SBCA TREE CONSULTING
1534 Rose Street, Crockett, CA 94525
Phone: (510) 787-3075
Fax: (510) 787-3065
Website: www.sbcatree.com
Steve Batchelder, Consulting Arborist Molly Batchelder, Consulting Arborist WC ISA Certified Arborist #228 WC ISA Certified Arborist #9613A CUFC Certified Urban Forester #134 ISA Tree Risk Assessment Qualified CA Contractor License #(C-27) 53367 E-mail: [email protected] E-mail: [email protected]
Date: June 29, 2023
To: Jeff Brown, Public Works Superintendent
City of Hercules
111 Civic Dr.
Hercules, Ca. 94547
Subject: Refugio Park Tree Risk Assessment
Assignment: SBCA Tree Consulting was requested to review trees within the Refugio Park perimeters in the context of tree safety.
2020 Scope: The scope of the 2020 survey was identified by Public Works and includes trees located within Refugio Park. Trees growing in the natural wooded area adjacent to the creek (mostly shrubs and willows) were not included. The Coast Live Oaks (Quercus agrifolia) which have developed naturally on the northeastern slope were not included in the survey since none pose a safety concern. Early structural pruning for the oaks is discussed in this report.
2023 Scope: A number of trees have been removed since the initial survey was conducted in July 2020. Recent limb failures on the remaining trees have prompted the City to have areas with higher target values reassessed. Trees in these high target areas were reviewed as stands. Removals that have occurred since 2020 have not been updated in the excel tree data sheets.
Introduction
The purpose of the tree survey is to understand the risks associated with the Eucalyptus trees, as well as other park trees located in significant target areas where tree safety is of greater concern. This report provides discussion of the present structural integrity of the Eucalyptus as well recommendations for action to enhance park safety.
This report provides data on the 388 trees surveyed in the context of the guidelines presented in the Tree Risk Assessment Manual, Second Edition published by the ISA (International Society of Arboriculture.
Appendix 1 provides the complete survey data for all trees. Data references trees by tree number, species, and diameter. An individual number tag has been attached to each tree on the north side and corresponds to the data entry.
Refugio Park Tree Risk Assessment Amended 6-29-23
City of Hercules 2 of 10
SBCA Tree Consulting Phone (510) 787-3075
1534 Rose St. Crockett, CA 94525 Fax (510) 787-3065 [email protected] www.sbcatree.com
Appendix 2 contains the tree location map. A yellow dot with a number corresponding to the metal tag locates each tree on a Google map. This is a basis for tree location and should be considered generally accurate. Due to the proximity of many of the trees, some areas do not show individual tree locations but instead provide numbers for the group.
The risk assessment is valid for two years. In that time, pruning and removals that have been recommended must be completed. If not completed, it is recommended that the trees designated for pruning or removal be reassessed.
Summary
Recommendations 2023 – Many of the Eucalyptus viminalis located in the park have reached the end of their safe and future life expectancies. The risk associated with large branch failures is higher than the City’s threshold for safety. Removal of all trees within a stand at one time is most practical from a cost perspective and future safety. When trees in a stand are only partially removed, the remaining trees will become subject to additional wind forces, thereby increasing the potential for failure during high wind events.
• Immediate action – It is recommended that 37 additional trees be removed and replaced to eliminate risk. This entails removal of all remaining trees numbered 12 through 69. These trees are concentrated in the highest target areas surrounding the parking lot and children’s play area. The history of failure shows that large branch failures can happen without warning and are difficult to predict. Tables and benches under these trees can be fenced off until tree removal is complete. Signs can be posted which read: “Eucalyptus trees are dropping large branches without warning. Keep Out.”
• Future action –A plan to remove and replace all Eucalyptus can be found under recommendations on pages 9-10 of this report.
