Aboveground and belowground competition between intercropped cabbage and young Eucalyptus torelliana

An expanding market for planted timber in the Philippines is providing a strong incentive 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 aboveground and belowground competition in a vegetable/timber intercropping system was investigated in the uplands of Mindanao, the Philippines. Eight 5-m² 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 m²) were also installed. Four tree/cabbage microplots and all cabbage-only plots were fertilized with ¹⁵ 5N-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 15N 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. This revised version was published online in June 2006 with corrections to the Cover Date.