Salem Willows Tree Inventory Report - November 2018`
Tree Inventory Summary Report
Salem Willows Park, Massachusetts
November 2018
Prepared for:
Hatch Associate Consultants
27 Congress Street
Salem, Massachusetts 01970
978-224-3122
Prepared by:
Davey Resource Group, Inc.
1500 North Mantua Street
Kent, Ohio 44240
800-828-8312
Davey Resource Group i November 2018
Executive Summary
Hatch commissioned an inventory and assessment of the trees, stumps, and planting sites located
in Salem Willows Park. Understanding an urban forest’s structure, function, and value can promote
management decisions that will improve the urban forest as well as human health and
environmental quality. DRG collected and analyzed the inventory data to understand species
composition and tree condition and to generate maintenance recommendations. Tree values and
benefits have been quantified using the i -Tree Streets benefits model (developed by the United
States Department of Agriculture Forest Service in partnership with The Davey Tree Expert
Company). This report will discuss the health and benefits of the inventoried tre e population
throughout the town.
Key Findings
● The overall condition of the tree population is Fair.
● Most common species: Acer platanoides (Norway maple), 49%; Salix alba (white willow),
9%; Prunus serotina (black cherry), 6%; Malus spp. (flowering crabapple), 6%; and
A. rubrum (red maple), 4%.
● The majority (30%) of the urban forest is rated as mature, having a diameter greater than 24
inches.
● 75% of the population is recommended for Pruning, 13% is recommended for Removal,
7% is recommended for Discretionary, and 4% is recommended for Training.
● Trees provide approximately $23,225 in the following annual environmental benefits:
o Air Quality: valued at $302 per year.
o Aesthetic and Other Tangible Benefits: valued at $10,312 per year.
o Carbon Sequestration: 4,171 pounds valued at $24 per year.
o Energy: 17,787 kilowatt-hours (kWh) and 6,397 British thermal units (therms) valued
at $11,499 per year.
o Stormwater: 136,058 gallons valued at $1,088 per year.
Notice of Disclaimer
Inventory data provided by Davey Resource Group, Inc. “DRG” are based on visual recording at
the time of inspection. Visual records do not include individual testing or analysis and do not
include aerial or subterranean inspection. DRG is not responsible for the discovery or identification
of hidden or otherwise non-observable hazards. Records may not remain accurate after inspection
due to the variable deterioration of inventoried material. DRG provides no warranty with respect
to the fitness of the urban forest for any use or purpose whatsoever. Clients may choose to accept
or disregard DRG’s recommendations, or to seek additional advice. Important: know and
understand that visual inspection is confined to the designated subject tree(s) and that the
inspections for this project are performed in the interest of facts of the tree(s) without prejudice to
or for any other service or any interested party.
Davey Resource Group ii November 2018
Table of Contents
Executive Summary .......................................................................................................................... i
Section 1: Tree Inventory Assessment ............................................................................................ 1
Section 2: i-Tree Streets Benefits .................................................................................................... 5
Conclusion and Recommendations .................................................................................................. 7
Glossary ........................................................................................................................................... 8
References...................................................................................................................................... 12
Tables
1. Annual Benefits Provided by Inventoried Tree Population....................................................... 5
2. Annual Energy Savings Provided by Inventoried Tree Population ........................................... 6
3. Annual Air Quality Benefits Provided by Inventoried Tree Population ................................... 6
4. Annual Carbon Dioxide Benefits Provided by Inventoried Tree Population ............................ 6
5. Annual Stormwater Benefits Provided by Inventoried Tree Population ................................... 7
6. Annual Aesthetic and Other Tangible Benefits Provided by Inventoried Tree Population ..... 7
Figures
1. Tree species composition in Salem Willows Park..................................................................... 1
2. Top five genera in Salem Willows Park, in relation to the 20% Rule. ...................................... 2
3. Age class distribution compared to Richards’ (1983) ideal. ..................................................... 3
4. Overall condition ratings. .......................................................................................................... 3
5. Tree condition by age class. ...................................................................................................... 4
6. Maintenance needs by risk rating. ............................................................................................. 4
7. Annual $ i-Tree benefits. ........................................................................................................... 5
Appendices
A. Data Collection and Site Location Methods
B. Risk Assessment/Priority and Proactive Maintenance
C. Suggested Tree Species
Davey Resource Group 1 November 2018
Section 1: Tree Inventory Assessment
Project Area
In October 2018, DRG arborists assessed and inventoried trees, stumps, and plantin g sites in Salem
Willows Park in the City of Salem, Massachusetts. Additional information about the inventory can
be found in Appendix B.
Species Diversity
Throughout Salem Willows Park, 287 sites were inventoried, including 270 trees, 13 stumps, and
4 proposed planting sites. Figure 1 shows the composition of the most populous species compared
to all inventoried species. The composition of a tree population should follow the 10-20-30 Rule
for species diversity: a single species should represent no more than 10% of the urban forest, a
single genus no more than 20%, and a single family no more than 30%.
Currently, Salem Willows Park has one species—Norway maple (comprising 49% of the
inventoried population), that surpasses the 10% rule for species.
Figure 1. Tree species composition in Salem Willows Park.
