The Governor’s Oaks
Careless Messing with Historical Trees
Excerpts from a report I prepared for the builder and St. Johns County in January, 2007 (not all footnotes included)
I was asked by a builder/developer to assess the damage to two large protected Southern live oaks (Quercus virginiana) by a trenching activities done within the tree protection zone. In the 1980’s then Governor Martinez declared both trees “Historic Trees” stopping planned state highway department widening project that would have had the trees removed. Since then I understand the county has designed both trees as specimen trees.
The north live oak has a diameter (DBH) of 57 inches and the south live oak has a DBH of 55 inches. Overall both trees are in very good condition with good foliage, branches, and, until recently, undisturbed root systems. Both trees have good branch structure. Excavation into the backfilled trench found torn and cut roots near the trunks of each tree. Such root damage can cause both structural problems and long-term health problems. The loss of roots close to the trunk can affect a tree in two ways:
• probable significant loss of a tree’s stability, which is the tree’s ability to
remain standing in a windstorm
• reduction in the root mass that supports the canopy resulting in stress and likely decline I also found fill soil had been added near the base of the two trees and some soil compaction had occurred, probably during the trenching process near the south tree. The trees, known as the Governor’s Oaks, have special historical and environmental significance to the residents in the area and are a significant part of the recently designated Wm. Bartram Scenic & Historic Highway. I used an Air Spade to move soil away from roots without harming the roots. And I used a penetrometer, which measures soil compaction, to determine if the soil near the tree has been compacted to the point that root growth is inhibited or roots are killed.
I used a soil probe – a 48-inch tall, t-shaped device also known as a tile probe – to locate the side of the trench nearest the tree. The trench was easy to find because the backfill soil was soft and the probe would easily sink its entire 48-inch length. I also tested disturbed soil where the existing turf had been covered or removed to determine if fill soil had been added over the native soil level. I examined the north oak first and then the south oak. After I located the edge of the trench, I used the Air Spade to remove soil until I found a root or root stub. I would then uncover the root or root stub and photograph it before moving the soil back into the trench.
The trench and subsequent directional boring to install a 7 to 8-inch diameter utility pipe near the trees is the cause of the concern for the Governor’s oaks and the main reason for this report. The north-south trench extends along the
east side of both trees. The trench is approximately 24 to 36 inches wide by
approximately 48 inches deep and had already been backfilled when I was called to the site for the first time on December 21, 2006. The trench is located approximately 6 feet from the trunk of the north tree at the closest point and curves about 4 feet to the east as it approaches the south oak where the trench is approximately 10.5 feet from the south tree trunk. The trench was dug near the north tree first and the south tree last.
After examining both trees and the site, I found the following problems that, in my opinion, can have a negative impact on the Governor’s Oaks:
• reduction of both trees’ stability, which is the ability to remain standing in a
• reduction in the root mass that supports the canopy resulting in stress and likely
• fill soil at a depth of 4 to 12 inches applied over large areas within the tree
• some compaction by the south tree
• entrance road construction close to the south side of the south tree
Effects of Cutting Roots on Carbohydrate Allocation
According to Roberts, et. al. severing large roots causes an imbalance between roots and shoots (foliage and canopy). Carbohydrates are reallocated to the root system for regrowth at the expense of shoot growth. For this reason, a commonly observed effect of root severance after trenching near trees is loss of vigor in above-ground growth. If carbohydrate availability within the tree is limited as it often is on large, old trees, re- establishment of the root-shoot equilibrium may not be possible and trenching can threaten the survival of the tree. They add, “A number of possibilities exist; for example the root system may be inadequate to supply the canopy with sufficient water and desiccation occurs. Alternatively, or in addition, the stability of the tree may be compromised and it may blow over in high winds.” 4, 5
Roberts, et. al., further discuss carbohydrate allocation as affected by a drought stress similar to the one-year drought we have been experiencing in St. Johns County since December, 2005, “The full extent of damage to a tree caused by root severance is unlikely to be immediately apparent, as the crown is intact and the foliage may appear healthy. If carbon assimilation by the tree has been impaired in the years prior to trenching, for example, by drought or disease, reserves of energy may have already been depleted, leaving the tree less able to recover. Thus, resilience of a tree to trenching depends on the state of its carbohydrate reserves prior to trenching and during recovery…and raises the risk of long-term injury or mortality.”6
