Many bottomland tree species are tolerant of compacted soil and perform well in urban environments; however, the mechanism underlying this tolerance is unknown. Increased soil water content has been shown to alleviate some of the effects of soil compaction on plant growth, presumably because increasing soil water reduces soil strength. We hypothesized that tree species tolerant of very wet soils would have opportunities for root growth in compacted soil when high soil water contents reduced soil strength, whereas species intolerant of bottomland conditions would not. We tested this hypothesis on flowering dogwood (Cornus florida L.), a mesic species intolerant of inundation, and silver maple (Acer saccharinum L.), a bottomland species. Seedlings of both species were grown in pots for 21 and 30 days, respectively, in a growth chamber in native loam soil maintained at various combinations of soil strength and soil water tension. Downward root growth rate decreased in response to increasing soil strength in both species. At low soil strength (0.6 MPa), downward root growth rate of dogwood seedlings slowed when soil was either excessively wet or dry, whereas root growth rate of silver maple seedlings increased linearly with soil water content. In moderately compacted soil (1.5 g cm(-3) bulk density), silver maple seedlings had greater root growth rate, root length per plant, and ratio of root length to root dry weight in wet soil (0.006 MPa soil water tension) than in moist and dry soils (0.026 and 0.06 MPa, respectively), even though mean oxygen diffusion rate (ODR) was only 0.28 &mgr;g cm(-2) (SE = 0.05). No such effect was detected in highly compacted soil (1.7 g cm(-3) bulk density) in either species. Mean ODR showed a weak positive correlation with soil water tension (r = 0.40, P = 0.07), but was unrelated to soil strength. We conclude that silver maple roots can grow in moderately compacted soil when high soil water content decreases soil strength, whereas dogwood is unable to take advantage of this opportunity.