49%
9%
6%
6%
4%
3%3%
2%18%
Norway maple (49%)
white willow (9%)
black cherry (6%)
flowering crabapple (6%)
red maple(4%)
pin oak (3%)
white ash (3%)
sugar (2%)
other (18%)
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Figure 2 represents the top five most common genera identified during the inventory in comparison
to the 20% Rule. The genera Acer (maple) far exceeds the recommended 20% threshold. Maple
makes up 56% of the park tree population.
Figure 2. Top five genera in Salem Willows Park, in relation to the 20% Rule.
Diameter Size Class Distribution
Analyzing the diameter size class distribution (measured as diameter at breast height [DBH])
provides an estimate of the relative age of a tree population and insight into maintenance practices
and needs.
The inventoried trees were categorized into the following diameter size classes: young trees (0 –8
inches DBH), established (9–17 inches DBH), maturing (18–24 inches DBH), and mature trees
(>24 inches DBH). These categories were chosen so that the population could be analyzed
following Richards’ ideal distribution (1983). Richards proposed an ideal diameter size class
distribution for street trees based on observations of well -adapted trees in Syracuse, New York.
Richards’ ideal distribution suggests that the largest fraction of tree s (approximately 40% of the
population) should be young (<8 inches DBH) with a smaller fraction (approximately 10%) in the
large-diameter size class (>24 inches DBH). A tree population with the ideal distribution would
have an abundance of newly planted and young trees, with established, maturing, and mature trees
present in lower numbers.
Figure 3 compares Salem Willows’ inventoried park tree diameter size class distribution to the
ideal proposed by Richards (1983). Salem Willows’ distribution is the opposite of the ideal; where
there are a greater number of mature trees (over the ideal by 20%) and the smaller number of young
trees (under the ideal by 24%). As the urban forest in Salem Willows ages and over mature trees
are removed, and new trees are planted, this ideal will begin to balance out.
maple willow apple cherry ash
Salem Willows 56%9%6%6%4%
Ideal 20%20%20%20%20%
0%
10%
20%
30%
40%
50%
60%Percent of PopulationSalem Willows Ideal
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Figure 3. Age class distribution compared to Richards’ (1983) ideal.
Condition
Several factors were considered for the condition of each
tree, including root characteristics; branch structure;
trunk, canopy, and foliage condition; as well as the
presence of pests. The condition of each inventoried tree
was rated Good, Fair, Poor, or Dead.
Most of the inventoried park trees were recorded to be in
Fair condition, 59% (Figure 4). The second most common
condition inventoried was Poor, 24%. Based on these
data, the general health of the overall inventoried tree
population is rated Fair.
Figure 5 illustrates the condition of the urban forest in
relation to the relative age classes. The majority of the
young, established, maturing, and mature trees were rated
to be in Fair condition. With an established maintenance
schedule and proactive care, this trend can be improved
to have a healthier urban forest.
16%
25%
29%30%
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
0–8"
(Young)
9–17"
(Established)
18–24"
(Maturing)
> 24"
(Mature)Percent of PopulationSize Class (inches)
Salem Willows Ideal
Number of Trees
Good 44
Fair 160
Poor 64
Dead 2
0
50
100
150
200
250
300
350
400
Number of TreesFigure 4. Overall condition ratings.
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Figure 5. Tree condition by age class.
Primary Maintenance and Risk
Primary maintenance refers to the task identified for a tree or site: Removal, Prune, Discretionary,
or Train. Risk is a graduated scale that measures potential tree-related hazardous conditions. A tree
is considered hazardous when the potential risks associated with it exceed an acceptable level.
DRG based the maintenance recommendations and risk values (Figure 6), in part, on the evaluation
of species, diameter class, condition, impact of hazard, and defects found in the individual tree.
Identifying and ranking the maintenance needs of a tree popu lation enables tree work to be assigned
priority based on observed risk. Once tree work is prioritized, it can be accomplished systematically
to eliminate the greatest risk and liability first (Stamen 2011).
The inventoried population in Salem Willows Park has a total of 36 recommended removals, 19
Discretionary prunes, 203 Prunes, and 12 Trains. Figure 6 expresses the risk values associated with
each maintenance need.
Figure 6. Maintenance needs by risk rating.
0%10%20%30%40%50%60%70%80%
Mature
Maturing
Established
Young
Good to Excellent
Fair
Poor to Dead
Removal Discretionary Prune Train
Low 17 19 179 12
Moderate 17 -21 0
High 2 -3 0
-
20
40
60
80
100
120
140
160
180
200
Number of Trees
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Section 2: i-Tree Streets Benefits
The i-Tree Streets application was used to assess the trees inventoried —this management and
analysis tool uses tree inventory data to quantify the dollar value of annual environmental and
aesthetic benefits provided by trees, including energy conservation, air quality improvement,
carbon dioxide (CO2) reduction, stormwater control, and increases in property value. It estimates
the costs and benefits of a street tree population and creates annual benefit reports that demonstrate
the value street trees have and give to a community.
The inventoried urban forest of Salem Willows Park has a recorded benefit savings of $23,225
annually from energy savings, stormwater reduction, increased property values, and overall air
quality improvements. Figure 7 and Table 1 provide a breakdown of the annual benefits provided
to Salem Willows Park.
