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Assessing street tree diversity in four Ohio communities using the weighted Simpson index
Landscape and Urban Planning 106 (2012) 44–50
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Assessing street tree diversity in four Ohio communities using the weighted Simpson index S. Subburayalu ∗ , T.D. Sydnor 1 School of Environment and Natural Resources, The Ohio State University, 2021 Coffey Road, Columbus, OH 43210-1085, United States
a r t i c l e
i n f o
Article history: Received 15 April 2011 Received in revised form 25 January 2012 Accepted 5 February 2012 Available online 3 March 2012 Keywords: Environmental benefit Pest resistance Diversity Urban forestry Species adaptability Simpson index
a b s t r a c t Diversity is a key factor which should be considered during planning street replacements and involves assessment of existing street population for various factors especially resistance of trees to pests. Street tree replacement histories in the United States reveal extensive replacement of American elms killed by Dutch elm disease (DED) with maples, or ashes. In turn, these trees are now under attack by Asian longhorn beetle (ALB) and emerald ash borer (EAB). Two commonly used evaluation measure of diversity namely the 10:20:30 rule and the Simpson index takes into account only the number and evenness of the taxa in a street tree population. However these indexes fail to consider the differential functional values of trees which are critical in building a sustainable urban forest while attempting to increase diversity. The proposed weighted Simpson index which weights the absolute abundance of tree taxa based on some functional variable could be used as a guide to identify areas of focus for a community. A case study involving four communities in Ohio namely Toledo, Westerville, Dublin and Yellow Springs was conducted to examine the usefulness of the weighted Simpson Index. Three functional variables including environmental benefits, pest vulnerability and taxon adaptability at three different taxonomic levels, family, genus and species, were used to arrive at the weighted Simpson indexes for the four communities. Results revealed the usefulness of weighted Simpson Index in exposing areas of concern for each community. Generally all the four communities were lacking larger statured trees with greater environmental benefits. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The need for diversity in street trees was reinforced by the recent invasion of the emerald ash borer (EAB), Agrilus planipennis Fairmaire, in the urban forests of Michigan and subsequently in more than eight other states in the United States and two provinces in Canada. Researchers in urban forestry have recommended that diversification of community street tree populations be considered before planting and replacement (e.g., Poland & McCullough, 2006). Biologically diverse tree plantings and replacements must be incorporated into community forests to reduce potential damage from pests. For example, after the loss of American elm, Ulmus americana, which was caused by Dutch elm disease (DED), Ophiostoma spp. Buism., many elms were replaced by either maple or ash trees, which are now under attack by the Asian longhorn beetle (ALB), Anoplophora glabripennis Motschulsky, or EAB, respectively. To further minimize risk of pests, researchers have also recommended
∗ Corresponding author. Tel.: +1 614 247 8035; fax: +1 614 292 7432. E-mail addresses: [email protected] (S. Subburayalu), [email protected] (T.D. Sydnor). 1 Tel.: +1 614 292 3865. 0169-2046/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.landurbplan.2012.02.004
diversifying community forests at higher taxonomic levels, as pests ´ & McBride, generally operate at the genus and family levels (Lacan 2008; Nowak, 2001; Raupp, Cumming & Raupp, 2006; Seigert & McCoullough, 2006). Achieving diversity in street tree plantings must involve an initial evaluation of diversity in the existing populations. A number of methods to evaluate diversity have been applied in urban forestry ´ & McBride, 2008; McPherson & Rowntree, (Galvin, 1999; Lacan 1989; Raupp et al., 2006; Sanders, 1981; Santamour, 1990; Sun, 1992). Two commonly used methods for assessing street tree diversity are (1) the target-based 10:20:30 heuristic guideline (Santamour, 1990) and (2) non-target-based mathematically computed indexes, such as the Simpson and Shannon-Weiner indexes (Sanders, 1981; Sun, 1992). According to the 10:20:30 guideline, the street tree population of a community should consist of no more than 10% of a single tree species, 20% of a single genus and 30% of a single family. This guideline was proposed to protect urban forests from serious pest outbreaks. Raupp et al. (2006) noted an important limitation of this target-based guideline, i.e., the guideline does not consider the fact that most pests attack more than one tree species, genus or family. These authors therefore recommended diversification at the genus, family and perhaps ordinal levels. Similarly, Richards (1983) argued
S. Subburayalu, T.D. Sydnor / Landscape and Urban Planning 106 (2012) 44–50
that this guideline lacks a scientific basis and that following the guideline without considering the adaptability of various taxa can result in a less stable population. A similar limitation of non-target-based indexes, such as the Shannon-Weiner index (Shannon, 1948), Simpson index (Simpson, 1949) or the inverse of Simpson index (Sun, 1992), is that they are based entirely on the number and relative abundance of all the taxa being evaluated (species, genus or family). Although the use of these indexes has a scientific basis, their use as an evaluation tool in the plant selection process is limited to the number and evenness of the taxonomic unit being evaluated. Richards (1983), in outlining reasonable guidelines for street tree diversity, correctly noted that such diversity should relate to the set of conditions and objectives for a given community. Urban foresters and municipal arborists should consider several factors or limitations that are specific to a location and/or tree species when planning plantings or replacements. These factors include, but are not limited to, pest vulnerability, environmental benefits, and adaptability.
