Choosing cover crops to enhance ecological services in orchards : A multiple criteria and systemic approach applied to tropical areas

Conventional agriculture is based on a high level of chemical inputs such as pesticides and fertilisers, leading to serious environmental impacts, health risks and loss of biodiversity. Pesticide reduction is a priority for intensive agricultural systems such as orchards. Reintroducing biodiversity in single crop systems can enhance biological regulations, and contribute to reduce the use of chemicals and to provide additional services such as run-off and erosion control. In tropical wet areas, weed control is difficult to manage without herbicides especially when orchards are not located in easily mechanised areas and when labour force is costly. Cover plants can be easily introduced in orchards and could be efficient in weed control and other functions. Based on this assumption, we developed a specific approach for the choice of adapted cover plants in single crop orchards to control weeds and provide additional ecological services. The approach was undertaken on citrus orchards in the French West Indies. A multicriteria evaluation grid was built to select an "optimal" cover crop. In both Martinique and Guadeloupe, 202 species were first selected in the local flora, and tested on vegetative characteristics. Specific criteria were secondly defined relating to seed availability (limitation of alien species introduction), farm constraints and regulations (no invasive species). Specific features were then determined according to the agronomic potential and ecological services for an optimal cover plant. Criteria included weed control, the ability to control runoff and erosion, water and nutrient competition, pests and natural enemies hosting capacity. The whole evaluation grid combines data from literature, expert assessment and experimental measurements. Optimum cover crop functional groups were defined according to agrosystem and associated objectives. In Guadeloupe, a participatory approach led first to the selection of Fabaceae (Neonotonia wightii) characterized by high auxiliaries hosting services. These services were assessed using a bio-indicator (family of phytoseiidae). In Martinique, the need for a high covering index associated with a low production led to the selection of grasses: Urochloa mozambicensis and three Paspalum species. The multicriteria grid can be used as a generic tool to select cover plants and can be easily adapted to apply to various cropping systems. Its use for banana cropping system is currently in progress. However, the concept of an optimal single cover plant remains difficult to achieve and the elaboration of a multi-specific cover system is often required to reach the desired efficiency