Figure 7. Annual $ i-Tree benefits.
Table 1. Annual Benefits Provided by Inventoried Tree Population
Benefits Total ($) Percent of Total
Aesthetics/Other 10,312 44.4%
Air Quality 302 1.3%
Carbon Dioxide 24 0.1%
Energy 11,499 49.5%
Stormwater 1,088 4.7%
Park Trees Total 23,225 100.0%
11,499
24 302
1,088
10,312
Energy
Carbon Dioxide
Air Quality
Stormwater
Aesthetics/Other
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Trees and Energy Use
The contribution of the public trees towards conserving energy is reflected in their ability to shade
structures and surfaces, reduce electricity use for air conditioning in the summer, and divert wind
in the winter to reduce natural gas use. Based on the inventoried trees, the annual electric and
natural gas savings are equivalent to 17,786.99 kWh of electricity and 6,396.80 therms of natural
gas. When converted into monetary values using default economic data, this accounts for a savings
of $11,498.66 in energy consumption each year. Large leafy canopies provide greater reductions
in energy use by providing shade and natural wind barriers. In contrast, smaller trees tend to have
smaller reductions in energy usage.
Table 2. Annual Energy Savings Provided by Inventoried Tree Population
Total Electricity (kWh) Total Natural
Gas (Therms) Total ($) Avg.
$/Tree
17,786.99 6,396.80 11,498.66 42.59
Air Quality Improvements
The inventoried tree population annually removes 42.63 pounds of air pollutants (including ozone,
nitrogen dioxide, sulfur dioxide, and particulate matter) through deposition. The specified tree
population provides a total annual benefit of $302.48 in overall air quality improvements. Each
tree provides an average of $1.12 in annual benefits.
Table 3. Annual Air Quality Benefits Provided by Inventoried Tree Population
Net Total
Avoided/Deposition (lbs) Total ($) Avg. $/tree
42.63 302.48 1.12
Carbon Dioxide Removal
Trees store some of the CO2 they absorb, thereby preventing it from reaching the upper atmosphere
where it can react with other compounds and form harmful gases like ozone, which adversely affect
air quality.
The i-Tree Street analysis found that the specified park trees i n Salem Willows have stored
4,171.22 pounds of carbon (measured in CO2 equivalents). This amount is equal to the amount of
carbon these trees have amassed during their lifetime. The inventoried trees provide an annual
carbon benefit of $23.59 with a per-tree average of $.09.
Table 4. Annual Carbon Dioxide Benefits Provided by Inventoried Tree Population
Net Total CO2
Sequestered (lbs.) Total ($) Avg. $/tree
4,171.22 23.59 0.09
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Stormwater Interception and Mitigation
Trees intercept rainfall, which reduces costs to manage stormwater runoff. Salem Willows’
inventoried trees intercept 136,058.18 gallons of rainfall annually. The estimated average savings
for Salem Willows Park in stormwater management is approximately $1,088.47 per year. On
average, each tree provides an annual stormwater benefit of $4.03.
Table 5. Annual Stormwater Benefits Provided by Inventoried Tree Population
Total Rainfall
Interception (gal.) Total ($) Avg.
$/tree
136,058.18 1,088.47 4.03
Aesthetic/Other Benefits
The total annual benefit associated with property value increases and other tangible and intangible
services trees provide is $10,312.09. Each tree provided an average benefit of $38.19.
Table 6. Annual Aesthetic and Other Tangible Benefits
Provided by Inventoried Tree Population
Conclusion and Recommendations
Managing trees in urban areas is often complicated. Dealing with the recommendations of experts,
the needs of residents, the pressures of local economics and politics, the concerns for public safety
and liability issues, the physical aspects of trees, the forces of nature and severe weather events,
and the expectation for all of these issues to be resolved simultaneously is a considerable challenge.
The managers of Salem Willows Park must carefully consider each specific issue and balance these
pressures with a knowledgeable understanding of trees and their needs. By completing a tree
inventory, the city has shown interest in not only preserving the urban forest, but maintaining it for
future generations. Maintaining an established planting program, routine pruning operation, and
public outreach program will aid in maintaining the many benefits the urban forest provides to the
community.
Currently, the urban forest of Salem Willows Park is in Fair condition and provides an annual
$23,225 in benefits. With the continued dedication to the urban forest resource, the park managers
can increase diversity, condition, and the annual benefits these trees provide .
Total ($) Avg. $/tree
10,312.09 38.19
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Glossary
Aesthetic/Other Report: The i-Tree Streets Aesthetic/Other Report presents the tangible and
intangible benefits of trees reflected in increases in property values in dollars ($).
Air Quality Report: The i-Tree Streets Air Quality Report quantifies the air pollutants (ozo ne
[O3], nitrogen dioxide [NO2], sulfur dioxide [SO2], coarse particulate matter less than 10
micrometers in diameter [PM10]) deposited on tree surfaces, and reduced emissions from power
plants (NO2, PM10, Volatile Oxygen Compounds [VOCs], SO2) due to reduced electricity use
measured in pounds (lbs.). Also reported are the potential negative effects of trees on air quality
due to Biogenic Volatile Organic Compounds (BVOC) emissions.
arboriculture: The art, science, technology, and business of commercial, public, and utility tree
care.
canopy: Branches and foliage that make up a tree’s crown.
