Nitrogen fixation in legume and non-legume trees

Nitrogen-fixing root nodules are found in 10 plant families containing trees and shrubs associated with rhizobia or Frankia. The nodulation status and nodule characteristics have been extensively studied for herbaceous legumes, while for many legume trees species these characters are unknown. Hardwood timber production represents the major economic use of both leguminous and actinorhizal trees, although other roles in providing shelter, land conservation, land reclamation and as medicinal plants are important. Their success relies not only on their ability to fix N-2 symbiotically, but also on a range of other adaptations including flooding and drought tolerance, mycorrhizal formation, cluster root production and herbivore defences, Future research to detail nodulation status, nitrogen fixation rates, physiological adaptations and genetic diversity in both legumes and actinorhizal trees will provide fundamental knowledge to further conservation and utilisation of these plants. (C) 2000 Elsevier Science B.V. All rights reserved.