Abstract
Trees provide an array of social, economic, and ecological benefits; furthermore, trees on public land are critical for providing those benefits to people who cannot afford their own trees. It is important to know how managers make trade-offs and prioritize different tree selection criteria in order to target educational campaigns at the state or regional level. Primary contacts for Tree City USA designated cities were surveyed across the Pacific Northwest. Of these municipalities, 79 out of 151 responded (52.3% response rate), with 6 municipalities providing responses from different departments for a total of 85 responses. Currently, there are primarily descriptive statistics in relation to tree species selection. This study provides a framework for future statistical analysis and greater exploration of how municipalities and managers are selecting tree species. Results were analyzed with a Mann-Whitney U test to compare International Society of Arboriculture (ISA) Certified Arborists® to those who are not certified across various tree species selection criteria. Another Mann-Whitney U test was used to compare small (≤ 50,000) and large (> 50,000) municipalities across the same criteria. ISA Certified Arborists® showed statistically significant differences from those who are not certified in a number of tree species selection criteria. ISA Certified Arborists® also differed in urban forest management on a city-wide scale, particularly in favoring greater tree species diversity. The differences in urban forest management between ISA Certified Arborists® and noncertified—and between municipality sizes—can help to influence future educational campaigns targeted toward increasing urban forest health and resiliency.
INTRODUCTION
Urban Forestry Background
Urban land is expected to more than double in the United States from 2000 to 2050, drastically reducing natural areas, native forests, and the ecosystem services they provide. As this trend continues, urban forests will be more important for providing critical services to urban residents (Nowak and Walton 2005). Municipalities receive a wide array of benefits from trees, equally as diverse are the reasons why managers select species. Selecting a tree suitable for the planting site increases the survival rate, which in turn increases the social, economic, and ecological value of the tree (Roy et al. 2012). With proper management, urban forests can provide a wide variety of benefits, including: reducing air pollution, reducing healthcare costs, and improving recreation (Dwyer et al. 1992). Urban forests provide a range of social benefits (e.g., sense of community, stress relief, and privacy) that contribute to the health of residents (Matsuoka and Kaplan 2008). Additionally, retrofitting urban areas to include more green space and trees creates a more livable city (Hagerman 2007).
The unique climates of urban areas are shaped by the buildings and green spaces within. Green spaces, including trees, play a critical role in moderating urban climates. With strategic planning and planting, green spaces can be used to mitigate the negative effects of urban heat and pollution (Sieghardt et al. 2005). Trees that are selected for the right site will grow quicker, live longer, and in turn sequester more carbon dioxide (McPherson and Simpson 1999). Planting trees can provide substantial energy savings, which typically start small and can nearly double after ten years. Energy savings from trees can be maximized by proper species selection, planting location, and orientation (Sawka et al. 2013). Maximizing benefits often starts with tree species selection; however, there are a variety of other factors that are crucial to a tree’s success (e.g., maintenance, citizen engagement, and proper planting techniques).
The cost of maintaining trees in urban areas can be reduced by selecting an appropriate tree species for the planting location. If the tree is too large for the site, it may limit the mature size and increase maintenance costs. It is important to invest in proper installation and maintenance in order to increase survival rates of newly planted trees (Koeser et al. 2014). Often trees planted in the public sector outgrow their initial space and conflict with utilities or other types of built infrastructure. Additionally, the perceptions of citizens can play an important role in the success of an urban forestry program. Trees with a large amount of leaf litter or allergens are often considered undesirable. Careful planning and the regular maintenance of the urban forest can alleviate many of these conflicts (Escobedo et al. 2011).