Carbon Dioxide Report: The i-Tree Streets Carbon Dioxide Report presents annual reductions in
atmospheric CO2 due to sequestration by trees and reduced emissions from p ower plants due to
reduced energy use in pounds. The model accounts for CO2+ released as trees die and decompose
and CO2 released during the care and maintenance of trees.
community forest: see urban forest.
condition (data field): The general condition of each tree rated during the inventory according to
the following categories adapted from the International Society of Arboriculture’s rating system:
Good (80%), Fair (60%), Poor, (40%), Dead (0%).
diameter at breast height (DBH): See tree size.
diameter: See tree size.
Discretionary (primary maintenance need): pruning for aesthetic appearance and/or tree health
Energy Report: The i-Tree Streets Energy Report presents the contribution of the urban forest
toward conserving energy in terms of reduced natural gas use in winter measured in therms [th]
and reduced electricity use for air conditioning in summer measured in megawatt -hours (MWh).
failure: In terms of tree management, failure is the breakage of stem or branches, or loss of
mechanical support of the tree’s root system.
genus: A taxonomic category ranking below a family and above a species and generally consisting
of a group of species exhibiting similar characteristics. In taxonomic nomenclature, the genus name
is used, either alone or followed by a Latin adjective or epithet, to form the name of a species.
geographic information system (GIS): A technology that is used to view and analyze data from
a geographic perspective. The technology is a piece of an organization’s overall information system
framework. GIS links location to information (such as people to addresses, buildings to parcels, or
streets within a network) and layers that information to give you a better understanding of how it
all interrelates.
global positioning system (GPS): GPS is a system of earth-orbiting satellites that make it possible
for people with ground receivers to pinpoint their geographic location.
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High Risk tree: Tree that cannot be cost-effectively or practically treated. Most High Risk trees
have multiple or significant defects affecting less th an 40% of the trunk, crown, or critical root
zone. Defective trees and/or tree parts are most likely between 4–20 inches in diameter and can be
found in areas of frequent occupation, such as a main thoroughfare, congested streets, and/or near
schools.
inventory: See tree inventory.
i-Tree Streets: i-Tree Streets is a street tree management and analysis tool that uses tree inventory
data to quantify the dollar value of annual environmental and aesthetic benefits: energy
conservation, air quality improvement, CO2 reduction, stormwater control, and property value
increase.
i-Tree Tools: State-of-the-art, peer-reviewed software suite from the USDA Forest Service that
provides urban forestry analysis and benefits assessment tools. The i-Tree Tools help communities
of all sizes to strengthen their urban forest management and advocacy efforts by quantifying the
structure of community trees and the environmental services that trees provide.
Low Risk tree: Tree with minor visible structural defects or wounds in areas with moderate to low
public access.
mapping coordinate (data field): Helps to locate a tree; X and Y coordinates were generated for
each tree using GPS.
Moderate Risk tree: Tree with defects that may be cost-effectively or practically treated. Most of
the trees in this category exhibit several moderate defects affecting more than 40% of a tree’s trunk,
crown, or critical root zone.
monoculture: A population dominated by one single species or very few species.
Net Annual Benefits: Specific data field for i-Tree Streets. Benefits and costs are calculated
according to category and summed. Net benefits are calculated as benefits minus costs.
Nitrogen Dioxide (NO2): Nitrogen dioxide is a compound typically created during the combustion
processes and is a major contributor to smog formation and acid deposition.
Ozone (O3): A strong-smelling, pale blue, reactive toxic chemical gas with molecules of three
oxygen atoms. It is a product of the photochemical process involving the Sun’s energy. Ozone
exists in the upper layer of the atmosphere as well as at the Earth’s surface. Ozone at the Earth’s
surface can cause numerous adverse human health effects. It is a major component of smog.
Particulate Matter (PM10): A major class of air pollutants consisting of tiny solid or liquid
particles of soot, dust, smoke, fumes, and mists.
primary maintenance need (data field): The type of tree work needed to reduce immediate risk.
Prune (primary maintenance need): Based on ANSI A300 (Part 1) standards, selective removal
of dead, dying, broken, and/or diseased wood to minimize potential risk.
pruning: The selective removal of plant parts to meet specific goals and objectives.
removal (primary maintenance need): Data field collected during the inventory identifying the
need to remove a tree. Trees designated for removal have defects that cannot be cost -effectively or
practically treated. Most of the trees in this category have a large percentage of dead crown.
right-of-way (ROW): See street right-of-way.
risk: Combination of the probability of an event occurring and its consequence.
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risk assessment (data fields): See Appendix B
risk rating: See Appendix B
Severe Risk tree: Tree rated to be Severe Risk cannot be cost-effectively or practically treated.
Most Severe Risk trees have multiple and significant defects present in the trunk, crown, or critical
root zone. Defective trees and/or tree parts are most likely larger than 20 inches in diameter and
can be found in areas of frequent occupation, such as a main thoroughfare, congested streets, and/or
near schools.
species: Fundamental category of taxonomic classification, ranking below a genus or subgenus,
and consisting of related organisms capable of interbreeding.
stem: A woody structure bearing buds and foliage, and giving rise to other stems.
stems (data field): Identifies the number of stems or trunks splitting less than one foot above
ground level.
