A comparison of root growth dynamics of silver maple and flowering dogwoodin compacted soil at differing soil water contents

Many bottomland tree species are tolerant of compacted soil and perform wel l in urban environments; however, the mechanism underlying this tolerance i s unknown. Increased soil water content has been shown to alleviate some of the effects of soil compaction on plant growth, presumably because increas ing soil water reduces soil strength. We hypothesized that tree species tol erant of very wet soils would have opportunities for root growth in compact ed soil when high soil water contents reduced soil strength, whereas specie s intolerant of bottomland conditions would not. We tested this hypothesis on flowering dogwood (Cornus florida L.), a mesic species intolerant of inu ndation, and silver maple (Acer saccharinum L.), a bottomland species. Seed lings 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 decrease d in response to increasing soil strength in both species. At low soil stre ngth (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 c ompacted soil (1.5 g cm(-3) bulk density), silver maple seedlings had great er root growth rate, root length per plant, and ratio of root length to roo t dry weight in wet soil (0.006 MPa soil water tension) than in moist and d ry soils (0.026 and 0.06 MPa, respectively), even though mean oxygen diffus ion rate (ODR) was only 0.28 mu g cm(-2) (SE = 0.05). No such effect was de tected in highly compacted soil (1.7 g cm(-3) bulk density) in either speci es. 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 sil ver maple roots can grow in moderately compacted soil when high soil water content decreases soil strength, whereas dogwood is unable to take advantag e of this opportunity.