Construction Development Versus Oak Trees: Can the Two Co-Exist?

I had the good fortune to be selected as the project arborist for a portion of the San Diego Gas and Electric Sunrise Power Link Project. This is a high voltage energy transmission project bringing an additional source of electricity to the San Diego County area and supplementing the California energy grid. The transmission lines bring energy from the desert southwest into and through San Diego County backcountry.

The portion of the project I consulted on is known as the Suncrest Substation, located at the edge of the Cleveland National Forest just east of Alpine, California. Beta Engineering was the design/build general contractor awarded the contract for constructing the substation for SDG&E. Since the substation was located three miles from the nearest paved road, a three mile construction and operations access road needed to be developed.

In order to build the access road, SDG&E obtained road easements with private land owners to grade and build the access road. However, due to the presence of hundreds of existing Coast Live Oaks and Engelmann Oaks, the property owners required a certified arborist be present before and during construction to make recommendations and implement best management practices to preserve and protect the oak trees during road grading and construction.

Beta Engineering selected Rappoport Development Consulting Services LLC as their independent certified consulting arborist for the Suncrest Substation construction project.

The following article describes the project and how Jeremy Rappoport, an ISA certified consulting arborist designed and implemented best management practices for oak tree construction preservation.

Construction Development Versus Oak Trees:

Can the Two Co-Exist?

Rappoport Development Consulting Services LLC recently completed a project providing independent certified arborist consulting services for Beta Engineering, a design build general contractor awarded the construction contract for the Suncrest Substation, a portion of the San Diego Gas and Electric Sunrise Power link Project. The Suncrest Substation is located east of Alpine, California adjacent to the Cleveland National Forest, approximately 40 miles east of San Diego.

The substation project included developing a 2.5-mile access road for constructing the substation and for ongoing operations and maintenance once completed. The access road runs 2.5 miles through private land used for ranching and recreation. The rugged hills and mountains contain many native California plant species and areas of relatively undisturbed native habitat.

To reach the future substation site, the access road had to be constructed on land containing many native oak trees, including Quercus engelmannii andQuercus agrifolia. The Engelmann oak and Coast Live Oak, both native to California, thrive in the hills, canyons and mountains in eastern San Diego County.

The Engelmann and Coast Live Oaks ranged from 50 to 250 years old, with some magnificent Engelmann specimens displaying 42-inch diameter trunks.

While the Engelmann oaks appeared in relatively good health, the Coast Live Oaks (Quercus agrifolia) showed symptoms of Gold Spotted Oak Borer (GSOB), infestation including crown thinning and dieback, with major trunk and vascular system damage. Fortunately, the Engelmann oaks did not display any signs of bark beetle infestation.

In an effort to minimize tree loss due to construction impacts, the road was surveyed and redesigned several times. Even with the best efforts of the design team, many oak trees had to be removed for construction. Additionally, there were several hundred oak trees adjacent to the construction zone and road easement that would be impacted by construction activities.

Since Beta Engineering was the GC responsible for grading and improving the access road, they were also responsible for protecting and preserving oak trees immediately adjacent to the construction zone. Beta Engineering was required to provide an experienced, certified consulting arborist to implement an oak tree construction preservation plan and provide monitoring and recommendations throughout the access road construction time period of approximately one-year.

Beta Engineering selected Rappoport Development Consulting Services LLC as the certified consulting project arborist. The first site visit revealed beautiful, relatively undisturbed natural terrain. Since the access road was built within an easement on private land, much of the area has been protected by development, resulting in large oak trees and other native species including Ceanothus, Manzanita, Rhus and Malosma.

Pre-Construction Panning

Pre-construction tree protection planning is mandatory to achieve success in the design and implementation of a protection plan. The scope of the assignment was:

  • Review and amend the existing tree inventory with current site data.
  • Analyze grading, improvement and tree survey exhibits, determine and rate the extent of adjacent construction impact to the trees.
  • Design best management practices (BMP’s) and specifications to minimize construction impacts.
  • Provide field inspections and observations prior to and throughout the construction period. Make job site recommendations as required.
  • Develop a final report summarizing the results of the project.

Using an existing tree inventory and survey maps identifying tree locations, pre-construction field inspections compared this information with the survey stakes located in the field. Current field status was updated in the tree inventory, including digital photographs and the tree condition.

The importance of pre-construction planning is crucial for a successful tree protection plan. Tree protection best management practices generally work far better when implemented before heavy equipment and grading operations begin.

With a complete understanding of the field and tree conditions and upcoming construction schedule, RDCS LLC developed best management practices for the general contractor to implement and distribute to their sub-contractors and consultants.