Arboriculture Certification
There is a long history of planting trees in urban areas and, with an increase in urbanization and a greater understanding of tree biology, there has been a growing need for tree management. Arboriculture is a major component of urban forestry, but is often unregulated. The International Society of Arboriculture (ISA) is one of the more prominent organizations and helps to set industry standards; these standards are optional and are driven by participation from consumers and arborists. There are comparable certifications in other states, such as the Massachusetts Certified Arborist (Rines et al. 2011). Having a professional certification from the ISA helps to differentiate arborists who have met basic competencies and are engaging in continuing education from those who are not. Elmendorf et al. (2005) showed that among the arboriculture community there was strong support for ISA certification, and it raised industry standards overall. Additionally, tree selection was ranked highly among education topics for arboriculture programs (Elmendorf et al. 2005). The Arbor Day Foundation is another organization that helps set guidelines for urban forestry and is responsible for the Tree City USA designation. These cities are required to spend $2 per capita on urban forestry, celebrate Arbor Day, maintain a tree board, and have a tree ordinance (Arbor Day Foundation 2017).
Statement of the Problem
There is an array of benefits for carefully weighing different criteria for tree species selection. Many of these are obvious, such as longevity and increased biodiversity; however, some managers may be basing their selections on outdated criteria or lack the information to make the best selection. There are new tools (e.g., electronic inventories) that can help provide managers with more information. Careful consideration of management activities and ecological diversity could become increasingly important as a greater percentage of the landscape becomes urbanized (Vogt et al. 2017). Both the benefits and threats to urban forests have been well-documented; however, it is important to determine how the public funds are being used for tree species selection and in turn how urban forest management is being conducted. Recognizing the importance of proper tree selection, this study sought to examine the criteria urban forest managers in the Pacific Northwest (PNW) of the United States were using.
It was important that all respondents used a consistent definition of what a tree is to avoid spurious data. Roy et al. (2012) conducted a literature review on urban trees and their benefits; this study similarly defines an urban tree as a woody perennial with lateral branches that originate some distance from the ground and reach a mature height of 12 feet (3.6 m) or greater. Additionally, both single and multi-stemmed trees were included. Only trees that originated within the boundaries of a municipality and were planted on public land were included—growing both in clusters and singularly. Furthermore, seedlings were excluded and only trees that were a height of 4 feet (1.2 m) upon planting were counted—this included bare root, container, and balled and burlap trees. The broad definition, and inclusion of multistem trees, was designed to assess the full potential for diversity of the urban forest. Urban forest describes all the plants and green spaces within the boundaries of the municipality. Urban trees are a highly visible component of the urban forest, and they contribute to the social, economic, and ecological health of the municipality.
Survey Context
Exploring how managers make decisions on tree species selection could improve education and potentially lead to an increase in benefits provided by those trees. Professional organizations such as the ISA could help to bridge this gap and increase awareness of tree species selection issues. Different tree species provide different benefits and are adapted for different sites; understanding how managers—those responsible for tree species selection—make these selections can inform regional education to improve tree selection by managers. Selecting species in urban areas that are well-suited to the existing conditions on the site will help mitigate loss of tree species to a changing climate. Forest ecosystems that are more diverse and are managed sustainably will be more resilient to climate change (Spittlehouse and Stewart 2004).
There has been a limited body of research examining how professional certification influences urban forest management. There have been a number of studies that have used empirical evidence to justify ISA certification (Carlson 1995; Green 2002; Elmendorf et al. 2005); however, little research investigates how this training is operationalized in arboriculture practices. Other studies have examined how different professional organizations play a role in the urban forest, such as the Society of Municipal Arborists (e.g., Young 2010). Ries (2017) found that participation in the Municipal Forestry Institute (MFI) leadership training increased the likelihood of various levels of leadership outcomes. While there are an increasing number of certifications, there has not been any explicit exploration of how ISA Certified Arborists® differ from those that are not. The use of quantitative statistical analysis can help to elucidate trends and explore relationships between variables that could lead to more informed tree species selection.