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of tree species in stressful urban sites are also key factors that must be considered to ensure the long-term survivorship and health of trees. 1.4. Weighted Simpson index
The losses caused by DED and EAB demonstrate the importance of diversity in street tree populations. An early study showed that the landscape and nursery industry in Michigan suffered about $11 million in damages from EAB, primarily to Fraxinus pennsylvanica Marsh (green ash) and Fraxinus americana (white ash) (Herms, McCullough, & Smitley, 2004), while losses in urban areas were much higher. A more recent study indicated that the losses caused by EAB in Ohio were between $1.8 and 7.5 billion (Sydnor, Bumgardner, & Todd, 2007). Understanding the current and probable future risks from pests is crucial in making planting and ´ and McBride (2008) outlined a matrix replacement decisions. Lacan for evaluating the effects of insects and diseases on an urban forest at a citywide scale that would help in developing lists of tree species for planting.
As discussed above, a variety of factors must be considered when determining appropriate street tree diversity. Focusing on the evenness or abundance of one taxon, e.g., species, and ignoring other factors can prove detrimental to the stability and functionality of a community’s urban forest. For example, if the nearly 20,000 urban forest trees in Dublin, OH consisted of 20 species of Acer, the area might be considered to be diverse using the Simpson index; however, the population would be susceptible to a pest such as ALB (Sydnor & Subburayalu, 2009). Similarly, if factors such as environmental benefits, adaptability, and management ease are emphasized in the process of considered for tree selection, a community may end up with a limited number of species, leaving a large proportion of the population susceptible to biotic events such as a DED or an EAB infestation (Guntenspergen, 1983). Guiasu and Guiasu (2003) outlined the use of a modified or weighted Simpson index in assessing the diversity of crayfish in 27 lakes, from Georgian Bay region, in the province of Ontario, Canada. The weighted Simpson index is essentially the same as the Simpson index except that it uses qualitative weights that reflect additional information about the taxon, e.g., its economic significance, ecological importance or biotic sensitivities. The aim of the present study was to explore the application and usefulness of the weighted Simpson index in evaluating diversity in street tree populations in four Ohio communities. Tree taxa in these communities were weighted according to pest vulnerability, environmental benefits, and taxon adaptability to provide a more robust means to identify areas of concerns for each community and to determine appropriate street tree diversity for planning additional plantings and replacement.
1.2. Environmental benefit
2. Materials and methods
In addition to their esthetic value, urban forests provide several critical ecosystem services to the more than 220 million people who live in the urban areas of the United States, a value which is often overlooked (Nowak et al., 2010). Innovative tools are available to quantify the ecosystem services provided by street trees, including air pollution removal, carbon sequestration, energy conservation, and storm water management. One such tool is “i-Tree streets” in the i-Tree software suite (Maco & Mcpherson, 2003; USDA Forest Service, 2006), a peer-reviewed suite of urban forestry tools. The ecosystem benefits offered by trees vary not only by tree species but also by size. For example, large trees (>30 in. DBH) remove an amount of air pollutants that is approximately 60–70 times greater than that removed by small trees (<3 in. DBH) (Nowak, 1994). Therefore, species that are larger at maturity will typically produce greater environmental benefits than do smaller trees. To achieve a sustainable urban forest, tree planting, and replacement decisions need to consider the community’s desire for the increase of associated ecosystem services offered by the trees.
