December 2015
Community Tree Inventory Project Report A project by the Washington Department of Natural Resources Urban and Community Forestry Program Completed December 2015
Prepared for: WA DNR Urban and Community Forestry Program Prepared by: Plan-It Geo, LLC
Acknowledgements: Funding provided by the WA DNR Review, editing, and contribution by Linden Lampman and Ben Thompson
Table of Contents Executive Summary Introduction Methods of Analysis Results and Trends: Statewide
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Structural Trends Species Richness and Composition Relative Age Distribution Canopy Cover Growing Space Management Trends Tree Condition Management Needs Management Recommendations Potential Risk Trends Defects Species Diversity Assessment for Identifying Potential Risk Trends Appraisal Values and Ecosystem Benefits Appraisal Values Background Information Appraisal Values Summary Ecosystem Services Overview Total Tree Benefits Energy Savings Carbon Sequestration Air Quality Improvements Stormwater Reduction Aesthetics/Other Summary of Ecosystem Benefits by the Top 10 Most Common Species
Future Analysis Guidance and Recommendations Conclusion References Appendices Appendix I. i-Tree Analysis Process and Assumptions Appendix II. Summaries by Population Range Structure Trends Common Species by Population Ranges Relative Age Distribution by Population Ranges Management Trends Tree Condition by Population Ranges Appraisal Values Appendix III. Microsoft Access Analysis Guidance
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Figures and Tables Figures Figure 1. Distribution of the top ten species statewide Figure 2. Map of the location of communities in relation to the Cascade Range and population size Figure 3. Distribution of the top ten species comprising the total population Figure 4. Distribution of the top ten genus comprising the total population Figure 5. Distribution of trees by diameter range Figure 6. Distribution of canopy spread among the population Figure 7. Distribution of the growing space types for total population (46,888) Figure 8. Distribution of condition classes among the population of trees inventoried Figure 9. Distribution of the three Defect Types Figure 10. Species with the highest percent of each Defect Type among total number of defects identified Figure 11. Distribution of the top ten Specific Defects identified (56,471 total) Figure 12. Distribution of the top 25 Appraisal Totals by species
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Appendix II Figures Figure 13. Top ten species for each population range Figure 14. Condition of trees in each population range
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Tables Table 1. Summary of ecosystem savings across regions Table 2. Summary of the number of trees inventoried for each community or geography organized by region Table 3. Summary of the diversity of trees in each community by family, genus, and species Table 4. Condition rating summaries for the top ten species Table 5. Relative Performance Index for the top ten species project-wide Table 6. Count and distribution of general management task prescribed Table 7. Distribution of the top 5 prescribed detailed tasks Table 8. Top ten species and the number one specific task prescribed Table 9. Location of inventory in relation to the Cascade Range for use in i-Tree Streets Table 10. Total annual benefits of public trees by climate zone (region) Table 11. Annual energy benefits of public trees by climate zone (region) Table 12. Annual CO2 benefits of public trees by climate zone (region) Table 13. Annual air quality benefits of public trees by climate zone (region) Table 14. Annual stormwater benefits of public trees by climate zone (region) Table 15. Annual aesthetics/other benefits of public trees by climate zone (region) Table 16. Ecosystem benefits totals for the three most common species
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Appendix II Tables Table 17. Distribution of DBH classes for each population range Table 18. Condition rating of trees in each population range Table 19. Average appraisal value summarized for population ranges
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Executive Summary Street and park trees are an important public asset in urban environments. As a buffer between transportation corridors and homes, urban trees filter stormwater runoff, reduce the effects of car emissions, increase property values, calm traffic, and regulate summer temperatures, among other benefits. Proper management of trees can ensure continued delivery of their benefits while minimizing tree-related risk and liability. An inventory of public trees is integral to the development of a sustainable urban forestry management program. Information on tree species, size, condition, location, and management needs can help cities to effectively prioritize the planning, planting, and maintenance of city trees, as well as the safe removal of trees when necessary. The Washington DNR Community Tree Inventory Project’s analysis compiled and summarized key information for 46,888 public trees across 21 communities. The inventories were divided regionally by the Cascade Range due to the vast differences in species, growth habits, and environmental conditions. The 15 inventories in the western portion of the state contain a total of 33,617 trees while the east contains 13,271 trees within 6 communities. Summary of Urban Forest Structure and Composition A structural analysis is the first step towards understanding the benefits provided by these trees as well as their management needs. Considering species composition, diversity, age distribution, condition, and canopy coverage, it was determined that the following information characterizes this urban forest resource:
There were 330 unique tree species identified in the inventories. The predominant tree species are red maple (Acer rubrum, 8.8%), ponderosa pine (Pinus ponderosa, 7.5%), and Douglas fir (Pseudotsuga menziesii, 6.9%). The urban forest age structure consists largely of established trees with 29% of trees measuring 612” DBH (diameter at breast height, measured at 4.5 feet above the ground) with an overall average diameter of 10.7”.
2.1%
2.0%
2.1% 8.8% 2.8% 3.0% 7.5% 5.0% 5.1%
maple, red douglas fir pear, flowering oak, red pine, austrian
6.9%
pine, ponderosa maple, norway linden, littleleaf plum, cherry or purple leaf ash, white
Figure 1. Distribution of the top ten species statewide
Summary of Urban Forest Management Needs and Risks Using the data from the inventories provided, an assessment of common management needs and potential risks was conducted. The following information highlights the trends found during the assessment of tree condition, Relative Performance Index (RPI) of species, maintenance needs, and presence of defects.
A majority of the inventoried trees (50%) are in good condition and 45% are in fair condition.
Red oaks have the highest percentage of trees rated as Good (72%) while trees classified as either plum, cherry or purple leaf have the highest percentage of trees rated as Poor (8%).
Red oak condition ratings are above average when compared to the overall public tree population while flowering pears are performing substantially lower than the population. 1
56% of all trees inventoried require some type of pruning to improve structure, branch strength, and longevity. The majority require crown cleaning (14%) and subordinate pruning (33%).
85% of the defects identified are classified as structural defects. Of all the defects identified, 27% were a concern of co-dominant stems.
Out of the 76 types of defects noted, four human caused defects appear in the top ten most prevalent recorded defect types. These include staked, root collar missing, topped, lawnmower, and trunk scar.
Summary of Urban Forest Benefits and Appraisal Values Annually, the trees analyzed provide ecosystem benefits to the community at an average value of $44.42 per tree in the eastern region and $123.71 per tree in the west, for a total value of $2.9M per year. Specific benefits are as follows: Table 1. Summary of ecosystem savings across regions
The trees provide a net energy Interior West North savings of $464,649 with an Ecosystem Benefit (east of (west of Benefit Total Cascades) Cascades) average of $12.57 (east) and $18.21 (west) per tree each year. Energy $65,243 $399,406 $464,649 CO2 $5,715 $69,640 In the eastern communities, trees $75,355 Air Quality $9,791 $59,960 sequester a net total of 1,731,806 $69,751 Stormwater $26,456 $487,992 $514,448 pounds of carbon and 9,285,271 Aesthetic/Other $123,423 $1,695,919 $1,819,342 pounds in the west for a total of Total ($) $230,628 $2,712,917 $2,943,545 11,017,077 pounds annually, valued at $75,354. Air quality improvements, including removal and avoidance of pollutants provided by the tree population, are valued at $69,751 annually with average per tree benefit values of $1.89 (east) and $2.73 (west). The tree canopy captures and reduces a total of 50,475,703 gallons of stormwater runoff (5.3 million in the east and 45.2 million in the west), saving communities an estimated $514,448 annually in stormwater management costs*.
