Forest types in MadagascarThe following is excerpted from the FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS. The text is copyright FAO.
The eastern rainforest accounts for over half the country's closed forests, extending mostly unbroken from north to south and rising from the coast to the central highlands and mountains. It tends to be a heterogeneous forest between 25 and 30 m in height, made up of over 150 species mixed together, with no dominating species. The large trees rarely grow straight; they have many branches and are covered with epiphytes, mosses and lichens. There are no well-defined layers but rather a very dense, closely-mixed complex. The intermediate species grow much straighter and have a better shape than the dominant species. The understory is very thick and often contains many lianas and bamboo-lianas. Generally speaking, the forest is wetter and richer toward the north and toward the coast. Its vigor and quality decline further south and especially further west, largely as a result of rainfall and temperature. Rainfall can reach 3 000 mm or even more in the north, but is considerably less than 2 000 mm in the south, and there is a similar difference moving from east to west on the eastern slope up to an altitude of 1 000 m. Four main types of eastern forest can be distinguished, depending on altitude:
Forests on alluvial soil and alongside watercourses have been almost totally destroyed, initially by clearing for crops, and then, after invasion by savannah grasses, by repeated fires during the dry season. The rare remnants have a high forest of 25 to 30 m containing both deciduous (Canarum multiflorum and Khaya madagascariensis) and evergreen (Eugenia sakalavanum) species. Terminalia mantaly, T. tricristata, Ficus sakalavanum, Albizia bernieri and A. boinensis are also found here. Two large palms, Medemia nobilis and Borassus madagascariensis, and Treculia perrieri are part of these formations. Undergrowth is sparse or absent.
On saline alluvial soils directly exposed to the tides, mangroves are found in the far north near Diégo-Suarez and then continue to just south of Morombe, almost exclusively on the western coast, with the following species: Rhizophora mucronata, Bruguiera gymnorhiza, Ceriops boiviniana, Avicennia officinalis and Sonneratio alba. These species are often mixed with Heritiera littoralis, Terminalia catappa, Thespesia populnea, Derris uliginosa and, on the edge of these stands, Casuarina equisetifolia, as well as a large fern, Acrostichum aureum. The largest stand is found along the Betsiboka estuary. Mangroves are of considerable economic value and are thus extensively exploited (tannin bark from the Rhizophora, Ceriops and Bruguiera genera, and firewood), easily regenerating under normal conditions.
Some swampy areas in the west often contain relatively pure stands, such as raffias on clay-based depressions, where the Raphia ruffia palm occurs in almost pure stands with two ferns, Nephrodium unitum and N. cucullatum. On crystalline soil or where swamps are temporary, Ficus sakalavanum is found together with Phragmites. On the other hand, Typha angustifolia and the fern Acrostichum aureum proliferate on calcareous ground.
Relics of gallery forests very similar to those along western rivers are found along water courses in the semi-arid southern zone. The main species found here include Colvillea racemosa, Eugenia sakalavanum, Protorhus grandidieri, Ficus spp. and Tamarindus indica, most of them evergreen.
The Malagasy term savoka refers to secondary formations varying widely in appearance and composition and following directly on closed forest destroyed by shifting cultivation (felling, followed by burning of what has been felled). There are few species capable of occupying the land immediately after the temporary cultivation has ceased, and they form almost impenetrable and generally nearly pure stands. In the east, they are mainly bamboos (Ochlandra capitata and Arundo madagascariensis), Afromomum angustifolium, Daniellia ensifolia, Psiadia altissima, Philippia spp., Solanum auriculatum and Harungana madagascariensis. The last is a small tree forming secondary stands that cover large areas and resembles an almost pure forest with an undergrowth of Pteridium aquilinum and Lycopodium clavatum, mixed with a small number of indigenous species such as Tristemma virusanum or naturalised species such as Psidium gujava and various species of Citrus, etc. Further toward the centre, medium-altitude (800 to 1 300 m) savokas contain Solanum auriculatum, Harungana madagascariensis, Ravenala madagascariensis, Trema orientalis, Dombeya spp. and bamboos. Savokas would revert to forest if farmers did not destroy them when primary forest starts to become scarce and they want to establish new crops or increase the area of savannah as grazing land for cattle. These secondary formations are very widespread in the eastern part of the country.
In the eastern zone, after prolonged cropping, savoka is invaded by grasses that burn very easily, leading to a reduction in secondary woody vegetation. The resulting savannah is therefore man-made and not due to climate. It is covered by tall grasses, Hyparrhenia rufa being dominant on good soils, and Eleusine indica and Pennisetum spp. on degraded land. It can reach heights of 1.5 to 3 m. There are a few species that resist repeated fires. For instance, Ravenala madagascariensis is able to withstand fires and is thus found dotted about the countryside. Some species from the low sclerophyllous forest found on western slopes, such as Uapaca bojeri, have spread into the fairly recent secondary savannah, although they eventually disappear because of the repeated fires that destroy the young trees especially Dicoma incana, Stereospermum euphorioides and Acidocarpus excelsus. However, older and more widespread exemplars of other species for example Brachylaena ramiflora and a Nuxia and a Ficus species have resisted the fires, thanks to their virtually incombustible bark and the absence of grasses under their leafy cover.
