1. Institute of Silviculture, Department of Forest and Soil Sciences, University of Natural Resources and Life Sciences, Vienna (BOKU), Peter-Jordan-Strasse 82, A-1190 Vienna, Austria
2. Department of Natural Resources, Egerton University; P.O. Box 536-20115 Egerton, Kenya
3. Independent Researcher, 17 Via Saletto, Storo, 38089 Trento, Italy