Stored Carbon Report: Whereas, the i-Tree Streets Carbon Dioxide Report quantifies annual CO2
reductions, and the i-Tree Streets Stored Carbon Report tallies all of the Carbon (C) stored in the
urban forest over the life of the trees as a result of sequestration measured in pounds as the CO 2
equivalent.
Stormwater Report: A report generated by i-Tree Streets that presents the reductions in annual
stormwater runoff due to rainfall interception by trees measured in gallons (gals.).
street name (data field): The name of a street right-of-way or road identified using posted signage
or parcel information.
street right-of-way (ROW): A strip of land generally owned by a publ ic entity over which
facilities, such as highways, railroads, or power lines, are built.
street tree: A street tree is defined as a tree within the right-of-way.
structural defect: A feature, condition, or deformity of a tree or tree part that indicates weak
structure and contributes to the likelihood of failure.
stump removal (primary maintenance need): Indicates a stump that should be removed.
Sulfur Dioxide (SO2): A strong-smelling, colorless gas that is formed by the combustion of fossil
fuels. Sulfur oxides contribute to the problem of acid rain.
Summary Report: The i-Tree Streets Summary report presents the annual total of energy,
stormwater, air quality, carbon dioxide, and aesthetic/other benefits. Values are dollars per tree or
total dollars.
Train (primary maintenance need): Data field based on ANSI A300 (Part 1) standards, pruning
of young trees to correct or eliminate weak, interfering, or objectionable branches to improve
structure. These trees, up to 20 feet in height, can be worked with a pole pruner by a person standing
on the ground.
tree benefit: An economic, environmental, or social improvement that benefits the community and
results mainly from the presence of a tree. The benefit received has real or intrinsic value associated
with it.
tree inventory: Comprehensive database containing information or records about individual trees
typically collected by an arborist.
Davey Resource Group 11 November 2018
tree size (data field): A tree’s diameter measured to the nearest inch in 1 -inch size classes at
4.5 feet above ground, also known as diameter at breast height (DBH) or diameter.
tree: A tree is defined as a perennial woody plant that may grow more than 20 feet tall.
Characteristically, it has one main stem, although many species may grow as multi -stemmed forms.
urban forest: All of the trees within a municipality or a community. This c an include the trees
along streets or rights-of-way, in parks and greenspaces, in forests, and on private property.
Volatile Organic Compounds (VOCs): Hydrocarbon compounds that exist in the ambient air and
are by-products of energy used to heat and cool buildings. Volatile organic compounds contribute
to the formation of smog and/or are toxic. Examples of VOCs are gasoline, alcohol, and solvents
used in paints.
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References
Richards, N.A. 1983. “Diversity and Stability in a Street Tree Population.” Urban Ecology
7(2):159–171.
Stamen, R.S. “Understanding and Preventing Arboriculture Lawsuits.” Presented at the Georgia
Urban Forest Council Annual Meeting, Madison, Georgia, November 2–3, 2011.
Davey Resource Group November 2018
Appendix A
Data Collection and Site Location Methods
Data Collection Methods
DRG collected tree inventory data using a system that utilizes a customized Rover program loaded
onto pen-based field computers equipped with geographic information system (GIS) and global
positioning system (GPS) receivers. The knowledge and professional judgment of DRG’s arborists
ensure the high quality of inventory data.
Data fields are defined in the glossary of the management plan. At each site, the following data
fields were collected:
• address/location
• species
• tree size
• multi-stem tree
• condition
• primary maintenance
• defects
• risk rating
• risk assessment complete
• residual risk
• further inspection
• overhead utilities
• date of inventory
Maintenance needs are based on Best Management Practices: Tree Risk Assessment (International
Society of Arboriculture [ISA] 2011).
Tree inventory data will be delivered in DRG’s TreeKeeper® software and as ESRI® shapefiles
and/or geodatabase, an Access™ database, and an Excel™ spreadsheet.
Site Location Methods
Equipment and Base Maps
Inventory arborists use FZ-G1 Panasonic Toughpad® unit(s)
Base map layers were loaded onto these unit(s) to help locate sites during the inventory. The table
below lists the base map layers, utilized along with source and format information for each layer.
Base Map Layers Utilized for Inventory
Imagery/Data Source Date Projection
Massachusetts GIS Clearing
House
https://www/mass/gov/service-
details/massgis-data-layers
2017-2018
NAD 1983
StatePlane
Massachusetts
Mainland,
Meters
* tree size is measured in inches in diameter at 4.5 feet above ground (or diameter at breast
height [DBH])
Davey Resource Group November 2018
Appendix B
Risk Assessment/Priority and Proactive Maintenance
Risk Assessment
Every tree has an inherent risk of tree failure or defective
tree part failure. During the inventory, DRG performed a
Level 2 qualitative risk assessment for each tree and
assigned a risk rating based on the ANSI A300 (Part 9),
and the companion publication Best Management
Practices: Tree Risk Assessment (ISA 2011). Trees can
have multiple failure modes with various risk ratings. One
risk rating per tree will be assigned during the inventory.
