Gibbons Sidewalks and Trees Assessment

Gibbons Tree Inventory, prepared by HortScience | Bartlett Consulting

Gibbons Drive in Alameda was laid out in the 1920s or 1930s, during the development of the Fernside neighborhood. The oldest sweetgums (Liquidambar styraciflua) on Gibbons Drive were likely planted during development, making them approximately 100 years old. The trees are an iconic feature and important to the community's character and urban forest. However, the roots of many trees have caused significant and longstanding damage to the adjacent sidewalks, curbs, and storm infrastructure, creating both safety hazards and accessibility challenges.

Managing these sweetgums in an urban environment has been and continues to be a complex issue which the City of Alameda has been monitoring for some time. As a step toward developing a comprehensive and thoughtful solution, the City reached out to HortScience | Bartlett Consulting, Divisions of the F.A. Bartlett Tree Expert Company, to assess trees and assist with creating an equitable and community-oriented management plan. Forthcoming public input and comment will be a foundation of the process. This letter responds to that request.

Description of Trees

Darya Barar, Managing Consulting Urban Forester and Arborist, and I visited the site and assessed the trees on July 29 and 30, 2025. One hundred and seventeen (117) sweetgums were street trees along either side of Gibbons Drive. The mature canopy of large, fused crowns defined the landscape of the neighborhood. Planting space was limited, with approximately 4 feet between the back of curb and the sidewalk on either side of the road. Most individual sweetgums were mature in development. According to arboricultural best management practices, this corresponds with having lived roughly 40 – 80% of their natural lifespan. Ninety-nine (99) trees had trunks that were 20 inches in diameter or larger. Eighteen (18) trees had trunks measuring between 6 – 19 inches in diameter, indicating a degree of successional planting of the species over time.

Of the 117 sweetgums, 88 were in good condition. These trees were generally vigorous and free of major structural conditions of concern such as cavities or decay columns. Most trees had been maintained using a combination of branch removal, branch reduction, and heading cuts over the years to create a candelabra form (Photo 1). Large scaffold branches often emerged at a single point on the trunk and swept upwards. Branch attachments appeared sound. 

Twenty-four (24) sweetgums were in fair condition. These trees often had lower vigor than those in good condition. Some had moderate structural defects such as well-compartmentalized cavities or weak branches that may be abated with treatment like pruning.

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For example, tree #1584 had a moderate vigor and slightly thin foliage (Photo 2). The lowest branch on the west side was weakly attached and damaged at the attachment point (inset), which could be remedied by pruning.

Five sweetgums (#1538, 1547, 1564, 1571, and 1582) were in poor condition. These were characterized by poor vigor or structural conditions of concerns that would be impossible to correct. Condition can be expected to decline, regardless of treatment. We recommend these be considered for removal.

For example, sweetgum #1571 leaned north into the road at the corner of Gibbons Dr and Cornell Dr (Photo 3). Buttress roots on the tension side, which offer most of the structural support, were heavily constrained. A large cavity with significant decay was present on the northeast stem. Both conditions will only worsen over time.

Mature Tree Management Discussion

The sweetgums along Gibbons Drive are a tremendously valuable asset to the neighborhood and City. Large trees offer considerable shading benefits and reduce ambient temperatures. The dense, closed canopy provides good habitat for nesting wildlife and improves air quality by filtering particulate matter. Additionally, the trees define the landscape in an aesthetic sense. 

As a population, the trees along Gibbons Drive were in remarkably good condition considering their location and mature development stage. It is rare that street trees reach this stage of maturity due to the difficult growing conditions and infrastructure conflicts. The Gibbons Drive trees are exceptional in that sense. The trees have been well cared for, and have the potential to thrive for many more years. That said, management of large, mature trees in urban areas presents significant challenges. Old trees, while having emotional and aesthetic appeal, are limited in their ability to respond to change. This group has outgrown the available rooting space and frequently damages surrounding hardscape. Ongoing repairs are costly and can and often accelerate tree decline or require removal.

If the City and community are committed to preserving these trees, additional space must be allocated for them. This may involve, but should not be limited to, removing hard scape, creating bulb-outs (potentially reducing parking), shifting sidewalks toward private property, or redesigning the street. Although no survey has been conducted, the City assumes a 60-foot right-of-way, which is believed to extend approximately 2 feet onto private property.

In shaping future management of this tree population, particularly relating to conflicts with hardscape, the City intends to meet with the community to listen to their experiences, perspectives, and goals which can help to inform recommendations. Preliminary recommendations contained in this letter are made with the goal of enhancing the quality of the existing tree resource. The key elements of tree maintenance for the sweetgums on Gibbons Drive would include:

Pruning

The City conducts an enhanced level of pruning for these sweetgums compared to other street trees. Typically, street trees in Alameda are pruned once every 5 years. The subject sweetgums are pruned on a 2.5-year cycle with the primary goal of reducing likelihood of branch failure. Trees along Gibbons Drive are being pruned regularly and with the following three objectives. Given the nature of the site and tree resources, the existing pruning regime should continue.

