John Lichter, M.S., Owner, Tree Associates Inc.,
ASCA Registered Consulting Arborist, ISA Board Certified Master Arborist

Steve Pro, Owner of Morningstar Builders, asked me to prepare an Arborist Report, including a tree evaluation, impact assessment and preservation plan, to meet City of Davis requirements prior to approval of his construction project. The project involved demolition of an existing residence and construction of a new one on a narrow lot. A concrete slab foundation with an 18-inch-deep stem wall was to be built 11 feet from the trunk of a mature valley oak and 13 feet from a London plane (Figure 1). A paved driveway was to be constructed between the trees and the foundation (Figure 2).

Figure 1
Figure 1. Looking southwest at property in Davis, California, post-demolition. The valley oak is on the left and the London plane is in the right side of the photo.
Figure 2
Figure 2. Site plan of property in Davis, California project. Subject trees are located to the left (south) of the ribbon driveway in the lower left side of plan.

I concluded that there was a high likelihood that the trees would topple or decline in health as a result of construction, considering the proximity of the trees to the proposed foundation and the likely damage caused by typical construction methods. This rating assumed an even distribution of roots at typical depths below grade. However, root systems vary considerably in size, depth, and spread, even among trees of the same species. I recommended that the driveway be installed on grade, with no scarification and minimal soil compaction, to avoid root injury and allow for adequate moisture penetration to the roots. In addition, because the foundation could not be located farther away from the trees, I recommended that the soil be excavated to expose tree roots where the foundation would be located. The information gained would help me to refine my prognosis for the tree and guide a more “tree friendly” foundation design.

With our Air Knife, a pneumatic excavator that uses compressed air to pulverize and remove soil safely and efficiently preserving roots as small as 1/8 inch diameter, I created a 38-foot-long and 14- to 20-inch-deep trench (Figure 3). The excavation uncovered several roots of one inch diameter and six larger roots. The larger roots were between 2 and 3.25 inches in diameter and from 4 to 13 inches deep. All of the larger roots were at least 9” below grade, except for one 2-inch diameter root.

Figure 3
Figure 3. View looking southwest at subject trees and pneumatically excavated trench. Exposing the roots allowed us to provide a more accurate prognosis and recommendations for effective tree preservation.

The proposed foundation construction plan would have required severing the six large roots. I discussed my concerns with the contractor, and we came up with a plan to “bridge” the foundation over the six roots so that they would not need to be cut and had adequate space for future root growth to occur without damaging the foundation (Figure 4).

Figure 4
Figure 4. Property in Davis, California post-construction with subject trees on left. Note the driveway was installed on-grade without scarification and soil compaction. The foundation “bridged over” all roots greater than 1” in diameter.

The tree preservation plan also called for avoiding construction vehicle traffic within as much of the trees’ root zones as possible, installing mulch over a geotextile fabric in areas where traffic was necessary, and irrigating the trees regularly during the construction process.

Approximately two and one half years after the project was built, the trees were thriving and exhibited no signs of stress (Figure 5). As this example shows, effective tree preservation that is compatible with building construction requires an understanding of tree biology and root architecture, as well as effective communication and cooperation between development professionals and the Arborist.

Figure 5
Figure 5. Looking west at property in Davis, California on April 24, 2012. The trees are healthy two and one-half years after the project.

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