Eucalyptus-wheat interaction on Ethiopian Nitosols

Over the past few years a single row of Eucalyptus globulus trees planted along the borders of cropland has come to dominate central highland agroforestry practices. Although evidence is scanty, there is a perception that this practice adversely affects crop productivity. An on-farm trial was therefore conducted at Ginchi to determine the biomass production potential of eucalypt boundaries and their effect on the productivity of the adjacent wheat crop (Triticum aestivum) on highland Nitosols. Three rotation cycles of 4 years each, two stand ages within each rotation, four field aspects and six yield strata perpendicular to the tree-crop interface were arranged in a split-split plot design with three replications. The annual wood production rate, which was 345-903 kg ha-1 yr-1 with two- and four-year-old stands in the first cycle, was increased more than 2-fold in the subsequent two rotation cycles. With these productivities, eucalypt boundaries on a hectare of land in the second and third cycle would satisfy about 70% of the annual biomass energy requirement of a rural household with a family size of five people for four consecutive years. However, adjacent wheat yields were substantially reduced because of the combined effects of water, light and nutrient competition. In the last two rotation cycles, significant yield depressions occurred over the first 16 m from the line of trees as opposed to only the first 8 m in the first cycle. The yield drop was 4.5-8.1%, and 8.1-10.4% in the first and last two rotation cycles, respectively. Nevertheless, the benefit accrued from the tree component adequately compensated for this reduction in wheat yield and generated additional income. The implications of these results are discussed in the context of the suitability of the practice in the region and its role in relieving the increasing pressure on indigenous forest and woodland.