Seventeen years' growth of street trees in structural soil compared with a tree lawn in New York City

In 1997, Quercus bicolor and Quercus phellos in a New York City streetscape were planted in CU-Structural Soil under a concrete sidewalk and in a tree lawn with the intention of observing long term plant response as a comparative study of an early installation of the new designed soil method. The trees have been measured nine times since the second year post-installation. We present the growth of trunk diameter, height and slenderness ratios. To verify root colonization in the structural soil under the sidewalk, tree root presence was measured using ground-penetrating radar in 2009, year 12. In the 17th year, trees in the sidewalk were similar in size with a higher level of survivorship as compared to the trees in the tree lawn. Adjusting for year of replacement in the tree lawn, model predictions for diameter of trees 15 years post-installation were no different for Q. bicolor in either the sidewalk (21.38 cm) or the tree lawn (22.05 cm). Q. phellos in the tree lawn were predicted to be slightly larger, 34.29 cm versus 31.34 cm in the sidewalk. Roots had colonized the structural soil under the sidewalk to 60 cm, its full depth. This provided evidence that the structural soil medium served as an acceptable rooting environment.