The Didiereaceae Drake is a small family endemic to Madagascar consisting of four genera, Alluaudia, Alluaudiopsis.Decaryia, and Didierea, with eleven species. The spiny cylindrical or conical pachycaul stems of the plants give a superficial resemblance to columnar species of Euphorbia or cacti and the Didiereaceae are sometimes known as "the cacti of the old world". The stems are woody, and scarcely succulent, with limited space for water storage. The plants have simple, deciduous leaves, except for Alluaudia dumosa, and small, unisexual flowers. Accounts of the family are given by Rauh (1963) and Rowley (1993).
Species of the Didiereaceae are ecologically important components of the dry thorny forest in the south and south-west of Madagascar. Much of this vegetation has been cleared for agriculture and most of the remaining stands are impenetrable or nearly so. Some remnant patches of dry thorny forest occur within protected areas. The Reserve naturelle integrale de Tsimanampetsotsa, for example, has vegetation dominated by Didiereaceae and Euphorbiaceae and covers part of the very restricted distribution of Alluaudia montagnacii. The Reserve naturelle integrale d'Andohahela has a good representation of Alluaudia and Didierea spp.
The whole family is threatened mainly by habitat destruction and utilisation of the wood for construction and charcoal production. The wood of Alluaudia procera, A. ascendens, and A. montagnacii is used locally in the areas where the species grow and is sold in the cities of Madagascar. Exploitation of the wood is a threat, for example, to the species and vegetation in the Reserve naturelle integrale d'Andohahela.
Another threat to Didiereaceae is collection for horticulture. All species are in demand by collectors, but they rarely flower in cultivation, and seed is in short supply. Some species are cultivated in Madagascar for export, but others such as Didierea madagascariensis and D. trollii are gathered from the wild and exported as seedlings. The whole family has been included in Appendix II of CITES since the Convention came into force in 1976. This has enabled the collection of trade data for the family, which revealed, for example, the import of thousands of wild plants of Didierea and Alluaudia into Europe in the mid-1980s erroneously labelled as artificially propagated plants.
of Madagascar, although a general identification guide to the common plants of the island is being prepared.
Madagascar can be divided into a number of phytogeographical regions reflecting local differences in topography, geology, soils, and climatic conditions. Within four broad regions — eastern, central highlands, western, and southern / south-western — seventeen vegetation types are recognised, within which there are many distinct plant communities. No systematic classification at the community level is available as yet. Most of Madagascar's vegetation has been extensively modified and only about 20 per cent of the natural vegetation remains.
lnselberg on the Central
Flore de Madagascar et des Comores (Humbert 1936) covers most of the major succulent families, but many of the early volumes are out of date. The genus Euphorbia has not yet been covered by the Flora. Some of the relatively well known succulent groups such as the genus Aloe and geophytic Euphorbia spp. are in need of taxonomic attention (Supthut and von Arx 1993). In the case of Aloe spp. their spatial separation on isolated inselbergs, long intervals of time between species description, and difficulty of access to sites have all contributed to taxonomic uncertainty. Professor Rauh has prepared the first volume of Succulent and Xerophytic Plants of Madagascar (1995) and volume two is expected in 1997. There is currently no local guide to the succulents
The eastern escarpment to the central highlands all the way to the coast, characterised by high rainfall and a variety of soil types, is the richest botanically. Vegetation consists of a band of evergreen rain forest below 800 m and along the coast, now extensively deforested and replaced by 'savoka', or secondary lowland forest.
The undulating central plateau region, from 500-1500 m in elevation, is characterised by lateritic clay soils and montane and cloud forests. The plains have been overgrazed and burnt for centuries resulting in impoverished grasslands with their upper layers strongly eroded and are now very poor in succulent species (Rauh 1983b). Rupicolous vegetation occurs on the inselbergs which occur in the plateau area of central Madagascar. The inselberg rock formations of volcanic origin represent local ecological niches in the middle of a montane forest zone. The initial colonists, after lichens and mosses, are plants of the genus Fimbristylis which produce a pure black neutral to acidic humus carpet. This becomes completely desiccated in the dry season (Rauh 1983b). Succulents can tolerate the soil conditions and are well represented in the rupicolous vegetation with, for example, species of Pachypodium, Aloe, Kalanchoe, Cynanchum, Euphorbia, and Tetradenia. Most of the highland Aloe species are inhabitants of the inselberg rock formations: A. haworthioides; A. parvula, which grows in the quartz Itremo rocks in cushions of Fimbristylis or in cracks in the rocks; A. laeta in the Massif du Mont Ibity
Didierea trolli, Madagascar.
growing in association with A. ibitensis; A. paraUelifolia; and A. trachyticola (Rauh 1983b). The floral composition of rupicolous vegetation on the western rocky outcrops is, in general, relatively poorly known.
The western region has climax vegetation of dry deciduous forest with woody endemic succulents of genera such as Adenia, Adansonia, Uncarina, Pachypodium, and Euphorbia. Most of the deciduous forest has been destroyed by human activity and is replaced by secondary or wooded grassland. The western escarpment of the central plateau consists of low evergreen sclerophyllous forest, and transitional forest in the Sambirano region of north-west Madagascar.
The southern and south-western region is characterised by arid climatic conditions. The climate is characterised by lack of seasonality with the slight precipitation (approximately 300 mm per year) falling irregularly throughout the year. Towards the coast high air humidity is a climatic feature, with heavy dew and fog precipitation. The vegetation consists of deciduous xerophytic thicket, sometimes known as spiny desert. This vegetation is dominated by Didiereaceae and Euphorbia spp. It has a sparsely developed ground layer consisting predominantly of succulents. Deciduous thicket has been replaced by grassland over much of its natural area of distribution.
