Aboveground and belowground competition between intercropped cabbage and young Eucalyptus torelliana T. M. NISSEN 1, * , D. J. MIDMORE 2 and M. L. CABRERA 1 1 Dept. of Crop and Soil Sciences, University of Georgia, Athens, GA 30602, USA; 2 School of Biological and Environmental Sciences, Central Queensland University, Rockhampton, Qld 4702, Australia (*Author for correspondence: E-mail: tnissen@uga.cc.uga.edu) Key words: farm forestry, nitrogen efficiency, nitrogen-15, nitrogen uptake, vegetables Abstract. An expanding market for planted timber in the Philippines is providing a strong incen- tive for upland farmers to incorporate trees into their farming systems. Farmers often intercrop young timber species with well-fertilized annuals in expectation that inter-species competition for nutrients and light will be minimal while the trees are small, and that the trees will benefit from intensive nutrient and weed management of the intercrop. The relative level of above- ground and belowground competition in a vegetable/timber intercropping system was investi- gated in the uplands of Mindanao, the Philippines. Eight 5-m 2 microplots were established containing one nine-month-old Eucalyptus torelliana and four rows of cabbage (two on each of the north and south sides of the tree, 0.5 and 1.0 m from the stem base). The tree canopy shaded north rows. Monocrop cabbage microplots (2 m 2 ) were also installed. Four tree/cabbage microplots and all cabbage-only plots were fertilized with 15 N-labeled ammonium sulfate (100 kg N ha –1 ); remaining microplots received unlabeled fertilizer. Cabbage yields were reduced by 16% in the north rows when compared to the south rows, and by 15% in rows closer to the tree when compared to rows further from the trees. Belowground competition in the first cabbage row, possibly for moisture, is supported by the high proportion of tree roots found in the top 30 cm of soil. Competition did not appear to be for N or other nutrients. Foliar analyses revealed no row differences in mineral concentrations in cabbage, uptake of applied N, or percent of N derived from fertilizer. The modest amount of 15 N found in aboveground tree parts (4.5% of N applied to four cabbage rows) improved overall N-use efficiency in the intercropped plots. An improved understanding of the tradeoffs between improved nutrient efficiency and depressed intercrop growth, as well as management options to reduce competition, will help farmers design systems to improve efficiency without increasing competition. Introduction Steadily rising prices for timber on Mindanao in the Philippines have given farmers an incentive to devote some of their land to fast-growing species such as Paraserianthes falcataria and Eucalyptus spp. Intercropping timber trees with annuals may provide advantages not realized by woodlots or industrial plantations, including the ability to charge timber establishment and mainte- nance costs to the intercrop and reduce the risk of catastrophic fires (Garrity and Mercado, 1997). The trees may also improve nutrient efficiency by recov- ering nutrients in soil before they have leached or eroded away, both during and between cropping seasons. Such advantages are offset by the increased competition for aboveground Agroforestry Systems 46: 83–93, 1999.  1999 Kluwer Academic Publishers. Printed in the Netherlands.