Location and General Description
This succulent woodland is located in southwestern and central western Madagascar. Its southern limit is the start of the spiny thicket and northern limit the lower end of the dry deciduous forest. The southern portion of the succulent woodland ecoregion is inland from the spiny thicket and further north this ecoregion meets the sea around Belo-Tsiribihina. The eastern extent of the ecoregion, at the 600 m to 800 m contour, is contiguous with the sub-humid forests ecoregion, and to a large extent coincides with the southwestern limit of the central highlands. The fauna and flora of the succulent woodlands have some overlap and similarities with the spiny thickets and dry deciduous forests. Previous descriptions of phytogeographical regions of Madagascar had grouped the succulent woodlands with the dry deciduous forests as a "western domain" (see Guillaumet 1984, Lowry et al. 1997), but here this domain is separated to better differentiate between the spiny thicket and the dry deciduous woodlands – in many ways the succulent woodland ecoregion is a transitional habitat type between these two other ecoregions.
The succulent woodland ecoregion has a tropical dry climate with a distinct dry season between May and October. During the wet season, November to April, rainfall may reach 750 mm, within a yearly range of 575 mm to 1,330 mm. The annual average daily temperature for the region is between 25o and 31o C.
The geology of the western part of the ecoregion comprises unconsolidated sands on the coast and Tertiary limestones and sandstones inland. In the southern part of the ecoregion, there are also metamorphic and igneous basement rocks (Du Puy and Moat 1996). The soils are generally sandy with richer alluvial soils around river areas. The terrain is relatively flat but there are some notable rock outcrops and deep precipitous valleys (e.g., in the Makay region). The vegetation of the region is similar to dry deciduous woodland but is characterized by more xerophytic species. These species often have water storage adaptations, stem photosynthesis and remain without leaves for long periods. Forests of the ecoregion may reach 15 m in height, with the endemic baobabs (Bombaceae family) Adansonia za and A. grandidieri as distinctive emergent species. Other canopy species belong to the families Euphorbiaceae and Leguminosae including several endemic species of Pachypodium. The shrub layer consists of the families Sapindaceae, Euphorbiaceae, Anacardiaceae, and Burseraceae (White 1983).
In the southwest the coastal Mikea Forest between Manombo and Morombe have some species characteristic of southern "spiny thickets," such as Didierea madagascariensis of Madagascar’s endemic family, Didiereaceae. These forests grow on sandy soils and in a semi-arid climate with annual precipitation as low as 350 mm. The canopy, which rarely exceeds 12 m, is shorter than that of the forests inland and of those further north. The plant families forming the canopy include Leguminosae, Euphorbiacae, Burseraceae, and Bombaceae. The shrub layer is comprised mainly of Croton sp. (Euphorbiaceae), Aloe vaombe (Aloaceae) and lianas such as Dioscorea sp. (Dioscoreaceae) (Nicoll and Langrand 1989).
The forests of the ecoregion represent important habitats for 8 species of lemurs and 60 to 90 species of birds. There is an overlap of species between the succulent woodlands and the spiny thicket to the south as well as the dry deciduous forests to the north. Within the succulent woodland ecoregion there are several areas of high local endemicity (e.g., the forests between the Tsiribihina and Mangoky rivers and around the National Park of Zombitse-Vohibasia. Five mammals are endemic to this ecoregion: narrow striped mongoose (Mungotictis decemlineata decemlineata), pale fork-marked lemur (Phaner furcifer pallescens), the giant jumping rat, Berthe’s mouse lemur (Microcebus berthae) and the red-tailed sportive lemur (Lepilemur ruficaudatus). Near-endemics include the large-eared tenrec (Geogale aurita), the lesser hedgehog tenrec (Echinops telfairi), and Coquerel’s dwarf lemur (Mirza coquerli). Verreaux’s sifaka (Propithecus verreauxi verreauxi), and the red-fronted brown lemur (Eulemur fulvus rufus) are both found in this ecoregion. Several animal species have the whole of their very localized ranges within this ecoregion.
