Flora of Madagascar

The flora of Madagascar is part of the island's unique wildlife. Madagascar has more than 10,000 plant species, of which more than 90 percent are only found here. The endemics include five unique plant families and such emblematic species as baobab trees or the traveler's palm.

The island has very contrasting vegetation types, with a notable distinction between the west, centre, and east. While the centre is dominated by dry forests and grasslands, the east, receiving most rain from the Indian Ocean, harbours mainly tropical rainforest, and the driest part of the country in the southwest features unique spiny forests.

Human use has changed Madagascar's natural flora dramatically. Rice paddies are a common feature, and other tropical crops such as bananas, yams, cassava, and vanilla are cultivated. Deforestation is a serious problem and has led to the decline of many native vegetation types.

Its high diversity coupled with dramatic decrease of its natural vegetation makes Madagascar one of the world's biodiversity hotspots. A number of areas are protected as nature reserves.

Diversity and endemism

Vascular plants

Madagascar has been described as "one of the most floristically unique places in the world".^[1] According to the Catalogue of the Vascular Plants of Madagascar, 243 vascular plant families with roughly 11,000 species are known, of which 83 per cent are only found on the island. This includes five endemic families: Asteropeiaceae, Barbeuiaceae, Physenaceae, Sarcolaenaceae and Sphaerosepalaceae.^[2] As many as 96 per cent of Madagascan trees and shrubs were estimated to be endemic.^[3]

Ferns and lycophytes have 272 described species in Madagascar. About half of these are endemic; in the scaly tree fern family Cyatheaceae, native to the eastern humid forests, all but three of 47 species are endemic. Six conifers in genus Podocarpus—five endemic—and one cycad, Cycas thouarsii, are native to the island.^[2]

In the flowering plants, basal groups account for some 320 Madagascan species, around 94% of which are endemic. The families most rich in species are Annonaceae, Lauraceae, Monimiaceae, and Myristicaceae, containing mainly trees, shrubs, and lianas, and the predominantly herbaceous pepper family (Piperaceae).^[2]

Monocots are highly diversified in Madagascar. The most species-rich plant family, orchids (Orchidaceae), fall in this lineage and have over 900 species, of which 85% are endemic. Palms (Arecaceae) have around 200 species in Madagascar (most in the large genus Dypsis), more than three times as many as in continental Africa; all but five are endemic. Other large monocot families include the Pandanaceae with 78 endemic pandan (Pandanus) species, mainly found in humid to wet habitats, and the Asphodelaceae with most species, over 130 endemics, in the succulent genus Aloe. Grasses (Poaceae, 541 species^[4]) and sedges (Cyperaceae, 314 species) are diverse, but have lower levels of endemism (40%^[4] and 37%, respectively). A national emblem and widely planted, the traveller's tree, endemic to humid habitats, is the sole Madagascan species in the family Strelitziaceae.^[2]

The eudicots account for most of Madagascar's plant diversity. Their most species-rich families on the island are:^[2]

• Fabaceae (662 species/77% endemic), accounting for many trees in humid and dry forests, including rosewood.
• Rubiaceae (632/92%), with notably over 100 endemic Psychotria and 60 endemic Coffea species.
• Asteraceae (533/81%), of which over 100 endemic species in Helichrysum.
• Acanthaceae (500/94%), with 90 endemic species in Hypoestes.
• Euphorbiaceae (460/94%), notably with the large genera Croton and Euphorbia.
• Malvaceae (485/87%), including the large genus Dombeya (177 species, 97% endemic) and seven out of nine baobabs (Adansonia), of which six are endemic.
• Apocynaceae (363/93%), including the Madagascar periwinkle (Catharanthus roseus).
• Melastomataceae (338/98%), mainly trees and shrubs.