Tree species selection has been examined through a variety of lenses: climate adaptability (Roloff et al. 2009), residential attitudes (Kirkpatrick et al. 2012), and those who plant on public land (Conway and Vander Vecht 2015). This study primarily builds on a study conducted in Toronto, Canada, by Conway and Vander Vecht (2015) exploring tree species selection on public land. Conway and Vander Vecht (2015) found the factors most influential in tree species selection for urban foresters were increasing canopy cover, native species, and increasing local and overall tree species diversity. They collected data on four groups: garden centers, the Toronto Urban Forestry Department, nonprofit organizations, and nurseries. They found some differences in selection criteria between organizations, but there was also a lot of overlap in species planted. The Toronto Urban Forestry Department had a general policy to diversify their canopy, plant native species, and increase canopy cover. By conducting a partial replication of the methods in Conway and Vander Vecht (2015) over a broader geographical scale, the goal of this study was to explore how municipalities may differ in tree species selection, rather than how stakeholders differ within a municipality. This survey was designed to explore how managers are using existing research, as well as how they are using social, ecological, and economic criteria, to influence tree species selection. As in Conway and Vander Vecht (2015), we surveyed managers regarding their tree species selection criteria. However, the focus of this study was on how these selection criteria differed based on ISA certification and municipality size.
METHODS
Study Area
The study population consisted of representatives from Tree City USA designated cities across the PNW states of Oregon and Washington. There are 61 Tree Cities in Oregon, where roughly half the population of the state resides within a Tree City. The largest Tree City in the state is Portland, with a population around 609,000, and the smallest is Rivergrove, with a population around 480 (Arbor Day Foundation 2016a). There are 90 Tree Cities in Washington, where about 47% of the population lives in a Tree City. The largest Tree City is Seattle, with a population around 653,000, and the smallest is Farmington, with a population around 140 (Arbor Day Foundation 2016b). Targeting Tree Cities helped to eliminate those cities that are not actively managing their urban forests.
Regional Context
The climate of the PNW is influenced by the Pacific Ocean, the Cascades, and the Olympic Mountains. These mountain ranges cast a rain shadow over the eastern high desert portions of Washington and Oregon. The substantial decline in precipitation is a dominant factor in influencing the composition of flora (Albright and Peterson 2013). The more moderate climate allows for a greater diversity of tree species in the western portions of the states (McPherson et al. 2002). The diverse climate led us to believe there would be a diverse selection of tree species. Additionally, this is an area where ISA certification is the primary professional certification for arborists.
Study Design
The sample frame was developed in conjunction with staff from the Oregon Department of Forestry and the Washington Department of Natural Resources. Primary contacts for each Tree City were provided, which consisted of urban foresters, park staff, and others (collectively referred to as “managers”). The initial 22-question survey instrument was developed using Qualtrics (Qualtrics 2017) and was approved by a University Institutional Review Board (IRB). Primarily closed-ended quantitative questions were used to examine tree species selection criteria. The survey was targeted toward managers who plant primarily on public lands.
An initial pilot survey was sent to Tree Cities in Idaho to validate the survey questions. The pilot survey included an extra question to test for clarity and ease (Vaske 2008). Responses from Idaho deemed the survey comprehensive and no further iterations were needed; therefore, the same survey instrument was sent to the primary contact for Tree Cities in the PNW. Out of 151 potential municipalities, 79 responded (52.3% response rate), with 6 municipalities providing responses from different departments, for a total of 85 responses. Sample size varied between questions, ranging from 60 to 85. Participants were asked to base their responses on tree species choices on public land and not on private property. Using methods outlined by Millar and Dillman (2011), an attempt was made to increase response rate. An initial contact was made via email, requesting participation in the survey. The relevant risks were explained, and the link to the Qualtrics survey was sent out. A reminder email with the link to the survey was sent out two weeks later, followed by a third email a week later.
This study incorporated similar criteria from Conway and Vander Vecht (2015) to quantify the criteria for tree species selection in PNW municipalities. This study differed from Conway and Vander Vecht (2015) due to the exclusive use of an online questionnaire and restricting the survey to public managers. This study incorporated additional statistical analysis beyond the descriptive statistics used in Conway and Vander Vecht (2015).
