The City of Bentonville’s urban forest is comprised of trees, forests, gardens, green spaces and other natural areas. Urban forest benefits reduce costs associated with managing storm water runoff, water quality, energy used for heating and cooling, and expenditures on physical and mental health. Evidence also suggests that healthy urban forests are associated with higher property values, increased consumer spending and reduced crime. This study was initiated to assess the City of Bentonville’s existing Urban Tree Canopy (UTC), or urban forests viewed from above. This report presents maps, tables, and graphs that will allow city officials and city staff to monitor and enhance the quality and stream of benefits received from its urban forests by continuing with existing forestry programs (like citywide tree planting events) and providing a benchmark for developing and implementation of an urban forest management plan. The study also produced numerous tools and data described in the Appendix.
Natural resource managers use top-down (aerial imagery) and bottom-up (field based) approaches to map and measure land cover, trees, other types of green infrastructure, and associated ecosystem services. Terms and methods for this UTC assessment are presented within Project Fundamentals on page 6.
Tree Canopy
This study encompasses 31.5 square miles defined by the city limits of Bentonville, Arkansas. Based on a land area (after excluding water) of 20,044 acres, the City has 4,982 acres (25%) of existing tree canopy. There are 9,621 acres (48%) of Possible Planting Areas (PPA), defined as vegetated areas where tree planting is feasible. Additionally, 1,768 acres (9%) of Bentonville is unsuitable for tree planting. The land cover data was used to assess similar metrics for the City’s zoning. See Results section on pages 13-22.
Figure 1: Tree Canopy in the City of Bentonville, Arkansas
EXECUTIVE SUMMARY
1
Urban trees and forests in communities provide many “ecosystem services”, or direct and indirect economic and environmental benefits. This assessment used computer software to estimate the values of some ecosystem services including the removal of air pollutants, storage and sequestering carbon, and mitigation of polluted stormwater runoff. Bentonville’s existing tree canopy provides an estimated $1.7M in air quality services each year, $500,000 in annual carbon sequestration benefit, and $32.6 million in total stormwater management savings. See Ecosystem Services section on page 19.
To support the City’s “Annual Tree Planting Blitz”, potential planting locations were generated from the land cover data using a GIS model, resulting in 176,258 available sites citywide. To help the City prioritize these locations for plantings that achieve specific benefits, numerous attributes were joined to the GIS points based on data overlays. Examples include proximity to wildlife linkages, urban heat islands, and transportation corridors where new plantings will impact social wellbeing, environmental health, and economic vitality. See pages 21-22.
Trees are widely beneficial and forest managers in Bentonville aim to increase UTC across the City. To support and facilitate future tree planting and canopy preservation policies, tree canopy goals were analyzed based on existing planting areas and tree canopy. A suggested city-wide canopy cover goal of 35% (an additional 2,060 acres of tree canopy) is presented along with goals for individual zoning types. These should be used as a starting point for an interdisciplinary goal setting process. See Suggested Canopy Cover Goals on page 24.
To balance future development and growth with effective urban forestry planning, city officials must value Bentonville’s canopy cover in environmental, social, and economic terms during planning processes. Maintaining and enhancing this green infrastructure will involve ongoing care and protection, strategic canopy increases, and education and awareness of UTC benefits. This report provides the benchmark data required for natural resources planning, monitoring, and policy-making.
EXECUTIVE SUMMARY
2
The City of Bentonville, Arkansas covers approximately 32 square miles and has a population of roughly 38,000 (2012 Census). With funding from Bentonville’s Tree & Landscape Committee and the US Forest Service, the City and Arkansas Forestry Commission contracted with Plan-It Geo LLC to conduct Bentonville’s first Urban Tree Canopy (UTC) Assessment. This project is part of the Canopy in the Mid-South grant, comprised of 16 cities throughout Mississippi, Arkansas, and Tennessee. Using Geographic Information Systems (GIS), the process measures existing canopy cover, provides a baseline of information for increasing environmental services from the urban forest, strengthens local/state partnerships, builds awareness of urban forest benefits, and enables Bentonville and the State of Arkansas to establish canopy cover goals, plans, and strategies.
Bentonville City Limits
Figure 2: City boundary of Bentonville, Arkansas and the study area for the UTC Assessment.
PROJECT BACKGROUND
3
Data from this assessment will be used to: Prioritize tree planting sites and strategies Monitor and assess the effectiveness of land use planning and natural resource protection policies Determine tree and forest canopy change over time Inform sustainability and urban forest management plans
Bentonville’s UTC Assessment is a top-down approach which provides data and tools to enhance planning, management, and value received from the City’s urban forest. Additional bottom-up inventory and assessment can ground-truth viable planting areas as well as provide data on species composition, condition/quality, safety, and forest structure to augment this assessment. The products and outcomes will support developing and monitoring of canopy objectives, data-driven resource management plans, refinement of policies and ordinances, and help to foster a greater understanding of benefits we all derive from the urban forest.