A less species-rich, but very characteristic eudicot family is Didiereaceae with eleven species, only found in the spiny forests of the southwest.^[2]

Mosses, liverworts, and hornworts

An checklist from 2012 record 751 moss species and infraspecific taxa, 390 liverworts, and three hornworts for Madagascar. About 34% of the mosses and 19% of the liverworts are endemic. It is unknown how many of those may have gone extinct since their discovery, and a number of species likely remain to be described.^[5]

Fungi

If fungi are included in the flora, it is assumed that many species remain to be described in Madagascar. An overview of edible mushrooms listed a number of species consumed in the country, especially from the genera Auricularia, the chanterelles (Cantharellus), and the brittlegills (Russula). Most of the ectomycorrhizal species are found in plantations of introduced eucalyptus and pine, but also in native tapia woodlands.^[6]

The pathogen Batrachochytrium dendrobatidis, responsible for chytridiomycosis which threatens amphibian populations worldwide, was long considered absent from Madagascar. It was however first recorded in 2010 and confirmed since for various areas and different frog families in the country, alerting scientists about the potential threat to Madagascar's already endangered frog fauna.^[7]

Exploration and documentation

Early naturalists

Madagascar and its natural history remained relatively unknown outside the island before the 17th century. Its only overseas connections were occasional Arab, Portuguese, Dutch, and English sailors, who brought home anecdotes and tales about the fabulous nature of Madagascar.^[8] With the growing influence of the French in the Indian Ocean, it was mainly French naturalists that documented Madagascar's flora in the following centuries.

Étienne de Flacourt, envoy of France at the military post of Fort Dauphin (Tolagnaro) in the south of Madagascar from 1648 to 1655, wrote the first detailed account of the island, Histoire de la grande isle Madagascar (1658), with a chapter dedicated to the flora. He was the first to mention the endemic pitcher plant Nepenthes madagascariensis and the Madagascar periwinkle.^[9][10] About one century later, in 1770, French naturalists and voyagers Philibert Commerson and Pierre Sonnerat visited the country from the Isle de France (now Mauritius).^[11] They collected and described a number of plant species, and many of Commerson's specimens were later worked up by Jean-Baptiste de Lamarck and Jean Louis Marie Poiret in France.^[12]:93–95 Sonnerat described, among others, the emblematic traveller's tree, Ravenala madagascariensis.^[13] Another contemporary, Louis-Marie Aubert du Petit-Thouars, also visited Madagascar from the Isle de France; he collected on the island for six months and wrote, among others, Histoire des végétaux recueillis dans les îles australes d'Afrique^[14] and a work on orchids of Madagascar and the Mascarenes.^{[12]:344–345[15]}

Nineteenth century to present

One of the most productive explorers of Malagasy wildlife in the 19th century was Alfred Grandidier. His first visit in 1865 was followed by several other expeditions. He produced an atlas of the island and, in 1885, published his work L'Histoire physique, naturelle et politique de Madagascar, which would comprise 39 volumes.^[16] Although his main contributions were in zoology, he was also a prolific plant collector; several plants were named after him, including Grandidier's baobab (Adansonia grandidieri) and the endemic succulent genus Didierea. His work was continued by his son, Guillaume.^{[12]:185–187} A contemporary, the British missionary Richard Baron, lived in Madagascar from 1872 to 1907 where he also collected plants and discovered up to 1000 new species;^[17] many of his specimens were described by Kew botanist John Gilbert Baker.^[18] Baron was the first to catalogue Madagascar's vascular flora in his Compendium des plantes malgaches, including over 4,700 species and varieties known at that time.^[17][19]

During the French colonial period, one of the major botanists in Madagascar was Henri Perrier de la Bâthie, who worked in the country from 1896 onwards. He compiled a large herbarium which he later donated to the National Museum of Natural History in Paris. Among his publications was notably the first classification of the island's vegetation, La végétation malgache (1921),^[20] and Biogéographie des plantes de Madagascar (1936),^[21] and he directed the publication of the Catalogue des plantes de Madagascar in 29 volumes.^{[12]:338–339} A contemporary and collaborator who equally made large contributions to botany in Madagascar was Henri Humbert, professor in Algiers and later in Paris: He undertook ten expeditions to Madagascar and, in 1936, initiated and edited the monograph series Flore de Madagascar et des Comores.^{[12]:214–215} Other important botanists from the colonial era until shortly after Madagascar's independence, describing more than 200 species each, include:^[2] Aimée Camus, living in France and specialised on grasses;^[2] René Capuron, a major contributor to the woody plant flora; and Jean Bosser, director of the French ORSTOM institute in Antananarivo, working on grasses, sedges, and orchids.^[12]:32–33

