Introduction of biodiversity in the cultivated field is a challenging issue of current agronomy. In this respect, we need tools to define and select plants of interest (cover-crop) and to determine their spatial and temporal arrangement in the cropping system. The concept of functional traits, originally defined in ecology as any trait at the plant level which impact its fitness (Violle et al., 2007), could be effectively applied to this objective. However, in an agrosystem, the definition of functional traits should be adapted towards traits which impact agro-environmental and production functions of the cropping system. To the best of our knowledge, there have been only few attempts to use the concept of functional traits as an agronomical purpose (Volaire, 2008, pasture survival against drought in farming systems). Most of the interactions between plants take place underground (Casper and Jackson, 1997, Hauggaard-Nielsen and Jensen, 2005). We thus hypothesized that root functional traits determined the main issues of a banana / cover-crop system: (i) the distribution of soil resources uptake by the cover-crop, which impacts on its ability to compete with banana, (ii) and the conservation of soil fertility (biological porosity, nutrients availability).