PROJECT BACKGROUND
4
Bentonville has 25% (4,982 acres) Urban Tree Canopy and 48% (9,621 acres) of additional area available for planting (vegetated areas where tree planting is feasible). Residential zoning accounts for 52% (2,578 acres) of the City’s UTC.
Residential zoning comprises 40% (8,025 acres) of the total land area, and with 2,578 acres of UTC, makes up 52% of all tree canopy. Planned Development zones in Bentonville have 29% (353 acres) of tree canopy and 58% (695 acres) of additional area for planting. Tree protection and preservation should be a focus in these areas. Street ROW covers nearly 10% of Bentonville, yet only comprises 4% (199 acres) of Bentonville’s tree canopy.
The current urban forest removes nearly 410,000 pounds of air pollutants from the air annually, valued at $1.7 million per year. Tree canopy provides a value of $2.8 million in avoided stormwater infrastructure costs annually. Bentonville’s canopy provides $500,000 in carbon storage and sequestration benefits annually.
Using the land cover data and GIS modeling, 176,000 potential tree planting sites were mapped. Further analysis shows that 37,000 of the potential planting sites will improve wildlife linkages and connectivity of the forest canopy. Additionally, shade from 8,900 specific tree planting locations mapped in this project will most effectively mitigate temperatures caused by the “urban heat island effect”.
2,060 acres of additional tree canopy would be required to reach the suggested citywide goal of 35% UTC, equal to roughly 24,000 new mature trees. The suggested canopy goal in residential zoning is 35% UTC, requiring an additional 2,809 acres of tree canopy. 5
MAJOR FINDINGS
This section describes the following fundamental components and methods of Bentonville’s UTC assessment project:
Mapping Land Cover & Urban Tree Canopy UTC Terminology Assessment Boundaries Urban Forest Ecosystem Benefits Planting Site Prioritization in Bentonville Goal Setting
The “top-down” UTC assessment conducted for Bentonville provides an accurate evaluation of canopy cover within the desired boundary. The following section describes the data and methods used for land cover classification, the terminology for defining and assessing Urban Tree Canopy (UTC) and Possible Planting Area (PPA Vegetation), and the boundaries (geographic units of scale) that were assessed.
Aerial photography from the 2013 National Agricultural Imagery Program (NAIP) with 1-meter pixel resolution was used as the basis for this UTC assessment. Object-based image classification results were combined with GIS data provided by the City to produce five (5) GIS-based land cover classes: (1) tree canopy, (2) other low-lying vegetation such as grass and open space, (3) impervious surfaces, (4) bare soil/dry vegetation, and (5) water.
Urban Tree Canopy:
Vegetation:
Impervious:
Bare Soil:
Water:
Tree cover, when viewed and mapped from above
Grass and open space vegetation
Artificial structures that are covered by impenetrable materials
Areas of dry grass/bare soil where possible planting area is omitted
Bodies of water which were removed from total land cover
Golf courses and ball fields were manually removed from existing vegetation to define possible planting areas. Once finalized, the land cover data was the input to assessing boundaries to provide UTC metrics at multiple scales.
PROJECT FUNDAMENTALS
6
Land cover classes were grouped into meaningful categories describing where UTC is present, where trees can be planted (Possible Planting Area – PPA), and where it is not feasible to plant (Unsuitable UTC). These “UTC Types” were then summarized across spatial scales selected by the City to provide input to meaningful planning and management.
Comprises forests and individual trees when viewed and mapped from above.
PROJECT FUNDAMENTALS
Area of grass and open space where tree canopy does not exist and it is biophysically possible to plant trees.
The combination of bare soil, roads, buildings, ball fields, airports, and water, where it is not feasible to plant trees.
7
The three UTC types – Existing UTC, Possible Planting Areas, and Unsuitable UTC – were then assessed and summarized for several geographic boundaries provided by the City. From the citywide scale to the parcel-level, the area and percent of each UTC type was calculated and used for map-making and tabular summaries. The UTC assessment boundaries provide a locational link to where we live, work and play. These summaries provide benchmark data for city officials and city staff at multiple spatial scales important for urban forest management.