Contemporary research

Today, national and international research institutions are documenting the flora of Madagascar. The Botanical and Zoological Garden of Tsimbazaza harbours a botanical garden as well as the country's largest herbarium (code TAN) with over 80,000 specimens, a number of which have been digitised and are available online through JSTOR and Tropicos.^[22] The University of Antananarivo has a research department for Plant Biology and Ecology.^[23] Outside the country, Royal Botanic Gardens, Kew are one of the leading institutions in the revision of Madagascar's plant families; Kew also maintains a permanent conservation centre in Madagascar and cooperates with the Silo National des Graines Forestières to build a seed bank of Malagasy Plants in the Millennium Seed Bank project.^[24] The National Museum of Natural History in Paris has traditionally been one of the centres for research on the flora of Madagascar. It holds a herbarium with roughly 700,000 Malagasy plant specimens as well as a seedbank and a living collection, and continues to edit the Flore de Madagascar et des Comores series begun by Humbert in 1936.^[25] Missouri Botanical Garden maintains the Catalogue of the Vascular Plants of Madagascar,^[2] a major online resource on the Malagasy flora, and is also present with a dependency in Madagascar.^[26]

Vegetation types

Madagascar features very contrasting and some unique vegetation types, determined mainly by topography, climate, and geology. A mountain range on the east, rising to 2,876 m (9,436 ft) at its highest point, captures most rainfall brought in by trade winds from the Indian Ocean. Consequently, the eastern belt harbours most of the humid forests, while precipitation decreases to the west. The rain shadow region in the southwest has a sub-arid climate. Temperatures are highest on the west coast, with annual means of up to 30 °C (86 °F), while the high massifs have a cool climate, with a 5 °C (41 °F) annual mean locally. Geology features mainly igneous and metamorphic basement rocks, with some lava and quartzite in the central and eastern plateaus, while the western part has belts of sandstone, limestone (including the tsingy formations), and unconsolidated sand.^[27]

The marked east–central–west distinction of the Madagascan flora was already described by Richard Baron in 1889.^[18] Authors of the 20th century, including Henri Perrier de la Bâthie and Henri Humbert, built up on this and proposed several similar classification systems, based on floristic and structural criteria.^[28] A recent classification is the Atlas of the Vegetation of Madagascar (2007), which resulted from the CEPF Madagascar Vegetation Mapping Project^[29] and distinguishes 15 vegetation types (including two degraded types) based on satellite imagery and ground surveys; these are defined mainly bases on vegetation structure and can have different species composition in different parts of the island.^[27] They partly correspond to the terrestrial ecoregions identified by the World Wildlife Fund (WWF) in Madagascar.^[30]

Humid forest

This tropical rainforest covers around 8 percent of the island, but used to encompass more than twice as much. It ranges from sea level to 2,750 m (9,020 ft) elevation and is mainly found on the eastern plateaus, on basement rocks with lateritic soils. Annual rainfall is 1,500–2,400 mm (59–94 in), the dry season short or absent. The predominantly evergreen forest, up to 35 m high (115 ft), is composed of diverse tree and understory species from various families such as Burseraceae, Ebenaceae, Fabaceae, and Myristaceae; bamboos and lianas are frequent.^[27] The WWF classifies the eastern belt, below 800 m (2,600 ft) elevation, in the Madagascar lowland forests^[31] and the montane forests of the central highlands in the Madagascar subhumid forests.^[32]

Degraded humid forest—savoka in Malagasy—covers ca. 10 percent of the land. It spans various states of degradation and is composed of forest remnants and planted or otherwise introduced species. It is primarily the result of slash-and-burn cultivation in primary forest. Some forest fragments still harbour a considerable amount of biodiversity.^[27]

Littoral forest

Found in several isolated areas along the eastern coast, this forest type covers less than 1 percent of the land area, on mainly sandy sediments. Climate is humid, with 1,300–3,200 mm (4.3–10.5 ft) annual rainfall. Littoral forest covers sandy soil forest, marsh forest, and grasslands. Its flora includes various tree families, lianas, and epiphytic orchids and ferns; in the marsh forests, pandans (Pandanus) and the traveler's tree (Ravenala madagascariensis) are common.^[27] They are part of the WWF's Madagascar lowland forests ecoregion.^[31]

