Phenotypic variation of agromorphological traits of the shea tree, Vitellaria paradoxa C.F. Gaertn., in Mali

Forty-one shea tree populations were sampled, spanning the main climatic zones of Vitellaria paradoxa Gaertn. in Mali and 10-35 adult trees were chosen randomly in the agroforestry parklands of each area. A total of 12 morphological traits, related to tree morphology, fruit size and leaf form were measured. The variance components showed that variation among populations represented the smaller percentage of the total variation with most of the values varying between 15 and 30%. The repeatability coefficient was generally high for tree within populations with values ranging between 0.23 and 0.78. Although genetic correlations cannot be accurately estimated, due to difficulties in separation from environmental effects, the results indicate that there is a very low genetic relation between the three kinds of traits, i.e., between those related to tree, those related to leaf and those related to fruit. Leaf and fruit size traits were positively and significantly correlated with rainfall, although tree circumference was negatively correlated with rainfall and the significantly larger shea trees were noted in the drier areas - an observation thought linked to human management of the parklands. Soil drainage and parkland density, however, did not explain differences between populations for fruit traits. This study offers preliminary information for the development of a breeding population for a shea tree improvement programme. The value of repeatability, the low correlation between sets of traits and the distribution of variation, suggest that selection of many individual trees within a few populations, would allow capture of large genetic gain especially for fruit traits. (Résumé d'auteur