• New tree planting – There are a number of exciting oak species available in nurseries that make excellent park trees. Many have drought tolerant characteristics and are better suited for the future than the present Eucalyptus. Some species to consider for replacement planting:
o Cork Oak (Quercus suber)
o Island Oak (Quercus tomentella)
o Netleaf Oak (Quercus rugosa)
o Mexican Blue Oak (Quercus oblongifolia)
o Engelman Oak (Quercus engelmannii)
o Chisos Oak (Quercus canbyi)
o Texas Red Oak (Quercus buckleyi)
o Graves Oak (Quercus gravesii)
o Silver Leaf Oak (Quercus hypoleucoides)
o Chinquapin Oak (Quercus muehlenbergii)
o Southern Live Oak (Quercus virginiana ‘Sky Climber’)
o Macedonian Oak (Quercus trojana)
Refugio Park Tree Risk Assessment Amended 6-29-23
City of Hercules 3 of 10
SBCA Tree Consulting Phone (510) 787-3075
1534 Rose St. Crockett, CA 94525 Fax (510) 787-3065 [email protected] www.sbcatree.com
Recommendations 2020 – The survey tagged and recorded data on 388 trees. The most numerous of the 13 species identified was the Eucalyptus camadulensis with 317 trees. The next three most numerous species are London Plane (Platanus x hispanica) with 20 trees and the two additional Eucalyptus species, Eucalyptus sideroxylon and Eucalyptus polyanthemos with 15 trees and nine trees, respectively.
Eleven trees were found to represent an Extreme-risk or High-risk and require prompt pruning or removal.
A total of 170 trees are recommended for removal for different reasons. Extreme-High-Moderate-risk trees are to be removed within the next two years. If Low-risk trees are not removed, these trees must be reviewed again in two years.
A total of 179 trees are recommended for pruning in the next two years. Primary pruning needs include end-weight reduction on heavy lateral stems and deadwood removal. The most critical are those trees in areas of high pedestrian use.
Risk Assessment Discussion
Tree removal and pruning was designated primarily to address health or structural concerns. Some smaller Eucalyptus are recommended for removal so that they do not grow large. Several Eucalyptus trees have been recommended for removal to reduce competition with native Coast Live Oaks trees, a species better suited to the area. Most are small seedling oaks; some are larger and quite valuable.
Low-risk trees are not considered to present an immediate safety concern. The recommendations note that several the trees can be either pruned or removed to attain the level of safety required.
Extreme and High-risk trees1 – The survey identified one (1) tree that was determined to be an Extreme- risk and is recommended for removal. Ten trees are considered a High-risk: six of these are recommended for removal; four are recommended for pruning. Ten of these trees are Eucalyptus; one is a Weeping Willow (Salix babylonica). Mitigation for Extreme and High-risk trees should be conducted as soon as is possible.
Moderate-risk trees2 – Thirty-four (34) trees were determined to be a Moderate-risk. Twenty (20) trees require removal within the next year to mitigate risk; Six trees can receive pruning to remove heavy branch end weight and/or dead wood; Eight trees have an option of either pruning or removal.
1 “Extreme-risk situations should be mitigated as soon as possible. Immediate action may be required to restrict access to the target zone. High-risk situations should be mitigated as soon as it is practical” (Dunster, Julian A. Tree Risk Assessment Manual, Second Edition. U.S.A.: International Society of Arboriculture, 2017. Print. 2 Moderate-risk situations may not require mitigation but, if deemed necessary, could be mitigated when budget, work schedule, or pruning cycle allows. If the risk is acceptable to the client, the tree(s) could be retained and monitored” (Dunster).
Refugio Park Tree Risk Assessment Amended 6-29-23
City of Hercules 4 of 10
SBCA Tree Consulting Phone (510) 787-3075
1534 Rose St. Crockett, CA 94525 Fax (510) 787-3065 [email protected] www.sbcatree.com
Low-risk3 – Three-hundred forty-three (343) are Low-risk trees. One-hundred twenty-eight (128) are recommended for removal; these trees require removal or reassessment in two-years. One-hundred and twenty-five (125) trees are recommended for safety pruning. Eight trees can either be pruned or removed.