Wooded grassland–bushland mosaic

This diverse mixture of different vegetation types covers roughly 23 percent of the surface. It ranges from sea level to 2,700 m (8,900 ft) elevation and is mainly found on the western and central plateaus and their escarpments, but also in areas of cleared vegetation in the south and east. Substrates are basement rock, sandstone, and lava; climate ranges from sub-humid to sub-arid. The flora has been strongly modified through human-induced fires and grazing, and includes exotic species such as plantations of pine, eucalypt, and cypress. At higher altitudes on thin soil, there is an indigenous, sclerophyllous vegetation that includes Asteraceae, Ericaceae, Lauraceae, and Podocarpaceae shrubs, among others.^[27] These forests and thickets on the high massifs are singled out by the WWF as Madagascar ericoid thickets,^[33] while most of the remainder falls in the Madagascar subhumid forests ecoregion.^[32]

Plateau grassland–wooded grassland mosaic

This most widespread type covers about 42 percent of the island in the west and the central plateaus, ranging from sea level to 2,700 m (8,900 ft). Geological substrate is variable but mainly igneous and metamorphic in the central highlands. Climate is variable likewise, ranging from sub-humid to sub-arid, with rainfall between 300 millimetres and 3,300 millimetres (12 in and 10.8 ft) per year. It is supposed to be of human origin in most parts, created and maintained through fire and grazing. Grasses, mainly Loudetia simplex and the endemic Aristida rufescens, are dominating the grassland parts, while wooded parts feature native and introduced trees.^[27] Most of this vegetation type falls in the Madagascar subhumid forests ecoregion.^[32]

Tapia forest

This evergreen forest type, dominated by its namesake tapia (Uapaca bojeri), is found on the western and central plateaus, at altitudes of 500–1,800 m (1,600–5,900 ft). It covers less than 1 percent of the surface. The broad regional climate is sub-humid to sub-arid, but tapia forest is mainly found in drier microclimates. Trees other than tapia include Asteropeiaceae, Sarcolaenaceae, and others, with a herbaceous understory. Tapia forest is subject to human pressure, but relatively well adapted to fire.^[27] The WWF includes tapia forests in the Madagascar subhumid forests.^[32]

Western humid forest

Confined to a very small plateau region in the south west, on the eastern slope of the Analavelona massif, on lavas and sand, at 700–1,300 m (2,300–4,300 ft) elevation. This isolated humid forest is maintained through condensating moisture from ascending air. The forest is unprotected but the local population considers it sacred.^[27] The WWF includes it in the Madagascar subhumid forests.^[32]

Western dry forest

Accounting for roughly 5 percent of the surface, Western dry forest is found in the west, from the northern tip of the island to the Mangoky river in the south. It ranges from sea level to 1,600 m (5,200 ft) in elevation. Climate is sub-humid to dry, with 600–1,500 mm (24–59 in) annual rainfall and a dry season of around six months. Geology is varied and can include limestone forming the eroded tsingy outcrops. Vegetation is diverse; it ranges from forest to bushland and includes trees of the Burseraceae, Fabaceae, Euphorbiaceae, and baobab species.^[27] The WWF classifies the northern part of this vegetation as Madagascar dry deciduous forest^[34] and the southern part, including the northernmost range of Didiereaceae, as Madagascar succulent woodlands.^[35]

Western sub-humid forest

This forest occurs inland in the south west and covers less than 1 percent surface, mainly on sandstone, at 70–100 m (230–330 ft) elevation. Climate is sub-humid to sub-arid, with 600–1,200 mm (24–47 in) annual rainfall. The vegetation, up to 20 m tall (66 ft) with a closed canopy, includes diverse trees with many endemics such as baobabs (Adansonia), Givotia madagascariensis, or the palm Ravenea madagascariensis. Cutting, clearing and invasive species such as opuntias and agaves threaten this vegetation type.^[27] It is part of the WWF's Madagascar subhumid forests.^[32]