Two matrices were the primary focus of analysis. The first was constructed to measure tree species selection on a site-by-site basis. Respondents were asked to rank 16 tree species selection criteria: “aesthetics,” “mature size,” “existing tree diversity,” “planting budget,” “availability,” “genetic diversity,” “maintenance costs,” “citizen preference,” “resistance to pests and disease,” “native species,” “soil type,” “root space,” “tree hardiness,” “water requirements,” “hours of sun,” and “proximity to infrastructure.” There were 81 responses recorded, and each criterion was ranked on a 5-point scale of 1, “not at all important,” to 5, “very important.” The second matrix was designed to measure tree species selection on a city scale; 78 responses were recorded. The following statements were used: “I strive to plant no more than 10% of a species, 20% of a genus, or 30% of a family,” “increasing canopy cover in the city I live in is important,” “my city’s street tree list strongly influences what I plant,” “my city generally plants the same 3 to 5 tree species year to year,” “the tree species my city plants have changed a lot over the course of my career,” “community engagement is a critical component of my department’s success,” and “I use the tree inventory to influence the trees I select.” A 7-point scale from 1, “strongly disagree,” to 7, “strongly agree”—with the inclusion of an eighth option, “no opinion”—was used to determine managers’ opinions on broader species selection. The category “no opinion” was coded as missing data because it is not on the continuous scale and would skew data. The “no opinion” response was included to reduce the number of respondents who selected a middle response when they truly did not have an opinion on the statement. Respondents were asked to categorize the status of their tree inventory. They were provided with four options: “no inventory,” “in progress,” “yes, not regularly,” and “yes, regularly.”
Analysis
All results were analyzed using IBM’s Statistical Package for the Social Sciences (SPSS)(IBM corp. 2016). Significance levels of ∝ = 0.05 were used for all statistical analysis. Due to small sample sizes and a lack of normality, primarily nonparametric statistics were used. This helps to reduce the influence of outliers, but potentially reduces the power of the statistics, and leads to a greater likelihood of type two errors. However, there were still statistically significant differences, providing a conservative approach to data analysis (Field 2013). Butler and Koontz (2005) used similar statistical analysis for a United States Department of Agriculture Forest Service survey. Effect size (r) was estimated by dividing z-score by the square root of the number of respondents in each question (Rosenthal 1991). A nonresponse bias check was conducted comparing respondents from the three different waves, which compares managers who responded earlier to those who responded later (Armstrong and Overton 1977; Atif and Bilgin 2012). There was a statistically significant difference (Kruskal-Wallis-H = −18.86, P < 0.01) only between the first and last wave based on planting budget. Third wave respondents were more likely to rate planting budget as important (median = 5, “very important”) than first wave respondents (median = 4, “important”). This is not surprising considering there was a greater number of small municipalities responding later in the survey. No nonresponse bias was conducted on nonrespondents due to a lack of phone contacts.
This study examines how tree species selection criteria (Table 1) and selection criteria on a city scale (Table 2) differ across ISA Certified Arborists® and non-ISA Certified Arborists® using the Mann-Whitney U test. This analysis meets all of the following assumptions for the Mann-Whitney U test. The dependent variables of tree species selection criteria were on a continuous scale, while the independent variable of ISA certification was measured in two mutually exclusive groups. Both groups had a similar, but skewed shape (Field 2013).
Tree species selection criteria by ISA Certified Arborist® (No = not a certified arborist; Yes = is a certified arborist).
Tree species selection on a city scale grouped by ISA Certified Arborist® (No = not a certified arborist; Yes = is a certified arborist).