Table 1. Table below shows the levels of risk posed by the different tree species.
Species
Common
Name
Total
Amount
Low
Risk
Moderate
Risk
High
Risk
Extreme
Risk Comments
1 Acacia
melanoxylon
Blackwood
Acacia 1 1 0 0 0 Nice tree, in contact with
adjacent Eucalyptus
2 Alnus
rhombifolia
White
Alder 1 1 0 0 0
Minor tip dieback, may be
running out of soil volume,
otherwise nice tree
3 Eucalyptus
camaldulensis
Red River
Gum 317 278 30 8 1
Planted as a monoculture in tight
spacing. Trees are nearing the
end of their Safe and Useful life
expectancies.
4 Eucalyptus
polyanthemos
Silver
Dollar
Gum
9 9 0 0 0 Likely mistakenly planted as E.
camadulensis
5 Eucalyptus
sideroxylon
Red Iron
Bark 15 14 0 1 0
All located at top of slope
adjacent to residences. Species is
notorious for poor structure.
6 Pinus radiata Monterey
Pine 6 5 1 0 0
#12 adjacent to Refugio Valley
drive has a moderate risk and
requires DW removal
7 Platanus x
hispanica
London
Plane 20 20 0 0 0
Trees located at entrance drive
are overall in good condition; #10
is an excellent specimen; Trees
located behind bathrooms are not
thriving likely due to inadequate
soil volume, have poor aesthetics,
and are recommended for
removal
8 Populus
fremontii
Fremont
Poplar 4 3 1 0 0
Located at NE corner of park near
creek; Not thriving with poor
structures.
9 Populus nigra
‘Italica’
Lombardy
Poplar 6 4 2 0 0
Two are recommended for
removal; Three require removal
of dead wood
10 Prunus
serrulata
Flowering
Cherry 1 1 0 0 0 Dedicated tree
11 Salix
babylonica
Weeping
Willow 2 1 0 1 0
#341 is in terrible structural
condition; #70 is iconic to the
park
3 Low-risk trees should be retained and monitored (if appropriate) and/or mitigated, if desired, when the budget, work schedule, or pruning cycle allows” (Dunster).
Refugio Park Tree Risk Assessment Amended 6-29-23
City of Hercules 5 of 10
SBCA Tree Consulting Phone (510) 787-3075
1534 Rose St. Crockett, CA 94525 Fax (510) 787-3065 [email protected] www.sbcatree.com
12 Salix
lasiolepis
Arroyo
Willow 5 5 0 0 0
Adjacent to Refugio Valley Rd.
Pruning recommended to remove
dead wood and lighten heavy
lateral branches
13 Washingtonia
robusta
Mexican
Fan Palm 1 1 0 0 0 No issues, Remove dead fronds
Totals: 388 343 34 10 1
Mitigation– A total of 170 trees are recommended for removal. One-hundred seventy-nine (179) trees are recommended for pruning: end weight reduction pruning is recommended for 101 trees to reduce branch failure potential; 76 trees require dead wood removal over paths, parking lots, streets, and residences; 2 trees are recommended to receive structural pruning to improve scaffold structure.
Table 2. Table below shows the breakdown by species with pruning and removal recommendations.
