Comparisons of Understorey Vegetation and Soil Fertility in Plantations and Adjacent Natural Forests in the Ethiopian Highlands

Abstract

Eighty-three plantations and adjacent natural forest stands in highland Ethiopia were analysed for herbaceous plant cover and biomass, species richness, and soil physical and chemical characteristics, in order to reveal the impact of plantation establishment. Climatic (temperature and rainfall) and altitudinal control over the distribution of herbaceous plant species in the forests was exposed by canonical correspondence analysis, and the areas of sampling were segregated on the basis of the gradients this revealed. Sites at higher altitudes were characterized by tall trees with large basal area, and by soil with a high organic matter, high N content and low pH. In contrast to the frequent view of eucalypts as harmful for understorey plants, the richness and biomass of herbaceous plant species in plantations of Eucalyptus 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 grassland. The herbaceous vegetation in Cupressus lusitanica plantations older than 9 years was distinct in composition, less diverse, and much reduced in terms of cover and biomass compared with similarly aged plantations of Eucalyptus globulus, E. grandis, E. saligna, Pinus patula and with natural forests. Cupressus lusitanica should therefore not be planted because of the risk of soil erosion. The overall soil characteristics of the natural forests differed from those of the five most common plantation tree species, as revealed by redundancy analysis. Natural forest soil had a higher content of total N, available P and exchangeable Ca (in the case of P as much as 4-25 times higher), possibly owing to a combination of loss of organic matter during conversion of natural forest to plantations, increased leaching in young plantations, and low nutrient demand by natural forest trees as compared with fast-growing exotics.