THE BENEFITS AND POTENTIAL RISKS OF WOODY LEGUME INTRODUCTIONS

Abstract

Woody members of the Leguminosae are being introduced and planted as exotics on an unprecedented scale throughout the tropics. These multiple-use trees have an important role in social forestry, agroforestry and land reclamation schemes. They are especially suited to planting on harsh degraded sites, being fast growing, readily established and managed. In many cases they fix nitrogen and improve the soil. Such introductions have not always been beneficial in the past since they caused many severe weediness problems that necessitated expensive control programmes. Poor performance and low species adaptability must also be regarded as costs. The biological factors which constitute the adaptive weedy syndrome are discussed and it is shown that some woody legumes exhibit particularly aggressive combinations of frequent early flowering, successful seed dispersal and seed longevity. It is shown that although plant biology may determine weediness potential, the actual perception of a plant as a weed or as an asset is controlled by a combination of site and socio-economic factors and land management practices. A more rational approach to species introductions through logical consideration of plant biology, 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. The weediness tendencies of 146 species are listed.