*It should be noted that the stormwater benefits model used for this study does not reflect the growing cycles and physiology of western Washington tree species which would effect their impact on stormwater management. These numbers provided are a means to illustrate that Washington’s urban trees have value and not representative of the total population.
Using the Council of Tree and Landscape Appraisers (CTLA) formula, the total compensatory (replacement) value of the inventoried trees is an estimated $153,140,020 with an average value of $3,200. Out of 330 unique species, ponderosa pines have the highest appraisal value, representing 17% of the total value followed by Douglas fir (16%) and Norway maple (6%). The top 25 appraisal values contribute 79% to the overall value of the inventoried trees. The following sections provide a comprehensive analysis of sample tree inventory data in Washington State. Trends identified do not necessarily represent trends and conditions of the entire state or regions. This report can be used as a protocol for future studies. Photo courtesy of WA DNR
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Introduction Communities of any size can benefit from inventorying public trees to improve how trees are managed for urban forest health, public safety and mitigation of tree-related risks and for community development purposes. Collecting ground inventory data provides information essential to developing a sustainable community forest management plan. Data analysis illustrates and defines the associated benefits of the resource and provides a baseline from which to develop strategies and goals leading to a sustainable program. Enacted in 2008, the Evergreen Communities Act, E2SHB 2844 (ECA) established protocols for a prioritized statewide inventory and assessment of urban trees throughout Washington State. Although ECA is currently in hiatus, Washington State’s Department of Natural Resources Urban and Community Forestry Program (UCF) continues work, on a smaller scale, to achieve the intended goals. The ECA authorizes the WA DNR to develop a statewide inventory and assessment of community forests in Washington State to assist local governments with forest planning and ordinance development. This analysis of tree population characteristics, ecosystem benefit values, and trends and vulnerabilities helps to assess maintenance costs and recommend management strategies. UCF’s Community Tree Inventory Project (CTIP) works in partnership with Washington municipalities, college campuses, and other public urban landowners to implement public tree inventories. To assure accurate and consistent information, inventory data is collected through contractual agreement with UCF by professional urban forestry consultants. Tree resource data is summarized and delivered to communities to use for resource management planning. With this information, communities are poised to create policy, plan for, and initiate a sustainable urban and community tree management program. This project aggregates community tree data collected through the CTIP over the past 4.5 years, with a goal to more clearly understand the structure, condition, and function of Washington urban forests. While this is just a small sample of Washington urban trees, analysis of the data begins to tell a compelling story of current conditions, and future risks associated with urban forests in the state. These outcomes will assist DNR in providing the resources for technical, educational, and financial assistance to communities striving to achieve ECA standards. The project included inventories from large and small communities and several campuses which offer a diverse sample for study and analysis. The 21 datasets include the communities of Auburn, Buckley, Cashmere, Chelan, Covington, DuPont, Fairfield, Kent, Kirkland, Mukilteo, Poulsbo, Shoreline, Snoqualmie, Spokane, Sumner, Tacoma, Toppenish, and Tukwila, as well as Clark College, Whitworth University, and State Capitol Campus (see Figure 2).
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Figure 2. Map of the location of communities in relation to the Cascade Range and population size
The communities range in population from 615 in Fairfield, to 211,300 in Spokane and range in tree numbers from 444 in Fairfield to 6,574 in Spokane. Collectively, 46,888 trees are included in the analysis (see Table 2 below) and are dispersed regionally with 13,271 trees in the communities east of the Cascade Range and 33,617 trees to the west. Table 2. Summary of the number of trees inventoried for each community or geography organized by region
Community Number of Trees Location 1,139 Cashmere East 1,623 Chelan East 444 Fairfield East 6,574 Spokane East 1,484 Toppenish East 2,007 Whitworth East 3,832 Auburn West 1,442 Buckley West 495 Capitol Campus West 1,134 Clark College West 2,188 Covington West 6,258 DuPont West 2,130 Kent West 2,581 Kirkland West 2,098 Mukilteo West 1,649 Poulsbo West 1,602 Shoreline West 2,143 Snoqualmie West 1,911 Sumner West 2,054 Tacoma West 2,100 Tukwila West Total 46,888
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Methods of Analysis To complete an analysis of the inventories collected across the State of Washington, the data was collected from the WA DNR UCF Program and added to Microsoft Access software in order to aggregate each community’s data into one database while still representing each entity with a unique identifier. This allows the data to be summarized by region and statewide as well as by individual community. Once the data was aggregated and the fields and values were reviewed for consistency, tiers of analysis were identified (statewide, by region, city, population range, and land use). These tiers were only listed if they provided valuable and statistically accurate results. Tiers were separated into Tier-1 and Tier-2 categories in which the latter required analysis and summary of more than one field. Within these tiers, the analyses were separated by their type; Structure, Management, and Potential Risk. After review, queries of the data were made in Access to deliver the analyses in a reportable format. The purpose of this report is to provide a protocol that could identify trends and conditions, given a statistically significant sample of inventoried trees. To view all tiers and queries completed for this study see the associated spreadsheet (“WA DNR Inventory Analysis Results“). Once all the queries were generated, the data deemed significant for reporting was exported to Microsoft Excel. Data was then further organized and analyzed to identify specific trends, format for reporting purposes, and generate charts and graphs for illustrative summaries of the data. The data is organized in Excel by geography (statewide, regions, and population ranges) and by analysis category (structure trends, management trends, and potential risk trends) in order to easily conduct further analyses or use as reference. Most tabs in the spreadsheets are exported data from the Access database of aggregated inventories and contain the query statement in order to cross-reference both databases. With nearly 50,000 trees and 330 unique species identified in the 21 datasets, most summaries presented in the report analyzed the top ten species that are present throughout the CTIP areas. These top ten species represent 45% of the overall population and have a great impact on the structure, management needs, and potential risks of the urban forest. The report is structured so that results are presented at a high-level of analysis to offer summaries and potential trends in Structure, Management Trends, Potential Risk Trends, Appraisal Value and Ecosystem Services. Further analyses summarize regional and population range potential trends in the report and the Access database provides the queries for individual city and community analyses. The information is intended for internal use but will help to foster data-driven discussions with communities about the importance of urban forest management and to identify priorities. The data and results can be provided to selected communities and interested government or non-governmental stakeholders in order to compare the initial trends with on-the-ground observations and management plans. The analysis of the inventories follows a protocol set by the WA DNR UCF Program as part of the Evergreen Communities Act. Appraisal value was collected during the inventory process using the Council of Tree and Landscape Appraisers formula, an industry accepted technique for assessing the value of trees. More information can be found in this Arboricultural Journal article1. As the CTIP continues to progress with new inventories, this process will serve as protocol for identifying realistic trends and needs and can then be disseminated to members and organizations in the communities to begin or improve their path towards urban forest sustainability. 5
Results and Trends: Statewide Using the DNR’s standardized data fields for each inventory, trends and summaries are presented below in the following categories; Structure, Management, and Potential Risk. While not statistically representative of the state, the inventories provided to date offer possible trends and repeatable methods.
