1. a Agroforestry and Novel Crops Unit , School of Tropical Biology, James Cook University , PO Box 6811, Cairns, Queensland, 4870, Australia
2. b World Agroforestry Centre , BP 2067, Yaoundé, Cameroon
3. c Overseas Development Institute , 111 Westminster Bridge Road, London, SE1 7JD, England
4. d Department of Environmental Science , Rhodes University , PO Box 94, Grahamstown, South Africa