Figure 3: Zoning attributes at the parcel level for optimal planning
Bentonville City Limits
The jurisdictional limits of Bentonville, Arkansas
PROJECT FUNDAMENTALS
Land Use by Parcel Classes: Agricultural, Commercial, Downtown, Industrial, Planned Development, Residential, Street Right-of-Way
8
Urban forests are an integral part of the character and function for many residents and policy makers in the City of Bentonville. Environmental benefits provided by trees are referred to as “ecosystem services” and describe the ways that urban forests impact our lives and the environment. The information below and the diagram on the following page list many reasons to promote, establish, manage, and maintain a robust, “working” urban forest in Bentonville.
PROJECT FUNDAMENTALS
9
Trees are often appreciated for their aesthetic appeal, but are often undervalued for their contributions to the health and vitality of resident’s daily life. While the net benefits of urban trees are usually positive, costs are also part of this study’s ecosystem service evaluation and scenario tools. CITYgreen1 and i-Tree Vue software were used to estimate the benefits of Bentonville’s existing urban tree canopy and scenarios with decreased and increased canopy cover. i-Tree Vue benefits relate to carbon storage and sequestration and air pollutant removal. CITYgreen was used to calculate stormwater mitigation benefits of urban forests. See complete details below and on page 19.
Current and future values were estimated for the following three ecosystem service types:
Air Quality: Trees naturally remove pollutants and lower air temperature
Stormwater mitigation: Trees intercept stormwater, reducing runoff and filtering out pollutants that would otherwise enter rivers and lakes
Energy Savings: lower energy demand through summer shade and winter wind block CO2 Sequestration: Through photosynthesis, trees absorb atmospheric carbon and use it for new growth (stems, branches, roots and leaves), acting as a natural carbon sink
1
Figure 4: Ecosystem Services that Trees Provide
CityGreen is a GIS-based ecosystem services analysis tool originally produced by American Forests.
PROJECT FUNDAMENTALS
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One of the objectives of Bentonville’s UTC assessment was to provide data and maps to inform strategic tree planting. This was performed using two steps, described below, to help the City prioritize planting locations and plan events. Step 1 – Locate realistic planting sites: using a GIS model and the Possible Planting Areas (PPA) from the land cover data, potential planting sites (GIS points) were auto-located where a mature tree could be established. Areas where planting is undesirable, such as golf courses, airports, or near existing tree canopy and buildings, were excluded. Step 2 – Assign attributes to potential planting sites: areas favorable for plantings were attributed based on their proximity to buildings, zoning type, parking lots, major road corridors, and more. See pages 17-18 for examples. Table 1 lists the GIS data inputs used to map potential planting sites. Figure 5 illustrates the input (PPA) at left, constraints/exclusions in the middle, and final outputs at right. Table 1: Constraints/exclusions in generating potential planting sites
GIS Layers
Rule Applied
Reason
Non-Tree Vegetation
Plant only in these areas
Where tree planting is feasible
Tree Canopy Polygons
Buffered by 4 meters
To allow room for spacing and growth of existing trees
Building Footprints
Buffered by 2 meters Remove Sites from golf courses and airports
To avoid tree and building conflicts
Unsuitable Areas
Tree planting will not occur in these areas
Figure 5 (from left to right): Input areas (PPA), exclusions, and final planting sites.
PROJECT FUNDAMENTALS
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Another objective of this assessment was to recommend challenging but attainable canopy cover goals for Bentonville. Suggested canopy goals help support communities in facilitating future tree planting and canopy preservation policies. Using the project’s GIS data and statistical methods, canopy goals were developed for the City as a whole and each zoning district (Agricultural, Commercial, Downtown, Industrial, Planned Development, Residential, and Street ROW). Plan-It Geo’s canopy goal setting process uses current canopy and potential planting areas to provide Bentonville with goals that are appropriate and attainable in the future. A detailed methodology of the canopy cover goal setting process is described here: UTC and PPA metrics are calculated for each parcel (tax assessor lot), providing area and percent statistics. Representative parcels are selected for the goal setting process by eliminating outlier parcels, or those with greater than 99% and less than 1% UTC and PPA represent areas that are extraordinary either because of extreme protections (e.g. 100% canopy) or because trees are very unlikely to survive in the location (e.g. 0% canopy). PPA Percent is summarized by zoning using the 50th Percentile Rule. The representative parcels are then sorted by their PPA percent and ranked from the lowest to highest PPA value. The percent PPA value at which half of parcels are below and half of parcels are above represents the 50th percentile. This ranking and determination is done for parcels in each zoning type separately (seven times in this case, plus citywide). An appropriate and attainable goal is selected for each zoning district. The challenge of attaining the 50th percentile goal may be appropriate for communities that are particularly active, but our purpose is to provide goals that are attainable under most circumstances across Bentonville. Growing tree canopy on parcels at the 50th percentile (either all of half the number of parcels or half the PPA on all parcels at the 50th percentile) will allow the City to meet the goal for each zoning district. Once the citywide goal was determined, Plan-It Geo’s “Canopy Calculator” spreadsheet tool (see page 27 in the Appendix) was used to determine the number of trees needed to reach specific goals. The City should use the suggested citywide and zoning districts goals as a starting point for an interdisciplinary goal setting process. The process can also be applied to other planning efforts.