The failure mode having the greatest risk will serve as the
overall tree risk rating. The specified time period for the
risk assessment is one year.
• Likelihood of Failure—Identifies the most
likely failure and rates the likelihood that the
structural defect(s) will result in failure based on observed, current conditions.
o Improbable—The tree or branch is not likely to fail during normal weather
conditions and may not fail in many severe weather conditions within the specified
time period.
o Possible—Failure could occur but is unlikely during normal weather conditions
within the specified time period.
o Probable—Failure may be expected under normal weather conditions within the
specified time period.
• Likelihood of Impacting a Target—The rate of occupancy of targets within the target
zone and any factors that could affect the failed tree as it falls towards the target.
o Very low—The chance of the failed tree or branch impacting the target is remote.
− Rarely used sites
− Examples include rarely used trails or trailheads
− Instances where target areas provide protection
o Low—It is not likely that the failed tree or branch will impact the target.
− Occasional use area fully exposed to tree
− Frequently used area partially exposed to tree
− Constant use area that is well protected
o Medium—The failed tree or branch may or may not impact the target.
− Frequently used areas that are partially exposed to the tree on one side
− Constantly occupied area partially protected from the tree
o High—The failed tree or branch will most likely impact the target.
− Fixed target is fully exposed to the tree or tree part
• Categorizing Likelihood of Tree Failure Impacting a Target—The likelihood for
failure and the likelihood of impacting a target are combined in the matrix below to
determine the likelihood of tree failure impacting a target.
Davey Resource Group November 2018
Likelihood of Failure
Likelihood of Impacting Target
Very Low Low Medium High
Imminent Unlikely Somewhat
likely Likely Very Likely
Probable Unlikely Unlikely Somewhat
likely Likely
Possible Unlikely Unlikely Unlikely Somewhat
likely
Improbable Unlikely Unlikely Unlikely Unlikely
• Consequence of Failure—The consequences of tree failure are based on the
categorization of target and potential harm that may occur. Consequences can vary
depending upon size of defect, distance of fall for tree or limb, and any other factors
that may protect a target from harm. Target values are subjective and should be
assessed from the client’s perspective.
o Negligible—Consequences involve low value damage and do not involve personal
injury.
− Small branch striking a fence
− Medium-sized branch striking a shrub bed
− Large tree part striking structure and causing monetary damage
− Disruption of power to landscape lights
o Minor—Consequences involve low to moderate property damage, small
disruptions to traffic or communication utility, or very minor injury.
− Small branch striking a house roof from a high height
− Medium-sized branch striking a deck from a moderate height
− Large tree part striking a structure, causing moderate monetary damage
− Short-term disruption of power at service drop to house
− Temporary disruption of traffic on neighborhood street
o Significant—Consequences involve property damage of moderate to high value,
considerable disruption, or personal injury.
− Medium-sized part striking a vehicle from a moderate or high height
− Large tree part striking a structure resulting in high monetary damage
− Disruption of distribution of primary or secondary voltage power lines,
including individual services and street-lighting circuits
− Disruption of traffic on a secondary street
o Severe—Consequences involve serious potential injury or death, damage to high -
value property, or disruption of important activities.
− Injury to a person that may result in hospitalization
− Medium-sized part striking an occupied vehicle
− Large tree part striking an occupied house
− Serious disruption of high-voltage distribution and transmission power line
disruption of arterial traffic or motorways
Davey Resource Group November 2018
• Risk Rating—The overall risk rating of the tree will be determined based on
combining the likelihood of tree failure impacting a target and the consequence of
failure in the matrix below.
Likelihood of Failure
Consequences
Negligible Minor Significant Severe
Very likely Low Moderate High Extreme
Likely Low Moderate High High
Somewhat likely Low Low Moderate Moderate
Unlikely Low Low Low Low
Trees have the potential to fail in more than one way and can affect multiple targets.
Tree risk assessors will identify the tree failure mode having the greatest risk, and
report that as the tree risk rating. Generally, trees with the highest qualitative risk
ratings should receive corrective treatment first. The following risk ratings will be
assigned:
o None—Used for planting and stump sites only.
o Low—The Low Risk category applies when consequences are “negligible” and
likelihood is “unlikely”; or consequences are “minor” and likelihood is “somewhat
likely.” Some trees with this level of risk may benefit from mitigation or
maintenance measures, but immediate action is not usually required.
o Moderate—The Moderate Risk category applies when consequences are “minor”
and likelihood is “very likely” or “likely”; or likelihood is “somewhat likely” and
consequences are “significant” or “severe.” In populations of trees, Mode rate Risk
trees represent a lower priority than High or Extreme Risk trees.
o High—The High Risk category applies when consequences are “significant” and
likelihood is “very likely” or “likely,” or consequences are “severe” and likelihood
is “likely.” In a population of trees, the priority of High Risk trees is second only
to Extreme Risk trees.
o Extreme—The Extreme Risk category applies in situations where tree failure is
imminent and there is a high likelihood of impacting the target, and the
consequences of the failure are “severe.” In some cases, this may mean immediate
restriction of access to the target zone area to avoid injury to people.
