Which Comes First, the Root or the Crack?

Roots and Sidewalk Condition

Trees were selected based on genera, date of planting, and tree lawn width. Data were obtained from the City of Cincinnati’s planting records. Four families and genera of trees were selected for the study: Gleditsia triacanthos (Fabaceae); Quercus, specifically trees in the red oak group including Q. rubra, Q. palustris, Q. shumardii, Q. phellos, Q. acutissima, Q. imbricaria, and Q. nigra (Fagaceae); Zelkova serrata (Ulmaceae); and Koelreuteria paniculata (Sapindaceae).

The genus Quercus is the only genus for which observations were made on multiple species. Quercus is a large genus, and small numbers of many different species in this genus have been planted on Cincinnati’s city streets. Species of Quercus were limited to the red oak group because many species in this group have similar uses in urban situations (Sydnor and Cowen 2001). The City of Cincinnati has regularly planted a single species from each of the other three genera in sufficient quantities for study.

The study focuses on trees planted in the past 4 to 20 years. The 20-year time frame was chosen because the City of Cincinnati expects sidewalks to have an effective service life of 20 to 25 years, and the city’s planting records covered roughly this period. Because the 1999 study found a very low rate of tree-related sidewalk failure during the first 20 years of a sidewalk’s life (Sydnor et al. 2000), tree-sidewalk interactions during this period became of primary interest.

The following data were recorded for each tree: name of the street and approximate address of the tree’s location; year planted; width of tree lawn; sidewalk condition as “not cracked,” “cracked but not displaced,” “displaced but not failed,” or “failed” (a “failed” sidewalk is defined as one that has been vertically and/or horizontally displaced 1/2 in. or more because this condition is considered condemnable by the City of Cincinnati); presence or absence of a root greater than 1 cm (3/8 in.) in diameter within 20 cm (8 in.) of the surface that was growing beneath the sidewalk; and diameter of main stem at 1.3 m (4.5 ft) (dbh).

For the purpose of this article, the term “sidewalk joint” refers to one of the designed failure points in the sidewalk or an expansion joint. To determine whether a root was growing under the sidewalk, joints located within 1.8 m (6 ft) of the trunk of the tree were examined. A nursery spade was used to dig to a depth of approximately 20 cm (8 in) immediately adjacent to the joint on the tree lawn side of the sidewalk, and the presence or absence of any root(s) at least 1 cm (3/8 in.) diameter growing underneath the walk was noted. A note was made as to whether the joint was cracked or not cracked. The term “cracked” refers to a fissure completely through the “joint.” Date of planting was determined using Cincinnati planting records and the assistance of employees from the Cincinnati Park Board. Tree lawns were limited in size to a width of less than 3.5 m (11.5 ft)

Soil Oxygen

Oxygen concentrations were measured under a single 100-m (328-ft) section of sidewalk. Twelve cracked sites were selected that appeared to be unaffected by adjacent trees. Pairs of holes 15 mm (5/8 in.) in diameter were drilled through the sidewalk on each side of the crack, 5 cm (2 in.) from it. The holes were made 15 cm (6 in.) from each edge of the sidewalk and in the center. A 1-cm (3/8-in.) steel bar was driven into the base to a depth of 15 or 30 cm (6 or 12 in.) from the sidewalk surface. A 15- or 30-cm-long polyethylene tube (10 mm o.d.) was inserted into the hole made with the bar. The tube was sealed into the sidewalk with a 2.5-cm (1-in.) sleeve of rubber tubing (15 mm o.d.) coated inside and out with vinyl caulk. The tube was capped with a rubber septum seal. Similar gas-sampling tubes were set in 12 intact regions of sidewalk, and further tubes were placed in the open ground at a distance of 15 and 30 cm from each edge of the sidewalk for both cracked and intact sidewalk locations. The sample tubes were set up in two batches in July and August, and they were allowed to equilibrate for about 30 days before gas samples were first collected. Further samples were collected in July one year later. A disposable plastic syringe was use to collect a 1-mL (0.034-oz) sample from each tube, and the syringes were sealed by insertion in a rubber stopper for transport back to the laboratory. Oxygen concentration was estimated by injection into a stream of nitrogen (20 mL min⁻¹) passing through a paramagnetic oxygen analyzer (Servomex, Crowborough, England) connected to a computing integrator (Shimadzu, Kyoto, Japan).

Statistical Analysis

Statistical analysis of root occurrence for the four genera growing on city streets was performed using SAS with the assistance of the Statistics Laboratory at the Ohio Agricultural Research and Development Center. The presence or absence of cracks and the presence or absence of roots growing beneath a sidewalk were used to tabulate trees in two-way frequency tables. Chi-square tests were done for each case by genus and for all trees. The tree root observations were made over a period of two years. Only the nearest joint to the tree was used for this analysis.

