Based on the recommendations of Master Arborist Vincent DeBrock

Draft Version 1

As of 6/15/23

For Discussion Purposes

In an ideal world, the critical root zone of the ancient live oak tree on North Vine Street would look like the area outlined in white below:

The tree is 64 inches in diameter. Since live oak trees grow about ¼ inch in diameter every year, this tree’s diameter would indicate that it’s approximately 250 years old (4 X 64 =  256 years). 

Generally accepted arborist guidelines specify that the radius of a tree’s critical root zone (CRZ) is defined as 1 foot for every one inch of diameter measured at breast height. For this particular tree, that guideline defines a CRZ having an area of approximately 12,860 square feet. (64 X 64 X 3.14 = 12,861 sq ft). 

Note: The above diagram was created using the Google Earth app. Square footage represented within the white outline was measured automatically using Google Earth. Linear measurements may not appear to scale even though they are accurate because Google Earth takes the Earth’s curvature into account.

In the perfect world postulated by the diagram above, the total surface area of this ideal CRZ would be free of large obstructions and impermeable areas. It would provide the tree with access to oxygen and water. Underneath the surface, it would be free of large obstructions down to a depth of 24 inches since the tree’s roots can penetrate that deep through soil. The topsoil layer of the CRZ would be permeable to oxygen and provide the tree with access to organic and inorganic soil nutrients. 

Unfortunately, this tree doesn’t live in a perfect world. Over time, its CRZ has been obstructed and it has had to adapt. It has done so successfully, despite what we humans have thrown at it, for at least the past 250 years. 

At some point in the past, the city installed North Vine Street about 10 feet from the base of the tree on the west side. The roadbed under the street and the  infrastructure under the street including water and sewer lines likely extends well below the 24 inch root depth; therefore, the street effectively cuts off the tree’s CRZ 10 feet from its base. If those assumptions are correct, the street eliminated approximately 4000 square feet of CRZ as shown in the following Google Earth diagram:

The arborists’ generally accepted guideline is that up to half of a healthy tree’s CRZ can be obstructed as long as the tree has sufficient space in which to rebuild its lost root system. In this case, the tree had plenty of area to the east in which to regrow new roots. After many years of recovery time the tree’s new CRZ may have looked something like the area outlined in green in the following diagram:

In 2013 the Bosque River Trail was routed across the tree’s CRZ. The trail route is shown in the above diagram by a dashed gray line. The concrete paved trail and its bed can be assumed to have severed the top 8 inches of the tree’s root system along its path. Since the tree’s root system extends down 24 inches, approximately ⅓ of the tree’s CRZ on the northeast side of the trail would have been eliminated. Impact of that intrusion on the tree’s total CRZ area is shown by the area outlined in red in the diagram below: 

As shown in the diagram above, the likely impact of the Bosque River Trail on the tree’s CRZ can reasonably be estimated as at least 2000 square feet. Since the tree recovered to full health after that impact, we can assume that the tree replaced the lost roots. The likely place for the replacement roots to occur is shown by the area outlined in yellow in the following diagram: 

In 2022 the City of Stephenville compacted 120 dump truck loads of fill dirt onto the tree’s CRZ and beyond. This fill dirt was to be the foundation for a new Senior Center building positioned near the tree. Although the City abandoned plans for the Senior Center building, the fill dirt remains in place as of June, 2023. The fill dirt is mostly inert clay with no organic content. It is virtually impervious to water penetration. Given it’s clay composition, inert nature, depth, compaction and expanse, it effectively cancels out the CRZ area underneath it. The following diagram provides an estimate of the fill dirt’s impact on the CRZ and the effective size of the CRZ as long as it remains in place: 

Based on the assumptions in the analysis above, it appears that the tree has been subsisting on a much-reduced CRZ of approximately 8360 sq ft since May, 2022. That’s about ⅔ of what the tree requires for near term survival. At present, it is using up reserve resources to stay alive. If it depletes its reserves, it will go into a death spiral and succumb to oak wilt or other disease.

As it has done in response to past intrusions on its root zone, the tree could recover on its own if it had an available area into which it could expand its root zone. Unfortunately, the latest intrusion has blocked any path it has for recovery. It is painted into a corner. Vine Street blocks it on the west. The Bosque River Trail blocks it on the northeast and the fill dirt blocks it on the southeast. Although it might be able to send out some deep roots under the Bosque River Trail, it’s rumored that the dirt in that area is low quality landfill from past history of dumping along the Bosque River. 

The City’s remediation plan at present is to clear the fill dirt out to 10 feet beyond the drip line of the tree. They may also do some air spading in that area to remediate the compaction. As the following diagram illustrates, that small amount of remediation, outlined in pink below, is grossly inadequate. 

Clearing the 10 foot strip, outlined in pink above, will only give back 1000 sq ft of root zone bringing the CRZ up to 9360 sq ft. This is approximately 3500 sq ft short of what the tree needs. The tree will still be in a state of depleting reserves with an insufficient CRZ and nowhere to grow beyond that. 

The following diagram shows an example of a remediation plan that would give the tree a better chance of recovering from recent depredations. 

This plan entails clearing the fill dirt out to 50 feet from the drip line. Clearing the area outlined in green above would restore the CRZ to the 12,860 sq ft target and provide some room for future growth. It will add 5000 sq feet to the area available for the tree’s root zone, making the total square footage open to the tree 13,860 sq ft. 

Once the 50 ft strip of fill dirt is removed, the entire cleared area needs to be air spaded to remediate the effects of compaction and encourage new root growth.

In addition, this proposed remediation plan opens a clear path for the root system into the river floodplain where the fill dirt is more shallow and easy to remove. The river floodplain is rich in organic matter that the tree needs. The river can provide water resources if the root system has unimpeded access to it. It’s likely that access to the river flood plain is what has sustained the tree for the past 250+ years. At present, the tree’s access is blocked by over 24 inches of fill dirt between it and the flood plain. 

To hold down the cost of implementing this plan, the city can leave the deepest section of the fill dirt in place without putting the tree at undue risk.