Species Common Name
Total
Amount
Structural
Pruning
Deadwood
Removal
End-
weight
removal
Remove
Tree
1 Acacia melanoxylon Blackwood Acacia 1 0 0 0 0
2 Alnus rhombifolia White Alder 1 0 0 0 0
3 Eucalyptus
camaldulensis Red River Gum 317 2 64 90 165
4 Eucalyptus
polyanthemos Silver Dollar Gum 9 0 3 2 5
5 Eucalyptus
sideroxylon Red Iron Bark 15 0 0 0 9
6 Pinus radiata Monterey Pine 6 0 1 3 1
7 Platanus x hispanica London Plane 20 0 0 1 8
8 Populus fremontii Fremont Poplar 4 0 0 0 3
9 Populus nigra
‘Italica’ Lombardy Poplar 6 0 3 1 2
10 Prunus serrulata Flowering Cherry 1 0 0 0 0
11 Salic babylonica Weeping Willow 2 0 0 0 1
12 Salix lasiolepis Arroyo Willow 5 0 4 4 1
13 Washingtonia
robusta Mexican Fan Palm 1 0 1 0 0
Totals: 388 2 76 101 170
TRAQ Basic Risk Assessment Investigation
Arborist investigation in June 2020 entailed the following:
Refugio Park Tree Risk Assessment Amended 6-29-23
City of Hercules 6 of 10
SBCA Tree Consulting Phone (510) 787-3075
1534 Rose St. Crockett, CA 94525 Fax (510) 787-3065 [email protected] www.sbcatree.com
• Level of assessment – The inspection procedure is The International Society of Arboriculture’s Best Management Practices: Basic Tree Risk Assessment (TRAQ). • Visual inspection of upper crown – Visual inspection of crown, trunk and surrounded soil area was conducted from the ground.
• Rubber mallet – The lower 8 feet of trunks were sounded with a rubber mallet to detect hollowness/internal decay.
Scope of Work and Risk Assessment Methodology
Identify target– People who could be injured, property that may be damaged, or activities that could be disrupted by tree failure.
➢ People on the path
➢ Cars and people in parking lot
➢ Cars and people on Refugio Valley Rd and Sycamore Ave. ➢ Residential structures and people within to the East
➢ People on slope
Identify conditions of concern – The tree parts that could fail and the likelihood of failure within a 2-year period. The follow abbreviations have been used in Appendix 1, Tree Survey Data. ➢ T is Trunk – Main stem failure
➢ PB is Primary Branch – Stem failure
➢ STB is Secondary or Tertiary Branch – Stem failure
➢ R is Roots – Root failure
Estimate likelihood of impacting the target – Determined by the amount of time one of more targets are within the target zone, as well as factors present that could protect the target from impact. ➢ Refugio Valley Road and Sycamore Ave have a Frequent Occupancy Rate ➢ Paved walking paths have a Frequent Occupancy Rate
➢ Dirt walking paths have an Occasional Occupancy Rate
➢ Park turf areas have a Frequent Occupancy Rate
➢ Steep slope areas have a Rare Occupancy Rate
➢ Residential structures to the East have a Constant Occupancy Rate
Estimate consequences of failure – The value of the target and the potential injury, damage, or disruption that could be caused by the impact of the failure. ➢ Consequences of failure should the tree impact a target. Based on likelihood of failure, likelihood of failed part impacting the target.
Refugio Park Tree Risk Assessment Amended 6-29-23
City of Hercules 7 of 10
SBCA Tree Consulting Phone (510) 787-3075
1534 Rose St. Crockett, CA 94525 Fax (510) 787-3065 [email protected] www.sbcatree.com
Designate risk – Using the Tree Risk Assessment Matrix to the right.
Time frame –This tree risk assessment allows 2 years of action provide pruning mitigation to lower tree risk or remove tree to eliminate risk. The assessment is based upon the failure potential under “normal” weather conditions.
Discussion of Eucalyptus Tree Structural Integrity
There are several factors that impact the structural safety of the Eucalyptus trees. Some are inherent and a number are the result of pruning care.
History of failure – Many Eucalyptus trees have experienced large mainstem or secondary stem failures, sometimes multiple times on the same tree. While conducting our survey over the past month, we witnessed two large branch failures during windy days. Understanding the reasons why failure occurs provides information to help assess the likelihood of failure on the same tree or adjacent tree when a similar defect is present. Trees that experience one type of failure are more likely to experience the same failure again.