South western dry spiny forest-thicket

This vegetation type unique to Madagascar covers ca. 3 percent of its area. It is confined to the sub-arid region in the southwest, at an elevation of up to 300 m (980 ft), on limestone and sandstone bedrocks. Mean annual rainfall is about 540 mm (21 in), and the rain season very short. It is a dense thicket composed of plants adapted to dry conditions, notably through succulent stems or leaves transformed into spines. The characteristic plants are the endemic Didiereaceae family, baobabs, and Euphorbia species.^[27] The WWF classifies it as Madagascar spiny thickets.^[36]

Degraded spiny forest accounts for ca. 1 percent of the surface and is the result of cutting, clearing, and encroachment. Introduced species associated to disturbed areas, such as agaves and opuntias, are found with remnants of the native flora.^[27]

South western coastal bushland

Located in the same larger southwest region as the spiny forest, this formation is only found along the coast, at an elevation of 0–50 m (160 ft). Climate is sub-arid, with only 370–380 mm (15–15 in) of rainfall per year, concentrated in one month or less. The substrate is mainly unconsolidated sediment. This vegetation type is an open bushland without closed canopy. It is dominated by several tree and shrubs, including Euphorbia species.^[27] It is part of the WWF's Madagascar spiny thickets.^[36]

Wetlands

Wetlands cover roughly 1 percent of the island, not counting running water. Marshes, swamp forests and lakes are found in all regions, along with rivers and streams. Typical species of wet habitats are several endemic Cyperus sedges, ferns, pandans (Pandanus), and the traveller's tree (Ravenala madagascariensis). Two species of water lilies, Nymphaea lotus and N. nouchali, are found in the west and east, respectively. Lagoons are mainly found on the east coast, but also occur in the west; they have a specialised halophyte flora. Peat bogs are restricted to highlands above 2,000 m (6,600 ft) elevation; their distinct vegetation includes, among others, Sphagnum moss and sundew species (Drosera). Many wetlands have been converted into rice paddies and are otherwise threatened by destruction and pollution.^[27]

Mangroves

Mangrove forests occur on the western, Mozambique channel coast, from the very north to just south of the Mangoky river delta. Eleven mangrove tree species are known from Madagascar, of which the most frequent belong to the families Acanthaceae, Lecythidaceae, Lythraceae, Combretaceae, and Rhizophoraceae. Mangrove forests are threatened by encroachment and cutting.^[27] The WWF lists them as Madagascar mangroves ecoregion.^[37]

Cultivation

Cultivation accounts for at least 3.9 percent of the surface, but its real extent is difficult to estimate. It includes especially rice fields, and there are plantations of sisal as well as pine and eucalyptus. Other important crops are cassava, sweet potatoes, sugar cane, coffee, and vanilla.^[27]

Origins and evolution

Madagascar's high species richness and endemicity are attributed to its long isolation as a continental island: Once part of the Gondwana supercontinent, Madagascar separated from continental Africa around 150–160, and from the Indian subcontinent 84–91 million years ago (mya).^[38] The Madagascan flora was therefore long seen as relict of an old Gondwanan vegetation, separated by vicariance through the continental break-up, although the possibility of some across-ocean dispersal was also admitted.^[39] Molecular clock analyses shifted support to immigration via dispersal for most plant and other organismal lineages, given their divergences estimated after Gondwana break-up;^[40][41] the only endemic plant lineage on Madagascar sufficiently old to be a possible Gondwana relict appears to be Takhtajania perrieri (Winteraceae).^[41] Most plant groups have African affinities, consistent with the relatively small distance to the continent, and strong similarities also exist with the Indian Ocean islands of the Comoros, Mascarenes, and Seychelles. There are however also links to other, more distant floras, such as those of India and Malesia.^[41]

After its separation from Africa, still connected with India, Madagascar moved polewards, to a position south of 30° latitude. It moved northwards again and crossed the subtropical ridge between the end of the Cretaceous (66 mya) and the start of the Oligocene (23 mya). This passage likely induced a dry, desert-like climate on the now–separate island, of which the sub-arid spiny forest–thicket in the southwest may be a remnant. Humid forests could only establish once Madagascar was north of the subtropical ridge by the Oligocene. Also, India, still close to the east, moved north towards Eurasia and cleared the eastern seaway in the Middle to Late Eocene, allowing more oceanic precipitation brought in by trade winds. The origin of the Indian Ocean monsoon system after around 8 mya is believed to have further favoured the expansion of humid and sub-humid forests in the Late Miocene.^[38]