Trees with elevated (Extreme or High) risk levels are usually recommended for removal or pruning
to eliminate the defects that warranted their risk rating. However, in some situations, risk may be
reduced by adding support (cabling or bracing) or by moving the target away from the tree. DRG
recommends only removal or pruning to alleviate risk. But in special situations, such as a memorial
tree or a tree in a historic area, Salem Willows may decide that cabling, bracing, or moving the
target may be the best option for reducing risk.
Davey Resource Group November 2018
Priority Maintenance
Identifying and ranking the maintenance needs of a tree population enables tree work to be assigned
priority based on observed risk. Once prioritized, tree work can be systematically addressed to
eliminate the greatest risk and liability first (Stamen 2011).
Risk is a graduated scale that measures potential tree-related hazardous conditions. A tree is
considered hazardous when its potential risks exceed an acceptable level. Managing trees for risk
reduction provides many benefits, including:
● Lower frequency and severity of accidents, damage, and injury
● Less expenditure for claims and legal expenses
● Healthier, long-lived trees
● Fewer tree removals over time
● Lower tree maintenance costs over time
Regularly inspecting trees and establishing tree maintenance cycles generally reduce the risk of
failure, as problems can be found and addressed before they escalate.
Proactive Maintenance
Proactive tree maintenance requires that trees are managed and maintained under the responsibility
of an individual, department, or agency. Tree work is typically performed during a cycle. Individual
tree health and form are routinely addressed during the cycle. When trees are planted, they are
planted selectively and with purpose. Ultimately, proactive tree maintenance should reduce crisis
situations in the urban forest, as every tree in the inventoried population is regularly visited,
assessed, and maintained. DRG recommends proactive tree maintenance that includes pruning
cycles, inspections, and planned tree planting.
Determination of acceptable risk ultimately lies with park
managers. Since there are inherent risks associated with
trees, the location of a tree is an important factor in the
determination and acceptability of risk for any given tree.
The level of risk associated with a tree increases as the
frequency of human occupation increases in the vicinity of
the tree. For example, a tree located next to a heavily
traveled street will have a higher level of risk than a similar
tree in an open field.
Davey Resource Group November 2018
Appendix C
Suggested Tree Species
Proper landscaping and tree planting are critical components of the atmosphere, livability, and
ecological quality of a community’s urban forest. The tree species listed below have been evaluated
for factors such as size, disease and pest resistance, seed or fruit set, and availability. The following
list is offered to assist all relevant community personnel in selecting appropriate tree species. These
trees have been selected because of their aesthetic and functional characteristics and their ability
to thrive in the soil and climate conditions throughout Zones 5 and 6 on the USDA Plant Hardiness
Zone Map.
Deciduous Trees
Large Trees: Greater than 45 Feet in Height at Maturity
Scientific Name Common Name Cultivar
Acer rubrum red maple Red Sunset®
Acer saccharum sugar maple ‘Legacy’
Acer nigrum black maple
Betula alleghaniensis* yellow birch
Betula lenta* sweet birch
Betula nigra river birch Heritage®
Carpinus betulus European hornbeam ‘Franz Fontaine’
Carya illinoensis* pecan
Carya lacinata* shellbark hickory
Carya ovata* shagbark hickory
Castanea mollissima* Chinese chestnut
Celtis laevigata sugarberry
Celtis occidentalis common hackberry ‘Prairie Pride’
Cercidiphyllum japonicum katsuratree ‘Aureum’
Diospyros virginiana* common persimmon
Fagus grandifolia* American beech
Fagus sylvatica* European beech (Numerous exist)
Ginkgo biloba ginkgo (Choose male trees only)
Gleditsia triacanthos inermis thornless honeylocust ‘Shademaster’
Gymnocladus dioica Kentucky coffeetree Prairie Titan®
Juglans nigra* black walnut
Larix decidua* European larch
Liquidambar styraciflua American sweetgum ‘Rotundiloba’
Liriodendron tulipifera* tuliptree ‘Fastigiatum’
Magnolia acuminata* cucumbertree magnolia (Numerous exist)
Magnolia macrophylla* bigleaf magnolia
Metasequoia glyptostroboides dawn redwood ‘Emerald Feathers’
Nyssa sylvatica blackgum
Platanus occidentalis* American sycamore
Platanus × acerifolia London planetree ‘Yarwood’
Quercus alba white oak
Davey Resource Group November 2018
Large Trees: Greater than 45 Feet in Height at Maturity (Continued)
Scientific Name Common Name Cultivar
Quercus bicolor swamp white oak
