The growth and distribution of root systems of an annual crop (French dwarf bean) and a young tree species (poplar) in large soil columns at three different soil water supply regimes were studied in a simulated agroforestry under a greenhouse condition at the University of Edinburgh, UK. The root biomass production of bean plants was found unaffected under the limited soil water supply whether grown in a monoculture or in mixed stands in contrast to that of poplar plants. The root system of bean explored only top 36 cm of soil column whereas poplar root system explored beyond the depth of 36 cm . These differential responses of an annual crop and a young tree species may be used as a basis for selecting complementary agroforestry components in a dry area.