The Relationship Between Structural Root Depth and Vigor of Urban Trees

Study Sites

Sites were chosen based on their geographic and climatic differences, 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 cultivar 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 typically less than two meters. A single species or cultivar was planted along each street. Streets with Quercus rubra, Fraxinus 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 valid 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.

All trees were planted as ball-and-burlap stock, and trees were surrounded by turfgrass beyond a small (1–1.5 m) mulch ring at the base of the tree. Sampling dates and species for each site are listed in Table 1.

Root Depth

The root flare is the commonly used reference point for root depth discussions, but is really a zone rather than a specific point that can be measured precisely. To be more specific in this study, and in compliance with best management practices (Watson and Himelick 2005), root depth was measured from the upper surface of individual roots at the point where the root became distinct from the trunk. Depth was measured to the nearest half-centimeter relative to the soil against the trunk. The depth of individual roots usually varied around the tree. The depth of the two uppermost woody structural roots was recorded for each tree and averaged. If the soil against the trunk was higher or lower than the surrounding grade, this measurement was recorded and root depth relative to surrounding grade was calculated.

Root collars were excavated with hand tools and pneumatic excavation, depending on circumstances at each site. At WA, hand digging was the only option, and root collars were excavated to a maximum depth of 20 cm. Further hand excavation would have caused more site disruption than was tolerable. If no roots were found within 20 cm, depth was recorded as >20 cm. At IL, root collars were first excavated by hand to a depth of 15 cm, and if roots were not exposed, a pneumatic excavation tool was used on a return visit to excavate until roots were located.

At the NC site, root collars had been excavated several months earlier to expose the structural roots because Phytophthora lesions were discovered on the lower portion of the trunk (pers. comm.: E. Thomas Smiley). The depressions were filled with coarse mulch that was easily removed by hand to re-expose the structural roots.

Tree Performance

A visual rating system was developed for crown vigor. Trees were rated during the dormant season at WA, and only twig growth could be rated.

Vigor ratings:

• 1 – Vigorous: Terminal twig growth; leaf size and color better than average for species.

• 2 – Normal: Terminal twig growth; leaf size and color typical of species.

• 3 – Acceptable: Terminal twig growth; leaf size and color less than typical, but acceptable.

• 4 – Stress overcome: History of subnormal twig growth or minor twig dieback, but now acceptable or better.

• 5 – Current stress: Subnormal twig growth; leaves small, scorched or off color; minor dieback.

• 6 – Significant dieback: Major portions of crown dead, still in decline.

• 7 – Replacement needed: Dead or nearly so.

In addition to crown evaluations, trunk condition was also rated at the North Carolina site. This tree–health variable was included because these trees had a history of Phytophthora lesions on the lower portion of the trunk. The scale was based on visual estimation of the portion of the trunk circumference damaged by the disease lesion: 0 = 0%, 1 = 1%–25%, 2 = 26%–50%, 3 = 51%–75%, 4 = 76%–100%.

Trunk diameter was measured with a diameter tape 15 cm above soil line prior to excavation. Trunk diameter was used to assess performance only on sites where trees of uniform size, from the same source, were all planted at the same time (NC, WA).

Statistical Procedures

Regression analysis was used to examine the relationship between root depth and tree performance, with depth as the independent variable, and crown vigor, trunk condition, or trunk diameter as the dependent variable. Linear regression was used for trunk diameter measurement data (SAS procedure PROC REG, SAS 9.2). Significance is reported for P < 0.05. Crown vigor and trunk condition ratings were analyzed with logistic regression (SAS procedure PROC LOGISTIC, SAS 9.2) with Somers’D used as an indicator of model fit. Significance is reported at χ² probability P < 0.05.