Root system size and distribution were measured for Raywood ash (Fraxinus oxycarpa 'Raywood') and Lombardy poplar (Populus nigra 'Italica') planted with and without circling root barriers. Trees with circling barriers had fewer numbers of roots than controls (no barriers), but mean root diameters were similar. Root depth 30 cm outside barriers was greater for trees with barriers, but at 90 and 150 cm away, depth was equivalent to controls. Roots tended to grow toward the soil surface after growing under the barriers. No consistent differences in root response to any of the four types of barriers tested were found for either species. Soil cultivation during the installation of a subsurface barrier (used to simulate a hardpan) resulted in lower soil bulk densities and a deeper distribution of roots in the soil profile than in plots which were not cultivated. Reducing soil bulk densities that are limiting to root growth may be an important consideration when using circling root barriers.



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