Municipalities with larger populations typically have better developed and funded urban forestry programs. Larger municipalities, greater than 50,000, were more likely to have a tree inventory than those under 50,000 (Kenney and Idziak 2000). Based on the results in Kenney and Idziak (2000), population size of the municipalities was split into two categories, those above 50,000 and those below 50,000. There were 67 municipalities with populations of 50,000 and under (small), and 14 over 50,001 (large). An X2 analysis was conducted to test the association between municipality size and the status of tree inventory. Additionally, a Mann-Whitney U test was conducted to test how municipality size impacted tree species selection criteria, and another was conducted to test municipality size vs. selection on a city scale.
RESULTS
Respondents were asked to indicate their highest level of education; overall, they were well-educated: 39.7% (n = 29) had a bachelor’s degree and 32.9% (n = 24) had a master’s degree or above. Males accounted for 58.9% (n = 43) of the responses and females accounted for 39.7% of responses (n = 29). This is generally consistent with research conducted by Kuhns et al. (2002), which found that the majority of professionals in urban forestry are male. There were a variety of professions that responded: urban forester, arborist, landscape architect, urban planner, park manager, public works specialist, and other. Backgrounds of respondents were similarly varied, and the two most frequent backgrounds were urban forestry and city planner; 23% of respondents indicated “other” as their professional or educational background (Table 3).
Respondents were asked, “Which of the following best describes your professional and/or educational background?”
ISA Certified Arborists®
ISA Certified Arborists® had statistically significant differences from those who were not certified on three tree species selection criteria: mature size, existing diversity, and native species. ISA Certified Arborists® were more likely to consider mature size an important criterion (median = 5, “very important”; mean = 4.8) than those who are not certified (median = 5, “very important”; mean = 4.4). These two groups showed statistically significant differences (U = 433; Z = −2.54; P = 0.01) when considering the mature size of tree species (Table 1). Effect size (r = −0.3) was medium (Cohen 1988; Rosenthal 1991).
ISA Certified Arborists® were more likely to consider existing diversity an important criterion (median = 4, “important”; mean = 4.2) than those who are not certified (median = 4, “important”; mean = 3.5). These two groups showed statistically significant differences (U = 400; Z = −2.70; P < 0.01) when considering the existing diversity of tree species (Table 1). Effect size (r = 0.31) was between medium and large (Cohen 1988; Rosenthal 1991). ISA Certified Arborists® were less likely (median = 3, “neither important nor unimportant”) to plant native species than those who are not certified (median = 4, “important”). These two groups showed statistically significantly difference (U = 837; Z = 2.49; P = 0.01), meaning those who are certified are less likely to consider this an important criterion in tree species selection (Table 1). Effect size (r = 0.29) was between small and medium (Cohen 1988; Rosenthal 1991).
Respondents were asked various statements pertaining to tree species selection across the entire municipality. These were grouped by those who are ISA Certified Arborists® and those who are not. ISA Certified Arborists® were generally more likely to agree with all the statements except, “my city generally plants the same 3 to 5 tree species year to year” (Table 2). ISA Certified Arborists® were more likely to agree (median = 6, “agree”) with the statement “I strive to plant no more than 10% of a species, 20% of a genus, or 30% of a family” than noncertified arborists (median = 4, “neither disagree nor agree”). These differ significantly (U = 197; Z = −4.03; P <0.01), meaning that ISA Certified Arborists® were more likely to follow Santamour’s rule of thumb to increase diversity (Table 2). Effect size (r = −0.51) was large (Cohen 1988; Rosenthal 1991).
ISA Certified Arborists® were more likely to disagree (median = 2, “disagree”) that their city plants the same 3 to 5 tree species every year than those without the certification (median = 4, “neither disagree nor agree”). This difference (U = 781; Z = 2.58; P = 0.01) indicates that ISA Certified Arborists® are more likely to change the tree species they plant from year to year (Table 2). Effect size (r = 0.31) was between medium and large (Cohen 1988; Rosenthal 1991).