Structure The urban forest resource is better understood through examination of structure, composition, and species diversity. Interpretations of the data can help tree managers understand the importance of individual trees as well as planting and maintenance cycles. Species Richness and Composition Of the combined 21 datasets across the state, 46,888 trees were inventoried which included 330 unique species, 96 Genera, and 52 Families. Despite the large number of species found in the inventories the top ten species represent 45% of the total population (Figure 3). The predominant tree species are red maple (Acer rubrum, 8.8%), ponderosa pine (Pinus ponderosa, 7.5%), and Douglas fir (Pseudotsuga menziesii, 6.9%). The following figure (Figure 3) shows the top ten species of the aggregated datasets and the percent in which they comprise the population.
Top Ten Species Distribution 10% 9% 8% 7% 6% 5% 4% 3% 2% 1% 0%
Figure 3. Distribution of the top ten species comprising the total population
Figure 3 above shows that each of the top 10 species comprises less than 10% of the total population, which is an indicator of rich diversity. Red maple (Acer rubrum) comprises the largest amount of the population with 4,130 trees inventoried or 8.8%. Of the top 10 species statewide, white ash (Fraxinus americana) comprises the least amount of the population with 939 trees or 2.0%. The top 10 species comprise 45% (21,211) of Washington’s urban forest. The high number of red maples in the inventories results in the genus Acer to comprise most of the public tree population with 22% (Figure 4) and contribute almost a third of the trees in the top 10 most abundant category. The top 10 Genera make up 71% of the total street and park tree population.
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Distribution of the Top Ten Genera 25% 20% 15% 10% 5% 0%
Figure 4. Distribution of the top ten genus comprising the total population
For maximum protection against pest outbreaks and other risks, the urban forest within communities and regions should contain no more than 10% of any single tree species, no more than 20% of any tree genus, and no more than 30% of any tree family2. Although, emerging research is suggesting even more conservative targets for species diversity, whereby tree populations should not contain greater than 10% of any particular genus. For more information, see research by Dr. John Ball3. The data collected from the inventories were tallied and summarized in the table below to assess each community’s susceptibility to large scale canopy loss. After adding the family name to each of the trees in the inventory, it was identified that 10 out of 21 communities have less than 30% of one family, 6 out of 21 have less than 20% of one genus, and only one community has less than 10% of any one species. Over 71% (2,007 trees) of Whitworth’s trees are in the Pinaceae family. Spokane and Whitworth have the largest percentage of the same species with 34% and 56%, respectively. The results show areas of concern across the state regarding lack of species diversity and risk of canopy loss. No community passed the 30-20-10 diversity rule. New plantings in the immediate future should limit these species to reduce overreliance using approved local species lists. Table 3. Summary of the diversity of trees in each community by family, genus, and species using the 30-20-10 rule
Community Auburn Buckley Capitol Campus Cashmere Chelan Clark College Covington Dupont Fairfield Kent Kirkland Mukilteo Poulsbo Shoreline Snoqualmie Spokane Sumner Tacoma Toppenish Tukwila Whitworth Yes/No Count
Family Percent 30.48% 34.54% 22.83% 38.54% 28.16% 26.01% 29.52% 25.14% 34.46% 24.51% 32.24% 29.65% 34.69% 45.51% 32.76% 49.22% 43.12% 20.69% 19.54% 27.52% 71.50%
Family Pass No No Yes No Yes Yes Yes Yes No Yes No Yes No No No No No Yes Yes Yes No 10/11
Genus Percent 18.22% 34.54% 22.83% 17.12% 15.40% 15.34% 22.44% 25.14% 34.46% 24.51% 18.60% 29.65% 34.69% 45.51% 32.76% 43.29% 30.46% 20.69% 18.13% 27.52% 58.69%
Genus Pass Yes No No Yes Yes Yes No No No No Yes No No No No No No No Yes No No 6/15
Species Percent 16.68% 19.28% 15.76% 11.33% 12.20% 12.43% 17.09% 13.01% 32.21% 15.07% 18.60% 19.54% 20.68% 23.22% 18.48% 33.95% 16.85% 8.96% 17.45% 13.86% 55.80%
Species Pass No No No No No No No No No No No No No No No No No Yes No No No 1/20
Overall Pass No No No No No No No No No No No No No No No No No No No No No
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Relative Age Distribution An additional measure of structure and resiliency of an urban forest is the distribution of diameter classes. Figure 5 shows the distribution of trees by diameter class as measured at diameter at breast height or 4.5’ DBH.
Distribution of Trees by Diameter Range 35% 30% 25% 20% 15% 10% 5% 0% 0-3
6-12
12-18
18-24
30-36
24-30
>36
Figure 5. Distribution of trees by diameter range
To optimize the value and benefit of trees, the community forest should have a high percentage of large canopy trees which provide more ecosystem benefits. At the same time, there must be a sufficient number of younger, smaller trees in the tree population to account for the loss of trees over time and thereby maintain a sustainable community forest. In traditional forest management, this is similar to an uneven aged stand or tree population. Figure 5 shows that the 6-12” DBH range comprises the majority of the urban forest with 29% and the 30-36” DBH range makes up the smallest portion of the urban forest with 2%. The average diameter among the population is 10.7”. According to Figure 5 the aggregated data represents a healthy structure of young to mature trees with a markedly high number of trees greater than 36” in diameter. The distribution of individual tree ages within a tree population influences present and future costs as well as the flow of benefits. An ideal age distribution in the tree population allows managers to allocate annual maintenance costs uniformly over many years and assures continuity in overall tree canopy coverage and associated benefits which are often dependent on the growing space of individual trees (e.g. open grown versus restricted growing areas). Canopy Cover The amount and distribution of leaf surface area is the driving force of the provisioning of urban forest benefits for the community. As canopy cover increases, so do the benefits afforded by leaf area. Figure 6 below shows the distribution of canopy spread ranges among the total trees inventoried.
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Distribution of Canopy Spread among the Population 40% 35% 30% 25% 20% 15% 10% 5% 0% 0-10'
10-20'
20-30'
30-40'
40-50'
0.4%
0.1%
0.05%
0.01%
0.004%
50-60'
60-70'
70-80'
80-90'
>90'
Canopy Spread (feet) Figure 6. Distribution of canopy spread among the population
Growing Space Growing space has a significant impact on the health and growth of individual trees as well as the structure of the entire urban forest. Figure 7 shows the distribution of growing space types for each of the 46,888 trees. This analysis includes street and park trees which have varying characteristics due to the growing space available and presence or absence of other stressors.
Distribution of Growing Space for Total Population 50% 40% 30% 20% 10% 0%
Figure 7. Distribution of growing space types for total population (46,888)
The 12 types of growing spaces recorded range from open/unrestricted planting area to small planting areas as well as constructed sites for improving tree health and growth such as tree wells and tree grates. Of all the growing space types, unrestricted growing space was most prevalent (45%). Of the constructed growing space types, tree grates were the most prevalent with 2%. The 4-8 feet, medium growing space type comprises 25% of the inventories followed by the 8+ feet, large growing spaces with 12%. Given proper planting procedures, good soils and environmental conditions, and continued care in accordance with best practices for pruning, for example, trees planted in larger spaces with ample root space will perform well and grow to their genetic potential.