Root Growth at the Site
Understanding tree root growth is important if we are to maintain healthy, long-living trees in our urban environment. There is a common misperception among people who are unfamiliar with recent research about tree growth and physiology. This misperception is tree roots grow downward and only out as far as the dripline. But we now know tree roots grow outward from the tree often as far as the tree is tall when soil conditions permit. Tree roots grow mainly in the upper 18 to 24 inches of soil where the oxygen content of the soil is higher.8, 9
It is likely that the highway, S.R. 13, restricts the significant westward root growth of both the Governor’s Oaks Soil beneath the road is highly compacted and is a hostile environment for significant root growth and survival of fine feeder roots. The location of the roots can be determined by surrounding infrastructure, According to Roberts, et. al., “A tree growing next to a road or building is likely to have virtually all of its roots growing on the opposing side of the trees, as observed by Cermak, et. al., using ground-penetrating radar to map coarse roots. Trenching close to such a tree could cause loss of almost the entire root system and would almost certainly have a severe impact on the viability and stability of the tree.10
I also found that fill soil had been added around both trees within the tree protection zone. This fill soil will cut off oxygen to the remaining roots and further exacerbate the trees’ problems. According to Roberts, et. al., “Filling creates the problem of poor soil aeration. Consequently, if fill is placed over the root zone of an existing tree, the growth and function of the root system may be impaired depending on the depth of fill and material used.”11
Multiple Stresses and “Tough” Trees
A single stress can cause problems for a tree especially an old tree. Multiple stresses can compound the problems. And the Governor’s Oaks have been exposed to multiple stresses. Even before the construction began, the Governor’s Oaks had been exposed to a severe drought for almost one year. Then the trenching followed. According to Dr. Watson, a trench dug near a tree being the “only stress will probably be less destructive than at a site where there has also been other disturbances such as soil compaction, a new pavement surface, or drainage changes.” Watson goes on to write, “So-called ‘tough’ species will often tolerate more root damage than ‘sensitive’ species on the same site.
The Southern live oak is a “tough” species based upon various researchers and my own experience. unfortunately, the Governor’s Oaks have a number of stress factors – severe root loss, drought stress for the past year, fill soil, root wounds where decay can start, compaction – that can complicate recovery of the trees’ health. Mitigation of the trees’ compromised stability is of greater and immediate concern.
Directional boring would have been a viable alternative to trenching if it had been used according to the County Landscape Development Code specifications. Even though the initial cost of directional boring seems higher than trenching, the long view of mitigation costs, the possible bracing and support of the trees, and damage and decline of the trees could make the cost of directional boring appear reasonable. And directional boring could have resulted “in a tremendous amount of public goodwill.19
There is the potential for 1 to 4 feet of fill and heavy compaction to damage roots in order to complete an entrance road on the south side of the south tree. There are no plans to save the tree roots beneath the street. And those roots could be critical to the survival of the south tree Older trees and stressed trees may be severely impacted by slight or moderate root loss. Broadly spreading trees may require more extensive boring than trees with small crowns.”17 Live oaks are generally considered to be a “tough” or tolerant species.18
Although I am pleased that a very large root on the south tree has been saved, the tree protection zone is unduly compressed on the east side of both trees. The east root protection zone boundary is only about half the distance to the east dripline. Within only 10 feet of the large saved root there has been severe soil disturbance which almost appears to be grubbing or root raking. So in spite of saving the large root, the area to where this root extends has been badly disturbed. This tree protection zone should be expanded and protected. Damage has already been done. Compaction occurs only above the geo-grid which diffuses the compaction over a wider area.
The difficulty in assessing damage is the Governor’s Oaks do not react to these stresses right away. It takes several years for the problem to become apparent. There are several possible outcomes. The trees could die but that is unlikely in the near future. However, it is quite likely the trees will begin to decline and possibly have some major branches die. Dieback will probably occur on the side the roots were cut causing an unbalanced canopy with more of the surviving canopy weight and mass hanging over the road exacerbating the trees’ instability.