Figure 6: Crosswalk of steps used for suggested goal setting
PROJECT FUNDAMENTALS
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The UTC boundaries assessed in Bentonville are governed, owned, managed, and used in different ways by diverse constituents in the community. Therefore, the UTC data and analysis results are presented for different land use classes to inform city officials, city staff, and citizens alike. The following sections present canopy cover and planting potential across several geographic assessment boundaries and describes how the City can use the data to develop management and implementation plans for enhancing Bentonville’s urban forests.
Tree Canopy in Bentonville This study encompasses 20,044 acres (after excluding water) defined by the City limits of Bentonville and provides a snapshot of land cover based on 2013 satellite imagery. Tree canopy covers 4,982 acres (25%) of Bentonville based on total land area after excluding water.
Ecosystem Services in Bentonville The analysis from the ecosystem services study revealed that Bentonville’s urban forest provides $1.7M in air quality services each year, $500,000 in annual carbon sequestration benefit, and $32.6 million in total stormwater management savings.
Possible Planting Area in Bentonville Other, non-tree vegetation (grass and open space) covers an additional 9,621 acres (48%) of Bentonville. These areas were used to prioritize planting sites throughout the city, based on various GIS criteria used. Goal Setting in Bentonville A suggested city-wide canopy cover goal of 35% (an additional 7,041 acres of tree canopy) is presented along with goals for individual zoning types. These should be used as a starting point for an interdisciplinary goal setting process.
ASSESSMENT RESULTS AND PRODUCTS
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5-class Land Cover Tree Canopy Vegetation Impervious Water Soil / Dry Vegetation
Figure 7: Map of 5-class Land Cover in Bentonville
Figure 8: Distribution of land cover in Bentonville
ASSESSMENT RESULTS AND PRODUCTS
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Project results include information about each individual parcel and summaries by zoning district. At the detailed parcel scale, UTC, PPA and prioritized planting sites can be queried and symbolized using GIS to drill down and identify specific owners by planting opportunities. Of the 19,018 parcels in Bentonville, 5,803 have UTC percent greater than 25% (the citywide average).
Percent UTC by Parcel < 10% 10 - 30% 30 - 70% > 70% City Limits
Figure 9: Percent UTC by parcel.
ASSESSMENT RESULTS AND PRODUCTS
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Many of the policies, regulations, ordinances, and actions influencing tree canopy in Bentonville are dependent on zoning districts. To provide data that advances urban forest policy and management, seven zoning categories were assessed for tree canopy and possible planting areas (Table 2). The following pages present the UTC results across Bentonvilleâ&#x20AC;&#x2122;s zoning classes, and examples of how to best utilize the data. Agricultural Commercial Downtown Industrial Planned Development Residential Street Right-of-Way
Figure 10: Distribution of total UTC by zoning class.
Figure 11: Map of zoning classes in Bentonville
Table 2: UTC metrics by zoning classes
ASSESSMENT RESULTS AND PRODUCTS
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Figure 12: Neighborhood with high planting potential, defined as less than 10% tree cover and greater than 50% plantable area, and energy conservation planting sites
th
SSW 10 St.
# *
Planting Sites for Energy Conservation UTC% < 10 & PPA > 50%
UTC% < 10 & PPA > 50% Residential Parcels
Figure 13: Residential parcels with high planting opportunities
Older neighborhoods in Bentonville exhibit the healthiest urban forest characteristics and provide the most ecosystem services to the city, while newer developments have many young trees which should one day contribute to much of Bentonvilleâ&#x20AC;&#x2122;s Urban Tree Canopy.
Newer developments will one day provide mature canopy
Figure 14: New to old neighborhood developments and the Urban Tree Canopy within each.
ASSESSMENT RESULTS AND PRODUCTS
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The City of Bentonville plants, manages, and maintains trees in the street rights-of-way (ROW). This is where the City has the most direct opportunity to increase tree cover and manage tree health. Using the UTC assessment data, the City may target strategic areas to increase tree canopy in streets and highway corridors where aesthetic, noise and air quality improvements will be most recognized. Bentonville’s total ROW represents 9% of the total land area (1,889 acres). Bentonville’s ROW is comprised of: Street Rights-of-Way offer forest managers public space for planting while providing environmental and aesthetic benefits.