Tree aspect ratio –Aspect ratio is the diameter of the branch relative to the diameter of the trunk, both measured immediately above the union. Branches with a small aspect ratio are very well attached to the trunk; those with a large aspect ratio separate more easily from the trunk (Ed Gilman) Over 98% of Eucalyptus trees surveyed were noted with a large aspect ratio, or branches with diameters that are over 50% the diameter of the main stem. It is our professional opinion that most of the previous stem failures were in part a result of large aspect ratios.
The branch aspect ratio can be made smaller by slowing the growth rate of the branch relative to the trunk. The best way to do this is by pruning to remove foliage from the outer portion of the branch.
Foliage should be removed from the
outer one-third of the stem, and
rarely interior stems.
Codominant stems – Trees having
codominant stems (two or more
primary stems of equal size) have a
greater potential for failure.
Codominance is a high aspect ratio.
Single stem dominance can be
encouraged through early structural
pruning.
Refugio Park Tree Risk Assessment Amended 6-29-23
City of Hercules 8 of 10
SBCA Tree Consulting Phone (510) 787-3075
1534 Rose St. Crockett, CA 94525 Fax (510) 787-3065 [email protected] www.sbcatree.com
Stem taper – Many of the Eucalyptus trees lack sufficient taper on long stems. Branch taper is impacted by the loss of interior stems that provide a source of carbohydrates for wood (xylem) generation. When taper is insufficient, branches typically fail at the weakest point in accordance with wood strength and laws of physics.
It is our professional opinion that many of the previous stem failures were in part a result of poor stem taper.
Tree spacing – Many of the Eucalyptus trees were planted closer than 10 feet apart. Planting large stature trees in such narrow spacing limits branching development which reduces tree wind buffering ability. Trees are forced to reach for the light to be competitive, and lower branches die from lack of sunlight. Trees that have long overextended branches with reduced secondary, tertiary, quaternary etc. branches (poor taper) are more subject to branch failure.
Lion’s tailing – This type of pruning entails the removal of inner branches, leaving foliage concentrated at branch ends. The loss of the interior branches reduces branch taper and leaves wounds that often lead to decay and branch failure. Lion’s Tailing is never a good practice and is exhibited in many of the Eucalyptus trees in Refugio Park. It is also the case that some trees lions tailed themselves due to light competition from adjacent trees in proximity.
Safety pruning and tree removal – End weight reduction pruning has been recommended for 101 trees.
One-hundred seventy trees have been recommended for removal. The bullet points below provide guidance on how best to carry out the safety mitigation. • Heading cuts are counterproductive. When stems are cut to a location where there is stub or the branch left to take the lead is less than half the size of the cut, the potential for future stem failures is increased. Pruning must avoid such cuts.
• Epicormic sprouts are new stems sprouts resulting from heading cuts or severe pruning. Such stem sprouts are weakly connected to the stem and often grow rapidly and fail due to the weak attachment. Epicormic sprouts should not be allowed to become large. • Included bark is common defect that is most serious when stems are codominant. This is a narrow angle of attachment between two stems that causes the bark to be included between the stems, precluding wood attachment. Codominant stems attached with included bark are structurally problematic and require pruning to remove or subordinate the branch likely to fail. • Unbalanced branch weight causing twisting. A twisting break is identifiable by the angle at which the brake occurs. Most stem failures occur due to a twisting action when unbalanced by the wind. Pruning must address balancing.
• Hazard Beam splitting, or a split in the middle of a branch longitudinally between tensile and compression wood. This defect is most common when a stem develops an upward sweep before growing more horizontally. The growth causes a crack in the middle of the stem between tensile and compression wood. Such branches have a higher risk of failure. • Fiber-Buckling is the buckling of compression wood below branches near the point of attachment. This shows how a tree is responding to an over-weighted branch. Branches that display such signs are pruned to subordinate.
