Arboriculture & Urban Forestry 38(1): January 2012 Arboriculture & Urban Forestry 2012. 38(1): 13–17 13 The Relationship Between Structural Root Depth and Vigor of Urban Trees Gary W. Watson and Angela M. Hewitt 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. Key Words. Buried Roots; Deep Planting; Root Architecture. Excessively deep root systems are common on trees in ur- ban and suburban landscapes. The structural woody roots that collectively form the root flare have been found to be more than 7.6 cm below the soil surface on up to two-thirds of street trees in several Illinois and Minnesota, U.S., cit- ies (Watson et al. 1990; Giblin et al. 2006). In Long Island, New York, U.S., the average depth of soil over the structural roots was slightly more than 15 cm (Smiley 2006). This is deeper than the best management practice (BMP) recommen- dation that “at least two structural roots should be within 2.5 to 7.6 cm of the soil surface” (Watson and Himelick 2005). In addition to simply planting trees too deeply in the land- scape, nursery field production practices can also contribute to deep root systems (Harris et al. 2001; Hewitt and Watson 2009). In the 1980s, deep roots were reported on New York City, New York, U.S., trees and were attributed to nursery practices prior to planting (Berrang et al. 1985). In a study of Ohio, U.S. nurseries, the average depth of structural roots in nursery fields and harvested root balls has been reported at 2–10 cm and 7–11 cm, respectively (Rathjens et al. 2007). The question of how the deep root systems are affecting tree performance has not been adequately answered. Controlled ex- periments in field plots showed that deep root systems reduce survival and growth under some conditions, especially poor drainage (Wells et al. 2005; Arnold et al. 2005; Arnold et al. 2007; Bryan et al. 2010; Day and Harris 2008). Similar studies are lacking on trees planted in urban and suburban landscapes where many factors can affect tree growth and vigor. This study attempts to understand whether root depth affects vigor of com- monly planted street trees in Greensboro, North Carolina (NC); Snoqualmie, Washington (WA); and Glen Ellyn, Illinois (IL). METHODS Arborists were contacted to locate suitable street tree plantings for the study. Knowledge of site history and consistency in site conditions were important consid- erations. Tree inventory records were examined from nine lo- cations, six were visited for evaluation, and three were chosen. Study Sites Sites were chosen based on their geographic and climatic differenc- es, species and number of trees available, and availability of local cooperators. The NC site was a large commercial property where the space between the pedestrian sidewalk and street curb was less than two meters wide and too narrow for planting. Therefore trees were planted one meter from the sidewalk on the side away from the street with 10 m or more open space beyond. A single culti- var of (Acer rubrum) trees was growing along the entire street. The WA site was a residential neighborhood. Planting space width between the pedestrian sidewalk and street curb was somewhat variable from street to street but was typi- cally less than two meters. A single species or cultivar was planted along each street. Streets with Quercus rubra, Fraxi- nus oxycarpa, and Tilia cordata were used in the study. The IL site also consisted of street trees in a residential neighborhood. Planting space was also of variable width. Plantings along each street were of mixed species. When there were insufficient numbers of trees of a species for val- id data analysis, the data were reported by genus. All trees were considered well-established. Trees in NC and WA were all planted as a group at the same time, 5 to 10 years earlier, but the exact year of planting was not available from records. ©2012 International Society of Arboriculture
January 2012
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