Pruning must have specific objectives as described in the American National Standards Institute (ANSI) A300 standard for tree pruning. All terms should be familiar to ISA Certified Arborists. The following are appropriate objectives of the current pruning that should be continued as needed:

1. Pruning to improve structure: Structural pruning is commonly defined as pruning to improve tree form and correct defects. The defect can be caused by a variety of different occurrences, both biotic and abiotic. The subject trees have been structurally pruned over time to achieve the candelabra-shaped branch structure.
   
   
2. Pruning to reduce likelihood of failure: End weight reduction can be defined as branch shortening and removal to reduce the load on a lateral or scaffold branch. End weight reduction is often recommended when limbs, stems and laterals have grown in length or weight leaving the exterior of the canopy heavier than is sustainable. Heavy end weight can cause branches to fail. In contrast to lion tailing, reduction works from the tip of the branch towards the interior.
   
   
3. Pruning for clearance: Clearance pruning involves removing branches that block or impede site lines along sidewalks, roadways, or driveways.

Planned Tree Removal

Tree removal is an integral part of a comprehensive street tree management program. Although large, mature trees are visually and emotionally appealing, they can present significant maintenance and safety problems as they age. Most importantly, the potential for trees to fail increases with age with large-growing species such as sweetgum.

A phased removal plan would be a reasonable approach to management over time on Gibbons Drive. For example, poor condition trees would be removed first. Fair condition trees would be reevaluated over time, and removed as they decline or meet other defined thresholds for action such as infeasibility of hardscape repair. For example, removal of a fair condition tree may be a prudent choice when proposed hardscape repair would sever roots that would exceed the biological tolerance of the tree or compromise stability.

To limit visual and environmental impact on the surrounding area, removal of mature trees is often staggered both in location and over time. Coordinated replanting allows time for new trees to develop before other trees are removed.

Sidewalk Repair

In high-density and compact urban areas such as the public right-of-way, trees and hardscape including concrete or asphalt paving often conflict. Planting space is limited to the parkway, otherwise known as the landscape strip, between the sidewalk and curb. In the City of Alameda, these parkway planting sites are often small square or rectangular basins.

The soil below conventional paving restricts root growth due to compaction, surface sealing, and inadequate soil volume. In response, roots tend to grow just below the pavement where more air and water are available. Subsequent root thickening may cause pavement cracking, heaving, and / or shifting. Above ground, roots and trunks can begin to pillow over the tops of curbs or sidewalks once the space within the planting strip has been filled. Such damage can create a tripping hazard and require expensive repairs. Sweetgums, as a species, are known to cause particularly severe damage due to large and fast-growing surface roots. Damage is already severe throughout much of this stretch of Gibbons Drive (Photo 4). 

During typical repair operations for root-damaged hardscape, roots in question are sometimes cut. This requires oversight from a qualified arborist. Depending on the size and species of the tree in question, these repairs may predispose trees to premature decline and removal. An example of a repair that would lead to tree removal is removing portions of the trunk or buttress roots. This amount of damage will usually exceed the tolerance of the tree and result in rapid tree decline or destabilization.

With careful coordination of repair, efforts can be made to prioritize safety and limit tree removals. However, tree removals may not be avoidable entirely. Repair may simply be incompatible with tree preservation in some scenarios. We do not have adequate detail currently to identify individual trees for preservation or removal until a repair design alternative, or alternatives, are identified. This is the crux of the management issue along this stretch of road. The City has stopped all repair work until a plan is created. Given the degree of damage and the size of the trees, creative solutions will be necessary to limit tree removal.

Innovative and creative repair strategies include raising the elevation of the sidewalk, moving the sidewalk or curb farther from the trunk or roots (e.g., bulb out or meandering sidewalk), and / or use of diamond plates or other low-profile materials in-lieu of sidewalk (Photo 5 - 7). Root barriers are ineffective regardless of when they are installed. Installing them adjacent to mature trees, furthermore, would require the removal of a significant number of roots and lead to premature decline and destabilization of trees.

These techniques must be evaluated on a tree-by-tree basis. For example, a bulb-out may preserve roots on the street side of the tree, but may be unsuccessful in fostering tree preservation if roots cannot be preserved on the sidewalk side. In our experience, there are typically fewer and smaller roots on the street side as compared to the sidewalk side, due to more available water and more favorable growing conditions on the sidewalk side. Even with these creative techniques, preservation of all trees may not be possible. A key objective of the community engagement process is to determine which techniques merit further exploration.

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Maintenance Recommendations

To enhance the quality of the existing tree canopy, I recommend a combination of removal and pruning.