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Management Trends The inventory data was analyzed to identify potential trends in tree management needs and condition. Local information on the condition of street and park trees plays an important role in community planning, municipal budgeting, and use of resources. Tree Condition Tree condition is an indication of how well trees are managed and how well they are performing. Each inventoried tree was rated for the condition of the wood and the foliage.
Distribution of Condition Classes among Total Population 60%
50%
50%
45%
40% 30% 20% 10%
1%
3%
0%
poor
severe
0% excellent
good
fair Condition Class
Figure 8. Distribution of condition classes among the population of trees inventoried
In Figure 8 above, the distribution of condition ratings across the total population is depicted. It is observed that the condition ratings of “Good” and “Fair” dominate the public trees inventoried. Trees in good condition have minor issues or defects that do not require immediate attention and maintenance could occur later in the city pruning cycle. Trees in fair condition have well defined issues (dead branches; co-dominant stems) that warrant some corrective pruning or maintenance within the next pruning cycle. It should be noted that of the 46,888 total trees, 46,517 (99.2%) were populated with a condition rating in the field. Table 4 below shows the summary of condition ratings for each of the top ten species in the aggregated data. The percentages are based on the total count of each species and the count of each condition class. The values in bold show the highest percentage for each condition class and the associated species. Note that the species grouping plum, cherry or purple leaf have the highest percent of “Poor” condition (8%) and “Severe” (0.41%). Flowering pear has the highest percent of trees classified as “Fair” (66%) while red oaks have the highest “Good” classification with 72%.
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Table 4. Condition rating summaries for the top ten species
Species ash, white Douglas fir linden, littleleaf maple, Norway maple, red oak, red pear, flowering pine, Austrian pine, ponderosa plum, cherry or purple leaf
Excellent 0% 1% 0% 0% 0% 0% 0% 2% 0% 0%
Condition Class Good Fair Poor 62% 34% 3% 58% 41% 1% 47% 51% 1% 51% 46% 2% 45% 53% 2% 72% 26% 2% 32% 66% 1% 41% 57% 1% 47% 51% 2% 36% 56% 8%
Severe 0.32% 0.35% 0.07% 0.34% 0.17% 0.08% 0.26% 0.00% 0.00% 0.41%
The ten most prevalent species, white ash, Austrian pine, plum/cherry/purple leaf, red oak, littleleaf linden, flowering pear, Norway maple, Douglas fir, ponderosa pine, and red maple, comprise 52% of the trees inventoried. Thus, an analysis of their condition below in Table 5 summarizes the Relative Performance Index (RPI) for the predominant tree species. RPI is based on the proportion of each public tree species given a Good condition rating divided by the proportion of the total population classified as Good. An index value of 1 indicates that 50% (population-wide average) of that particular species are in Good condition, reflecting the average condition of all species in the city. A value higher than 1 indicates that there is proportionately more individuals classified as Good. An index value below 1 indicates that that species has a below average condition rating when compared with other public trees (CUFR4). Table 5. Relative Performance Index for the top ten species project-wide
Species ash, white Douglas fir linden, littleleaf maple, Norway maple, red oak, red pear, flowering pine, Austrian pine, ponderosa plum, cherry or purple leaf # Within Top 10 # Statewide
Total Trees by Species # of Good % Good RPI of Total Population 934 581 62.2% 1.24 3,187 1,844 57.9% 1.16 1,417 670 47.3% 0.94 2,375 1,220 51.4% 1.03 4,108 1,842 44.8% 0.90 1,311 943 71.9% 1.44 2,336 753 32.2% 0.64 951 386 40.6% 0.81 3,485 1,636 46.9% 0.94 986 356 36.1% 0.72 21,090* 10,231 48.5% 46,517* 23,279 50.2%
*Value differs from total of top 10 species reported because some trees did not receive condition rating (46,517 of the total 46,888 trees were given a condition rating. 121 ratings omitted within the top ten species).
As an example, 62.2% of all white ash (Fraxinus americana) have a condition rating of Good. The statewide percent of trees rated as Good is 50.2%. This shows that the entire white ash population has a better condition rating than the statewide average, equating to a RPI value above 1.0 (1.24). Table 5 above shows that of the top ten species, white ash, Douglas fir, and red oak have a higher RPI than the statewide average condition rating. Of all the species, red oak is performing the best with a RPI of 1.44 and 72% of the species is classified as Good. Littleleaf lindens, Norway maples, and ponderosa pines are performing close to the project-wide Good condition rating average of 50.2%. The remaining species; red maple, Austrian pine, plum, cherry or purple leaf, 11
and flowering pear are performing below average based on this data analyzed. It should be noted that the lowest performing species is flowering pear with only 32.2% of the species classified as Good resulting in a RPI of 0.64. The RPI results can be used to help guide future planting recommendations and management strategies. A tree’s condition is the best indicator of whether or not it is suited or best adapted to the given stressors. Factors to consider when reviewing these values include the possibility of an aging species (e.g. elms) as well as overplanting a low performing species or a lack of maintenance causing a low RPI value. Management Needs The table below (Table 6) summarizes the management needs (“Task”) identified in the inventories. Over half (56%) of the tree population requires pruning. Additionally, 68% of all trees in the 21 datasets require some type of management described in the “Task” field of the inventories. Table 6. Count and distribution of general management task prescribed
Task prune root collar remove stake removal replant root treatment pest treatment water inspect monitor remove object plant cable stump removal brace fertilize Total*
Count 26,342 1,977 1,492 1,415 108 102 76 75 67 48 32 17 10 7 4 1 31,773
% of Population (46,888) 56% 4% 3% 3.0% 0.2% 0.2% 0.2% 0.2% 0.1% 0.1% 0.1% 0.04% 0.02% 0.01% 0.01% 0.002% 68%
*31,773 out of 46,888 (68%) total trees were assigned a management task.
Each management task is described in more detail and summarized in the following table (Table 7). The task required for the most trees is subordinate pruning with 33% of the total population. In a landscaped area, trees are planted farther apart than in a naturally occurring forest setting. In the absence of competition from nearby trees, urban trees have a tendency to grow wider than they do in the forest. Often, several branches from the same tree will compete with each other for vertical space. These competing branches often lead to trees with multiple leaders and poor structure which can lead to branch failure. The large percentage of trees requiring subordinate pruning may be associated with a high proportion of the tree population found in the smaller diameter ranges (refer to Figure 5).