Municipality Size
Large and small municipalities showed statistically significant differences in the status of their inventories, X2 (df = 3; n = 74) = 13.65, P < 0.01. Of the small municipalities, 32% did not have a tree inventory, while all the large municipalities had some level of inventory. Fifty percent of large municipalities had an inventory and update it regularly, whereas only twenty-four percent of small municipalities reported as such (Table 4). Effect size was 0.385, which was between medium (0.3) to large (0.5)(Cohen 1988). This analysis was conducted using Cramer’s V, using likelihood ratio which is appropriate for comparing a dichotomous and categorical variable (Vaske 2008). Differences in status of tree inventories based on municipality size were consistent with past research (Kenney and Idziak 2000).
Relationship of city size to status of tree inventory.
Managers from large municipalities were more likely (U = 574; Z = 2.74; P < 0.01) to consider existing diversity important (median = 5, “very important”) than small municipalities (median = 4, “important”). Effect size (r = 0.31) was between medium and large (Cohen 1988; Rosenthal 1991). Managers from large municipalities were more likely (U = 570; Z = 2.67; P < 0.01) to consider availability of plant material important (median = 5, “very important”) compared to small municipalities (median = 4, “important”) (Table 5). Effect size (r = 0.30) was medium (Cohen 1988; Rosenthal 1991). There were no other differences across other variables.
Tree species selection criteria by municipality size.
Large municipalities were generally more likely to agree with all of these statements except for “my city plants the same 3 to 5 tree species year to year.” Statistically significant differences occurred between small and large municipalities for four statements (Table 6). Respondents from large municipalities were more likely to agree (median = 6, “agree”) with the 10-20-30 rule than small municipalities (median = 4, “neither disagree nor agree”); this was statistically significant (U = 469; Z = 4.09; P < 0.01). Effect size (r = 0.53) was large (Cohen 1988; Rosenthal 1991). Larger municipalities were more likely to agree (median = 7, “strongly agree”) that increasing canopy cover was important than smaller municipalities (median = 6, “agree”); this was statistically significant (U = 478; Z = 2.44; P = 0.02). Effect size (r = 0.29) was between small and medium (Cohen 1988; Rosenthal 1991)(Table 6). Respondents from large municipalities are more likely to agree (U = 494; Z = 2.92; P < 0.01) that tree species have changed over the course of their career (median = 6, “agree”) than small municipalities (median = 4.5, “somewhat agree”). Effect size (r = 0.35) was between medium and large (Cohen 1988; Rosenthal 1991)(Table 6).
Tree species selection on a city scale grouped by municipality size.
Respondents from large municipalities are more likely to agree that tree inventories influenced their tree species selection (median = 6, “agree”) than small municipalities (median = 4.5, “somewhat agree”); this was statistically significant (U = 421; Z = 2.20; P = 0.03). Effect size (r = 0.27) was between small and medium (Cohen 1988; Rosenthal 1991)(Table 6).
DISCUSSION
Overall, this study found that municipalities of different sizes and ISA certification are factors influencing how managers are making tree species selection decisions. These variables are critical in terms of how these managers are prioritizing various criteria, particularly in regard to increasing tree species diversity. Exploring the differences in tree species selection and tree inventory based on municipality size can help tailor state funding and assistance to municipalities of different sizes (Kenney and Idziak 2000). There has been an increase in the use of tree inventories across municipalities. Tree inventories can help managers make decisions based on existing tree species diversity, prioritize maintenance, and calculate ecosystem services, and they are largely considered important components of successful urban forestry programs (Clark et al. 1997; Kenney et al. 2011). Unfortunately, tree inventories can be expensive and labor intensive. This study can help demonstrate how municipalities of different sizes use tree inventories and how that influences tree species selection.