In the short-term:

1. Remove sweetgums #1538, 1547, 1564, 1571, and 1582. Significant decay was present in the main trunk of each tree, and compounded by a pronounced lean on trees #1547 and 1571. I know of no arboriculturally sound treatment to reverse or correct the damage and loss in structural stability. Condition will only worsen over time. Removal is a prudent management option.
   
   
2. Prune trees #1537 and 1584. Remove the low, decayed branch on #1537 on the northeast side. Remove the low, poorly attached branch on tree #1584 on the west side. I expect no negative impact to the trees.
   
   
3. Continue the scheduled pruning cycles for all other sweetgums.
   All pruning shall be done by a State of California Licensed Tree Contractor (C61/D49). All pruning shall be done by an ISA Certified Arborist or Certified Tree Worker in accordance with the latest edition of the Best Management Practices for Pruning and adhere to the most recent editions of the American National Standard for Tree Care Operations (Z133.1), and pruning (A300).

In the long-term:

1. Encourage root growth in the parkway by enhancing the soil and rooting conditions. Add drip irrigation at a slow rate at or as close to the dripline (in the right-of-way) as possible. Remove other plant material and use mulch instead.
   
   
2. Use the subject tree assessment and community input to shape future sidewalk repair efforts. Most trees have outgrown the current space between the curb and the sidewalk. Damage throughout the stretch of road was significant. Repair efforts, depending on the proposed work, have the potential to damage and remove many trees.
   Creative solutions, such as bulb-outs into the street, a modified street section, and / or meandering the sidewalk further from the trees on the property side, may allow for better tree preservation and must be investigated in further detail. Trees in good condition would be better candidates for creative preservation efforts than those in fair condition or limiting factors such as driveways or storm drains nearby. These features severely limit the ability to provide more space for the trees. H|BC is working with City of Alameda Public Works to coordinate a public meeting and begin the planning process.
   
   
3. Most trees were semi-mature or mature in development. A diverse mix of young, mature, and old trees makes for a more resilient tree population. Where space allows in response to any removals, the street would benefit from new plantings to replace mature trees as they are removed from the landscape.
   A phased removal plan would be a reasonable approach to management over time on Gibbons Drive. For example, poor condition trees would be removed first. Fair condition trees would be reevaluated over time, and removed as they decline or meet other defined thresholds for action such as infeasibility of hardscape repair. For example, removal of a tree in fair condition would be a prudent choice when hardscape repairs would sever roots that would exceed the biological tolerance or destabilize the tree.
   Proactive planting allows time for new trees to develop before other trees are removed. Given the history of maintenance difficulties, a species other than sweetgum should be prioritized. This is an opportunity for the City and community to engage in a thoughtful process to create a tree palette for new plantings. It also presents an opportunity to diversify trees along this street in terms of species and age distribution.
   
   
4. Plan tree preservation and new plantings in coordination for future irrigation design, including intensity of irrigation and water quality, and to allow for development of mature trees. Do not plant high water-use species or those with invasive potential. Do not plant species, such as sweetgum, which are known to cause significant hardscape damage. Ensure adequate spacing between plantings for development of mature trees.
   
   
5. Continue with the current pruning regime for the trees. The trees have been well maintained over time, and continued care will help maintain the asset they present to the City of Alameda. Ensure that future tree maintenance adheres to the most recent editions of the American National Standard for Tree Care Operations (Z133.1) and tree pruning (A300).

Summary

One hundred and seventeen (117) sweetgums were assessed 88 were in good condition, 24 were fair, and five were poor. Condition largely varied with overall vigor and foliage density along with the presence and severity of structural conditions of concern such as a lean or decay in the trunk.

I recommend #1538, 1547, 1564, 1571, and 1582 be removed. The trees had extensive, uncorrectable decay in the main stems, which was compounded by a lean in the case of #1547 and 1571. Conditions will not improve over time.

Sweetgums #1537 and 1584 would benefit from pruning to remove a decayed and damaged branch, respectively.

H|BC is working with City of Alameda Public Works to coordinate a public meeting and begin the planning process to inform tree management and sidewalk repair efforts in the future. This letter and inventory are intended to be an early step in that process.

If you have any questions regarding my observations or recommendations, please feel free to contact me.

Sincerely,

HortScience | Bartlett Consulting

Ryan Suttle, Consulting Arborist & Urban Forester

ISA Board Certified Master Arborist, Utility Specialist No. WE-12647BUISA Tree Risk Assessment Qualified
ASCA Registered Consulting Arborist #813
ASCA Tree and Plant Appraisal Qualified

Originally Enclosed (view in the PDF report):

Tree Assessment Form (detailed grid of each tree's identification number, condition, and impact to surrounding infrastructure)
Tree Assessment Map

If you need an alternative or translated format, please contact ada@alamedaca.gov.