Existing UTC in the ROW is 11% (199 acres) representing 4% of the City’s total UTC. PPA in the ROW is 24% (447 acres) representing 5% of the City’s total PPA.
Figure 15: Planting sites for air quality benefits in Street Rights-of-Way
ASSESSMENT RESULTS AND PRODUCTS
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Bentonville’s urban forests provide huge monetary savings to the city. Protecting and enhancing the urban forest by investing in urban forest management strategies that capitalize on understanding how trees contribute to the functioning of the city will save precious dollars by the public and private sectors.
Carbon and Air Quality Findings: Carbon storage and air quality provides both global and local benefits. Existing canopy in Bentonville stores approximately 770,000 tons of carbon, valued at $15 million, and each year sequesters approximately 25 thousand tons of carbon dioxide, valued at nearly $500,000. An increase in tree canopy to 30% UTC would result in almost 900,000 tons of carbon stored, increasing the City’s total savings to $17.3 million. Findings from the i-Tree Vue software also reveal that tree canopy in Bentonville removes over 400,000 pounds of air pollution annually, valued at $1.7 million. Increasing tree canopy to 30% would remove 473,000 pounds of air pollution annually; a value of $2 million.
Stormwater Mitigation Findings: The construction of storage basins to manage municipal runoff and filter water for consumption is expensive and can be reduced through proper urban forest management. Bentonville’s current tree canopy intercepts 10.9 million cubic ft. of stormwater runoff every year, a benefit valued at $2.8 million in annually avoided stormwater infrastructure costs, or a total stormwater value of $32.6 million. An increase of canopy from 25% to 30% overall UTC would intercept roughly 13.5 million cubic ft. of stormwater runoff annually; a total stormwater value of $41.3 million. A decrease in canopy cover to 20% would intercept nearly 6 million cubic ft. of stormwater runoff annually; a total stormwater value loss of $14 million.
Figure 16: Ecosystem benefits by canopy cover scenario
ASSESSMENT RESULTS AND PRODUCTS
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In Bentonville’s temperate climate, a tree canopy goal should be attainable physically, fiscally, and desired socially. Using methods described in Fundamentals an overall UTC goal of 35% meets these criteria. Zoning-specific UTC goals should be incorporated into the subdivision code, including Article 1300 Tree Protection and Article 1400 Landscaping, Screening and Buffering, tree preservation policies and planning documents such as the General Plan. Due to large variances in tree cover across broad zoning types, goals are suggested citywide and for each zoning type. Suggested practical and attainable UTC goals for Bentonville are presented here for the City to use as a starting point in canopy goal setting discussions and planning.
Table 3: Suggested goal setting by zoning districts in Bentonville
In order to effectively increase UTC in Bentonville, the City may consider the following suggestions: 1) Greater tree planting, 2) Preservation of existing trees, and 3) Establishing areas suitable for forest regeneration. Areas suitable for regeneration include: Areas adjacent to large patches of tree canopy and forest Areas within a riparian buffer where soils will experience conditions of flooding or high saturation Vacant or abandoned properties
ASSESSMENT RESULTS AND PRODUCTS
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Using the potential planting sites, we can count the number of mature trees that can physically grow within possible planting areas. Planting sites can be prioritized by resources managers using GIS criteria to estimate the number of available planting locations by priority type. “Other planting sites” (Table 4) do not fall into one of the assessed criteria but may still represent favorable locations to plant. Conditions at each specific planting site should be considered when selecting tree species for planting. However, general recommendations are provided below and greater diversity among species, genus, and family is always the best defense against today’s forest threats and future unknowns. Table 4: Criteria developed to prioritize Bentonville’s available planting sites.
Prioritization Type
Criterion Definition
Energy Conservation Air Quality Urban Heat Islands
Within 50 feet of buildings Within Street Rights-of-Way Within 25 feet of parking lots Within 100 feet of forest tracts (tree canopy >1 acre) Outside above criteria
Wildlife Linkages Other Planting Sites CITYWIDE TOTAL
No. of Planting Sites* 35,659 10,580 8,914 36,995 104,623 176,258
* A single planting site can be assigned to multiple criteria; therefore the total number of sites is not equal to the column sum of each prioritization type count.
# *
Urban Heat Heat Islands Urban Heat Islands (UHI) are pockets of concrete and other impervious surfaces that trap Urban Heat–Island Island Urban and reradiate heat from the sun’s energy. This phenomenon causes unnatural rises in air temperatures around these # * types. Air Quality Quality surface Trees planted near and around parking lots will reduce the negative impacts of UHI. Trees planted in and Air around parking lots must be tolerant of a variety of conditions from extreme heat and dryness to very wet conditions # * Wildlife Linkages Wildlife Linkages resulting from stormwater runoff.