These are my recommendations for the Governor’s Oaks:
Repairing a damaged tree is difficult at best. Older trees do not handle damage well and the impact may be greater and recovery difficult if at all. Decline may take several years to become apparent. My recommendations are as follows:
1. Clear away the added fill dirt as soon as possible and extremely carefully. Do not
drive heavy equipment over the roots. Most of this is hand work. If a track hoe is
used, the track hoe should be parked outside the dripline and carefully pull the
excess soil with the help of laborers away from the tree. A probe should be used
to determine the native soil depth so the track hoe does not damage existing
surface feeder and secondary roots.
2. Begin irrigating the two trees under the entire canopy. Irrigation should be once a week for the next three weeks and then once every two to three weeks in the absence of at least ½-inch of rainfall at the site until May. Then from May until completion of the project, irrigate once a week. A rain gauge should be installed at the site or nearby and monitored by an interested homeowner/neighbor. A post-construction irrigation schedule should also be set up based upon the condition of the trees at that time.
3. Establish a tree protection zone of the maximum size possible based upon county code criteria and the county approved construction plans. A tree protection zone boundary that is only half the distance to the dripline is inadequate especially
since there is additional space to enlarge the zone to the east.
4. Construct a sturdy wooden or chain link fence around the tree protection zone with signs indicating the tree(s) are protected and any construction activity (driving, parking, dumping, excavation, fill, temporary storage, concrete cleanout, etc.) is prohibited within the fenced area.
5. Organic mulch is probably the most useful and least expensive treatment for tree roots. The larger the mulched area the better. Mulch should be maintained at the 10 to 12 inch depth through the construction period and then reduced to about a three inch depth when construction is completed and landscaping begins. Only small equipment should be used to move mulch within the tree protection zone. This mulch will provide some cushioning protection to the tree roots in the likely event the tree protection zone is encroached by heavy equipment.
6. The mulch layer should be retained instead of installing turf at the end of the
project. Turf will compete against the weakened tree roots. It is better if there is no competition. Some negotiation with the Department of Transportation may be
required to get them to accept mulch instead of turf.
7. Do not treat the trees with fertilizer. Fertilizer will stimulate new foliage which will further stress the compromised root system. A growth regulator is recommended in these cases. See recommendation 10 below.
8. Prune dead branches only. The removal of these branches is a safety issue. Dead
branches are in various stages of decay and can fall. No live branches or sprouts
should be removed unless it is for clearance purposes only. Pruning should be
done only under the supervision of an ISA Certified Arborist on the site during
the entire pruning process.
9. Retain an ISA Certified Arborist trained in tree protection techniques during
construction for bi-weekly visits to the site to be sure all procedures and
protective measures are being followed. The arborist should have authority to call
County Code Enforcement officials if site managers are not following prescribed
tree protection procedures.
10. Apply a tree growth regulator treatment with paclobutrazol. The cost of
paclobutrazol for the two Governor’s Oaks would be $975. The treatment lasts
three years and has been shown to divert limited carbohydrate reserves into root
growth rather than shoot growth. 20
11. Redo entrance road by carefully removing fill, mitigating any compaction that has occurred so far and using a geo-grid at the natural soil level before the roadbed is set in place.
Post Construction Maintenance
A post construction maintenance plan should be set up and implemented at the end of construction.
1. There should be an assigned responsible party whom the arborist can contact to
assure the post-construction maintenance plans are being honored for at least five
years. Responsibility must be assigned to one person (builder, developer, owner,
condo association, property manager) who will answer to the County and respond
to requests by the arborist in charge of the trees.
2. There should be a tree and site inspection by an ISA Certified Arborist at the end of construction and before the post-construction maintenance plan goes into effect to document the condition and structural stability of the Governor’s Oaks and any other trees on the site that the County decides should be monitored. An
assessment of construction impacts should be made at this time.
3. The landscape plan should be coordinated with the landscape architect regarding plant and shrub placement near or beneath the canopy of the Governor’s Oaks. This plan should be reviewed by the ISA Certified Arborist responsible for the trees at the site.
