The benefits and risks of woody legume introductions.
Woody members of the Leguminosae are being planted as exotics on an unprecedented scale in all but a few tropical countries. Legumes are well suited to planting on harsh, degraded sites as they are fast-growing, readily established and managed, and often nitrogen-fixing. They provide such products as fuelwood and livestock fodder to rural communities, and play a leading role in agroforestry and in reforestation efforts to rehabilitate degraded lands, control soil erosion and prevent desertification. Woody legume introductions have turned weedy in places, however, necessitating expensive control programmes. Additional drawbacks can be poor performance and low species acceptability. Some woody legumes exhibit particularly aggressive adaptively weedy combinations of frequent early flowering, successful seed dispersal and seed longevity. Such aggressive species likely to become weeds are actively sought and used in reforestation. It is shown that although the biology of a species may determine its potential weediness, the perception of a plant as a weed vs. as an asset is controlled by a combination of site and socioeconomic factors and land management practices. A more rational approach to introductions through consideration of biological and site factors and management is outlined, and ideas for indices of weediness hazard and introduction potential are discussed. Lack of knowledge prior to introduction often presents an obstacle to implementing these ideas. Some concrete recommendations of immediate practical importance are given.