollination, since pollinia end up sticking to their beaks. Pollination by Zosterops was observed for both A. bracteosum and A. striatum in Réunion ( Micheneau et al. 2006, 2008b). As A. jeannineanum has an aperture ratio in-between of these two species ( Fig. 4 ), it is expected that it is probably pollinated by a Zosterops , but this has not been observed yet.

Etymology: —Because this new species is sympatric with and closely related to A. cadetii , we decided to name it after Mrs. Jeannine Cadet, the wife of the late Thérésien Cadet, an eminent botanist and ecologist from Réunion. Mrs. Cadet is a skilful botanical artist, who shared a passion for native plants with her husband, and dedicated a large part of her art to the orchid family. Part of Jeannine’s orchid illustrations were published in a book ( Cadet 1989), which included the recently described A. cadetii . She also kindly illustrated this new species.

Phenology: —Flowering in February to April; fruiting: April to May.

Distribution and Habitat: — Angraecum jeannineanum occurs between 200 24’– 200 28’ S and 570 26’– 570 28’ E ( Fig. 5 ), at (250) 500–690 m elev. The soil in this region is ferralitic and formed from volcanic rocks ( Willaime 1984), and located in the humid to super-humid zones of Mauritius, receiving annual rainfall between 2,000 mm (Bel Ombre) and 3,000 mm (Pétrin) ( Willaime 1984). The species is currently known to exist in the heath vegetation of Pétrin (~ 650 m elev.), growing chiefly on the stem of Erica brachyphylla (Bentham in DC. 1839: 697) Oliver (1993: 769) ( Ericaceae ), at 0.5 to 2 m from the ground. The locality of the type specimen has a vegetation , which is somehow similar to that of Pétrin, although the forest of Plaine Champagne can be taller, denser and dominated by species of the genus Sideroxylon Linnaeus (1753: 192) ( Sapotaceae ) ( Vaughan & Wiehe 1937). At Pétrin, this vegetation type is intermingled with the heath vegetation. The specimen collected from Bel Ombre (located on the windward slope near the southern extremity of the Black River Gorges National Park) carries no indication of the habitat type. Bel Ombre is a relatively large area, mostly dominated by a lowland wet forest with high tree density (around 1,000 trees per hectare that reach diameter at breast height ≥ 10 cm) ( Florens et al. 2012), and a canopy varying from 8–12 m high (C. Baider, pers. obs.), also dominated by Sapotaceae .

Apart from the Conservation Management Area (hereafter CMA) of Pétrin, and some parts of Bel Ombre forest, the original area of distribution of A. jeannineanum is mostly under native forests that are highly invaded by invasive alien plants, which comprise mainly Psidium cattleianum Sabine (1821: 317) ( Myrtaceae ) and to a lesser extent the Oleaceae Ligustrum robustum ( Roxburgh 1820: 101–102) Blume (1850: 313) subsp. walkeri ( Descaine 1879: 27) Green (1985: 130) . The regions of Pétrin and Plaine Champagne are also being invaded by two other major alien species, namely Ravenala madagascariensis Sonnerat (1782: 223) ( Strelitziaceae ) and Pinus elliottii Engelmann (1880: 186) ( Pinaceae ).

The sympatric and closely related species A. cadetii is also found to grow in the heath vegetation at Pétrin and around. Herbarium records indicate that this species was much more widespread than A. jeannineanum . Its sub-populations have been clearly sharply declining in Mauritius, such that only a handful of individuals are now known to remain in Pétrin, and at the nearby area of Grand Bassin. There are also seven plants in the Nature Reserve of Perrier, but these were introduced there in the 1970s ( Roberts 2001). Altogether, the known population of A. cadetii in Mauritius comprises less than 50 individuals. On Réunion, where the species is considered as Near Threatened (UICN– FRANCE et al. 2010), A. cadetii is more widespread and has better-preserved sub-populations.