4. Irrigation should be available with a permanent irrigation installation at ground
level. No trenching. A time clock should be provided with a rain shutoff. This
time clock should be set to irrigate the Governor’s Oaks up to once weekly as
needed According to Dr. Watson writing about root damage from trenching, “If
special maintenance, especially watering, can be continued for several years, then
the tree may be able to survive the stress associated with greater root loss. The
larger the tree, the longer it will need watering during dry periods….”21
5. Monthly monitoring from April through October and bi-monthly monitoring
November through March by an ISA Certified Arborist responsible for the
Governor’s Oaks. The Certified Arborist should also monitor the following during
regular visits to the site:
a. Pruning prescription should be made and monitored by an ISA Certified
b. Removal of fill soil or mitigation of any compaction
c. irrigation adjusted for rainfall and season
d. pest management
g. paclobutrazol treatments
i. weed control
j. any other treatments deemed necessary to the health and structure of the
Arborist’s Note: St. Johns County made the developer follow my recommendations. In 2015 the Governor’s Oaks are doing fine
Not all footnotes are included above because the report has been abbreviated.
1 L.R. Costell, K.S. Jones, Reducing Infrastructure Damage By Tree Roots: A Compendium of Strategies, Wester Chapter of the International Society of Arboriculture, 2003, p. 15.
2 Thomas Smiley, Bruce Faedrich, Neil Hendrickson, Tree Risk Management, Bartlett Tree Research Laboratories, Charlotte, NC 28278, 2002, p. 16.
3 John Roberts, Nick Jackson and Mark Smith, Tree Roots in the Built Environment, Centre for Ecology& Hydrology, Natural Environment Research Council, London, 2006, pp. 259-260.
4 Ibid., p. 256.
5 Ibid.,. p. 249.
6 Ibid., p. 254.
7 F.W.M.R. Schwarze, J. Engels, and C. Mattcheck, Fungal Strategies of Wood Decay in Trees, Springer, Germany, 2000, pp. 43-46.
8 Dr. Ed Gilman, Trees for Urban and Suburban Landscape, Delmar Publisher, New York, 1996, pp. 92 – 103.
9 Nelda Matheny and James Clark, Trees and Development: A Technical Guide to Preservation of Trees During Land Development, International Society of Arboriculture, Champaign, IL, 1998, pp. 16 – 17.
10 John Roberts, Nick Jacksonv and Mark Smith, Tree Roots in the Built Environment, Centre for Ecology & Hydrology, Natural Environment Research Council, London, 2006, p. 245.
11 Ibid., p.286.
12 Gary Watson and Dan Neely, Tree Root Damage from Utility Trenching,Trees & Building Sites, International Society of Arboriculture, 1995, p. 35.
13 According to Dr. Gary Watson referring to a study the costs of directional boring by J.D. Morell in 1984, comparison of the estimated cost of tunneling (directional boring) to the actual costs of removing and
replacing trees that died after trenches were dug to install water mains near large trees demonstrated that removal and replacement of trees killed by trenching cost $29,000 while the cost of directional boring with
no trees lost would be $32,000 for a net cost of directional boring of $3,000. Watson goes on to suggest that the Morell study shows a major problem with the way construction projects are often organized. “Not all
the indirect costs of a project are included in the project budget. If the cost of repair, removal and replacement of damaged trees were part of the project budget, the economy of directional boring would be
14 City of Palo Alto, Tree Technical Manual, Protection of Tree Suring Construction, Section 2.00, p. 2- 11, Table 2-1.
15 James R. Fazio, National Arbor Day Foundation, Trenching & Tunneling Near Trees, Nebraska Dr. City, NE, 1998.
16 Colorado State University, Telecommunications Design Standards, Revision 9, October, 2005.
17 Gary Watson and Dan Neely, Tree Root Damage from Utility Trenching, Trees & Building Sites, International Society of Arboriculture, 1995, pp. 40-41.
18 Nelda Matheny and James Clark, Trees and Development: A Technical Guide to Preservation of Trees During Land Development, International Society of Arboriculture, Champaign, IL, 1998, p. 176.
20 John Roberts, Nick Jackson and Mark Smith, Tree Roots in the Built Environment, Centre for Ecology
& Hydrology, Natural Environment Research Council, London, 2006, pp. 116-117.
21 Gary Watson and Dan Neely, Tree Root Damage from Utility Trenching,Trees & Building Sites, International Society of Arboriculture, 1995, p. 41.