IMPACTS OF TREE PLANTATIONS IN THE ETHIOPIAN HIGHLAND ON SOIL FERTILITY, SHOOT AND ROOT-GROWTH, NUTRIENT UTILIZATION AND MYCORRHIZAL COLONIZATION

In order to elucidate ecological effects of plantation establishment i n Ethiopia, soil physical and chemical characteristics, above-ground h erbaceous biomass and nutrient content, fine root biomass and producti vity and mycorrhizal colonisation were studied in a natural montane fo rest and in adjacent 28-40 year old plantations of the exotic species Cupressus lusitanica and Eucalyptus globulus and the indigenous Junipe rus procera. The field studies were combined with bioassays of growth and nutrient uptake if Acacua abyssinica, Chloris virgata and Eragrost is tef in soils derived from each site. The Cupressus and Eucalyptus s oils had lower nutrient content than Juniperus soil and that of the na tural forest. The number of forbs and graminoids was high in all sites , but their coverage was poor in the Cupressus site. The production of fine roots in topsoil was twice as large in the Eucalyptus and Junipe rus sites than in the other sites, whereas the standing crop of fine r oots was higher in the Cupressus site. Vesicular-arbuscular mycorrhiza l (VAM) fungi were abundant in roots and soil of all sites. Their colo nisation of roots of some of the most common forbs and graminoids was generally lower in the dry season than in the rainy season. In the bio assay, growth of Chloris virgata and Eragrostis tef was reduced in soi l of all the plantations, most strongly in Eucalyptus sea, compared wi th their growth in soil of the natural forest. Nutrient concentration and pool sizes in herbaceous plants varied strongly between sites beca use of differences in species composition and herbaceous standing crop . Owing to the difference between herbaceous biomass harvested in the forest and biomass accumulation in the bioassay, it is suggested that vegetation and soil analyses are combined with bioassays in surveys of the fertility of soils after plantation establishment. The negative e ffect of Eucalyptus on the growth of Eragrostis tef in the bioassay sh ould be considered when plans for future land use in Ethiopia are elab orated as respectively these are the most widely planted tree and crop species planted here.