Detailed Knowledge on local farming systems and farmers’ strategies in various situations such as pioneer zones, rehabilitation areas or traditional tree-crop belts) may contribute to build better and more adapted alternatives, solutions and proposals to help farmers to make the right decision at the right time concerning their future investment. CIRAD has developed (with INRA1 and IAMM) a software called “ Olympe ” that enable the modelling of farming systems [Penot, 2003 #1151]. There is also a module that permit the analysis at the level of farms groups. Positive or negative externalities can be integrated as well therefore enabling an approach taking into account C sequestration from tree crops of pollution effects.