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Two other pruning needs are identified within the inventoried trees; crown cleaning (14%) and clearance (7%). Crown cleaning consists of selective removal of dead, dying, diseased, and weak branches from a tree’s canopy and clearance is the removal of branches or directionally pruning to clear an obstruction of signs, utilities, buildings, etc. Table 7. Distribution of the top 10 prescribed detailed tasks
Task Detail subordinate crown clean clearance excavate remove stake removl restoration structural expose flair replant Total of Top 10
Count 15,343 6,763 3,365 1,987 1,431 1,414 468 351 100 93 31,315
% of Population (46,888) 33% 14% 7% 4% 3% 3% 1% 1% 0.2% 0.2% 66%
*Total reflects the count of the top 10 detailed tasks prescribed to the inventoried trees
Table 8 below summarizes the most numerous “Specific Task” prescribed for each of the top ten species. As Table 7 above has shown, subordinate and crown clean pruning is the dominant Specific Task required for each of the top ten species. Of the entire top ten species count, 55% require pruning. The majority of ponderosa pines (80%) require maintenance while only 29% of the Austrian pine population requires a Specific Task. 70% of the flowering pear population require subordinate pruning. Table 8. Top ten species and the number one specific task identified
Species
Specific Task
ash, white Douglas fir linden, littleleaf maple, Norway maple, red oak, red pear, flowering pine, Austrian pine, ponderosa plum, cherry or purple leaf Total
subordinate crown clean subordinate subordinate subordinate subordinate subordinate crown clean crown clean subordinate
Count requiring Specific Task 546 1,023 744 1,334 2,293 535 1,644 284 2,788 450 11,641
Total Spp Count 939 3,215 1,419 2,387 4,130 1,313 2,348 967 3,500 993 21,211
% Requiring Specific Task 58% 32% 52% 56% 56% 41% 70% 29% 80% 45% 55% 13
Management Recommendations
Maintain an appropriate age distribution by continuing to plant new trees to improve long-term resource sustainability and greater canopy coverage. To maximize benefits, focus on medium to large-stature trees where planting sites allow. Maximize the condition of the existing tree resource through continuing comprehensive tree maintenance and a cyclical pruning schedule. Implement a structural pruning program for young and establishing trees to promote healthy structure, extend life expectancy, and reduce future costs and liability. Maintain and update the tree inventory database. Discontinue or greatly reduce the planting of overrepresented species and genera in favor of less common trees.
The value of Washington’s inventoried tree resource will continue to increase as existing trees mature and new trees are planted. As the resource grows, investment in management is critical to ensuring that residents will continue receiving a high return on the investment in the future. Planning and funding for tree care and tree management must complement planting efforts in order to ensure the long-term success and health of the urban forest. Existing mature trees should be evaluated and removed as they begin to exceed thresholds of unacceptable risk. Trees in good condition should be maintained and protected whenever possible since the greatest benefits accrue from the continued growth and longevity of the existing canopy.
Potential Risk Trends An evaluation of potential risks to Washington’s urban forest was conducted by analyzing types of defects in the data collected. This evaluation reflects common issues facing urban trees and urban forests in Washington State. Immediate risks were identified when gathering the tree inventory data by documenting the trees that were given a Poor or Severe condition rating. Community tree managers were promptly notified if a tree posed an immediate risk. Defects In an urban environment, biotic and abiotic factors can impact a tree’s growth and condition by causing certain defects. For each inventory, trees were evaluated for any type of defect and classified under one of three categories; structural, cultural, or insects/disease. Structural defects are often a result of a tree’s physiology, genetics, or influences from the surrounding environment. For example, a tree’s growth pattern can result in defects or extreme temperatures can cause defects such as frost cracks that affect a tree’s condition and growth. Cultural defects can be a result of human factors such as poor planting and pruning practices and can also include maintenance negligence such as lack of water. The third defect type that was identified in the inventories was the presence of insects and diseases. Figure 9 below shows that the most common Defect Type identified across all inventories is structural. In many cases multiple defects were identified on a single tree, resulting in 56,471 defects reported. Of this total, 85% of the defects noted were structural issues.
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Distribution of Defect Types 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% insects/disease
cultural
structural
Figure 9. Distribution of the three Defect Types
As a result of red maples and ponderosa pines dominating the urban forest with 9% and 8% respectively, these two species have the highest count of the three defect types. As seen in the figure below, red maples have the greatest number of structural defects comprising over 10% of the total number of defects identified (56,471). These two species contain 13% of the total defects recorded.
Species with the Highest % of Each Defect Type 12%
10% 8% 6% 4% 2% 0% maple, red (4,130 total)
maple, red (4,130 total)
pine, ponderosa (3,500 total)
Structural
Cultural
Insects/disease
Figure 10. Species with the highest percent of each Defect Type among total number of defects identified (56,471)
Among the 21 datasets, a total of 76 specific defects were identified. These are the specific types of cultural, structural, and insect/disease defects found on each tree. Examples include mechanical damage, decay, dieback, included bark, cracks, fungus, and fall webworm. Figure 11 below shows the ten most recurring defects among the population and the percentage within the total number of specific defects identified (56,471).
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Distribution of Top 10 Specific Defects among Total Count 30% 25% 20% 15% 10% 5% 0%
Figure 11. Distribution of the top ten Specific Defects identified (56,471 total)
Figure 11 shows the most prevalent specific defects identified across all inventories. Of the 56,471 total specific defects identified, 1% was a result of staking issues. Co-dominant stems topped the chart with 27% distribution. It should be noted that of the top ten specific defects, four are a result of anthropogenic actions or inaction. These include the values; staked, root collar missing, topped, lawnmower, and quite possibly, trunk scar. The most prevalent structural defects include basal scar, deadwood, branch architecture, included bark, and co-dominant stems. The top ten specific defects did not include any insects or diseases. Of the 76 specific defects identified, rust has the highest count (ranked 20th) of any insect or disease identified but only encompasses 0.6% of the total count of defects. Species Diversity Assessment for Identifying Potential Risk Trends The evaluation of the population’s species diversity is another measure of a community’s risk potential to its street and park trees. As identified in the previous section detailing the structure of the urban forest, the potential risk to the health and vitality is low because of the diversity. According to the 30-20-10 rule and Table 3 under the Structure Trends section, the communities that need to improve the diversity of the family of trees planted are Auburn, Buckley, Cashmere, Fairfield, Kirkland, Poulsbo, Shoreline, Snoqualmie, Spokane, Sumner, and Whitworth. Those communities that need to increase diversity of the genus planted include Buckley, Capitol Campus, Covington, DuPont, Fairfield, Kent, Mukilteo, Poulsbo, Shoreline, Snoqualmie, Spokane, Sumner, Tacoma, Tukwila, and Whitworth. Only Tacoma has less than 10% of any particular species. In other words, of the 21 datasets, 10 (48%) fall under the family diversity threshold of 30%, 6 (28%) fall under the genus diversity threshold of 20%, and 1 (5%) fall under the species diversity threshold of 10%. Communities should consider this evaluation in future plantings and when updating recommended tree planting species lists.
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Appraisal Values and Ecosystem Benefits Appraisal Values Background Information The community forest is a public asset that, when properly cared for, has the potential to appreciate in value as the trees mature over time. A measure of individual tree worth was completed for each tree in the inventories. Appraisal values or replacement value accounts for the investment in trees over their lifetime. Replacement value is also a way of describing the value of a tree population (and/or average value per tree) at a given time. The replacement value reflects current population numbers, stature, placement, and condition. There are several methods available for obtaining appraisal (aka, replacement or compensatory) values. (Cullen1) For these inventories, an equation from the Council of Tree and Landscape Appraisers (CTLA) was used. (Cullen1). CTLA’s equations are industry accepted and standardized techniques for assessing value of trees. There are several appraisal techniques to determine amenity value. The appraisal value for each tree was determined using the Trunk Formula Method. This is a cost approach to determine the value of trees that are too large to transplant. The formula utilizes the following factors to calculate a tree’s value:
Size – larger trees tend to be of more value than smaller trees. Species – certain characteristics of a particular species of tree may be worth more than another. Condition Rating – a tree in good condition is worth more than a tree in poor condition. Location Rating – a tree in a very noticeable spot in a well-planned landscape typically has more value.