Refugio Park Tree Risk Assessment Amended 6-29-23
City of Hercules 9 of 10
SBCA Tree Consulting Phone (510) 787-3075
1534 Rose St. Crockett, CA 94525 Fax (510) 787-3065 [email protected] www.sbcatree.com
• Stand dynamics and changes in wind forces is a significant concern when planning to remove multiple trees in a closely planted stand. The wind forces on the trees that remain are altered from what was normal; the remaining trees become vulnerable to increased wind forces not previously experienced. It is often the case that the remaining trees must receive increased scrutiny regarding removal or pruning needs.
Observations and Recommendations
Tree removal and replacement – Other than trees recommended for prompt removal or pruning, issues can be addressed over the next two years. Areas recommended to be focused on first are those with the highest target value: streets, parking lot, paved walking paths and structures.
Replacement planting is most easily accomplished when all trees from the planting area are removed.
This will allow for a higher level of site preparation, more sunlight for the new trees, and less chance of damage to the new trees when the present trees are finally removed.
Tree spacing – It cannot be expected that the same number of trees removed will be planted due to the previous tight spacing. Spacing should consider the mature spread of the species. For closer planting, it is best to consider small stature trees or trees with a “fastigiate” or upright form.
Tree species diversity – Here is an excellent article by Greg McPherson about healthy urban forests- https://www.fs.fed.us/psw/publications/mcpherson/psw_2013_mcpherson002.pdf. McPherson has participated in urban forest studies for many cities, including Chicago and Sacramento. His findings are validated by statistical analysis. He says healthy urban forests are dependent on a diversity in “species dominance, age structure and pest threat”. In other words, diversity in tree species, mature stature size, lifetime expectancies, initial box sizes (which includes seedlings), and genus, are all equally important. It is recommended that diversity be championed in replacement planting for Refugio Park.
Northeastern slope – It is recommended that the native seedling oaks be encouraged to grow on the steep slope. Oaks will benefit from the removal of adjacent Eucalyptus due to less competition for light, water, and nutrients. It was noted than many young Coast Live Oak trees had been cut during the previous fire clearance efforts.
Structural pruning of young oaks – Early structural pruning is likely the most cost-effective maintenance a tree will ever receive. It determines the height of the first permanent branch, removes branches with defective attachments when they are small, and creates a tree with a low “aspect ratio” that is less likely to fail in later years.
Concluding Remarks
Renovation of Refugio Park’s canopy cover appears to be in order. Many of the existing trees are beyond their safe and useful life expectancy and, if not dealt with now, will continue to increase in associated risk.
Refugio Park Tree Risk Assessment Amended 6-29-23
City of Hercules 10 of 10
SBCA Tree Consulting Phone (510) 787-3075
1534 Rose St. Crockett, CA 94525 Fax (510) 787-3065 [email protected] www.sbcatree.com
Tree removal and replacement planting must consider the long-term implications on the park’s aesthetics and safety needs. The initial focus of work should be in areas having higher “target” values, such as the parking area, walkways, and seating areas. Tree removal and replanting can be coordinated so that as large areas are cleared of trees, site preparation and new tree planting can be carried out.
This method will pose less risk to newly planted trees when existing trees must be removed later.
This initial large-scale removal of problematic park trees can be understood as in opportunity to replant with more appropriate tree species and increased species diversity.
Replacement tree planting is most effective when done correctly: appropriate species selection, selecting good quality nursery stock, planting site preparation, and proper root ball preparation. The original park architect, Peter Walker and Partners, should be contacted regarding any park renovation plans.
Most efficient long-term tree management occurs when the most problematic trees are removed every year, some mature trees are pruned for safety, new trees are planted, and newly planted trees receive early structural pruning. Early structural pruning is likely the most important and cost-effective maintenance a tree will ever receive.
End Report
Report submitted by:
Steve Batchelder, Consulting Arborist
ISA Certified Arborist WE 228A
CaUFC Certified Urban Forester #138
Calif. Contractor Lic. (C-27) 533675
Molly Batchelder, Consulting Arborist
WC ISA Certified Arborist #9613A
Tree Risk Assessment Qualified (TRAQ)
Appendix items
1. Tree Survey Data
2. Tree Location Map
3. Photos