# *
Energy Conservation – Trees contribute to energy conservation within buildings by blocking wind in the winter Energy Conservation Energy Conservation and summer, thereby reducing spending and energy consumption. Pine trees offer the most # * providing Urbanshade Heatin Island protection from cold winds in the winter. Columnar and fastigiated varieties of tree species are a popular choice # * Airin narrow Quality especially spaces between buildings or along property lines.
# *
Wildlife Linkages – The most common wildlife species found in urban areas enjoy “forest edge” habitat. Expanding Wildlife Linkages “interior” forest spaces will invite migratory song birds and other rare and desirable wildlife species. Fruit and nut# * Energy bearing native treeConservation species such as oaks, hickories and cherries provide ample food mass and voluminous canopy to support wildlife. Heat Island # * Urban
# *
Air Quality – Noise reduction, visual buffering, air pollution removal and stormwater infiltration are all benefits Air Quality provided by trees located along Bentonville’s street rights-of-ways. Street trees also experience the greatest amount of # * from Wildlife Linkages stress structural and chemical soil modification and mechanical damage. Hardy, time-tested street trees are the
# *
Energy Conservation
ASSESSMENT RESULTS AND PRODUCTS
21
best candidates to plant in the street ROW. Consideration should be given to fall foliage, ease of pruning, and production of fruit or other forms of mass.
# *
Urban Heat Island
# *
Air Quality
# *
Wildlife Linkages
# *
Energy Conservation
Figure 17: An example of priority planting sites in Bentonville color-coded by the type of ecosystem service or benefit they can provide.
ASSESSMENT RESULTS AND PRODUCTS
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This project mapped land cover across the City of Bentonville, assessed benefit values of current and future canopy cover, and developed data and tools allowing city staff to identify where trees can be planted for specific community and environmental outcomes. Based on the results of this work, the following broad recommendations are provided on how to best use these data, tools, and information to implement urban tree canopy objectives in Bentonville.
LAND COVER ANALYSIS Disseminate the land cover data to diverse partners for urban forestry and other applications while the data is current and most useful for decision-making and implementation planning. Re-assess canopy cover in no less than 10-year intervals, and use LiDAR data if available, aiming for 95% minimum overall accuracy.
ECOSYSTEMS SERVICES Tree Canopy Overall Bentonville’s urban forest green infrastructure is providing a great deal of benefits to the City. Use these results to encourage investment in the forest monitoring, maintenance, and management. Use the results to develop targeted presentations for city leaders, planners, engineers, resource managers, and the public on the functional benefits of trees in addressing environmental issues. Tree Canopy for Air Quality This study identified 447 acres of possible planting area (PPA Vegetation) within street rights-of-way. Officials can partner to target canopy increases in the corridors as an air pollution mitigation strategy while increasing walkability and biking. Prioritize tree planting in areas with high impervious surface cover to mitigate the urban heat island effect and improve air quality. Tree Canopy for Energy Conservation Develop strategies to plant trees for energy efficiency. Consider the Arbor Day Foundation’s “Energy Saving Trees” program. Many new residential developments throughout Bentonville have newly planted trees. If properly cared for, these trees will contribute greatly to the urban forest.
PLANNING Use the information presented here to work with city staff to establish canopy cover goals for the short and long term. “No net loss” may be a reasonable near-term canopy goal. Continue “Tree Planting Blitz” tree plantings & provide volunteers with information about this assessment. Develop an urban forest management plan focusing on implementation strategies to reach established goals. Host the UTC data and potential planting sites in a web-mapping application focused on urban forest planning. Use the tool to engage public/private partners to advance urban forest benefits. Capitalize on the Plan-It Geo’s Canopy Viewer tool (see Appendix).
RECOMMENDATIONS
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Bentonville’s urban forest is a living resource that changes through natural and human processes. Managing UTC requires an understanding of where trees are, where they can be planted equitably to maximize benefits, and how to work within regulatory and physical restrictions to achieve goals. Bentonville’s UTC Assessment provides an accurate benchmark and tools to assist in the City’s near and long-term vision of urban natural resource management. Some highlights from this study include: 25% (4,982 acres) of Bentonville land area is covered by tree canopy with 48% (9,621 acres) available for addition tree planting (PPA Vegetation). Residential land use comprises 40% (8,025 acres) of total land area. With 2,578 acres of UTC, this makes up 52% of all UTC. Additionally, residential districts average 44% (3,513 acres) PPA. Commercial and industrial zones have 18% (444 acres) and 11% (78 acres) UTC respectively, implying that landscaping requirements in these areas may need to be refined. Available planting sites in Bentonville totaled 176,258, with 8,914 sites available for urban heat island mitigation, and 36,995 sites to improve wildlife linkages and canopy density. Bentonville’s tree canopy currently removes over 400,000 pounds of air pollutants annually, valued at $1.7M per year, and provides an estimated value of $2.8 million in avoided stormwater infrastructure costs annually. A detailed and accurate baseline of land cover has been thoroughly analyzed and presented in this report. An ongoing challenge will be to balance new development with the protection and conservation of environmental values related to forest cover such as energy conservation, air quality, and climate adaptation related to carbon storage and urban heat island mitigation. The social, environmental, and economic benefits provided by urban trees and forests are largely undervalued by most laymen. Use these tools to communicate to a large audience on how the beauty and livability of Bentonville is intimately tied to its urban forest resources.