Therefore, Value = Basic Tree Cost x Species Rating % x Condition Rating % x Location Rating % Factors influencing the values include the prevalence of the species, growth habits, unknown species, and specific use (e.g. aesthetics).
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Appraisal Values Summary Using the trunk formula method, the total appraisal value (or replacement value) of the inventoried trees was estimated at $153,140,020 with an average value of $3,200. As seen in Figure 12 below, of the 330 unique species inventoried, ponderosa pines have the highest appraisal value, representing 17% of the total appraisal value followed by Douglas fir (16%) and Norway maple (6%). The top 25 appraisal totals contribute 79% to the overall value of the inventoried trees and collectively, the top ten species (by count) contribute 52% of the total appraisal value ($7.9 million of $15.3 million)
Distribution of Top 25 Appraisal Totals by Species 18% 16% 14% 12% 10% 8% 6% 4% 2% 0%
Figure 12. Distribution of the top 25 Appraisal Totals by species
Ecosystem Services Overview Urban trees provide many “ecosystem services” which impact our lives and the environment. Trees can be valued in terms of public health, energy demand, and public infrastructure savings, which helps to justify the many reasons to promote, establish, manage, and maintain a robust, “working” urban forest throughout Washington. Quantifying these services helps to demonstrate the value of urban forests beyond aesthetics. To do this, i-Tree Streets was used. As part of the i-Tree suite of tools developed by the USDA Forest Service, Streets is an “analysis tool for urban forest managers 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” (www.itreetools.org5). The ecosystem services were assessed by two separate regions; communities east and west of the Cascade Range, as seen in the table below. Communities east of the Cascade Range fall within the Interior West climate zone and communities west of the Cascade Range are classified as North climate zone. The reference cities used for the i-Tree analysis were Seattle and Spokane. For a complete listing of the values and results see the associated spreadsheet titled, “WA DNR Inventory Analysis Results”. Assumptions and decisions made for completing the i-Tree analysis can be found in Appendix I.
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Table 9. Location of inventory in relation to the Cascade Range for use in i-Tree Streets
East Cashmere Chelan Fairfield Spokane Toppenish Whitworth University
West Auburn Buckley Capitol Campus Clark College Covington DuPont Kent Kirkland Mukilteo Poulsbo Shoreline Snoqualmie Sumner Tacoma Tukwila Total Tree Count 13,271 33,617 Total Tree Benefits
The benefits of the trees in terms of energy savings, carbon sequestration, air quality, stormwater, and aesthetics/other are summarized in Table 10 below by region and as a whole. More inventory data was collected from the western region and should not be used as a comparison to the eastern region’s results. The combined total benefit of trees within the 21 datasets is nearly $3 million dollars, annually. Table 10. Total annual benefits of public trees by climate zone (region)
Climate Zone Interior West (east of Cascades) North (west of Cascades) Total
Energy ($) 65,243 399,406 $464,649
CO2 ($) 5,715 69,640 $75,354
Air Quality ($) 9,791 59,960 $69,751
Stormwater ($) 26,456 487,992 $514,448
Aesthetic/Other ($) 123,423 1,695,919 $1,819,341
Total ($) 230,628 2,712,916 $2,943,544
The following sections summarize the individual ecosystem services for each region and statewide. Energy Savings Trees modify climate and conserve building energy use in three principal ways:
Shading reduces the amount of heat absorbed and stored by built surfaces. Evapotranspiration converts liquid water to water vapor and thus cools the air by using solar energy that would otherwise result in heating of the air. Windspeed reduction reduces the infiltration of outside air into interior spaces and reduces heat loss, especially where conductivity is relatively high (e.g., glass windows).
As a result of the important role that trees play in reducing energy costs, an analysis of the energy savings per region was conducted. It is estimated that the tree population from the inventories saves an estimated $464,649 annually in local energy costs (Table 11). 19
Table 11. Annual energy benefits of public trees by climate zone (region)
Climate Zone Interior West North Total
Total Electricity (MWh) 562 2,790 3,352
Electricity ($) 44,310 177,418 $221,727
Total Natural Gas (Therms) 19,031 244,857 263,888
Natural Gas ($) 20,934 221,988 $ 242,922
Total ($) 65,243 399,406 $464,649
Avg. $/Tree 13 18
Carbon Sequestration Human activities, primarily fossil-fuel consumption, are adding greenhouse gases to the atmosphere. At the same time higher global temperatures are being reported and expected to have a number of adverse effects. The physiology of trees enables them to be an effective means to sequester carbon. Table 12 below shows that over 11 million pounds of carbon is sequestered annually by trees in the inventoried data, valued at $75,354. These estimates consider the carbon sequestered, carbon emitted during decomposition of organic matter, and the amount emitted during maintenance practices. Table 12. Annual CO2 benefits of public trees by climate zone (region)
Climate Zone
Net Total (lb)
Total ($)
Avg. $/tree
Interior West
1,731,806
5,715
1
North
9,285,271
69,640
3
Total
11,017,077
$75,354
Air Quality Improvements Urban forests provide air quality benefits in six ways:
Absorb gaseous pollutants (e.g., ozone, nitrogen dioxide, and sulfur dioxide through leaf surfaces. Intercept PM10 (e.g., dust, ash, pollen, smoke) and reduce PM2.5 (e.g. fuel combustion exhaust). Release oxygen through photosynthesis. Transpire water and shade surfaces, which lowers air temperatures, thereby reducing ozone levels. Reduce energy use, which reduces emissions of pollutants from power plants. Reduce ozone formation by shading paved surfaces and parked cars.
Table 13 summarizes the annual air quality benefits by region and as a total. Taking into account deposition and avoided air pollutants such as O3, NO2, PM10, and SO2 as well as the biogenic volatile organic compounds (BVOC’s) emitted by trees, the two regions’ trees absorb a net annual total of 41,105 pounds, saving almost $70,000 annually. Table 13. Annual air quality benefits of public trees by climate zone (region)
Climate Zone Total (lb) Total ($) Avg. $/tree Interior West 9,719 9,791 2 North 31,386 59,960 3 Total 41,105 $69,751
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Stormwater Reduction Trees can reduce stormwater runoff in several ways:
Leaves and branch surfaces intercept and store rainfall, thereby reducing runoff volumes and delaying the onset of peak flows. Roots increase the rate at which rainfall infiltrates soil and the capacity of soil to store water, reducing overland flow. Tree canopies reduce soil erosion by diminishing the impact of raindrops on barren surfaces. Transpiration through tree leaves reduces soil moisture, increasing the soil’s capacity to store rainfall.