ISA Certified Arborists®
There are some studies exploring professional development in arboriculture (Carlson 1995; Green 2002; Young 2010; Ries 2017), but there is a lack of research exploring how ISA Certified Arborists® differ in urban forest management from those who are not certified. ISA Certified Arborists® are prioritizing different criteria when selecting tree species than those managers who are not certified. ISA Certified Arborists® are more likely to consider “mature size of the tree” and “existing tree species diversity” important criteria. The mature size of the tree accounts for a variety of benefits and costs. Larger trees typically provide more benefits overall, but they also incur more costs in infrastructure damage if poorly sited. It is important to carefully consider the limitations of the site and select an appropriately sized tree (Mullaney et al. 2015). Increasing existing diversity can help to reduce potential pest outbreaks and promote overall urban forest health (Santamour 1990). However, ISA Certified Arborists® are less likely to consider “native species” as an important criterion. As more land is converted into urban use, planting native species is thought to help preserve biodiversity (McKinney 2002). While there are advantages of native tree species in many contexts (e.g., well-suited for the climate), there is also research suggesting that native species are not always the best choice in urban areas. The unique challenges posed by urban areas preclude the use of native trees in all situations. Generally, the literature suggests that noninvasive nonnative species are good additions to the urban forest. Managers should focus on function and diversity as opposed to exclusively native species (Chalker-Scott 2015). ISA Certified Arborists® are presumably making an educated decision by weighing native species less heavily than those that are not certified.
In addition to ranking existing diversity as more important, ISA Certified Arborists® are more likely to consider the 10-20-30 rule of thumb (Santamour 1990) more important than those who are not certified. Over the course of their careers, ISA Certified Arborists® are also more likely to have changed the tree species they plant. This fits with a general trend toward increasing tree species diversity and it could be a result of continuing education. There has been a trend in literature to increase diversity in urban forests to minimize loss from pests (Santamour 1990; Alvey 2006; Chalker-Scott 2015). Certification seems to be increasing awareness of the importance of increasing tree species diversity, which could lead to an increase in policies to promote biodiversity. A major limitation in this study is that there was no exploration of how managers may be considering diversity on a neighborhood scale. Increasing tree species diversity on a municipality scale or landscape scale is good; however, urban forests are not distributed equitably in terms of socioeconomic criteria (Heynen et al. 2006). After dieback or removal of trees, neighborhoods with renters are less likely to be reforested, further contributing to the inequitable distribution of benefits provided by the urban forest (Perkins et al. 2004).
Surprisingly, certification did not have a statistically significant impact on the prioritization of genetic diversity. This could have been caused by a lack of understanding of the question, or perhaps the question did not fully measure how managers view intraspecific diversity. Regardless, there likely needs to be an increase in focusing on diversity within species and a shift away from exclusive reliance on single cultivars. It is advantageous to plant cultivars when one needs a specific form and function, but in other situations, preference should be given to increasing intraspecific genetic diversity (Santamour 2004). It appears that ISA Certified Arborists® are more likely to follow emergent trends in urban forestry literature and education. This could provide some justification to hiring a certified arborist in management positions. In addition, these results may suggest that this is a successful means of conveying new knowledge; however, it is critical to ensure that those who are providing education are providing the correct information.
Municipality Size
Municipality size influences the status of tree inventory, tree species selection criteria, and urban forest management. All the large municipalities have a tree inventory in some level of completeness. This could be because it is difficult to manage an urban forest in a larger municipality without an inventory. Furthermore, status of inventory is used as an important monitoring criterion in urban forestry (Clark et al. 1997; Kenney et al. 2011). This is important when looking at how tree inventory impacts tree species selection; larger municipalities are statistically significantly more likely to use a tree inventory to influence tree species selection. This difference could occur based on differences in funding, or perhaps these smaller municipalities do not require a tree inventory. Smaller municipalities may need assistance in developing other components of their urban forest programs such as tree species selection, funding, or tree species diversification. It is also possible that smaller municipalities could benefit from tree inventories in other ways (e.g., raising public awareness). There could be further research conducted to see how tree inventories are being used and how this is operationalized in urban forest management.