Conservation status: —The only currently known population of A. jeannineanum is located in a remnant of native heath vegetation. It occurs in the 6 ha CMA of Pétrin, in the Black River Gorges National Park. CMAs are areas where invasive alien plants are controlled and fenced against large ground vertebrates (alien pigs and java deer mainly). In this locality, around 100 plants, including juvenile and adults, have been recorded. They all grow on stems of Erica trees, in a relatively clumped fashion. However, the species is also able to grow on other substrates. For example, the type specimen was found on Sideroxylon cinereum Lamarck (1783: 244–245) . Erica trees are absent from Bel Ombre.

In Mauritius, around 30% of the native orchid flora appears to be extinct (i.e. 27–29 species, Roberts & Wilcock 2003), while one native species (out of ca. 150) is believed to be extinct in Réunion (UICN-FRANCE et al. 2010). When comparing species shared by both islands, it appears, as a general pattern, that species from Mauritius have a lower number of sub-populations and usually with far fewer individuals than those in Réunion. There are many reasons for this pattern. The native forest remnants of Mauritius are much more restricted, covering around 5% of the total landmass ( Safford 1997); while around 30% of the native cover remains on Réunion ( Strasberg et al. 2005). However, only about one third of the remnant native vegetation on Mauritius harbour vegetation that has at least 50% of its canopy comprising native plants ( Page & D’Argent 1997). The main forest invader in Mauritius, P. cattleianum (Lorence & Sussman 1986, Florens 2008, Baider & Florens 2011), sheds its bark often, being therefore not a suitable support for most epiphytic orchids. Habitat degradation by invasive plants is an ongoing and progressing threat to the survival of most native species ( Baider & Florens 2006, 2011, Lorence & Sussman 1986, Florens 2008). Another problem faced by larger orchids is herbivory by alien monkeys, the long-tailed macaque Macaca fascicularis ( Raffles, 1821: 246) , which is absent on Réunion. Monkeys often rip and chew on the leaves of the larger epiphytic orchids before dropping the remains to the forest floor where conditions are often not suitable for them to survive. For example, a single monkey attack led to the destruction of 15–25% of leaves of a very large clump of Jumellea Schlechter (1914: 609) ( Baider et al. 2012), a genus closed to the genus Angraecum .

Angraecum jeannineanum has been confused with A. cadetii , in part because the two species are sympatric. Roberts (2001) recorded the reproductive output of the A. cadetii population of Pétrin, which was most probably A. jeannineanum , as Roberts said that the plants were growing in the heathland, in two groups of 43 and 80 individuals. From a sample of 57 orchids, 10 individuals were flowering and only one had fruits ( Roberts 2001), suggesting that the fruiting success of A. jeannineanum is probably very low. In 2008, of 30 flowering individuals 10 had fruits ( Fournel 2008), indicating that pollination and fruiting success are variable. Angraecum jeannineanum is likely to be pollinated by a Zosterops (accordingly to the ratio of the flower aperture, Fig. 4 ), although no pollinator visit was observed during 42 hours of video observations ( Fournel 2008). The two species of Zosterops found in Mauritius co-occur in the range of A. jeannineanum , however the Mauritius olive white-eye, Z. chloronothos ( Vieillot, 1817: 408) , is rare and declining, with an estimated population of less than 150 pairs (BirdLife 2012). Mauritius grey white-eye ( Z. borbonicus mauritianus Gmelin, 1789: 981 ) is, on the other hand, quite common. If A. jeannineanum is instead pollinated by an insect, there may be cause for concern in the face of invasion by alien plants because invaded vegetation have both a lower species richness and smaller populations of potential insect pollinators ( Florens et al. 2010, Hugel 2012, Kaiser et al. 2008; Kaiser-Bunbury et al. 2009).

