COMPARISONS OF UNDERSTOREY VEGETATION AND SOIL FERTILITY IN PLANTATIONS AND ADJACENT NATURAL FORESTS IN THE ETHIOPIAN HIGHLANDS

1. Eighty-three plantations and adjacent natural forest stands in high land Ethiopia were analysed for herbaceous plant cover and biomass, sp ecies richness, and soil physical and chemical characteristics, in ord er to reveal the impact of plantation establishment. 2. Climatic (temp erature and rainfall) and altitudinal control over the distribution of herbaceous plant species in the forests was exposed by canonical corr espondence analysis, and the areas of sampling were segregated on the basis of the gradients this revealed. 3. Sites at higher altitudes wer e characterized by tall trees with large basal area, and by soil with a high organic matter, high N content and low pH. 4. In contrast to th e frequent view of eucalypts as harmful for understorey plants, the ri chness and biomass of herbaceous plant species in plantations of Eucal yptus spp.,, and also of Pinus patula, was as high as that of natural forests. However, most of the herbs in the plantations were widespread species, mainly weeds or species invading from montane or wooded gras sland. 5. The herbaceous vegetation in Cupressus lusitanica plantation s older than 9 years was distinct in composition, less diverse, and mu ch reduced in terms of cover and biomass compared with similarly aged plantations of Eucalyptus globulus, E. grandis, E. saligna, Pinus patu la and with natural forests. Cupressus lusitanica should therefore not be planted because of the risk of soil erosion. 6. The overall soil c haracteristics of the natural forests differed from those of the five most common plantation tree species, as revealed by redundancy analysi s, Natural forest soil had a higher content of total N, available P an d exchangeable Ca (in the case of P as much as 4-25 times higher), pos sibly owing to a combination of loss of organic matter during conversi on of natural forest to plantations, increased leaching in young plant ations, and low nutrient demand by natural forest trees as compared wi th fast-growing exotics.