As seen in Table 14, it is estimated that the trees in both regions intercept an average of over 50 million gallons of precipitation annually. This results in an estimated $514,000 in savings by using regional estimates of the costs of managing stormwater in communities. It should be noted that these values come from using the available science and research of i-Tree. The i-Tree Streets analysis assumes a mostly deciduous forest with the rainy season occurring ‘leafon’ although, western WA has mixed forests and rainy season occurs ‘leaf-off’. These values are only meant to imply the benefits of tree canopy and further analysis is recommended. Table 14. Annual stormwater benefits of public trees by climate zone (region)
Climate Zone Interior West North Total
Total rainfall interception(Gal) 5,291,297 45,184,406 50,475,703
Total ($) 26,456 487,992 $514,448
Avg. $/tree 5 22
Aesthetics/Other Aside from the ecosystem benefits and importance of aesthetics, trees provide a myriad of other benefits that while difficult to assign value, have inherent impacts on the community and its residents. Examples of other benefits include:
Beautification - Trees add color, texture, line, and form to the landscape. In this way, trees soften the hard geometry that dominates built environments. Increased patronage to commercial sectors by providing a pleasant and cooling atmosphere resulting in longer consumer visits. Increased public safety by encouraging interactions among residents and create a more sociable neighborhood. Well-maintained trees increase the “curb appeal” of properties and as a result, increasing property values. Social and psychological benefits of green spaces have been studied which show they improve overall human health. Tree canopy has the ability to buffer and reduce noise pollution. Trees create a habitat and ecosystem for wildlife. Trees create a series of jobs through examples such as tree nursery production, maintenance, and management. Reduced pavement wear by reducing extreme temperatures and changes in temperature. Table 15. Annual aesthetic/other benefits of public trees by climate zone (region)
Climate Zone Interior West North Total
Total ($) Avg. $/tree 123,423 24 1,695,919 77 $1,819,341
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Summary of Ecosystem Benefits for the Three Most Common Species It is also possible using i-Tree Streets to summarize the ecosystem services provided by specific tree species. Table 16 presents the benefits by ecosystem service type for the top three predominant species across the state. Table 16. Ecosystem benefits totals for the three most common species
Species* Energy CO2 Air Quality Stormwater Aesthetic/Other Total ($) Red Maple $88,441 $15,847 $16,041 $93,293 $375,132 $558,754 Ponderosa Pine $3,939 $323 $560 $3,000 $9,075 $16,897 Douglas Fir $7,946 $930 -$663 $17,352 $19,793 $45,359 Top Three Total $100,327 $17,101 $15,938 $113,645 $404,000 $621,011 *Only the top three species were used because there were species from the top ten most common that weren't on the species list from i-Tree (e.g. no "flowering pear" or “plum, cherry or purple leaf�).
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Future Analysis Guidance and Recommendations This analysis of inventories from the Community Tree Inventory Project serves as an initial protocol for identifying trends within varying geographies. As the data was aggregated and synthesized there were issues that arose, hindering analysis or representative results. For future analysis, it is recommended that the following issues or methods presented in this section be addressed. Data Errors or Inconsistencies To aggregate and summarize multiple tree inventories, it is important to ensure data errors or inconsistencies are minimal. For example the field that rates the condition of each tree has a range of values that represent a severe through excellent rating. In a few cases, a range that wasn’t consistent with all inventories was observed. For example, Fairfield and Spokane had a condition rating of “severe” listed as “25”, whereas all other communities used “30”. Other issues that were encountered during analysis were omitted values, resulting in nulls or blanks in the inventory data sheets. A few nulls that were identified occurred in the Task Specific, Growspace, Appraise, and Condition fields. These null values can be found in the following Access queries:
Count_Task_Specific_LimitTree – null values Count_Growspace_All_LimitTree – null values Sum_Appraise_All_Species_LimitTrees – null values Count_Condition_All_LimitTree – does not add up to 46,888 total trees
Ecosystem Services Analysis To complete an ecosystem services analysis several fields are needed which include a species code that matches the species list for the given climate zone used in i-Tree. In some cases an error “Unmatched Species Codes” occurred because the species name was omitted or incorrect. One large limitation of Streets is the number of species for each climate zone that i-Tree was able to model and research. The species lists are very limited; sometimes only containing 25 to 30 different species. Because of this software limitation, many species of trees from each database were added to the lists and assigned a reference species for their ecosystem benefits to be populated. For example, if many different species from the populus genus were present and only one existed in the existing i-Tree species list, the remaining populus species were assigned to either the existing populus species or general "Other" categories like "Broadleaf Deciduous Other". Appraisals Giving an appraisal value is a difficult decision to make because of the varying parameters included in the formula and process. A protocol was issued for calculating an appraisal value when the species is “Unknown”. TreeWorks software cannot calculate the appraised value without a defined species so it defaults to zero. In nearly every one of these cases, the actual species name for these “unknown trees” could be found in the Notes field. The reason is because less common species are not accounted for in TreeWorks, so unknown becomes the default species name and the contractor identifies the tree in the notes field. This resulted in 41 trees not included in the appraisal summaries. Within the scope of the project, the analysts weren’t able to manually enter values and determine appraisal values. After initial review of some of the appraisal value results, unusual findings were identified. For example, there is gray birch (Betula populifolia) valued at approximately $10,000. Understanding the potential growth, uses, and appearance 23
of gray birch, it would seem as though this is an unusually high appraisal value. It is recommended that the tree be identified. Perhaps it holds a record or has historical importance. Other examples include the average appraisal value of California laurel (Laurus nobilus) which was rated at $34,600 and a bigleaf magnolia (Magnolia macrophylla) in the system. After further review, it appears that it was an input error for which Acer macrophylla should have been selected rather than Magnolia macrophylla. It is recommended that the processes for entering species and other data associated with appraisal value should be understood by all project partners. Notes to specific reasons for a high or low appraisal value are also recommended and should be referenced during analyses. Future Analysis and Inventory Suggestions As the CTIP progresses it is recommended to look at additional analyses for a thorough understanding of WA’s urban forest. An analysis of trees on varying land uses should be conducted especially where it compares street versus park trees. Park trees can skew the data due to the availability of large planting spaces and a tree’s ability to grow larger. Other analyses include a closer look at the correlation between a species height in comparison to its DBH and how it ranks among others of the same species. To continue to measure the diversity of new inventories of the project using the 30-20-10 rule, the family name of each species should be included. A measure of Stocking Levels could be conducted by using a community’s street miles in comparison to the available planting sites noted and the current canopy cover. Importance values (IV) are particularly meaningful to managers because they indicate a community’s reliance on the functional capacity of particular species. It takes into account total numbers, canopy cover, and leaf area. Importance Values of 100 implies total reliance on one species and an IV of 0 suggests no reliance. One last recommendation for further studies includes a benefits-costs analysis in which estimates of costs can be obtained from U.S. Forest Service research.
