Madagascar is located in the indian Ocean, separated from Africa by the broad Mozambique channel, and crossed by the Capricorn tropic in its southern part, close to Toleara (formerly Tulear). The island is thus located in a tropical climate and stretches over 1500 km from north to south, its width reaching almost 600 km.
Its landscapes are quite diverse and one can find there :
- mesas of an average altitude between 800 and 1200 meters, in the central part (Ankaratra mountains) up to the north (Tsaratanana range), the highlands are separated and marked by deep valleys, the landscape culminating at nearly 2000 m in the north.
- the oriental side falls in steep slopes down to the indian Ocean, delimitating a narrow coastal stripe
- the occidental side, along the Mozambique channel, is less steep and lowers gently towards large littoral plains
Many streams are born from the central range, in particular the Mananje, Betsiboka and the Fiheranna all ending in the Mozambique channel, while the Lokoho, Simiana, Mangoro and Mananara rives, much shorter, reach the indian Ocean to the west.
The island is thus separated in two areas distinct by their elevation, a separation also reflected roughly by geology :
- The precambrian platform (gneiss, schists, « cipolins ») constituting the central plateau is surrounding granitic islands (Masoala peninsula) and basaltic areas (remainings of volcanic activity during cretaceous and quaternary periods)
- around this central part, limestone sediments can be found in the West (Ankarana and Bemaraha ranges) and in the south, liasic sandstones (especially ind the Isalo range)
As for the geology, the climate is quite diverse : Madagascar is located between the equatorial lows in the North, and the Indian Ocean anticyclone southwestwards. The alizé winds rule the East and monsoons the North-west. As a general rule, the austral winter from april to october is rather cool and dry, whilst the summer from november to march is warm and humid. However, the insular effect, the dissymetric landscape and the lenght of the island determine several climatic areas :
- The east coast. Submitted to the southwestern alizé winds, the coast receives strong rainfall almost all year round (3500 mm at Toamasina (form. Tamatave), 6000 mm sometimes at Maroantsetra !). Wet season from november to march. and heavy rainfall between June and the end of August, no marked dry season in this area where temperatures remain highall year round (23°<t<30° in december and 18°<t<25° in july in Toamasina
- The North : rainfall less abundat than on the east °<t<coast, but the north is wet and very warm, the dry season is more marked (22°<t<33° in december, 18°<t<31° in july in the Sambirano)
- The central highlands : their climate is influenced by the elevation with less rainfall, and a marked dry season, with lower temperatures. The daily thermical range is broad, and the night temperatures are close to freezing during winter time (18°<t<25°in december, and 10°<t<20° in July in Antananarivo
- The occidental side : It is submitted to low rainfall, the wet season covers 7 months with a crescently marked dry season from north to south, the temperatures are elevated.
- The South-west ant the south : with a semi-arid climate, the south and south-western part receive less than 500 mm rainfall a year, the wet season stretches over a few weeks in december-january, the dry season lasts for then months or so and is quite hot (22°<t<33° in december, 18°<t<27° in july in Toleara)
Torn apart from the continent for more than 50 million years, the island followed evolution processes on its own. Thus, most of the flora an fauna is endemic. Madagascar represents an unique haven for fauna and flora. Un fortunately, the malagasies have to face drastic poverty an their way of life, especially their need for fuelwood, sets the forest and wildlife under threat. Very few of the original malagasy forest still remains.
One can make out :
- the primary structures including primary forests and thickets : the latter represent only 16% of the islands area with a high level of endemism. These are :
- the dense sempervirent (evergreen )
- wet forests (Sambirano, northernmost part, and oriental side) and
- the deciduous forests : tropophilic or dry moutain forest in the central highlands, noth_west west and southwest, sclerophilic or mediterranean types in average elevations (western side of the central part),
- the thickets submitted to severe climatic conditions (cold or alternatively, hot and dry) located in the moutaneous areas of the central highlands, also in the southwest and the south (xerophilic thickets and bush)
- the secondary structures :
- they represent 63% of the islands area and originate from the degraded primary forests. These are the secondary
- forests (oriental side, Sambirano, settling after the slash-and-burn practice), the savannahs following the destruction of secondary forests (western areas,central highlands ans some areas of the western part) and the southern steppes.
- The distinctive structures : such as rocky ridges, granitic domes of the central highlands or the tsingy limestones in the Northwest : each area harbours a high level of microendemism, as well as the swamps and mangroves of the littoral. In order to better understand the upper explanations, a map of Madagascar can be found here :
The begonias of Madagascar : As can be noticed easily, Madagascar does not form one but many biotopes. The begonias of Madagascar can be encountered from sea level to an elevation of 1800 m approximatively. They can be found on almost any type of soils in the island.Apart from the North and the East, they endure a dry season that may be very constraining as in the West, where they enter dormancy. A great number of them are tuberous. The wet season marks the beginning of the new growth with a flowering period at its end and seed setting at the beginning of the new dry season.
The Madagascar flora is one of the more profuse in the world with 80% endemic* plants. Some Madagascar Begonia species grow in the whole of tropical Africa but many ones grow only to Madagascar. We are seeing an very close endemism, often limited at a forest, a range, a cliff or a river, especially on the east coast.
The Monique Keraudren-Aymonin flora (1983) describes about fifty Begonia species from Madagascar and the islands close to Madagascar (La Réunion, Maurice, Seychelles, Comores). Some ones are a bit known. Other species were discovered since some years ago, in particular by Henri Laporte, explorer, collector and begonias grower. With eleven expéditions to Madagascar, Henri met many new species. Sure, there are still many ones to discover.
To Madagascar, we can see begonias :
- in shady rain forests in plains in north-west and north areas, on a close band along the east coast, and in remant forests in south : Sambirano, Sambava, Mandritsara, Masoala and near Mananara and Betampona :
MUSCIBEGONIA : B. kalabenonensis ; B. perpusilla
The plants of these two sections are often small tuberous ones ; they grow on rocks near streams in the north and east of Madagascar .
Other ones are climbing ones, rhizomatous or sarmentous with persistant leaves, in east rain forests.
- in some remnants forests high up : Français Mountain, Ambre Mountain, Tsaratanana massif, Marojezy massif, Alaotra lake, Antsirabe and Fianarantsoa areas and along the streams Mananjeba, Lokoho, Simianaand Mandrare .
These plants are tuberous or rhizomatous and grow in decdeou*s forests in west,, in limestone Tsingy , near the streams in north ; they lost leaves and stems just before the dry season.
- in the scattered west dry forests : Mahajanga area (Betsiboka river) and l’Ankaratra massif ; in spite of a hard and long dry season, we can find some Begonia in damp rockery places : Isalo massif, high Stream Firehanana
QUADRILOBARIA : B. isalensis
MEZEIRA : B. humbertii
These bégonias are often high ones, on tall stems, in west of Madagascar ; leaves fall before the dry season and so, only stems are visible.
( HL means Henri Laporte and JDBS means Jacky Duruisseau / Bernadette Sénéchal)