1. Kenya Forestry Research Institute (KEFRI) P.O. Box 20412‐00200 Nairobi Kenya
2. MacArthur Agro‐Ecology Research Centre 300 Buck Island Ranch Rd. Lake Placid FL 33852
3. Institute of Crop Science and Resource Conservation (INRES), Dep. of Plant Nutrition Univ. of Bonn Karlrobert‐Kreiten‐Strasse 13 53115 Bonn Germany
4. Plant Production Systems, Dep. of Plant Science Wageningen Univ. P.O. Box 430 6700 AK Wageningen the Netherlands
5. Inst. of Plant Production and Agroecology Univ. of Hohenheim 70593 Stuttgart Germany