Didiereaceae is one of the smallest families in the succulent world, yet few match its visual drama. Its Madagascan members are the architects of the island’s famous spiny forest: leafless, densely armed, cactus-like trees and shrubs that dominate a habitat found nowhere else on Earth. For decades the family was thought to be a Madagascar exclusive, until molecular studies revealed that two unrelated-looking African succulent lineages — the popular garden shrub Portulacaria afra among them — actually belong inside it. This page introduces the family as a whole and indexes its six genera.
Taxonomy and circumscription
Didiereaceae sits in the order Caryophyllales, the same broad lineage that contains cacti, ice plants, portulacas and amaranths. For most of the twentieth century the family was defined narrowly, comprising only the four spiny, tree-like genera endemic to southern and southwestern Madagascar: Didierea, Alluaudia, Alluaudiopsis and Decarya.
That picture changed with a series of molecular phylogenetic studies beginning around 2000. Work by Applequist and Wallace, followed by more detailed analyses from Bruyns and colleagues in 2014, showed that the southern African genus Portulacaria — previously classified in the unrelated family Portulacaceae — and the small East African genus Calyptrotheca both belong within Didiereaceae. The same 2014 study also resolved the southern African genus Ceraria as nested inside Portulacaria rather than standing apart from it, so every former Ceraria species has since been formally transferred into Portulacaria.
Didiereaceae is now understood to comprise three subfamilies, corresponding neatly to its three centres of distribution:
- Didiereoideae — the four spiny, dioecious, tree-forming genera of Madagascar (Didierea, Alluaudia, Alluaudiopsis, Decarya).
- Portulacarioideae — the single genus Portulacaria, of southern Africa.
- Calyptrothecoideae — the single genus Calyptrotheca, of tropical East Africa.
Across these three subfamilies and six genera, Plants of the World Online currently accepts around 20 species in total, making Didiereaceae one of the smallest families associated with succulent horticulture — smaller, for instance, than many single genera within Cactaceae or Euphorbia.
Morphology and biology
The Madagascan Didiereoideae define the family’s public image: densely spiny, semi-succulent shrubs and trees with a distinctive stem architecture in which short lateral shoots (brachyblasts) bear whorls of spines together with small, deciduous assimilation leaves, all borne along woody long-shoots. This construction converges strikingly on the growth form of columnar cacti and of the unrelated Fouquieria (ocotillo) of the Sonoran Desert, even though none of the three lineages are closely related to one another outside their shared order. All four Madagascan genera are strictly dioecious, with small male and female flowers borne on separate plants.
The two African subfamilies look, and largely behave, quite differently. Portulacaria species are soft-wooded, sparsely or non-spiny shrubs with fleshy, rounded leaves rather than the reduced, deciduous foliage of their Madagascan relatives; most are hermaphroditic rather than dioecious, and several — most famously Portulacaria afra — are able to switch between ordinary C3 photosynthesis and Crassulacean Acid Metabolism (CAM) under drought stress. Calyptrotheca, the family’s most obscure genus, consists of non-succulent, herbaceous to lightly woody shrubs with hermaphroditic flowers; its inclusion in Didiereaceae rests on molecular evidence rather than any obvious shared succulent morphology, and it is a useful reminder that this family, like most in Caryophyllales, is defined as much by DNA as by appearance.
One shared, family-wide trait links even the most dissimilar-looking members: like all Caryophyllales, Didiereaceae produces betalain rather than anthocyanin pigments, the same pigment class responsible for the reds and yellows of beetroot, bougainvillea and, indeed, cactus flowers. The family’s kinship with Cactaceae within Caryophyllales is close enough that Didiereaceae species can, in practice, be grafted onto certain cactus rootstocks.
Distribution and ecology
The three subfamilies occupy three separate, non-overlapping regions of the African-Madagascan landmass. Didiereoideae is confined to the spiny thickets of southern and southwestern Madagascar, an ecoregion of exceptional endemism that also hosts baobabs, spiny Euphorbias and several lemur species, and that is now considered one of the most threatened dry forest habitats on the planet. Portulacarioideae is centred on the arid western and southern regions of southern Africa, from the Richtersveld and Namaqualand through to the subtropical thicket vegetation of South Africa’s Eastern Cape, where Portulacaria afra is the dominant, keystone shrub. Calyptrothecoideae occupies dry bushland and rocky grassland in the Horn of Africa and East Africa, from Somalia through Kenya to Tanzania.
Genus index
Subfamily Didiereoideae (Madagascar)
- Didierea — two species, both known as “octopus trees”: Didierea madagascariensis, a columnar tree to 10 m with spines arranged in whorls of four, and Didierea trollii, a lower, more sprawling species with distinctively horizontal, creeping stems.
