Arboriculture & Urban Forestry 45(6): November 2019 rubra (Figure 4a-d). A similar trend was apparent in Acer saccharum (Figure 4e), but the very low level of root growth throughout the profile impeded the development of significant differences. Root growth of Ulmus Americana (Figure 4f) was consistent throughout the profile with no significant differences. In the loam-over-compacted-clay profile, there was a gradual reduction in root density with depth in Catalpa speciosa (Figure 4b) and Quercus rubra (Figure 4d), and an abrupt reduction at the loam/clay interface in Acer negundo (Figure 4a) and Gleditsia triacanthos (Figure 4c). Root density did seem to decrease with depth in Acer sac- charum (Figure 4e) as well, but again the very low level of root growth impeded the development of significant differ- ences. Ulmus americana (Figure 4f) root density was con- sistent throughout the profile, similar to the loam soil. Catalpa speciosa (Figure 4b) was the only species showing a difference in root growth between soil types throughout the profile. The other species showed no differ- ence at any level, with the one exception of the uppermost layer of Acer negundo (Figure 4f). Root growth seemed to be reduced more by high soil moisture and reduced aeration than soil texture and com- paction. Seasonally high moisture levels have been associ- ated with reduced root development throughout the year (Watson 2012). Day et al. (2000) found that the underlying mechanism of tolerance of some species to compacted soil was that their roots are able to grow under conditions of high soil moisture when soil strength is reduced. Implications for Urban Tree Growth and Longevity Three of the four urban soil tolerant species showed a sig- nificant reduction in root growth with increased depth and reduced aeration. Though generally considered “tough” species, they do still seem to be affected by challenging urban soil conditions. Ulmus americana showed no decrease and may be the most truly urban soil tolerant. The very low densities of Acer saccharum roots may be a characteristic of the species, since growth in the upper well-drained layers of the loam soil was also low. Slower growth may explain this species difficulty in adapting to urban soils. Catalpa speciosa had up to seven times the root density of other species at the surface and up to four times at the bottom. This species may be a good prospect for more fre- quent use in urban landscapes, despite concerns over fruit production. With perhaps the exception of Ulmus americana, all of the species tested showed reduced root growth deeper in the soil profile and would likely be affected if the adventi- tious root flare was too deep. It supports pruning the seed- ling taproot no more than 10 cm deep when preparing them for replanting in liner production. LITERATURE CITED Brady, N.C., and R.R. Wiel. 1996. The Nature and Properties of Soils. Prentice Hall, Upper Saddle River, New Jersey, U.S.A. 740 pp. Craul, P.J. 1992. Urban Soil in Landscape Design. John Wiley & Sons, Inc. New York, New York, U.S.A. 396 pp. Day, S.D., J.R. Seiler, and N. Persaud. 2000. A comparison of root growth dynamics of silver maple and flowering dogwood in compacted soil at differing soil water contents. Tree Physi- ology 20 257-263. Hewitt, A., and G.W. Watson. 2009. Bare root liner production can alter tree root architecture. J. Environ. Hort. 27: 99-104. Roloff, A., S. Korn, and S. Gillner. 2009. The climate-species- matrix to select tree species for urban habitats considering climate change. Urban Forestry & Urban Greening 8: 295-308. Watson, G. 2012. Establishing a relationship between soil aeration and fine root development of seven tree species using the steel rod technique. Arboricultural Journal 29: 161-171 ACKNOWLEDGEMENTS The authors acknowledge the J. Frank Schmidt Family Charita- ble Foundation for support for this and other related projects Angela Hewitt The Morton Arboretum Lisle, Illinois, U.S.A. Frank Balestri The Morton Arboretum Lisle, Illinois, U.S.A. College of DuPage* Glen Ellyn, Illinois, U.S.A. Marvin Lo The Morton Arboretum Lisle, Illinois, U.S.A. Gary Watson (corresponding author) The Morton Arboretum Lisle, Illinois, U.S.A.
[email protected] *Current affiliation Résumé. Deux profils de sols (loam sur argile compactée puis loam seul) furent créés dans des pots de culture de 10 cm × 10 cm × 25 cm. Ces pots furent placés dans des plateaux d’eau afin de simuler une situation de mauvais drainage souterrain. Quatre espèces tolérantes en milieu urbain (Acer negundo, Catalpa spe- ciosa, Gleditsia triacanthos et Ulmus americana) et deux espèces moins tolérantes de ce milieu (Quercus rubra et Acer saccharum) furent directement semées dans les pots. Le contenu volumétrique du sol en eau et le taux de diffusion de l’oxygène furent con- trôlés. À la fin de la recherche, la longueur des radicelles (diamètre < 2mm) fut mesurée partout dans les profils de sol. Avec la pro- fondeur, le taux d’oxygène diminuait et l’humidité augmentait. La densité des radicelles de toutes les espèces s’amoindrissait avec la profondeur à l’exception de Ulmus americana. Catalpa ©2019 International Society of Arboriculture 257
November 2019
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