Les ignames (Dioscorea spp.) de Madagascar : espèces endémiques et formes introduites ; diversité, perception, valeur nutritionnelle et systèmes de gestion durable

Abstract

The yams (Dioscorea spp.) of Madagascar : wild endemic and cultivated species ; diversity, perception, nutritional value, and sustainable management. — Endemic and introduced species of Malagasy yams (Dioscorea spp.) have been studied in the dry western area (Menabe central, near Morondava) where diversity is high, taking into account the occurrence of new species recently described. A comparison with a humid area of the eastern slopes of the country (around Brickaville) shows a striking difference in terms of diversity. However, in the humid area, the cultivated yams (especially Dioscorea alata) introduced by the fi rst Indo-Malay settlers are abundant in shifting cultivations, together with the wild species, and the various cultivars have been maintained thanks to a sociocultural system that implies an obligation of replanting the head of the tuber after cropping. Such an agro-ecosystem has a global production of the same order of magnitude as those of central Africa. Densities of the different species of yams have been measured on plots of the study areas. The sustainable utilization of the various yam species may differ, especially in the dry area, where regeneration occurs when pieces of broken tubers are left after harvesting, and the soil is subject to important restructuration due to frequent digging of holes for cropping. Local knowledge and traditions related to yams have been studied in villages of the two study areas. The yam tubers have been analysed to determine nutritional value, and comparative tests related to taste (hedonic value) conducted in various places, including Antananarivo, for cross-cultural and sensory comparisons. The results of this study, implying a possible introduction of new cultivated yam species, are discussed in terms of species differentiation in various bioclimatic areas and according to the specificity of the agro-ecosystem.