Larger municipalities are more likely to consider “existing diversity” and “availability” important criteria in tree species selection. If large municipalities aim to increase tree species diversity, availability may be more important. It can be difficult for nurseries to anticipate the needs of urban foresters, which may be perpetuated by a reduction in communication between stakeholders (Sydnor et al. 2010). Additional differences occur when comparing municipality size to tree selection on a city scale. Larger municipalities are more likely to utilize the 10-20-30 rule of thumb (Santamour 1990), prioritize increasing canopy cover, change the tree species they are planting, and utilize the tree inventory to make tree species selection. This is consistent with prior research that larger cities have more active management (Kenney and Idziak 2000). Further statistical analysis should be conducted to explore how municipalities of different sizes prioritize tree species selection and urban forest management.
Tree planting campaigns have been popular in many municipalities; however, urban forests are complicated and require a unique combination of social and ecological management. Thoughtlessly increasing canopy cover can lead to a lower return on ecosystem services and an inequitable distribution of those services. In addition to considering ecological criteria such as water use, it is important to engage the general public in the decision-making process (Pincetl et al. 2013). Understanding how managers are making these decisions and the factors that influence them are important in furthering the effective management of the urban forest. There have been a number of documents providing recommendations regarding maximizing ecosystem services (Escobedo et al. 2011), community tree planting guidelines (McPherson et al. 2002), site limitations (Sieghardt et al. 2005), and inequitable distribution of the urban forest (Heynen et al. 2006). This study explored how those are operationalized in the PNW and differ based on municipality size and certification. As new literature emerges in relation to how tree species perform in urban areas, there should be corresponding research to explore how this is operationalized.
Educational Campaign
These results indicate that ISA Certified Arborists® are using more current criteria for tree species selection. While this is encouraging, there are still many stakeholders that are not ISA Certified Arborists®. Additional education could be provided to help municipal arborists effectively disseminate the importance of tree species selection to a broader audience. Providing additional training on communication and outreach for these arborists could help make these forests more resilient and increase benefits for the entire community. It is important to train and involve more arborists in tree species selection. Future research could focus on how to improve partnerships between arborists, landscape architects, and nurseries.
Education for municipal arborists should vary based on the municipality size and objectives of the program. While there is some overlap in management objectives between smaller municipalities and larger municipalities, our results potentially indicate that education and resources should be allocated differently in smaller vs. larger municipalities. Smaller municipalities could try to maximize benefits provided by the trees they are planting and minimize those species that are susceptible to pests. Smaller municipalities should consider increasing diversity even if they are only planting a small number of trees each year. Larger municipalities likely have a greater need for active management due to the amount of land that is being converted into urban use. Whenever possible, smaller municipalities should partner with a larger municipality nearby. Resources and information regarding tree species selection could be shared to minimize input and maximize results for smaller municipalities.
Limitations
This study was limited by the small sample size. Nonparametric statistics were used in an effort to account for this and provide conservative results. Future studies should aim for a larger sample size and an expanded geographic area. There are a lot of common principles in urban forestry that are applied across a broad range of geographic, social, ecological, and economic areas without sufficient research to determine if these hold true across such broad categories. It would likely be beneficial to incorporate interviews or a qualitative component to add additional context to the difficulties of simultaneously balancing an array of tree species selection criteria. Specifically, it would be helpful to know how managers view the impact of ISA certification and municipal resources in how they select tree species. A question regarding professional background was included in the survey instrument; however, backgrounds of respondents were diverse and left too few people in each category to use this as a variable in statistical analysis.
CONCLUSION
Despite these limitations, this study provides a framework for future statistical analysis and greater exploration of how municipalities and managers are selecting tree species. ISA certification and municipality size impact individual tree species selection and influence levels of urban forest management. Exploring how urban forest management is operationalized by ISA Certified Arborists® vs. non-certified arborists—and different municipality sizes—can help to focus state or regional assistance to different professionals and municipalities.
Footnotes
Conflicts of Interest:
The authors reported no conflicts of interest.
- © 2020, International Society of Arboriculture. All rights reserved.
LITERATURE CITED
Jump to section
Related Articles
Cited By...
- No citing articles found.