Dioscorea originated in the tropical regions of both Africa and Asia depending on the species, of which there are more than 600 in existence. D. rotundata is known to have originated from West Africa while D. alata is known to have originated in the South Pacific and to have ‘migrated’ through trade to West Africa ~500 years ago. Trade has spurred yam growth around the world, even with D. bulbifera and D. alata being classified as a noxious weed in Florida and around the Gulf of Mexico and the Chinese yam being reported as ‘escaping’ cultivation in Illinois in 1986 (Gucker, 2009). In fact, the USDA’s Invasive Plant Research Laboratory conducted studies on D. bulbifera’s biological control (Rohrig, 2014). Below is a map of Dioscorea according to BONAP, the Biota of North America Program. For species-specific maps, follow this link: < a href="http://bonap.net/NAPA/TaxonMaps/Genus/County/Dioscorea" http://bonap.net/NAPA/TaxonMaps/Genus/County/Dioscorea.
//BONAP PHOTOPrimarily, however, it continues to be grown in West Africa with the ‘yam belt’ considered to be the five West African countries of Nigeria, Ghana, Cote d’Ivoire, Benin, and Togo. Statistics vary somewhat, but Nigeria is the world’s largest producer by a landslide and Ghana the second most. Shehu et. al. (2010) reported that 95% of the world’s white yam production, accounting for 48M tons, is grown in Nigeria while the 2013 First Global Conference on Yam gave reports that 65% of the world’s production takes place in Nigeria while Ghana used 6.32% of its arable land (Quain et. al) to cultivate 6.3M tons of yam (Sikpa). The Democratic Republic of Congo is actively seeking to promote yam cultivation, specifically with the aim of increasing yam production eight-fold to account for 8% of its food production by 2021. It is also an economically important crop to Costa Rica, Jamaica, and Colombia, however phytosanitary regulations for import/export have recently favored Costa Rica (Kumar et al., 2013). The following are a list of countries where Dioscorea grows:
Diosocorea grows in savannas, mesic forests, and their intermediaries. A small number of varieties are cultivated (since only ~10% Dioscorea are edible) with most species growing wildly in forests. Farmers do domesticate wild cultivars ‘experimentally’ out of curiosity and in an effort to provide more biodiversity. Pygmies in the Democratic Republic of Congo (DRC)are known to practice forest proto-farming [Paulus].
Nwosu et. al. () reported yams growing in ‘olokoro’ soils with the FAO classification of haplic nitisol, the soil type prevalent in Eastern Highlands, tropical islands, and coastal India, also known by the USDA as ‘clayey’, kaolinitic, and isohyperthermic soil. Others have reported yams growing in the alfisol soil order, similarly clay-rich but more specifically rich in Aluminum and Iron. Its composition is noted by 30% clay (by mass). The map below provided by YIIFSWA (Yam Improvement for Income and Food Security in West Africa) depicts the agroecological regions of West Africa (Gucker, 2009; Maroya et al., 2014), notably where the Niger floodplain provides rich soil for yam. While yam benefits from its relative drought tolerance, climate change could have a significant impact and researchers are working to anticipate and curb its negative impacts.
//Nigerian Ecologies PHOTOAvailable through Mendeley)