The Family Dipterocarpaceae of mainland Africa, Seychelles, South and Southeast Asia, and New Guinea has its closest relatives marooned on the island of Madagascar. They are members of a family known as Sarcolaenaceae, a group of some 80 species endemic to the island nation. Evergreen shrubs and trees, they have several traits in common with the Dipterocarps – secretory canals, fibrous bark, overlapping sepals, distinctive seed and wood anatomy, starchy endosperm, and ectomycorrhiza. It is the last trait that is of the greatest interest.
Ectomycorrhiza are structures produced by the symbiotic association of some species of plants with fungi. The latter form a network of hyphae (fungal filaments) around the root cells of the former, helping them absorb water and nutrients in marginal soils or adverse climatic conditions. The fungi receive nourishment (carbohydrates from the plant’s root cells) in return. Botanists have discovered that the Sarcolaenaceae, along with the Dipterocarpaceae, are among the rare tropical angiosperm lineages to have ectomycorrhiza.
The evolution of ectomycorrhiza might have played a crucial role in the success of these related families. Outside of Southeast Asia, members of the two groups survive in uplands (South America), seasonal dry forests (Africa) and relict forests (Madagascar). The Sarcolaenaceae, along with their relatives, most likely evolved from Early Cretaceous ancestors in Africa. Fossilized Sarcolaenaceae pollen has been discovered in Miocene sites of Noordhoek and Knysna, in the Western Cape Province of South Africa. At that point of time, the Western Cape had subtropical rainforests. These disappeared with changes in the global climate (glaciation of Antarctica and drying up of the Cape).
But before they went extinct in mainland Africa (by the Pliocene), ancestors of the Family Sarcolaenaceae had spread to Madagascar (that had become an island by 90 million years ago) via India, the Mascarene Plateau (a submarine plateau that includes the islands of Seychelles, Mauritius, Reunion and Rodrigues) and Antarctica. In the neighborhood of Knysna lie the Knysna-Amatole Montane Forests which might bear some resemblance to those Miocene subtropical forests. In Southern Africa, they gave way to sclerophyllous shrublands known as Fynbos. Today, the Sarcolaenaceae survive in the humid forests of eastern and central Madagascar, comprising the following genera:
Image Attribution: The image above, sourced from Wikimedia Commons, is the photograph of a Sohisika Tree (Schizolaena tampoketsana). A member of the genus Schizolaena, it is a Critically Endangered species found only in the Analamanga Region of the Central Highlands. According to the IUCN Red List of Threatened Species, only 240 individuals of this species survive. The clearing of the region’s grasslands for farming, and forests for timber and charcoal has devastated the species. It is, like every other member of the Sarcolaenaceae, found nowhere else in the world but the island nation of Madagascar. The photograph was uploaded by Mr. James Lucas.
- ‘On the Origin of the Sarcolaenaceae with reference to Pollen Morphological Evidence’ (by Siwert Nilsson, Joanna Coetzee and Elisabeth Grafstrom, 1996)
- ‘Dipterocarp Biology, Ecology, and Conservation’ (by Jaboury Ghazoul)
- Schizolaena tampoketsana (IUCN Red List of Threatened Species)