Figure 18: Comparing UTC in the Region
SUMMARY
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Additional details on the City of Bentonvilleâ&#x20AC;&#x2122;s 2014 Urban Tree Canopy assessment are provided including supporting information on:
A1: Glossary of Terms A2: Canopy Calculator and Other Products A3: Land Cover Classification Accuracy Assessment
APPENDIX
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A1: GLOSSARY OF TERMS Air Quality – The quantity of particulates and other pollutants present in a volume of air relative to necessary compounds such as oxygen. Trees improve air quality by absorbing and trapping air pollutants such as particulate matter, ozone, sulfur dioxide, carbon monoxide, and CO2 and by decreasing volatility by lowering air temperatures. Carbon Sequestration – The rate that carbon is removed from the atmosphere by trees. Carbon is considered a very important element because of its recognized influence on climate regulation as a greenhouse gas. Carbon Storage – Cumulative amount of carbon stored in the stems, branches and roots of trees over time. Ecosystem Services – Direct and indirect benefits provided by natural systems. The most common ecosystem services associated with urban trees and forests are air quality improvement, carbon sequestration and storage, energy conservation, and storm water mitigation. Energy Conservation – The amount of energy saved due to the presence of trees. Summer cooling through shade, and by wind blocking in the winter reduces total energy used. Geographic Information Systems – Computer mapping systems used to understand how resources are distributed across the Earth’s surface. i-Tree Canopy – As part of the i-Tree suite developed by the USDA Forest Service, this tool estimates tree cover and tree benefits for a given area with a random sampling process that lets you easily classify ground cover types. i-Tree Eco – As part of the i-Tree suite developed by the USDA Forest Service, this tool is a software application designed to use field data from complete inventories or randomly located plots throughout a community along with local hourly air pollution and meteorological data to quantify urban forest structure, environmental effects, and value to communities Possible Planting Area (PPA) – low-lying vegetation, grass, and open space. These are areas where tree planting is possible. Urban Heat Island Effect – refers to developed areas that are hotter than surrounding rural areas due to the abundance of man-made materials which absorb the sun’s energy much more than trees or other plants, and in turn warm the air around them (Center for Environmental Studies, Brown University, “Trees and the Urban Heat Island Effect”, 2010). Urban Tree Canopy (UTC) – defined as the layer of leaves and stems of trees that cover the ground when viewed from above. Stormwater Runoff Mitigation and Water Quality – Important ecosystem services related to precipitation events, hydrologic cycles, and urban forests. Trees reduce stormwater runoff and improve water quality by intercepting rainfall, increasing soil permeability, and evapotranspiration.
APPENDIX
26
A2: CANOPY CALCULATOR AND OTHER PRODUCTS To assist in advanced UTC goal setting, Plan-It Geo’s Canopy Calculator (MS Excel) tool is provided as an advanced but easy-to-use tool to the City of Bentonville. The City may quickly and easily enter various goals and determine future tree canopy cover and tree planting scenarios.
Figure 19: Screenshots of Bentonville’s UTC Calculator
APPENDIX
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Plan-It Geo’s web-based Canopy Viewer application for Bentonville will provide city officials, city staff, and citizens with a snapshot of Bentonville’s urban forest cover and can also support planning initiatives, implementation, and project tracking/monitoring.
Figure 20: Screenshot of Bentonville’s Canopy Viewer web-mapping application. For more information ,visit www.planitgeo.com.
APPENDIX
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A3: LAND COVER CLASSIFICATION ACCURACY ASSESSMENT
Bentonville Land Cover Classifications This report describes the methods used and generated results in mapping land cover types across the City of Bentonville, Arkansas. National Agricultural Inventory Program (NAIP) 2013 aerial photography and GIS data were combined to map five land cover classes: (1) Tree Canopy, (2) Impervious Surface, (3) Green Vegetation, (4) Soil and Dry Vegetation, and (5) Water. Object-based image analysis (OBIA) was used to map trees, other vegetation, impervious surfaces and soil/dry vegetation areas. Impervious surfaces were augmented with existing buildings (provided by Bentonville GIS staff). Water was mapped using features provided in addition to manual digitizing at 1:1,000 (or finer) scale.