Weighted Simpson indexes were calculated for the communities of Dublin, Toledo, Westerville, and Yellow Springs, Ohio in two steps. The first step assigned weights to the various taxa for the factors of environmental benefits, pest vulnerability, and taxon adaptability. The second step computed the weighted Simpson index values at three taxonomic levels (species, genus, and family) as described below.
1.1. Pest vulnerability
1.3. Taxon adaptability Richards (1983) emphasized the importance of tree adaptability and concluded that taxonomic diversity should relate to the diversity of site conditions and the functional requirements of the community’s streets. The Council of Tree and Landscape Appraisers (CTLA, 2000) presented guidelines for rating species adaptability as one of three factors to be used in calculating values for landscape trees in a particular state or region. The adaptability and longevity
2.1. Computation of weights 2.1.1. Environmental benefits The total environmental benefits at maturity (energy, storm water, carbon sequestration, air quality, and esthetic benefits in dollars) for 229 different tree species commonly found in Ohio were calculated using the i-Streets software. The benefits were expressed as hundredths of a dollar per tree at maturity. The size at maturity for each species was an estimate on the basis of expert knowledge (Anonymous, 1979; Sydnor et al., 2010). To standardize the weights to range between 0 and 1 (Appendix 1), the computed dollar benefits were then transformed using the following formula:
Wi =
Bi − Bmin Bmax − Bmin
where Wi is the weighting for the ith taxon, Bi is the computed benefit of the ith taxon, and Bmin and Bmax are the lowest and the highest benefit values computed for the commonly found species in Ohio.
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2.1.2. Pest vulnerability Biotic and abiotic pest vulnerability weights for each of the 229 species were based on nearly 60 years of experience (Anonymous, 1979; Sydnor et al., 2010). Ratings ranged from 0 to 1 (Appendix 1), with 0 representing certain attack and certain failure and 1 representing no probability of attack or impact from biotic or abiotic factors. These pest vulnerability weightings were based on factors present in Ohio. If this approach were to be extended to other regions of the country, multidisciplinary panels of experts would be needed to establish vulnerability weights based on the dominant plant communities. In this sense, Ohio is remarkably homogeneous when compared with California and many other states.
adaptability). To aid in comparing indexes and because the Simpson index can produce varying ranges of values, indexes were normalized to range between 0 and 1 using the following formula: dw =
Dw max Dw
and Maximum Dw (max Dw ) is computed as follows: 1 4 m
Wi −
i=0
(1 − (m/2))
m
i=0
2
(1/Wi )
3. Results and discussion 2.1.3. Taxon adaptability Species weights were developed using the accumulated knowledge of a panel of experts. Species adaptability ratings for more than 200 tree species have been published in a valuation guide for the state of Ohio (Sydnor, Gooding, & Bishop, 2007). The ratings are composites of opinion on species performance from a group of professionals, including arborists, nursery producers, landscape managers, and urban foresters. The ratings range from 0 to 100% and are generally presented as a 20 percent range, e.g., 80–100% for Quercus alba (white oak). Larger ranges are also seen in the ratings for certain trees, such as Acer platanoides (Norway maple), with a range of 40–80% recorded for this species of which the performance and functionality vary in differing situations in the state. Norway maple becomes invasive in natural settings in the northeastern part of Ohio. To calculate the species weight, the percentage was expressed as a decimal fraction (Appendix 1), and the mid-point of the range was used (e.g., 0.9 for white oak and 0.6 for Norway maple in the examples above). 2.1.4. Weighting at the genus and family levels for all factors Individual weights at the species level computed for various factors as outlined in Sections 2.1.1–2.1.3 were weighted according to the number of trees in each species within a particular genus or family in a community to compute weights at the genus and/or family levels using the following formula:
m
i=0
Wj =
Wi × Ni m
where Wj is the weight for the jth genus or family, Wi is the weight at the species level computed for the various factors, i = 1, 2, 3, . . ., m is the number of different species within each genus or family, Ni is the number of trees in the ith species and m is the total number of species in the jth genus or family. 2.2. Calculation of indexes The Simpson index (D) is calculated as follows: D=1−
m
p2i
i=0
where m is the number of species and pi is the proportion of the ith species in the given population. The weighted Simpson index (Dw ) is calculated or weighted as follows: Dw =
m
Wi pi [(1 − pi )]
i=0
where Wi is the weight of the ith species, m is the number of species and pi is the proportion of the ith species in the given population (Guiasu & Guiasu, 2003). A weighted Simpson index was produced for each factor (pest vulnerability, environmental benefits, and taxa
3.1. Yellow Springs The Yellow Springs street tree inventory was conducted by Ohio State University’s Greene County Extension with the assistance of the Yellow Springs tree committee and citizen volunteers. A total of 1646 trees were inventoried. The tree inventory was conducted only to the genus level, as that was the plant identification level with which the volunteers felt confident. There were 1646 trees in 40 genera and 26 families. The computed indexes are presented in Table 1. The Simpson index values at the genus and family levels were 0.89 and 0.85, respectively. The weighted indexes ranged between 0.17 and 0.61 across the taxonomic levels. The lowest value (0.17) was computed at the genus level weighted by environmental benefit, with a corresponding normalized index value of 0.07 (a comparison of the community against itself). As indicated by this value, although the Simpson index values suggest a reasonable diversity in street tree populations, the diversity from the point of view of environmental benefits was very low. Yellow Springs has a large number of small statured trees, such as Malus Mill. (crabapple), Picea A. Dieter (blue spruce), Cercis L. (eastern redbud), Prunus L. (cherry) and Amelanchier Medik. (serviceberry). Such small statured trees comprised approximately 25% of the population. Future plantings in Yellow Springs could increase the environmental benefit by planting larger statured trees, such as Gleditsia L. (honeylocust), Catalpa Scop. (catalpa), Liquidambar L. (sweetgum), and Celtis L. (northern hackberry), which are part of the existing inventory. There was a similar reduction in the diversity index value when it was weighted by pest vulnerability (0.61) and by taxa adaptability (0.55) at the genus level. This result is due partly to the presence in the population of 5.2% Fraxinus L. (ash), which is declining because of EAB. The corresponding normalized indexes of 0.68 and 0.62, respectively, indicate that Yellow Springs would benefit by modifying the proportions of the existing inventory to increase taxa adaptability and reduce pest vulnerability. 3.2. Dublin A total of 18,662 public trees were inventoried in Dublin and were identified as 58 species, 33 genera, and 18 families. The Simpson index values ranged from 0.86 at the family level to 0.95 at the species level (Table 2). Increased diversity at the species level is expected because the number of species was greater than the number of genera, which was in turn greater than the number of families. The weighted diversity measures showed a decline across different factors and levels, with the lowest value being 0.30 at the genus level for environmental benefits with a corresponding normalized value of 0.12. This result indicates that Dublin has yet to utilize their existing inventory of trees to maximize the environmental benefits. Small statured trees, such as Malus, Crateagus L. (hawthorn), Syringa L. (lilac), and Amelanchier, comprise 7.7% of the total population but deliver only 4% of the air pollution benefits and 1.2% of the esthetic benefits (Sydnor & Subburayalu, 2011).