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Conclusion The methods used in this analysis provide guidance for future analyses of Washington’s urban tree population as additional data is obtained. As new inventories are collected through the Community Tree Inventory Project, an assessment of the ecosystem services that trees provide can be enhanced and provide communities and residents within the State of Washington a picture of the impact that trees have on their economic and social wellbeing. This analysis looked at 21 datasets which included a total of 46,888 public trees. Of this total, there exists 330 unique species, with the majority consisting of red maple (Acer rubrum, 8.8%), ponderosa pine (Pinus ponderosa, 7.5%), and Douglas fir (Pseudotsuga menziesii, 6.9%). The age structure of the inventoried tree population is weighted in established trees with 29% of trees measuring 6-12” DBH (diameter at breast height, measured at 4.5 feet above the ground) with an overall average diameter of 10.7”. The majority of the inventoried trees have a condition rated as Good although 56% of all trees require some type of pruning. Of the defects identified, 85% of them were a result of structural issues but four of the ten most common defects are a result of human interaction (e.g. topped, lawnmower damage, etc.). Washington’s street and park tree population is a dynamic resource that requires continued investment to maintain and realize its full benefit potential. Appropriate and timely tree care can substantially increase lifespan. When trees live longer, they provide greater benefits. As individual trees continue to mature and aging trees are replaced, the overall value of the community forest and the amount of benefits provided grow as well. This vital, living resource is, however, vulnerable to a host of stressors and requires ecologically sound and sustainable best management practices to ensure a continued flow of benefits for future generations. The communities in Washington have the benefit of an established tree population in good condition. It was determined through the Council of Tree and Landscape Appraisers formula that the total replacement value of the inventoried trees is an estimated $153,140,020 with an average value of $3,200. Furthermore, these trees provide benefits to the community at an average value of $44.42 per tree in the eastern region and $123.71 per tree in the west, for a total gross value of $2,943,544 per year (i-Tree results). As more community inventories are added to the project, urban forest trends by land use and population ranges can be better understood, and the benefits and value of trees will be better portrayed to the general public and communities. This information supports the efforts and goals of WA DNR’s UCF Program in pursuit of healthy, sustainable urban forests. While the inventory data are not statistically representative of urban forests across Washington, staff at the Washington State Department of Natural Resources’ Urban and Community Forestry program contend Photo courtesy of WA DNR that results outlined in this report support their anecdotal observations of the composition, structure, and needs of urban forests in Washington State. 25
References
1.
2. 3.
4.
5.
Cullen, Scott. Putting a Value on Trees – CTLA Guidance and Methods. Arboricultural Journal 2007, Vol. 30, pp. 21–43. Clark, J.R., Matheny, N.P., Cross, G., Wake, V. A Model for Urban Forest Sustainability. Journal of Arboriculture 23(1): January 1997, pp. 17-30. Ball, J., Mason, S., Kiesz, A., McCormick, D., Brown, C. Assessing the Hazard of Emerald Ash Borer and Other Exotic Stressors to Community Forests Arboriculture & Urban Forestry 2007 Vol. 33(5), pp 350–359. http://www.amerinursery.com/americannurseryman/the-5-percent-rule/ Center for Urban Forest Research; Urban Forest Research, Pacific Southwest Research Station, USDA Forest Service. What Are Your Trees Relative Performance Index? Summer 2004. http://www.itreetools.org/streets/resources/cufr521_UF_Sum04FINAL_RPI_report.pdf USDA Forest Service i-Tree Tools; www.itreetools.org
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Appendix I. i-Tree Analysis Process and Assumptions i-Tree Streets is a program best used at a community level. It takes into account data such as the DBH, species, and condition of a tree and develops localized values for ecosystem benefits summarized by each different tree species. For the purposes of this statewide study, a slightly different approach had to be taken. For this project, i-Tree Streets was used to calculate the benefits for both street and park trees. Because the climates, species, and tree growth rates are so different depending on which side of the Cascade Range the community is located on, Washington DNR's comprehensive tree inventory was split into two separate i-Tree Streets databases; one for East of the Cascades and one for West of the Cascades. Each new database was assigned an i-Tree climate zone based on existing boundaries available in the i-Tree Streets manual and through many other sources. The area east of the Cascades was assigned the climate zone Interior West, and the area west of the Cascades was assigned the climate zone North (see Streets manual for a complete map of climate zones). After the databases were assigned climate zones, they were imported into Streets in order to approximate ecosystem benefits for each species of tree. One large limitation of Streets is the number of species for each climate zone that iTree was able to model and research. The species lists are very limited; sometimes only containing 25 to 30 different species. Because of this software limitation, many species of trees from each database were added to the lists and assigned a reference species for their ecosystem benefits to be populated. For example, if many different species from the populus genus were present and only one existed in the existing i-Tree species list, the remaining populus species were assigned to either the existing populus species or general "Other" categories like "Broadleaf Deciduous Other". After confirming with developers at i-Tree, this course of action was deemed viable and a necessity to complete the analysis. The results from the two i-Tree Streets projects were then summarized in the report and include annual estimated benefits such as carbon sequestration, stormwater mitigation, and air quality benefits. These can be summarized as either units of mass and volume, or as monetary estimates.
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Appendix II: Summaries by Population Range Population size plays an important role in urban forest structure, composition, condition, and management needs. This analysis organized the 21 datasets by the following population ranges; less than 10,000; 10,000 – 100,000; >100,000 and Other Inventories (N/A). These include three inventory datasets from the Capitol Campus, Clark College, and Whitworth.
Structure Trends Common Species by Population Ranges
Top 10 Spp for Population Range >100,000 50% 40% 30% 20% 10% 0%
Top 10 Spp for Population Range 10,000-100,000 25% 20% 15% 10% 5% 0%
Top 10 Spp for Population Range <10,000 25% 20% 15% 10% 5% 0%
Top 10 Spp for Other Inventories 60% 50% 40% 30% 20% 10% 0%
Figure 13. Top ten species for each population range
Table 17. Distribution of DBH classes for each population range
Population Range Large (>100,000) Medium (10,000 - 100,000) Small (<10,000) N/A
0-3 6-12 12-18 18-24 24-30 24-36 >36 23% 24% 18% 5% 8% 2% 20% 21% 32% 13% 3% 4% 2% 25% 25% 31% 12% 3% 4% 1% 24% 13% 22% 16% 7% 17% 3% 22% 28
Management Trends
Population Range Figure 14. Condition of trees in each population range
Table 18. Condition rating of trees in each population range*
Population Range excellent good fair poor severe Large (>100,000) 1% 49% 46% 4% 0% Medium (10,000 - 100,000) 1% 35% 60% 3% 0% Small (<10,000) 0% 70% 27% 2% 0% N/A 5% 32% 62% 2% 0% *Each percent represents the distribution within each population range Table 19. Average appraisal value summarized for population ranges
Population Range Average Appraisal Value N/A $5,037.96 Large (>100,000) $4,072.58 Medium (10,000 - 100,000) $3,046.39 Small (<10,000) $2,351.87
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Appendix III: Microsoft Access Analysis Guidance Inconsistencies and omissions in data resulted in varying methods for measuring the proportion an inventory field encompasses in a certain population. For example in some cases, the summary charts display percentages of the whole population (46,888 trees) and others display percentages of the total assigned a field (e.g. 46,517 of 46,888 trees were assigned a condition rating). It was also discovered after doing some initial analyses that Sites contain records for planting sites and stumps in addition to trees (e.g. n_ values are default values for new tree sites, pc_ and pd_ values are used to define parameters for a new planting site). Therefore, queries had to be rerun to ensure that they filtered data to only include tree sites. Otherwise, when summarizing condition ratings, stumps and planting sites would have been included which have a value of â&#x20AC;&#x153;Oâ&#x20AC;?. These instances indicate the need for thorough data checks and measure of an inventory managers understanding of fields and values to record. Also, it should be understood what value to enter or process should be conducted if a value is unknown (e.g. species, pest/disease, etc.). This became an issue where the species code was unavailable or unknown. Most species were identified but had to be recorded in the Notes section. These processes should also be described to the data analysts.
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