STEP ON A CRACK

The debate continues on whether tree roots destroy sidewalks. Researchers from The Ohio State University continued the quest for answers with their study of street trees in Cincinnati. It is generally accepted that correctly installed concrete sidewalks are designed to last 20 to 25 years. Following the lead of a previous study that found no difference between the failure rate of sidewalk panels next to trees and the failure rate of sidewalk panels not next to trees, a new study was performed with more than 600 street trees. Using City of Cincinnati planting records to date the installation of the street trees, scientists selected street trees planted in the 4- to 20-year period prior to the study. For each tree, researchers noted whether the sidewalk was cracked or not and whether roots were growing underneath the sidewalk. To locate the roots, the soil was excavated 8 inches deep between the tree and the sidewalk joint. Of 351 sidewalk joints observed with no roots underneath, 61 percent were cracked and 39 percent were not cracked. Of 260 joints observed with roots, 84 percent were cracked and 16 percent were not cracked. In a number of instances, roots had grown up to the intact edge of a block, where they proceeded to grow along the block until they found a failed joint, where they then grew underneath. Of great interest were the 81 blocks adjacent to crabapple trees planted a few months prior to the study — they found no roots under the sidewalk, but 39 percent of blocks were cracked.

In addition, oxygen concentrations were measured under a 328-foot stretch of sidewalk that was unaffected by trees. They inserted air-sampling tubes under both non-cracked and cracked areas of the sidewalk. Under the non-cracked sidewalks, they found very low oxygen levels, possibly too low to support root growth. As might be expected, they found slightly higher levels of oxygen under cracked sidewalks, but still the oxygen level was relatively low.

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Journal of Arboriculture 28(6)

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