Several hypotheses exist as to how plants and other organisms have diversified into so many species in Madagascar. They mainly assume either that species diverged in parapatry by gradually adapting to different environmental conditions on the island, for example dry versus humid, or lowland versus montane habitats, or that barriers such as large rivers, mountain ranges, or open land between forest fragments, favoured allopatric speciation.^[42]

Human impact

Humans colonised Madagascar relatively recently, with estimates ranging from 2000 to 4000 years before present for the first arrivals. While remainders of ancient settlements on the coast feature African, Asian and Arab traces, colonisation of the interior and the Central Highlands happened later and involved predominantly Austronesian settlers. Human settlement and expansion had a profound impact on the long-isolated flora of Madagascar.

Uses of native plants

Many native plant species are used as herbal remedies for a variety of afflictions. The drugs vinblastine and vincristine, used to treat Hodgkin's disease, leukemia and other cancers, were derived from the Madagascar periwinkle.^[43]

Agriculture

The most characteristic feature of agriculture in Madagascar is the widespread cultivation of rice in terraced fields, marking the cultural landscape of the Central Highlands. Rice is the staple food of the population and also an important crop for export; it was introduced, along with the cultivation technique, with the early Austronesian settlers. Conversion for rice cultivation has been one of the causes for the loss of natural wetlands in Madagascar.

A second important form of agriculture is zebu farming, mostly taking place in a half-wild, pastoralist fashion. Zebus have tremendous cultural importance in Madagascar and are the most valuable possession in many rural households. Land clearing for zebu farming through fire and-tree cutting, and the grazing of the zebus themselves, are widely believed to have been involved in the loss of forests in the Central Highlands and the expansion of grasslands, most of which are impoverished and dominated only by a few grass species.

Other agricultural crops include widely grown vegetables such as courgettes, carrots, onions, potato, or sweet potato. Tropical fruits, e.g. mango, bananas, papaya, or soursop, are grown as well as temperate fruits such as apples, pears, or peaches, depending on local climate. Most of these fruits and vegetable crops have origins outside Madagascar and were introduced with the earliest settlers or later traders and colonists (see also below, Introduced plants). The traditional slash-and-burn agriculture (tavy) practised for centuries, with a growing population today contributes to the loss of forests (see below, Threats and conservation.

Introduced plants

More than 1,300 exotic plants, around 10 percent compared to the native flora, have been reported for Madagascar, with the legumes (Fabaceae) as most frequent family. Many of them have been introduced for agriculture (see above) or have other uses. Around 600 exotic plant species have also naturalised and some are considered invasive.^[44] A notorious example is the water hyacinth, which widely spread through tropical regions including Madagascar and is considered one of the most important plant pests in wetlands. Several species of prickly pear cacti were introduced in southwest Madagascar. While many authors see introduced plants as a threat to the native flora,^[27] it has also been argued that they have not yet been directly linked to an extinction, and that many may actually provide economic or ecological benefits.^[44]

Threats and conservation

Habitat destruction threatens many of Madagascar's endemic species and has driven others to extinction. In 2003 Ravalomanana announced the Durban Vision, an initiative to more than triple the island's protected natural areas to over 60,000 km² (23,000 sq mi) or 10 percent of Madagascar's land surface. As of 2011, areas protected by the state included five Strict Nature Reserves (Réserves Naturelles Intégrales), 21 Wildlife Reserves (Réserves Spéciales) and 21 National Parks (Parcs Nationaux).^[45] In 2007 six of the national parks were declared a joint World Heritage Site under the name Rainforests of the Atsinanana. These parks are Marojejy, Masoala, Ranomafana, Zahamena, Andohahela and Andringitra.^[46] Local timber merchants are harvesting scarce species of rosewood trees from protected rainforests within Marojejy National Park and exporting the wood to China for the production of luxury furniture and musical instruments.^[47] To raise public awareness of Madagascar's environmental challenges, the Wildlife Conservation Society opened an exhibit entitled "Madagascar!" in June 2008 at the Bronx Zoo in New York.^[48]

The island has been classified by Conservation International as a biodiversity hotspot.^[49]

References