Urban Forestry Associates March 2, 2026
Town Hall Valley Oak Assessment
Assignment
Jacob Solis asked me to inspect a mature valley oak (Quercus lobata) across the street from Town Hall following a large limb failure and to provide management recommendations. While onsite, we also inspected two nearby valley oaks. All three trees were labeled with numbered tags, suggesting they are on the Town’s list of Heritage Trees.
Observations
Tree 1 – Valley Oak (Quercus lobata)
Metal tags: 158 and 75
The trunk of the primary tree measured 42 inches in diameter at standard measuring height. I was able to observe the point of failure from the ground. The failed portion had been removed prior to my inspection, though debris from the failure remained on-site.
Review of Google Street View imagery from 2024 suggests that the portion that failed was the dominant vertical leader and the tallest portion of the tree (Figure 1). Historic imagery dating back to 2011 shows progressive canopy decline, including repeated deadwood removal. This long-term pattern suggests chronic stress rather than an acute event.
Client: Town of Yountville
Project Location: 1981 Finnell Rd, Yountville, CA
Inspection Date: February 20, 2026
Arborist: Ben Anderson
Figure 1. Left: Tree after failure. Red arrow indicates failure point. Right: Tree before failure. Red circle highlights the portion that fell.
Urban Forestry Associates March 2, 2026
Town Hall Valley Oak Assessment
Debris from the failed limb displayed black
staining and internal boring consistent with
ambrosia beetle activity (Figure 2). I was not
able to collect samples sufficient to definitively
identify Mediterranean oak borer (Xyleborus
monographus, MOB). However, the pattern of
boring, associated staining, and host species
strongly suggests MOB. Mediterranean oak
borer, first detected in Napa County in 2019,
is now established in portions of Napa and
Sonoma Counties and is known to infest
valley oak and other native oaks. The beetle
introduces symbiotic fungi into the sapwood.
In the case of Mediterranean oak borer, the
associated fungi include species capable of
contributing to vascular dysfunction and
structural decline.
The trunk of the tree appeared generally
sound at the time of inspection, though I
observed what appeared to be a decayed
fungal fruiting body at grade on the street side
of the trunk. Due to its advanced state of
deterioration, I was unable to identify it. Its presence suggests a possible root or butt rot pathogen affecting the lower trunk and root system.
MOB typically infests upper canopy branches first, with infestations progressing downward over time. By the time beetle activity becomes evident in the main trunk, structural compromise is often advanced and management options are limited.
The canopy currently contains significant remaining deadwood. The loss of the dominant leader substantially alters canopy architecture and further compromises structural stability.
In my professional opinion, this tree is in advanced decline, structurally compromised, and located immediately adjacent to a public roadway. Given the high likelihood of additional large limb failures in the coming months and years, continued retention in this location is no longer reasonable.
I recommend immediate whole-tree removal and proper disposal of infested material to reduce potential pest pressure on adjacent valley oaks. While research is ongoing, preventative systemic treatments are currently being used experimentally on high-value oaks in Northern California, modeled in part on management strategies for invasive ambrosia beetles in Southern California. These treatments appear most effective as preventative measures in uninfested or minimally infested trees and are not considered effective remediation for heavily infested trees.
Tree 2 – Valley Oak
Metal tags: 78 and 160
This tree is located on the same side of the street as Tree 1. The trunk measured 33.5 inches in diameter.
Historic Google Street View imagery from 2024 shows upper canopy dieback. At the time of inspection, I observed significant deadwood in the upper canopy. The lower trunk appeared healthy and structurally sound.
This tree does not currently warrant removal. However, proactive pruning is justified to reduce the likelihood of Figure 2. Feeding galleries of ambrosia beetles in a chunk of wood near the tree.
Urban Forestry Associates March 2, 2026
Town Hall Valley Oak Assessment
limb failure and to monitor for signs of
ambrosia beetle infestation.
Tree 3 – Valley Oak
Metal tag number illegible due to rodent
damage
This tree is located across the street from
Trees 1 and 2 on the Town Hall side. It
appeared healthier than Tree 1 and
somewhat healthier than Tree 2, though
significant deadwood was present in the
canopy.
The trunk and root flare appeared sound at
the time of inspection.
This tree should receive the same pruning
treatment prescribed for Tree 2.
Pruning Prescription for Trees 2 and 3
All pruning shall conform to ANSI A300
(Part 1) Pruning Standards and ISA Best Management Practices for Tree Pruning.
Objectives
• Reduce the likelihood of branch failure over the roadway • Remove dead and structurally compromised branches
• Improve canopy structure while preserving natural form • Provide opportunity to inspect upper canopy for signs of ambrosia beetle activity Pruning Method
The trees shall be pruned using a natural pruning system.
Crown Cleaning
• Remove dead, dying, and broken branches throughout the canopy. • Remove deadwood greater than approximately 2 inches in diameter. • Remove damaged, rubbing, or crossing branches where appropriate. • Avoid interior thinning.
Crown Reduction
• Perform selective reduction cuts to shorten the longest branches, particularly those extending over the roadway.
• Reduction cuts shall be made to lateral branches at least one-third the diameter of the parent stem. • Reduction cuts should generally not exceed approximately 3 inches in diameter. • Total live canopy removed shall not exceed approximately 15 to 20 percent during this pruning cycle.
Monitoring
During pruning operations, crews shall monitor for:
Figure 3. Oblique map showing the location of the subject trees.
Urban Forestry Associates March 2, 2026
Town Hall Valley Oak Assessment
• Fine boring dust (frass)
• Entry holes in upper canopy branches
• Black staining or vascular discoloration consistent with ambrosia beetle colonization If widespread upper canopy infestation is observed, the Town should be notified prior to completion of work for reassessment.
Climbing spurs shall not be used. All cuts shall be made just outside the branch collar to preserve natural defense mechanisms and minimize decay.
Conclusions
Tree 1 is in advanced decline with evidence of ambrosia beetle activity and probable root decay. Given its location adjacent to a public roadway and the high likelihood of future large limb failures, immediate removal is recommended.
Trees 2 and 3 remain viable but require proactive crown cleaning and selective reduction to reduce risk and allow for continued monitoring. Preventative chemical treatment of high-value valley oaks throughout the Town should be considered as part of a broader management strategy, particularly in light of Mediterranean oak borer presence in the region.
I can provide additional information regarding current experimental treatment protocols upon request.
SCOPE OF WORK AND LIMITATIONS
Urban Forestry Associates has no personal or monetary interest in the outcome of this investigation. All observations regarding trees in this report were made by UFA, independently, based on our education and experience. All determinations of health condition, structural condition, or hazard potential of a tree or trees at issue are based on our best professional judgment. The health and hazard assessments in this report are limited by the visual nature of the assessment. Arborists cannot detect every condition that could possibly lead to the structural failure of a tree. Since trees are living organisms, conditions are often hidden within the tree and below ground. Arborists cannot guarantee that a tree will be healthy or safe under all circumstances, or for a specific period of time. Likewise, remedial treatments cannot be guaranteed. Trees can be managed but they cannot be controlled. To live near trees is to accept some degree of risk and the only way to eliminate all risk associated with trees is to eliminate all trees.
Benjamin Anderson, Urban Forester
ISA Board Certified Master Arborist & TRAQ
RCA #686, WE #10160B
(415) 454-4212