Location and General Description
The Madagascar Subhumid Forests are scattered in several "islands" of montane humid forest throughout the central highlands of Madagascar, the zone generally defined as above the coastal plain and escarpment starting at 900 m. The remaining large areas of the forest habitat are in the Sambirano region in the northwest, portions of Amber Mountain (Montagne d’Ambre) in the north, significant areas of the northern highlands (sensu Carleton and Goodman 1998), and the middle elevational portions of certain massifs in the central highlands (e.g., Ankaratra and Andringitra). Also included are some wetlands and lakes (e.g., Lake Alaotra) and the "tapia" forest of the central highlands between 800-1600 m and in the dry southwest portion of the ecoregion. Also, a few remnant regions of forest, isolated and highly fragmented, remain scattered across the central highlands (e.g., Ambohitantely, Ambohijanahary, Isalo). In the extreme southwest of the island, the isolated mountain of Analavelona retains in the summital area (1000-1300 m) a remnant subhumid forest surrounded by much drier vegetation. Degraded forests, huge expanses of secondary grasslands and exotic tree plantations surround these habitats. This secondary vegetation is the result of human activity; forest clearing and exploitation.
The subhumid forest ecoregion has been previously mapped as part of the eastern Madagascar regional center of endemism (White 1983). To the east, these subhumid forests meet moist forests, in the lowland forest ecoregion around 800 m elevation, and to the west they merge into the dry deciduous forest ecoregion around 600 m elevation. At higher elevations (generally above 1,800 to 2,000 m) these habitats are replaced by ericoid thickets.
The rainfall is approximately 1,500 mm per year, although it may total as much as 2,000 mm in the Sambirano area in the northwest and as little as 600 mm in the southwest. The temperatures at higher elevations are mainly moderate, between 15° and 25°C. There is a cool, dry season between July and September and a warmer wet season during the rest of the year.
The underlying geology of the ecoregion is mainly ancient Precambrian basement rocks that have been deformed and uplifted over millions of years. There are a few areas of more recent lava flows, and some alluvial deposits associated with wetlands (Du Puy and Moat 1996).
Vast grasslands now cover the central highlands at elevations ranging from 1,000 to 1,500 m (Jolly et al. 1984, Du Puy and Moat 1996). There is some debate regarding the degree to which this upland area was formerly forested and the degrees to which humans have affected the fauna and flora (Burney 1997, Lowry et al. 1997). However, it is clear there have been very significant anthropogenic changes in the ecoregion. The central highlands was once home to a remarkable array of endemic species. These included several species of elephant birds (Aepyornithidae), including the world’s largest bird species (Aepyornis maximus), a giant tortoise, and several species of lemurs most of which were large bodied species, some larger than female gorillas today. All of these species have become extinct since the arrival of humans on the island around 2,000 years ago.
The secondary grasslands that cover most of the high, central highlands are composed of alien or pantropical grass and tree species. There are only 3 or 4 species of grass over vast areas, resulting in a virtually sterile landscape with extremely low species diversity and endemism. The dominant grasses are Aristida similis and A. rufescens, interspersed with a few herbaceaous species. Several species of Eucalyptus and Acacia trees have been introduced and are now the most common trees in the highlands. Some native, fire-resistant trees persist in areas of the central highlands, including the endemic palms Bismarckia nobilis and Ravenala madagscariensis and the tapia tree (Uapaca bojeri). Other native trees of the genera Sarcolaena, Tambourissa and Weinmannia are also found.
The Sambirano region in the northwest is a center of endemism and a transition zone between the species compositions (both plant and animal) of the western and the eastern regions of Madagascar (Gautier and Goodman in press). The montane forests of the Sambirano, starting at between 600 and 1000 m, are very similar in structure to more eastern subhumid forests (Gautier in press). Relatively little remains of the lowland forest between sea level and about 800 m, the zone that was separated as the Sambirano Domain . There are subhumid forests up to 1,800 m. This habitat changes to a more sclerophyllous forest at higher elevations. This higher ground, above 2000 m, including the Tsaratanana Massif, is included in the ericold thicket ecoregion.