Quercus coccinea scarlet oak
Quercus lyrata overcup oak
Quercus macrocarpa bur oak
Quercus montana chestnut oak
Quercus muehlenbergii chinkapin oak
Quercus palustris pin oak
Quercus imbricaria shingle oak
Quercus phellos willow oak
Quercus robur English oak Heritage®
Quercus rubra northern red oak ‘Splendens’
Quercus shumardii Shumard oak
Styphnolobium japonicum Japanese pagodatree ‘Regent’
Taxodium distichum common baldcypress ‘Shawnee Brave’
Tilia americana American linden ‘Redmond’
Tilia cordata littleleaf linden ‘Greenspire’
Tilia × euchlora Crimean linden
Tilia tomentosa silver linden ‘Sterling’
Ulmus parvifolia Chinese elm Allée®
Zelkova serrata Japanese zelkova ‘Green Vase’
Medium Trees: 31 to 45 Feet in Height at Maturity
Scientific Name Common Name Cultivar
Aesculus × carnea red horsechestnut
Alnus cordata Italian alder
Asimina triloba* pawpaw
Cladrastis kentukea American yellowwood ‘Rosea’
Corylus colurna Turkish filbert
Eucommia ulmoides hardy rubber tree
Koelreuteria paniculata goldenraintree
Ostrya virginiana American hophornbeam
Parrotia persica Persian parrotia ‘Vanessa’
Phellodendron amurense Amur corktree ‘Macho’
Pistacia chinensis Chinese pistache
Prunus maackii Amur chokecherry ‘Amber Beauty’
Prunus sargentii Sargent cherry
Pterocarya fraxinifolia* Caucasian wingnut
Quercus acutissima sawtooth oak
Quercus cerris European turkey oak
Sassafras albidum* sassafras
Davey Resource Group November 2018
Small Trees: 15 to 30 Feet in Height at Maturity
Scientific Name Common Name Cultivar
Acer buergerianum trident maple Streetwise®
Acer campestre hedge maple Queen Elizabeth™
Acer cappadocicum coliseum maple ‘Aureum’
Acer ginnala Amur maple Red Rhapsody™
Acer griseum paperbark maple
Acer oliverianum Chinese maple
Acer pensylvanicum* striped maple
Acer triflorum three-flower maple
Aesculus pavia* red buckeye
Amelanchier arborea downy serviceberry (Numerous exist)
Amelanchier laevis Allegheny serviceberry
Carpinus caroliniana* American hornbeam
Cercis canadensis eastern redbud ‘Forest Pansy’
Chionanthus virginicus white fringetree
Cornus alternifolia pagoda dogwood
Cornus kousa kousa dogwood (Numerous exist)
Cornus mas corneliancherry dogwood ‘Spring Sun’
Corylus avellana European filbert ‘Contorta’
Cotinus coggygria* common smoketree ‘Flame’
Cotinus obovata* American smoketree
Crataegus phaenopyrum* Washington hawthorn Princeton Sentry™
Crataegus viridis green hawthorn ‘Winter King’
Franklinia alatamaha* Franklinia
Halesia tetraptera* Carolina silverbell ‘Arnold Pink’
Laburnum × watereri goldenchain tree
Maackia amurensis Amur maackia
Magnolia × soulangiana* saucer magnolia ‘Alexandrina’
Magnolia stellata* star magnolia ‘Centennial’
Magnolia tripetala* umbrella magnolia
Magnolia virginiana* sweetbay magnolia Moonglow®
Malus spp. flowering crabapple (Disease resistant only)
Oxydendrum arboreum sourwood ‘Mt. Charm’
Prunus subhirtella Higan cherry ‘Pendula’
Prunus virginiana common chokecherry ‘Schubert’
Staphylea trifolia* American bladdernut
Stewartia ovata mountain stewartia
Styrax japonicus* Japanese snowbell ‘Emerald Pagoda’
Syringa reticulata Japanese tree lilac ‘Ivory Silk’
Note: * denotes species that are not recommended for use as street trees.
Davey Resource Group November 2018
Coniferous and Evergreen Trees
Large Trees: Greater than 45 Feet in Height at Maturity
Scientific Name Common Name Cultivar
Abies balsamea balsam fir
Abies concolor white fir ‘Violacea’
Cedrus libani cedar-of-Lebanon
Chamaecyparis nootkatensis Nootka falsecypress ‘Pendula’
Cryptomeria japonica Japanese cryptomeria ‘Sekkan-sugi’
× Cupressocyparis leylandii Leyland cypress
Ilex opaca American holly
Picea omorika Serbian spruce
Picea orientalis Oriental spruce
Pinus densiflora Japanese red pine
Pinus strobus eastern white pine
Pinus sylvestris Scotch pine
Pinus taeda loblolly pine
Pinus virginiana Virginia pine
Psedotsuga menziesii Douglas-fir
Thuja plicata western arborvitae (Numerous exist)
Tsuga canadensis eastern hemlock
Medium Trees: 31 to 45 Feet in Height at Maturity
Scientific Name Common Name Cultivar
Chamaecyparis thyoides Atlantic whitecedar (Numerous exist)
Juniperus virginiana eastern redcedar
Pinus bungeana lacebark pine
Pinus flexilis limber pine
Pinus parviflora Japanese white pine
Thuja occidentalis eastern arborvitae (Numerous exist)
Small Trees: 15 to 30 Feet in Height at Maturity
Scientific Name Common Name Cultivar
Ilex × attenuata Foster's holly
Pinus aristata bristlecone pine
Pinus mugo mugo mugo pine
Dirr’s Hardy Trees and Shrubs (Dirr 2013) and Manual of Woody Landscape Plants (5th
Edition) (Dirr 1988) were consulted to compile this suggested species list. Cultivar selections
are recommendations only and are based on DRG’s experience. Tree availability will vary
based on availability in the nursery trade.