Forest types in Madagascar
The 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:
- Coastal forest occupies a fairly narrow strip from a few hundred meters to several kilometers wide, rarely over 10 m in altitude, almost completely on sand, mostly flat or slightly undulating, with swamps and rivers. Many species typical of the eastern region in general are found here, but also Afzelia bijuga, Calophyllum inophyllum, Trachylobium verrucosum and Diospyros spp. as well as the Chrysalidocarpus lutescens palm. Only a few dozen hectares of this type of forest are now left.
- Hill and cliff forest stretches from the low valleys to an average altitude of 800 m. In the lower areas � up to 300 to 400 m � it is receding rapidly except for some areas in the north-east. The clay soil and the gentle, fairly unbroken undulation of the wide valleys and plains have allowed intensive cropping (coffee, banana, etc.). Species found in the few hundred thousand hectares of intact or only slightly degraded forest are very much the same as those on the coast, although Afzelia bijuga, Trachylobium verruscosum, Calophyllum spp. and Uapaca spp. are somewhat rarer, and Canarium spp. and Dalbergia spp. are more frequent. Higher altitude species found on cliffs between 400 and 800 m become increasingly frequent; given its relief � V-shaped valleys with many rivers, rapids and waterfalls. The higher part is still basically untouched and represents the most unexploited forest of the eastern strip. Ocotea spp., Revensara spp., Gambeya spp., Canarium madagascariensis, Syzygium spp., Symphonia spp., Sloanea spp., Dalbergia spp., Uapaca spp., Sarcolaena codomoclamys, Leptolaena bernieri and Schizolaena viscosa are the most frequent species, along with the traveller's tree (Ravanala madagascariensis) and the Ochlandra capitata bamboo.
- Highland forest (between 800 and 1 300 m) survives only in a few patches, most often in mountain depressions or along water courses. It differs especially from the hill and cliff forest in the presence of a single upper story that forms a high forest about 20 to 25 m tall. The most frequent genera are Tambourissa, Weinmannia, Symphonia, Dombeya, Dilobeia, Dalbergia, Canarium, Vernonia, Diospyros, Eugenia, Protorhus, Grewia and Brachylaena. The undergrowth is also thicker and more impenetrable, with a well-developed moss and lichen layer, interspersed with low shrubs, large lianas and monocarpic bamboos. This forest has suffered extensive exploitation and clearing.
- Lichen woodland, found at altitudes that are usually cloudy between about 1 300 and 2 000 m, and mainly with an eastern exposure, is made up of a high forest of fairly low (10 to 12 m) sclerophyllous trees with many branches forming a single story above a shrubby undergrowth, often blending into it. The only surviving examples are now scattered as a result of frequent fires. The most common species here are Dicorypha viticoid, Tina isoneura, Alberta minor and Rhus taratana. The tree story also contains Ilex mitis, bamboos such as Arundinaria spp. and Ochlandra spp., and conifers (Podocarpus). The moss and lichen story is very thick, often several dozen centimetres. Trees and shrubs are draped in epiphytes.
The low sclerophyllous forest on the western slopes of the eastern mountain chain has almost completely disappeared as a result of periodic fires. The rare and shrinking examples of this evergreen forest have a low upper story (10 to 12 m), composed mainly of a large number of arborescent species such as Uapaca bojeri, Leptolaena pauciflora, Sarcolaena oblongifolia, Asteropeia densiflora, Agauria salicifolia, Dodonaea madagascariensis, Faurea forficuliflora, Dicoma incana, Rhus taratana, Protorhus buxifolia and Cussonia bojeri. The shrubs in the undergrowth are mainly Philippia spp., Vaccinium spp., Helichrysum spp., etc. Lianas are fairly numerous, but tend to lack vigour, epiphytes are rare, and there are no bamboos. There is no mossy layer, and only scattered low perennial plants.
The western tropophilous forest comprises several large stands (Bara, Manasamody, Bongolava, Ankarafantsika, between Analalava and Ambato-Boeni and between Antsalova and the Mangoky, as well as along this river) and many small stands surrounded by expanding savannah. Bush fires and uncontrolled and often excessive logging are steadily eating away at this forest, which is rapidly disappearing everywhere. Rainfall is about 1 500 mm in the north-east, steadily decreasing to 600 mm in the south-west, but the special feature of this low-altitude "dry" zone is a long dry season lasting between five and seven months. The leaves fall during this period, unlike those of the closed forests of the eastern zone. One of the most common species, especially on calcareous soil, is Hildegardia erythrosiphon, a tall tree, easily recognisable by its habit, with its outspread, almost horizontal branches. As in the eastern forest, the high forest is generally thick, heterogeneous and rich in species, though to a lesser degree. The dominant trees do not exceed 20 to 25 m in the north and 15 to 20 m further south, although they can be quite broad. Nevertheless, their numbers and volume per hectare are lower than in the north-eastern forests. The undergrowth is very thick, but the herbaceous vegetation tends to be poor. Epiphytes are rare. The western forests have been divided into four categories, based on the wide variety in terrain:
- On gneiss or basalt lateritic clay, where humus is thicker than in the east or centre, the native forest is a fairly open high forest reaching 12 to 15 m, with large trees scattered here and there. Dalbergia, Stereospermum, Givotia madagascariensis, Xylia hildebrandtii and Cordyla madagascariensis are found here. There are many lianas, but no mosses or epiphytes. These stands are mainly confined to the north-east.
- Forests on sandy soils are fairly similar to those on crystalline ground, varying according to soil moisture. Tamarindus indica is frequent species on relatively moist soils and can cover large areas, for example on the Cretaceous sand of Ankarafantsika. On the usually dry soils of the southern zone, leafless arborescent species of Euphorbia that can reach 15 to 20 m (E. enterophora) are mixed with high-forest deciduous species (Chlorophora greveana, Securinega seyrigii, Hernandia voyroni, Protorhus deflexa and Flacourtia ramontchii), notably in the central Fiherenana valley.
- Forests on the calcareous highlands contain many species exclusive to them. The high forest, reaching 12 to 15 m and dominated by some large trees of the Adansonia, Diospyros and Acacia genera, is made up particularly of Protorhus perrieri, P. humberti, Erythrophysa spp., Albizia polyphylla, A. greveana, Sideroxylon collinum and Poinciana regia. There are no palms, ferns or epiphytes. The undergrowth is made up of shrubs and the lianas are often vigorous.
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 Di�go-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.