Succulent plants are represented in all regions of Madagascar. The two most important regions are the Central Plateau, on the inselbergs, and the dry forests in the north-west, west, and south. The remarkable vegetation of the southern coastal region supports the highest percentage of endemic succulent plant species for the country as a whole.
Burning — Large areas of Madagascar have been burned since the first settlers arrived on the island 1500 years ago. The extensive areas of grassland are burned each year to provide pasture for zebu cattle. Some succulent species such as Aloe macroclada and various species of Euphorbia such as E. primulifolia are relatively fire-resistant, and the inselberg communities initially escape damage from fire, but in general burning is a major threat to the succulent plant flora. Pachypodium spp. are for example particularly susceptible to fire damage as are some rare Aloe spp.
Other Madagascan succulents considered to be endangered by fire include the asclepiads: Ceropegia dimorpha, C. armandii, both of which are also sought after by succulent collectors, C. bosseri, C. leroyi, and Cynanchum rossii. The latter two have not been seen in the wild for the past twenty years. The genus Stapelianthus which is confined to the south-west of the island, is also susceptible to fire, with the following species endangered | by burning and clearance of the xerophytic forest: ^ Stapelianthus decaryi, S. pilosus, S. madagascariensis, S.
montagnacii, S. insignis, S. hardyi, S. keraudrenae, S. Q arenarius, and S. calcarophilus.
Didierea trolli, Madagascar.
Grazing — The zebu cattle of Madagascar and their impact on the flora and fauna are well known. There are an estimated 10 million zebu on the island and the cattle are of great cultural importance.
Clearance for agriculture — Traditional agriculture in Madagascar consists of various forms of shifting and settled cultivation producing predominantly rice, with cassava, sweet potatoes, and maize. The main cash crops are coffee, grown mainly along the east coast and in the north-west; cocoa, also grown in the north-west; vanilla, grown mainly in the north; and cloves grown on the eastern coastal plains. Large plantations with oil palms, green peppers, cinnamon, and cloves are found in the area.
Agricultural development has been less important in the arid succulent-rich areas of southern Madagascar. Nevertheless, a large area of Didiereaceae vegetation has been cleared in the Amboasary region for sisal, Agave sisalana, plantations. Production of sisal declined until recently but is now expanding again and remains an important industry. Two new large sisal plantations have been laid out in the vicinity of Amboasary and another new sisal plantation exists north of Morandava. The rare species, Euphorbia cylindrifolia, E. ambovombense, E. ampanihense, together with some Stapelianthus and small Aloe species grow in Alluaudia-Euphorbia forests close to sisal plantations and could be destroyed through intensified agricultural development together with extensive clearance of forests for charcoal production. Opuntia plantations are also established in this region.
Introduced species —Agave sisalana is an invasive introduced species which continues to have a detrimental impact on the ecology of the xerophytic thicket
Euphorbia quartziticola, a Rare plant restricted to quartz substrates; threatened by habitat destruction.
vegetation. The species spreads by prolific vegetative reproduction, destroying primary vegetation.
The prickly pear, Opuntia dillenii, first introduced to Fort-Dauphin as fencing around houses in around 1770, quickly became established to the detriment of indigenous vegetation. Brought under control in the 1920s, the species is again established in southern Madagascar. Various other Opuntia spp. are planted for fodder throughout southern Madagascar.
Furcraea is another invasive, and increasingly common, introduced species, particularly on the Central High Plateau.
Alluaudia-Euphorbia forest, since destroyed, featuring E. plagiantha, A. ascendens, and A. procera.
Alluaudia montagnacii, used for construction and charcoal production.
Local use — Collection of fuelwood affects woody succulent species in the vicinity of towns such as Tulear, Sakahara, and Fort-Dauphin. The production of charcoal also consumes significant quantities of woody succulents particularly close to Tulear, where there is a commercial charcoal operation. Charcoal production is also common to the north of Tulear and at Ampanihy, Tsihombe, Amboasary, and Ambovombe. Species used for charcoal production include Euphorbia spp., Alluaudia spp., and Decaryia madagascariensis. Charcoal production is both for domestic consumption and export overseas.
Woody succulent species are utilised as timber for construction of dwellings. This places a strain on wild populations of various species, notably Alluaudia ascendens and A. montagnacii. Another species of Didiereaceae, Decaryia madagascariensis, is heavily exploited by local people. Its trunk forms dense, firm sections of charcoal which are highly valued, and the thinner twigs are used for hedging and corral building. Species of Didiereaceae are also used as live fencing.
A considerable number of succulent species are utilised locally for medicinal purposes as documented, for example, by Jenkins (1987). Certain species are rare, but it is not known to what extent collection for medicinal use has a detrimental impact on wild populations. Some succulent species are also considered to be sacred in some parts of Madagascar; for example, Pachypodium is avoided by local inhabitants.
Collecting for horticulture — Madagascan succulents arc of horticultural interest to specialist collectors worldwide. The international demand for succulent species from Madagascar has led to the wholesale removal of wild plants for export primarily to Europe, Japan, and the USA. Around 100 succulent species have been exported on a fairly regular basis, the principal genera being Pachypodium, Aloe, and Euphorbia. It is clear that where there is a monetary value to plants as a result of international horticultural demand, there is a strong incentive to collect and export the material which is not seen as having any intrinsic value locally. Both bulk removal of common species for the 'supermarket' trade and collection of very rare species for the specialist market continue to take place. The latter is probably more threatening or damaging to local populations.
The actual species collected for export change according to fashions in succulent trade. At present there is a strong demand for caudiciform plants, most of which are not listed on the Appendices of CITES. At the same time newly described species of all genera will be subject to particular threat.
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