There are few remaining patches of subhumid forest on the central highlands. Small patches are found on Ankaratra Massif, and some larger forest blocks are on the slopes of Andringitra and Tsaratanana massifs. At the higher elevations, the subhumid forest, also referred to as sclerophyllous montane forest, holds canopy trees that are 10 to 12 m in height, including species from the families Rubiaceae, Lauraceae, Verbenaceae and Ericaceae. At lower elevations, from 1,400 to 1,600 m, the forest has a 15 m canopy and includes species in the families Cunoniaceae, Araliaceae, Cornaceae, Celestraceae, Anacardiaceae, Burseraceae, Euphorbiaceae, Lauraceae and Ebenaceae.
Amber Mountain (Montagne D’Ambre) contains a significant area of humid forest. An isolated basaltic mountain with a humid microclimate above 1000 m, it is surrounded by dry deciduous forests. At sea level, the annual precipitation is approximately 980 mm, while at the peak of Amber Mountain the precipitation averages 2,378 mm. The humid forest on the mountain slopes has a canopy 40 meters high, dominated by plants from the families Sapotaceae, Burseraceae, Monimiaceae, Lauraceae, Flacourtiaceae, Sterculiaceae, Myrtaceae, Annonaceae, Apocynaceae, Potaliaceae and Elaeocarpaceae (Nicoll and Langrand 1989). In addition to the floral diversity of the forest, the faunal diversity is high with 8 species of primates and nearly 80 species of birds, including the endemic species the Ambre Mountain Rock Thrush (Monticola erythronotus). As the natural habitats of this ecoregion experienced numerous vegetational shifts associated with Pleistocene climatic vicissitudes many species are endemic and have very narrow altitudinal or isolated distributions.
The remaining "tapia" woodlands, in the southwest of the ecoregion are restricted in distribution. The largest intact areas of this habitat are found in the Isalo and Itremo massifs on sandstone and quartzite. They are characterized by a relatively open canopy dominated by members of the family Sarcolaenaceae and Euphorbiaceae, including the fire-resistant Uapaca bojeri and the genus Sarcolaena. The shrub layer consists of Asteraceae, Rubiaceae, and Leguminosae. There are also some endemic Kalanchoe and Aloe species.
This ecoregion is similar to others in Madagascar in that most natural vegetation cover has been destroyed. The remaining small and isolated patches or distinctly larger blocks are biodiversity "jewels" essential for a variety endemic species.
Numerous forms of mammals are considered strictly endemic to this ecoregion, including the Alaotran gentle lemur (Hapalemur griseus aloatrensis) (Mittermeier et al. 1994), and a number of shrews, tenrecs, and rodents. A far larger number of species are nearly endemic, with the majority shared with the lowland forests to the east. At least 45 species of mammals are found only in the subhumid forest ecoregion and the lowland forest ecoregion on Madagascar and these include, for example, two species of bamboo lemurs (Hapalemur aureus and H. simus) (Harcourt 1990, Mittermeier et al. 1994). Of the endemic and near-endemic mammal species in the ecoregion, 12 species listed are on the IUCN Red Data List (Hilton-Taylor 2000); 9 species are considered vulnerable; 2 are endangered and 1 (the Alaotran gentle lemur) is critical. In the Analavelona forest a species of small mammal was recently discovered, Microgale nasoloi, that is only known from this site and the nearby Zombitse-Vohibasia Forest, the latter being classified in the Madagascar succulent woodlands ecoregion.
Two endemic bird species are found in the wetlands of this ecoregion, and others are confined to the subhumid forests or shared with other Madagascar ecoregions. In the wetlands, both the Alaotra little grebe (Tachybaptus rufolavatus) and the Madagascar pochard (Aythya innotata), are considered critically endangered (Hilton-Taylor 2000) and may be extinct. In the forests the endemic species include, for example a new genus and species only named a few years ago called the cryptic warbler (Cryptosylvicola randrianasoloi), the yellow-browed oxylabes (Crossleyia xanthophrys), and the brown emutail (Dromaeocercus brunneus). Several other species of birds found here are limited to marshland habitats on Madagascar, including the slender-billed flufftail (Sarothrura watersi), Madagascar snipe (Gallinago macrodactyla), and Madagascar rail (Rallus madagascariensis) (Langrand 1990). Further, Appert’s greenbul (Xanthomixis apperti), an endemic species with a very limited geographical distribution, is abundant on the upper reaches of the Analavelona Massif. More than 20 other bird species that occur in the subhumid forests of this ecoregion are shared only with the eastern lowland forests ecoregion.