Figure 21: Sample land cover map from Bentonville, AR illustrating the input imagery (left), base 5-class (right) and some of the reference point locations (left and right).
Accuracy Assessment The main purpose of classification accuracy assessment is to measure how well the land cover classification estimates actual land cover on the ground. Even with high resolution imagery, very small differences in classification methodology and image quality can have a large impact on overall map area estimations.
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The internal accuracy assessment was completed in five (5) steps: 1. Five hundred (500) sample points were randomly distributed across the study area and assigned a random numeric value. 2. Sorting from lowest random value to highest (to ensure sequential randomized locations of reference), each sample point was referenced using the NAIP imagery and assigned one of the five land cover classes (“Ref_ID”). In the event that the reference value could not be discerned from the imagery, the point was dropped from the accuracy analysis (no points were dropped for this assessment). 3. An automated script is then used to assign values from the classification raster to each point (“Eval_ID”). 4. The classification supervisor provides unbiased feedback to quality control technicians regarding the types of corrections required (for example, “The classification is currently underestimating forest” would instruct the QC technician to focus on adding more tree canopy to the current classification across the entire study areai.2 5. Accuracy is re-evaluated (repeat steps 3 & 4) until an acceptable classification accuracy is achieved.
Reference Data
Classification Data
Tree Canopy Tree Canopy Vegetation Impervious Soil / Dry Veg. Water Total
Vegetation / Impervious Open Space
129 0 0 0 0 129
6 227 4 1 0 238
Overall Accuracy = Producer's Accuracy Tree Canopy Veg. / Open Space Impervious Bare Ground / Soil Water
100% 95% 92% 100% 100%
Soil / Dry Veg.
2 8 116 0 0 126
Water 0 0 0 6 0 6
Total Reference Pixels 0 137 0 235 0 120 0 7 1 1 1 500
96% User's Accuracy Tree Canopy Open Space / Grass Impervious Bare Ground / Soil Water
94% 97% 97% 86% 100%
Figure 22: Sample Error Matrix Interpretation
Statistical relationships between the reference pixels (representing the true conditions on the ground; “Reference Data”) and the intersecting classified pixels (“Classification Data”)are used to understand how closely the entire
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This is “unbiased information” because there is no spatial component to the instructions. The QC technician applies edits across the entire study area.
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classified map represents the Bentonville landscape. The sample error matrix represents the intersection of reference pixels manually identified by a human observer (columns) and classification category of pixels in the classified image (rows). The white boxes along the diagonals of the matrix represent agreement between the two pixel maps. Offdiagonal values represent the number pixels manually referenced to the column class that were classified as another category in the classification image. Overall accuracy is computed by dividing the total number of correct pixels by the total number of pixels reported in the matrix (129+227+116+6+1 = 479 / 500 = 96%), and the matrix can be used to calculate per class accuracy percent’s. For example, 129 points were manually identified in the reference map as Tree Canopy, and all 129 of those pixels were classified as Tree Canopy in the classification map. This relationship is called the “Producer’s Accuracy” and is calculated by dividing the agreement pixel total (diagonal) by the reference pixel total (column total). Therefore, the Producer’s Accuracy for Tree Canopy is calculated as: (129/129 = 1.00), meaning that we can expect that ~100% of all tree canopy in the Bentonville study area is covered by the “Tree Canopy” classification map. Conversely, the “User’s Accuracy” is calculated by dividing the number agreement pixel total by the total number of classified pixels in the row category. For example, 137 classification pixels intersecting reference pixels were classified as Tree Canopy, but 6 pixels were identified as Vegetation, and 2 as Impervious in the reference map. Therefore, the User’s Accuracy for Tree Canopy is calculated as: (129/137 = 0.94), meaning that 94% of pixels classified as Tree Canopy in the classification were actual tree canopy in the Bentonville. It is important to recognize the Producer’s and User’s accuracy percent values are based on a sample of the true ground cover, represented by the reference pixels at each sample point (it is cost-prohibitive to sample every pixel, so we use randomly selected points). Results Interpretation of the sample error matrix results indicate this land cover does a very good job of mapping land cover accurately in the City of Bentonville. The most reliable class is tree canopy cover (even though water appears more accurate, only one reference point fell on water). The largest source of classification confusion exists between impervious surfaces, vegetation, and soil / dry vegetation. This confusion is largely the result of human interpretation in that the interpreter must determine when a gravel parking lot should be considered pervious or impervious, or whether existing vegetation is living (green) or dry (brown).
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