General pre-construction recommendations included erecting barriers and fencing to define the workspace from the tree protection zone. A primary concern was protecting the root zone from mechanical damage and soil compaction. Warning signs were specified at regular intervals. Crown and canopy pruning specifications were designed and implemented to achieve vertical and horizontal clearance for large trucks and equipment while avoiding mechanical damage to oak tree limbs and branches.

Best management practices to minimize impacts to the oak trees during construction included placing mulch layers to reduce soil compaction, adjustments to changes in grade to minimize root disturbance and root pruning to reduce mechanical damage to roots caused by grading equipment. During the summer months, the trees canopies were rinsed by water truck to remove accumulated dust from construction activities.

Grading Impacts:

Due to site topography, the access road was constructed along slope side contours. To achieve a level roadbed through a hillside contour, one side of the road was a cut resulting in a 2:1 upslope from the edge of road to the top of slope. A concrete lined brow ditch was constructed at the top of the slope.

The other side of the road required a fill slope that resulted in a down-slope from the edge of the road to the bottom of the slope.

There were several storm drain crossing that conveyed water under the road in storm drain culverts to the downhill side of the road where the water “day-lighted” out of the toe of slope and ran in free flowing creeks. Therefore, construction impacts were anticipated where storm drain inlets and outlets were constructed.

Due to slope cuts and fills, the existing oak trees were threatened with several impacts. The trees on the cut slope side of the road would suffer from mechanical damage to their roots from grading and brow ditch cuts into the slope. Many trees contained low scaffold branches that would be damaged by grading equipment. Tree impacts on the fill slope side of the road included root disturbance and suffocation by placing fills soils over the existing surface grade.

During grading construction, very heavy grading equipment would result in soil compaction in the adjacent tree root zone.

Construction Monitoring and Supervision

In addition to developing and implementing best management practices for oak tree preservation, another project assignment included supervising the grading sub-contractor responsible for oak tree crown and root pruning activities. The grading contractor sub-contracted the tree pruning work to a professional tree service. The grading contractor utilized in house employees for root pruning laborers.

Rappoport Development Consulting Services LLC supervised both above grade crown and canopy pruning and well as below grade root pruning. With the advent of the raptor-mating season, all tree crown pruning had to be completed no later than December 31, 2010. Gaining vertical vehicular access road clearance was the main objective and reason for crown pruning. Therefore, crown pruning included skirting up low lying limbs to achieve seventeen feet vertical clearance from the future finish road surface. Horizontal clearance required pruning back any growth to prevent encroachment into the right of way. Crown reduction and selective limb removal were the main crown pruning techniques used to satisfy access specifications. One had to be experienced in reading survey stakes to understand where the finished road surface would be in relation to the existing tree limbs.

Due to the age and maturity of the oaks, many trees had very large scaffold branches ranging up to sixteen inches in diameter. Unfortunately, several large limbs had to be removed to achieve the required clearances. In those instances, cuts were carefully selected and made well outside the branch collar to provide as much protection for the tree to compartmentalize the wound. Dead wood was removed as required.

The root pruning work included both mechanical and hand trench excavation starting from the outside edge (drip line) of an individual tree crown or a stand of oak tree canopies. The trench was located between the outside edges of the grading work adjacent to the trees. Most of the tree rooting occurs in the upper twenty-four inches of the soil profile. Therefore, the trench specifications were approximately eighteen inches wide (wide enough to accommodate a laborer and shovel) with an average depth of thirty inches. In some cases the trench excavation extended to thirty-six inches. Approximately 80% of the roots were encountered above twenty-four inch depth.

The goal with root pruning is to avoid construction and mechanical damage to the root system by trenching and cutting roots prior to heavy equipment making grading cuts that would severely damage adjacent tree roots. Once the root pruning is employed, grading equipment can then excavate and pull the cut roots out without mechanically tearing the roots, thereby minimizing root disturbance and tree shock.

The objective was to expose roots, cut and remove roots crossing through (perpendicular) to the trench. As the trench deepened, roots up to six and eight inches in diameter were cut throughout the entire trench profile. Large diameter roots were cut with a sharp chainsaw, smaller size roots were cut with sharp tree loppers. Once all of the roots were cut, the trench was backfilled and grading proceeded up to the trench cut.

Monitoring and Data Collection

The tree crown pruning started in December of 2010 with grading operations beginning in February of 2011. Root pruning began with grading and was completed by March of 2011. Oak tree monitoring began prior to construction in November of 2010 and the final monitoring occurred prior to paving in September of 2011.

