Aboveground and belowground competition between intercropped cabbage and young Eucalyptus torelliana

An expanding market for planted timber in the Philippines is providing a st rong incentive for upland farmers to incorporate trees into their farming s ystems. Farmers often intercrop young timber species with well-fertilized a nnuals in expectation that inter- species competition for nutrients and lig ht will be minimal while the trees are small, and that the trees will benef it from intensive nutrient and weed management of the intercrop. The relati ve level of aboveground and belowground competition in a vegetable/timber i ntercropping system was investigated in the uplands of Mindanao, the Philip pines. Eight 5-m(2) microplots were established containing one nine-month-o ld 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 c anopy shaded north rows. Monocrop cabbage microplots (2 m(2)) were also ins talled. Four tree/cabbage microplots and all cabbage-only plots were fertil ized with N-15-labeled ammonium sulfate (100 kg N ha(-1)); remaining microp lots received unlabeled fertilizer. Cabbage yields were reduced by 16% in t he 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 competi tion in the first cabbage row, possibly for moisture, is supported by the h igh proportion of tree roots found in the top 30 cm of soil. Competition di d not appear to be for N or other nutrients. Foliar analyses revealed no ro w differences in mineral concentrations in cabbage, uptake of applied N, or percent of N derived from fertilizer. The modest amount of N-15 found in a boveground tree parts (4.5% of N applied to four cabbage rows) improved ove rall N-use efficiency in the intercropped plots. An improved understanding of the tradeoffs between improved nutrient efficiency and depressed intercr op growth, as well as management options to reduce competition, will help f armers design systems to improve efficiency without increasing competition.