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Table 1 Simpson index and weighted Simpson index values computed for public trees in Yellow Springs, Ohio at three taxonomic levels. Yellow Springs
Simpson index
Species Genus Family
NA 0.89 0.85
Environmental benefit
Pest vulnerability
Weighted
Normalized
Weighted
Normalized
Taxa adaptability Weighted
Normalized
NA 0.17 0.36
NA 0.07 0.13
NA 0.61 0.59
NA 0.68 0.69
NA 0.55 0.53
NA 0.62 0.69
Table 2 Simpson index and weighted Simpson index values computed for public trees in Dublin, Ohio at three taxonomic levels. Dublin
Species Genus Family
Simpson index
0.95 0.87 0.86
Environmental benefit
Pest vulnerability
Weighted
Normalized
Weighted
Normalized
Weighted
Normalized
0.33 0.30 0.63
0.10 0.12 0.43
0.61 0.56 0.56
0.59 0.68 0.76
0.64 0.59 0.58
0.54 0.65 0.74
These examples demonstrate the value of larger statured trees, such as Gleditsia, Catalpa, and Quercus L. (oak), if the environmental and political situations of an area permit larger statured trees. The decline in the diversity measures weighted by pest vulnerability and species adaptability rating can be explained in part by the higher percentages of green ash (11.8%) and white ash populations (4.6%) in Dublin, the sensitivity of these native ashes to EAB, and their low weightings for pest vulnerability. 3.3. Westerville A total of 12,176 public trees were inventoried, representing 102 different species in 54 genera and 30 families. The Simpson index values ranged from 0.81 at the family level to 0.92 at the species level (Table 3). The higher species diversity ratings resulted from the higher numbers of species in Westerville. Again, however, the weighted diversity measures showed a decline across different factors and taxonomic levels, with the lowest value being 0.25 at the genus level for environmental benefits, with a corresponding normalized value of 0.08. Smaller statured trees, such as Amelanchier, Cornus L., Crataegus, Maackia Rupr, Malus, Syringa, Cercis, Picea, Cercidiphyllum Siebold & Zucc., Eucommia Engl., and Ostrya Scop., comprise 22.7% of the total population. The decline in potential environmental benefits could therefore be offset by planting larger statured species, genera, or families. The decline in the diversity measures weighted by pest vulnerability and taxa adaptability resulted from the presence of 9.9% ashes in the population and the lower ratings of ashes for these two characteristics. 3.4. Toledo A total of 84,782 public trees were inventoried by the Toledo Division of Parks and Forestry and identified as 170 species, 73 genera, and 34 families. The computed indexes are presented in Table 4. The high Simpson index value of 0.91 recorded for Toledo at the species level is due to the presence of 170 different species in the population. The sharp declines in the Simpson index value from 0.91 at the species level to 0.75 at the genus level and 0.74 at the family level are attributed to the 51% of the street trees that belong to a small family, Aceraceae Juss., which is represented in Ohio by the single genus Acer L. (maple). The community of Toledo is advised to limit any future plantings of maples, regardless of size, because of maple’s sensitivity to ALB, as demonstrated elsewhere in Ohio. The weighted diversity measures show a decline across different factors and levels when compared with the unweighted Simpson index. The lowest value was recorded at the genus level (0.24) when weighted by environmental benefits with a corresponding normalized value of 0.06, demonstrating the need to plant larger statured
Taxa adaptability
trees. A substantial decline at the genus level was also observed in the diversity measures when weighted by pest vulnerability (0.50) and taxa adaptability (0.49), again indicating the overreliance on a single genus and family and the sensitivity of those taxa to ALB. 3.5. Comparing the communities When making comparisons between the communities, it is more appropriate to use the weighted Simpson indexes as opposed to the normalized Simpson index because the use of latter can be misleading since the absolute number of taxa can differ between the communities (Guiasu & Guiasu, 2003). The Simpson index can be considered to be the probability that the next plant encountered will belong to a different taxon at the level being investigated (species, genus, or family). No differences in the Simpson index values were found for the four communities, except for the decline noted for Toledo at the genus and family levels, resulting primarily from the street tree population that consists of more than 50% of maples (Fig. 1). When the Simpson index is weighted for environmental benefits or taxa adaptability, Dublin clearly displays greater diversity across all levels (Figs. 2 and 4, respectively). When the Simpson index is weighted by pest vulnerability, Yellow Springs displayed the highest pest resistance ratings at both the genus and species levels (Fig. 3). 3.6. How to use weighted indexes Weighted indexes should be used as a means to identify areas of focus and not as a final result. Maximizing one particular weighted index may lead to weaknesses in other areas. Preferably, communities should assess their performance in more than one functional
Fig. 1. Simpson index values for four different communities in Ohio (Yellow Springs, Dublin, Westerville, Toledo) at three taxonomic levels.
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Table 3 Simpson index and weighted Simpson index values computed for public trees in Westerville, Ohio at three taxonomic levels. Westerville
Species Genus Family
Simpson index
0.92 0.87 0.81
Environmental benefit
Pest vulnerability
Weighted
Normalized
Weighted
Normalized
Taxa adaptability Weighted
Normalized
0.28 0.25 0.53
0.05 0.08 0.14
0.59 0.56 0.52
0.44 0.55 0.67
0.59 0.55 0.51
0.36 0.51 0.64
Table 4 Simpson index and weighted Simpson index values computed for public trees in Toledo, Ohio at three taxonomic levels. Toledo
Species Genus Family
Simpson index
0.91 0.75 0.74
Environmental benefit
Pest vulnerability
Weighted
Normalized
Weighted
Normalized
Weighted
Normalized
0.30 0.24 0.51
0.04 0.06 0.13
0.60 0.50 0.49
0.32 0.47 0.58
0.61 0.49 0.48
0.25 0.40 0.57
Fig. 2. Simpson index value weighted by environmental benefit for four different communities in Ohio (Yellow Springs, Dublin, Westerville, and Toledo) at three taxonomic levels.