Roots vs. Cracks

There appears to be a strong relationship between the presence of a crack and whether or not a tree root is likely to grow under the sidewalk. Table 2 shows that in the case of all genera in the study, where roots were found at the nearest joint, the sidewalk was approximately five times more likely to be cracked than intact, compared to only 1.6 times as likely when roots were not found. Columns 6 and 7 of Table 2 report that of the 351 joints observed with no roots, 39% were intact and 61% were cracked. Of the 260 joints where roots were observed, only 16.2% were intact while 83.9% were cracked. An examination of 81 blocks adjacent to crabapple trees (Malus spp.) that were planted a few months earlier revealed no roots underneath adjacent sidewalks and that 39.5% of the blocks had already cracked (data not shown).

The data in this study were compared to the data from the 2000 study (Sydnor et al. 2000), which were also collected in Cincinnati. Sydnor et al. showed that in sidewalk blocks selected at random that were less than 20 years old, the rate of cracked joints was 17%, and there was no statistical difference in the failure rate between blocks next to trees and blocks not next to trees.

The present study examines only sidewalk joints adjacent to trees. Sidewalks of varying ages were included from new sidewalks as well as from many that were much greater than 20 years old. However, all the trees were examined 20 years after planting or sooner. Approximately 72% of the joints observed in this study had cracked, while 28% remained intact. In addition, it was noted that about 13% of the joints nearest the tree were condemnable, which is comparable to the 11% citywide rate for all blocks noted in Sydnor et al. (2000).

This finding suggests that roots have a fairly strong association with cracked joints versus intact joints and that cracked joints are often present at planting. Chisquare tests indicate that these findings are statistically significant, with P-values less than 0.01 (Table 2). The roots of Gleditsia appeared to have the strongest affinity for cracks followed by Zelkova, Koelreuteria, and Quercus. The differences among the latter three were fairly small. When roots were observed for all genera, they were nearly five times as likely to be present beneath a cracked joint than under an intact joint. Where no roots were found, the likelihood of a cracked joint was less than 1.6 times (61% ÷ 39%) that of an intact one for all genera. Chi-square tests demonstrate statistical significance with P-values less than 0.01 (Table 2).

Soil Oxygen

Soil oxygen concentrations underneath cracks in the sidewalk were higher when compared with similar areas underneath intact sidewalks (Table 3). As expected, oxygen concentrations were generally lower at 30 cm depth than at 15 cm. Under cracked blocks, oxygen concentrations increased from the tree lawn through the sidewalk to the adjacent park area. The lowest oxygen concentrations were observed at 30 cm depth in the tree lawn adjacent to an intact sidewalk.

While diffusion of oxygen into soils varies by soil texture, compaction levels, and drainage (Kramer and Kozlowski. 1960), the presence of an intact sidewalk appears to further limit the diffusion of oxygen into the soil underneath the sidewalk. Conversely, a crack appears to permit increased diffusion of oxygen underneath the sidewalk. Increased oxygen levels would be expected to increase root growth relative to areas with lower oxygen concentrations. One might also expect some accumulation of duff and nutrients that have been carried into the crack by runoff, which also might be expected to improve root growth. Wagar and Franklin (1994) found that temperature and moisture levels are higher underneath sidewalk blocks than in adjacent sod panels. Higher temperature and moisture levels and reduced oxygen diffusion rates underneath an intact block might be expected to lead to depressed root growth. Roots’ response to decreased oxygen is significantly impacted by temperature, with higher temperatures leading to increased effects from lowered oxygen (Glinski and Stepniewski 1985). Yet most agree that root growth of plants adapted to aerobic conditions begins to decline at oxygen levels below 20.9% (20.5 kPa) (Glinski and Stepniewski 1985). Studies have shown that root growth of corn decreased to 50% as oxygen levels were halved. (Saglio et al. 1984). A progressive decrease in mineral uptake in apple roots was noted at partial oxygen pressures below 14.7 kPa, and root initiation was suppressed below 11.8 kPa (Boynton et al. 1938). “There is no generally accepted means for determining whether growth on any given soil is being restricted by poor aeration” (Craul 1992). However, Craul notes, “the replacement [rate] of the oxygen may be more important [to root growth] than the actual concentration in the soil. The apparent length of the diffusion pathway in the liquid phase surrounding the soil roots is more important as a barrier to diffusion than the oxygen concentration.” As a root grows underneath an intact sidewalk, the diffusion pathway for oxygen will lengthen considerably relative to the diffusion pathway of a root that grows beneath a crack. The diffusion pathway beneath a cracked sidewalk would remain relatively constant. Craul further notes that poor aeration always restricts root penetration before foliage growth.