The savannah in the western zone is made up chiefly of grasses that are taller and generally thicker than those in the eastern or central zones (Hyparrhenia spp.). On fertile soil, where grasses grow more strongly and fires have thus been fiercer, there are no trees or shrubs. On dry, sandy or stony soils, where grasses are thinner, the savannah is dotted with trees or shrubs. Again, this is a man-made formation. Even so, the western forest does contain a number of fire-resistant woody species that are even capable of multiplying within grassy formations when the latter start to spread. Various species of palm are particularly fire-resistant, the most remarkable and the most common in most of the zone being Medemia nobilis. Another palm that proliferates especially on sandy soil is Hyphaena shatan, while Borassus madagascariensis is more restricted in range. Apart from these palms, the other trees most commonly found in the western savannahs are (in decreasing order) Sclerocarya caffra, Acridocarpus excelsus, Stereospermum euphorioides and Incoma incana, as well as some shrubs, although most of the latter are eliminated by the frequent fires: Strychnos spinosa, Gymnosperia linearis, Bridelia pervilleana, Terminalia seyrigii and Grewia triflora.
Dense high-mountain (over 2 000 m) thickets form communities of varying sizes found from the north to the south of the island, with certain common features which lend them a definite similarity in appearance and composition wherever they are found. This vegetation is made up, for the most part, of many species of heath (Ericaceae) for example, briar (Philippia) and Compositae (Psiadia, Senecio and Vernonia). Some species are common to several mountain areas, while some genera have different endemic species in almost every area, for example Heteromorpha spp. and Helichrysum spp. The genera Syncephalum, Vaccinium, Kosteletzkia, Dialypetalum, Nicodemia, Gentianothamnus, Radamaea, Terminalia, Aristea and Arthropodium are each represented by one or two endemic species. Here and there, some trees rise slightly above these thickets: Agauria salicifolia (Ericaceae), Ilex mitis, Cussonia bojeri, Alberta minor, Dodonaea madagascariensis, Tambourissa gracilis, Podocarpus rostratus, Vitex humbertii, Pittosporum chysophyllum, Faurea forficuliflora, Weinmannia spp., etc. There are almost no lianas and very few epiphytes. The moss and lichen layer is patchy and herbaceous plants fairly rare.
Frequent clearing and repeated fires have degraded the original forest into heath-like bushland of Helichrysum, Philippia, Agauria and Pteridium. On the other hand, degradation of the tropophilous forest results in "thicket woodland" with a broken dominant tree layer and a bushy layer, sometimes closed, sometimes less so.
The semi-arid southern zone, where annual rainfall is only 300 to 500 mm, is composed for the most part of plains and low plateaux of 200 to 400 m. It is covered by a xerophilous vegetation with a large number of species, although two main groups can be distinguished: one is the endemic family of Didiereaceae (Didierea madagascariensis, D. trollii, Alluaudia procea, Decaryia madagascariensis and Alluaudiopsis spp.), the other the Euphorbia genus (E. stenoclada and E. laroo). These are fairly small trees, not growing to more than 10 or 12 m, and do not form forests. Rather, together with some other more scattered species Adansonia za, A. fony, Tetrapterocapon geayi, Dicoma incana, D. carbonaria, Gyrocarpus americanus, Maerua filiformis and Ficus marmorata they tend to dominate thickets, which are fairly open on rocky soils, but more closed and impenetrable elsewhere. The woody species making up these thickets belong to many genera, including Acacia, Chadsia, Commiphora, Grewia, Solanum, Dichrostachys, Iphiona, Uncarina, Jatropha, Gardenia, Rhigozum, Cadaba, Megistostegium and Sclerocarya. Lianas are abundant, but not large. Trees, shrubs and lianas are partly leafless or have sparse evergreen or deciduous foliage. There are many thorn species, as well as species with thick, succulent leaves. Low plants are scattered over the ground, including grasses and various members of the Abutilon, small Aloe and Senecio genera. An unusual palm with tristichous leaves, Neodypsis decaryi, is confined to the eastern part of the southern zone, growing on the lower slopes of the first mountains along its edge.
The salt flats contain various genera typical of such ecological conditions, represented here by shrubs or grasses: Erblichia, Turnera, Flumbago, Salicornia, Salvadora angustifolia, Cryptostegia pluchea, etc. Scaevola koenigii, S. pluieri and Lumnitzera racemosa are common shrubs on dunes, helping to hold them in place.
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