During the access road design, the consultant team rated potential damage to the tree based on the percentage system below:

Status of the Tree:

% Damage to the Tree:

Preservation Category:

1. Save the tree No damage Tree is considered preserved
2. Save the tree, minor pruning required Up to 25% damage Tree is considered preserved but affected
3. Save the tree, major pruning required Over 25% damage Tree is not considered preserved
4. Remove the tree 100% damage Tree is considered a loss

The consultant team defined tree damage as any physical alteration of the tree caused by proposed construction, including branch and root pruning, grading within the root zone, removal of one or more multiple trunks. The determination of the percent damage is a judgment made by the arborist at the site during construction and the amount of disturbance to the individual trees and considers roots, trunks, branches, and crown.

Tree monitoring was implemented based on seasonality and for job specific construction events. All of the monitoring utilized visual observation and digital photographic record keeping. While monitoring, many types of data were collected. Much of the data was relative, comparing the tree health and condition from the original tree inventory to the current monitoring event.

The main categories in the tree-monitoring log examined items such as:

  • Tree category designation
  • Whether crown pruning occurred
  • Whether root pruning occurred
  • Trunk or root damage occurred
  • Spring defoliation and flush
  • Crown and leaf appearance
  • Comments on edge condition, wound response, presence of Gold Spotted Oak Borer (gsob) and general appearance
  • Photographic identification number

Comparisons between monitoring events examined:

  • Reaction to crown and root pruning
  • Signs of crown decline or dieback
  • Signs of new buds and growth
  • Spring flush, color and density
  • Leans or stability issues from root pruning
  • Overall tree health, vigor or decline
  • Current construction impacts
  • Punch list and corrective actions

The oak tree preservation and monitoring program was designed to minimize construction impacts to the trees and monitor the tree response over a period of time. The program did not include tree risk assessment for structural integrity, risk to the construction workers, the public or property, nor did the program address recommendations for the tree Owners to correct noted existing deficiencies.

The project construction timing may have benefitted the trees. The crown and root pruning occurred during the winter months, a time of slow metabolic activity for the oak trees. The grading disturbance began in early spring, approximately the same time many of the Engelmann oaks dropped their old leaves from the previous season and began their spring flush of new growth. The weather also cooperated in that San Diego county experienced above normal rainfall for the year, resulting in elevated water tables for the trees, keeping soil moisture conditions relatively high, even throughout the hot, dry summer months.

Results

Of the 369 trees inventoried and monitored during the program, no trees were lost. However, at least three to five trees exhibited signs of crown decline and foliage discoloration. Symptoms of tree decline from construction impacts may take up to three to five years to fully manifest. Therefore, full knowledge of successful survivorship will not be known for another two to three years.

That being said, it was very encouraging to see these ancient oaks survive at least during the heavy construction phase. Pruning cuts on limbs from four to twelve inches in diameter showed signs of healing and minimal evidence of decay. The trees dropped their foliage in the spring and displayed a nice spring flush of growth by the June monitoring. Yellowing and discolored foliage with browning of tips and margins was a typical foliage symptom. The fact the trees put out a healthy spring flush following crown, root pruning and grading operations, followed by a successful growing season was a positive sign.

Trees that appeared stressed in March re-foliated and displayed improved general health in June. More importantly, the trees survived throughout the hot summer months and did not defoliate as of observations made in September of 2011.

The ultimate success of the oak tree preservation program cannot be fully assessed until a sufficient time period has elapsed whereby the trees exhibit typical healthy growth patterns without signs of increasing or ongoing decline. The minimum elapsed time period for a final health assessment would be three years from date of construction impacts or March of 2014. This would provide three full years of “non disturbed” growth and allow trees to either fully recover or decline. Trees that continue to decline after three years most likely have entered the “tree death spiral”, a condition whereby the tree is unable to recover from injuries incurred during construction and are a total loss.

Unfortunately, the contract with Beta Engineering expired and the continuing status of the oak trees is no longer available to RDCS. Based on current survivorship and tree health, upwards of 369 Quercus engelmannii and Quercus agrifolia were persevered due to protection and preservation best management practices.

While final results will not be known for another year or so, the oak tree preservation program was determined to be a major success by both the general contractor and the client, San Diego Gas and Electric.

It was very gratifying to see the various project stakeholders acknowledge the importance and value in preserving as many native oak trees as possible balanced with the project transmission and sub-station objectives. While a certain percentage of the oaks may fail from the construction impacts over a period of time, a great majority of the trees were saved through careful preservation and protection best management practices.Therefore, the answer to the question, can development and oak tree preservation co-exist, I believe the answer is yes.

The importance and value in preserving and protecting trees has gained awareness with the “green movement” and creating sustainable environments. Tree benefits and their inherent value figure prominently in construction and development projects. Hopefully, public, owner and agency awareness about the positive value of saving, preserving and protecting native and heritage trees will continue to grow.

We all need to be responsible stewards of our planets diminishing resources. Knowing these oak trees survived during construction of a major electrical transmission and substation construction project and will be seen by future generations is a very rewarding experience.

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