- Alluaudia — six species, all Madagascan endemics and the most commonly cultivated genus of the subfamily. Alluaudia procera, sometimes called the Madagascar ocotillo for its superficial resemblance to Fouquieria splendens, is the most widely grown; Alluaudia ascendens and Alluaudia montagnacii form tall single trunks with vertical branching, while Alluaudia comosa and Alluaudia humbertii stay shorter and form dense tangles of horizontal branches.
- Alluaudiopsis — two species, Alluaudiopsis fiherenensis and Alluaudiopsis marnieriana, both restricted to the Toliara province of southwestern Madagascar and rarely seen in collections outside specialist growers.
- Decarya — a monotypic genus consisting solely of Decarya madagascariensis, immediately recognisable among the family’s Madagascan members for its strikingly zigzagging branch pattern.
Subfamily Portulacarioideae (southern Africa)
- Portulacaria — seven species following the 2014 absorption of the genus Ceraria. Portulacaria afra, the elephant bush, porkbush or spekboom, is by far the best known, both as an easy-care houseplant and bonsai subject worldwide and as the ecologically critical, carbon-sequestering keystone shrub of South Africa’s subtropical thicket, now the focus of large-scale habitat restoration programmes. The remaining species — Portulacaria armiana, Portulacaria carrissoana, Portulacaria fruticulosa, Portulacaria longipedunculata, Portulacaria namaquensis and Portulacaria pygmaea — are smaller, more localised shrubs of the arid far west of southern Africa, most still traded and grown under their former Ceraria names.
Subfamily Calyptrothecoideae (East Africa)
- Calyptrotheca — two species, Calyptrotheca somalensis and Calyptrotheca taitensis, non-succulent shrubs of dry rocky habitats from Somalia to Tanzania, occasionally grown locally as ornamentals or hedging but essentially unknown in international succulent cultivation.
Cultivation notes
Cultivation requirements differ substantially between the subfamilies and are covered in more depth wherever individual genera or species are treated in full elsewhere on this site. In general terms, the Madagascan Didiereoideae want maximum sun, a fast-draining mineral substrate, and a pronounced dry season corresponding to their natural winter dormancy, during which the deciduous leaves are shed; they are frost-tender and best suited to greenhouse or warm-climate cultivation. Portulacaria species are considerably more forgiving, tolerating a wider range of light and watering regimes and rooting readily from cuttings, which is why Portulacaria afra in particular has become one of the most widely traded and easy-going succulents in general cultivation. Calyptrotheca species remain essentially unavailable in the horticultural trade.
Conservation
Conservation concern within Didiereaceae is concentrated overwhelmingly in Madagascar. IUCN Red List assessments across the Didiereoideae range from Least Concern (Didierea madagascariensis, Alluaudia dumosa) through Near Threatened (Decarya madagascariensis) to Vulnerable (Alluaudia ascendens, Alluaudia comosa) and Endangered (Alluaudia montagnacii), reflecting the rapid clearance of the spiny forest for charcoal production and subsistence agriculture across its narrow southern Madagascan range. By contrast, Portulacaria afra is itself the subject of one of Africa’s best-known ecological restoration efforts: since 2004, tens of millions of spekboom cuttings have been planted across South Africa’s Eastern Cape to rebuild degraded subtropical thicket, restore biodiversity and sequester atmospheric carbon, making this one Didiereaceae species a conservation tool in its own right rather than merely a conservation concern.
Reference sites
Plants of the World Online (Kew) — https://powo.science.kew.org — the nomenclatural authority used for every accepted genus and species figure cited on this page, including the current six-genus, three-subfamily circumscription of the family and the Ceraria-into-Portulacaria synonymy.
IUCN Red List of Threatened Species — https://www.iucnredlist.org — source for the individual conservation assessments of the Madagascan Didiereoideae cited in the conservation section.
Bibliography
Applequist, W.L. & Wallace, R.S. (2000). “Phylogeny of the Madagascan endemic family Didiereaceae.” Plant Systematics and Evolution 221(3–4): 157–166. [The first molecular study to test the family’s traditional four-genus, Madagascar-only circumscription, laying the groundwork for the later inclusion of the African genera.]
Bruyns, P.V., Oliveira-Neto, M., Melo de Pinna, G.F. & Klak, C. (2014). “Phylogenetic relationships in the Didiereaceae, with special reference to subfamily Portulacarioideae.” Taxon 63(5): 1053–1064. [The study that confirmed Calyptrotheca and Portulacaria within Didiereaceae and resolved Ceraria as nested inside Portulacaria, the basis for the current six-genus family treatment used on this page.]
Van der Vyver, M.L., Mills, A.J., Difford, M. & Cowling, R.M. (2021). “Herbivory and misidentification of target habitat constrain region-wide restoration success of spekboom (Portulacaria afra) in South African subtropical succulent thicket.” PeerJ 9: e11944. [Source for the scale and aims of the South African spekboom restoration programme referenced in the conservation section.]