The relationship between structural root depth and vigor of urban trees

Abstract

The prevalence of deep root systems on urban trees has been well documented, but the consequences are not well understood. The re-lationship between structural root depth and vigor of street trees was investigated in Greensboro, North Carolina; Snoqualmie, Washington; and Glen Ellyn, Illinois, United States. Regression analysis was used to explore the relationship between root depth and crown vigor, trunk diameter growth, and trunk condition as indicators of tree performance. The average depth of structural roots for most species was less than three centime-ters. However, in 10 of the 14 species included in the study, the structural roots of 20%-60% of the individual trees were more than eight centime-ters deep. Regression analysis showed a significant relationship between root depth and indicators of tree performance for Acer rubrum, Quercus bicolor, Fraxinus oxycarpa, and Tilia cordata, but no relationship was identified for other species measured. Root depth explained less than half of the reduction in tree performance of these species, however, and is apparently only one of several factors affecting the growth of street trees. © 2012 International Society of Arboriculture.