Among the birds, Appert’s greenbul (Xanthomixis apperti) and the white-breasted mesite (Mesitornis variegata), are considered endemic to this ecoregion. The following species are near-endemic: Madagascar teal (Anas bernieri), Madagascar plover (Charadrius thoracicus), and long-tailed ground-roller (Uratelornis chimaera). Out of the above birds, one is threatened (Madagascar teal), and three are considered vulnerable (white-breasted mesite, long-tailed ground-roller, Madagascar plover). The red-capped coua (Coua ruficeps) is found throughout this ecoregion.
Some of the local endemic reptiles include Oplurus cuvieri, Chalarodon madagascariensis, and the gecko Phelsuma standingi. Pyxis planicauda has a narrow distribution range within the ecoregion. One gecko species, Paroedura vazimba, is only known from Zombitse-Vohibasia National Park. A least two frog species are endemic to this region: the hyperollid Heterixalus luteostriatus and the mycrohylid Dyscophus insularis. The rare snake Liophidium chabaudi occurs in this ecoregion, as well as numerous other species with limited distributions such as Mabuya tandrefana, Furcifer antimena, and Brookesia brygooi. The ecoregion also includes the headwaters for the important southern rivers Taheza, Fiheranana and Onilahy.
A number of protected areas fall within the succulent woodlands ecoregion, including Zombitse-Vohibasia National Park, Andranomena Special Reserve, and Kirindy-Mitea National Park. Further, the Kirindy forest, north of Marofandilia, is managed as a private reserve. These protected zones, as well as several classified forests, comprise a relatively small area of the remaining habitat within the ecoregion. The degree to which the forest and wildlife is protected varies considerably among reserves. The classified forests offer relatively little protection as logging continues in these areas. Some of the most important habitats, such as the Mikea forest, have no protection.
Types and Severity of Threats
As with many of the habitats of Madagascar, the major threat to the succulent woodlands is fire - both intentional burning for expansion of agricultural lands and unintentional wildfires. Further, the conversion of hardwoods into charcoal occurs in this region. Although it is not known whether the ecoregion comprised only dry woodland, or a mosaic of woodland and grassland prior to human settlement, the increased incidence of fire in recent times has certainly led increasingly fragmented and isolated patches of native vegetation.
The Malagasy endemic tree Hazomalania voyroni is at risk of becoming extinct through traditional forest exploitation for construction wood. Some attempts have been made to propagate and replant this species (Randrianasolo et al. 1996). In addition to this species, there are several other endemic trees that are removed from the forests, mainly for construction purposes including Givotia madagascariense, Cedrelopsis grevei, and Commifora arofy. Various endemic species are also collected for charcoal production, which is considered a grave problem here and in the spiny thicket ecoregion.
Traditional hunting continues in the ecoregion even within protected areas. With increasing populations and greater movement of people, these traditional activities are possibly becoming locally unsustainable. The main species threatened by hunting are the tenrec (Tenrec ecaudatus), fruit bats (Pteropus rufus and Eidolon helvum), and the red-fronted brown lemur. In many parts of the ecoregion, cattle and goat grazing are degrading the forests. Wood exploitation for charcoal production has caused massive deforestation. Honey collection, through the felling of trees, is a traditional activity that threatens the forests of the ecoregion to lesser degree.
Justification of Ecoregion Delineation
This ecoregion forms the northern part of the ‘subarid’ bioclimate zone of Cornet (1974). Humbert’s (1955) ‘southern vegetation domain’ was used to delineate the Spiny Thicket ecoregion, with the remaining extent of Cornet’s subarid bioclimate defined as the Succulent Woodland. It shares floral affinities with both the spiny thicket and western dry-deciduous forest, yet contains distinct assemblages of plants and mammals and represents the northern limit of Didieraceae.
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Prepared by: Helen Crowley
Reviewed by: In progress