There are at least 25 strictly endemic reptiles and more than 20 strictly endemic amphibians in this ecoregion.These numbers include described species and newly identified forms. Numerous forms of chameleon and dwarf chameleon only occur in this ecoregion, including Calumma oshaughnessyi ambreensis, C. tsaratananensis, Furcifer petteri, Brookesia ambreesis, B. antakarana, B. lineata, and B. lolontany in the northern and northwestern portion; and C. fallax, F. campani, and F. minor in the central and southern portions. Other lizard species endemic to the ecoregion include the skinks Mabuya grnadidieri, M. madagascariensis, M. nancycoutouae, Amphiglossus meva, and Androngo crenni; the geckos Lygodactylus blanci and Phelsuma klemmeri, and the plated lizard Zonosaurus ornatus. There are also a few endemic species of snakes including Pseudoxyrhopus ankafinensis, Liopholidophis grandidieri, and L. sexlineatus. A few groups of amphibians include more than one endemic species, such as the microhylids Rhombophryne testudo, Scaphiophryne goettliebi, the mantellids Mantella crocea, M. cowani, and Mantidactylus domerguei; and the rhacophorids Boophis laurenti and B. microtympanum.
The remaining natural habitats of the central highlands are extremely fragmented except for the zone spanning its eastern edge and the upper portion of the eastern escarpment. On the central highlands proper, the few patches of natural vegetation continue to be fragmented by grassland fires. Habitats of the ecoregion are partially protected with the remaining central highland forests of Ambohijanahary and Ambohitantely being protected areas and the eastern slopes of Andringitra and the upper slopes of Ranomafana being national parks. However, the degree to which the protected areas can maintain and manage the integrity of these habitats varies. Lack of resources, inadequate training and limited personnel, in addition to the absence of clear management plans, all contribute to the difficulty of preventing habitat destruction within the reserves.
Types and Severity of Threats
Remaining patches of forest and woodlands of the central highlands face continuous and intensive pressure from encroaching agriculture, increasing exploitation by growing human populations, and fire. Introduced plants and animals are affecting the integrity of habitats. Nearly all of the ecoregion has been modified either directly or indirectly by humans (see Lowry et al. 1997).
The wetland areas on the central highlands are threatened by conversion to rice farming, siltation, and pollution. Most marshlands and wetlands of Madagascar have already been degraded or converted to rice cultivation. A few relatively undisturbed areas remain, including the marshlands and associated forest of the Torotorofotsy area near Andasibe and other sites scattered throughout the ecoregion. Many of these marshlands still tend to be polluted with runoff from surrounding agricultural lands. Extinctions of freshwater fish have occurred in these wetlands in the last few years (P. Loiselle, 2000 pers. comm.). Lake Alaotra, in particular, is an important wetland possessing the endemic Alaotra little grebe and Madagascar pochard. The lake is also home to an endemic primate subspecies, the Alaotra gentle lemur (Hapalemur griseus alaotrensis), and several species of endemic fish. Lake Itasy used to be a significant habitat for water birds, but has been very degraded by intensive rice cultivation.
Justification of Ecoregion Delineation
The Madagascar Subhumid Forest, located in central Madagascar, is based on Cornet’s subhumid bioclimatic division (Cornet 1974), with the eastern boundary delineated at 800 m elevation and the western boundary delineated at 600 m elevation. Although the western boundary differs from Humbert’s vegetation map (Humbert 1959), the 600 m contour better reflects climatic patterns which distinguish moist evergreen forest from dry deciduous forest (Schatz per. comm.). The ecoregion also includes disjunct subhumid areas such as Mt. d’Ambre in the north and the Analavelona and Isalo massifs in the southwest.
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Prepared by: Helen Crowley
Reviewed by: In progress