Apart from these threats, A. jeannineanum is now known from around 100 individuals, including juveniles, from a single site (Pétrin). Although the species has been collected previously in two other localities, it appears unlikely than it still occurs there. Forests of Bel Ombre that were outside protected areas were deforested in 1950s ( Page & D’Argent 1997). Parts of Plaine Champagne were deforested in mid 1970s, not long after the type specimen was collected. The remaining vegetation at Plaine Champagne is now severely invaded by alien plants. This situation is known to elicit major declines in populations of native trees ( Florens 2008). The road crossing Plaine Champagne was enlarged in 1960–70s, and again in 2000s, increasing access to poachers of orchids and of wood, and also favouring further spread of invasive alien plants. Angraecum cadetii was considered as the rarest orchid of Mauritius ( Roberts 2001), and there were fears that it had gone extinct ( Strahm 1993), especially after the large deforestation of 1970s, during which most of the area where the species was known to occur was destroyed. For decades, many local botanists and amateurs have been looking for the species in native forest remnants from Mare aux Vacoas to Plaine Champagne, Florin, Grand Bassin, etc, which seems also suitable for A. jeannineanum , with almost no success, apart from one plant of A. cadetii found in 2014.

Today the species is known from a single site. When including the previous collections, the extent of occurrence (EOO) of the species would vary between of 2–3 km 2 and the area of occupancy (AOO) would be 12 km 2, but the AOO today would be only 4 km 2, representing a reduction by 67%. The species should thus be assessed as Critically Endangered CR B1ab(i,ii,iii,iv) + B2ab(i,ii,iii,iv); C2a(ii); D as per IUCN Red List criteria ( IUCN 2001).

Additional specimens examined (paratypes): — MAURITIUS. Bel Ombre, growing in garden in Curepipe, 17 March 1953, fl., O’Connor s.n. ( MAU 3192 ) ; Pétrin, Parc National des Gorges de la Rivière Noire, 600 m, 12 March 2008, fl., Allet in Fournel 700 ( REU12107 !, spirit) .

Examined specimens of Angraecum cadetii : — MAURITIUS. Les Mares, 15 March 1933, fl., Vaughan 837 ( K000364620 , MAU 1867 ) ; Les Mares, 15 May 1938, fl., Vaughan s.n. ( MAU 1601 ) ; Plaine Paul, 06 February 1972, fl., Lorence s.n. ( MAU 10673 ) ; Mt Cocotte , 14 March 1963, fl., Guého s.n. ( MAU 10673 ) ; forêts du centre, 1971, fl., Bosser 21146 ( P00107195 ) ; no loc., growing in garden in Curepipe, 2 April 1962, fl., Edgerley s.n. ( MAU 10352 ) . REUNION. Basse Vallée, 900 m, 15 February 1971, fl., Cadet 3036 ( REU06323 , REU06324 , P spirit) ; Hauts du Baril, 700 m, végétation preforestière, 19 January 1972, fl., Cadet 3507 ( REU06325 , REU06326 ) ; St Philippe, Mare Longue, 16 March 1972, fl., Cadet 3577 ( REU06322 ) ; St Philippe, Sentier Bras Plat, 730 m, 26 January 1990, fl., Lavergne 1738 ( REU07800 ) ; Sainte Marguerite, 6 June 2007, fr., Martos 116 ( REU13120 ) ; Bébour, Sentier Takamaka, 1150 m, 16 January 2008, Martos 190 ( REU10112 ) .

Figures

FIGURE 1. Flowers of the four species of Angraecum sect. Hadrangis. A. Angraecum striatum, showing aperture pattern of the flower with the vertical arrow indicating the height and the horizontal, the width.B. Angraecum cadetii. C. Angraecum bracteosum. D. Angraecum jeannineanum. Photos: J. Fournel and C. Micheneau.

FIGURE 2. Angraecum jeannineanum. A. Aspect of a flowering plant, B. Flower, C. Sepals, D. Labellum, E. Anther cap, F. Front view of the column, G. Longitudinal section of the ovary. Drawing by Jeannine Cadet.

FIGURE 3. Floral parts comparison (preserved in alcohol) between: A. Angraecum jeannineanum and B. Angraecum cadetii. a. Adaxial sepal. b. Lateral petals. c. Lateral sepals. d. Labellum. e. Ovary. f. Bracteole. Photos: J. Fournel.

FIGURE 4. Two-dimensional opening (distance between the labellum to the base of the pollinia and the distance between the edges of the labellum) of the flowers of Angraecum sect. Hadrangis.

FIGURE 5. Distribution of the new species Angraecum jeannineanum in Mauritius. Each dot is 1 km across. Close circle: actual known range. Open circle: recorded herbarium samples.