Fig. 3. Simpson index weighted by pest vulnerability for four different communities in Ohio (Yellow Springs, Dublin, Westerville, and Toledo) at three taxonomic levels.
Fig. 4. Simpson index weighted by taxa adaptability rating for four different communities in Ohio (Yellow Springs, Dublin, Westerville, and Toledo) at three taxonomic levels.
Taxa adaptability
area, such as storm water, air pollution, energy benefits, pest vulnerability, and adaptability. Weighted ratings could and perhaps should be established by one or more experts in a particular field, e.g., entomology, if they are to be applied on an area or regional basis. Alternatively, if a community forester feels sufficiently confident in making judgments for his/her community, he/she could weigh various characteristics on the basis of his/her own experience. We recommend the use of composite ratings across disciplines for a characteristic such as pest vulnerability, including at least the aspects of plant pathology, entomology, and plant physiology. The normalized weighted index is useful as a tool for evaluating the performance of a community against itself and as a guide in selecting priorities when planning tree replacement. A smaller normalized index suggests that there is room for improvement using plants that are already in the community’s plant list. Using additional taxa would also add diversity; however, because a normalized index is a comparison against itself, it uses only plants currently in the inventory. The weightings used for pest vulnerability and adaptability in the present study are subjective and can change quickly in situations such as the introduction of a new exotic pest to a region, e.g., ALB in Ohio. It is preferable to incorporate the collective wisdom of a group of multidisciplinary evaluators in the decision-making process, as difference evaluators tend to view the problem from differing perspectives. Community personnel could determine their own indexes if they were sufficiently knowledgeable/experienced, or regional weightings could be used. Another issue in the use of weighted indexes to increase diversity is defining the spatial scale to be used. Heimlich, Sydnor, Bumgardner, and O’Brien (2008) found that the citizens surveyed in Toledo did not value uniformity in their street tree planting; however, most city planners and landscape architects design single-species plantings along streets or street segments to impart a sense of unity. Community foresters normally control only public plantings within their jurisdiction, although they may offer suggested lists for private consideration. The communities in the present study range in population and size from Toledo (with a human population of 287,201, a street tree population of 87,782 trees, and an area of 224.6 km2 divided into 63 separate management units) to Yellow Springs (with a human population of 3487, a street tree population of 1676, and an area of 4.9 km2 in one management unit). Using political units as in this study, single-species plantings along a street or street segment can be performed. We therefore suggest that either an entire political unit or a management unit as defined by a larger community are appropriate units for the evaluation of diversity, assuming that the larger unit wishes to perform additional analyses to support its position for an initiative.
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Weighted Simpson indexes can also be used to weigh desirable characteristics or functions for consideration in an existing tree inventory or as a guide to identify additional trees that could be used to enhance the selected functionality with a program such as i-Tree Species. In most instances, a community does not have a number of diversity indexes from similar-sized communities within a state or region that can be used to make comparisons. Thus, for many communities, the most useful index will be the normalized index in which the community compares its current rating against the rating that it can maximally expect to attain. Weightings for a variety of desirable traits can be combined and evaluated to develop a more sustainable landscape over time. Any single rating is based on historical experience and is thus backward looking. The goal is a sustainable landscape or ecosystem over time, which requires reduced inputs such as labor, pesticides, and fertilizers to remain viable in the future.
4. Conclusion One of the commonly used measures of diversity in street tree population is the Simpson index. This index considers only the proportions of various taxa. However, when planning future street tree replacements for enhancement of long-term functionality, several other factors must also be considered, including the environmental benefits from trees, pest vulnerability, and taxa adaptability. Communities face differing problems over time. For example, another agency might identify an area of concern and request that the community step up its efforts to enhance benefits in an area such as reduction of storm water runoff. The taxa of street trees can be identified and weighted to assist in reducing runoff or reducing pest vulnerability. These functional values can be attached as weights to the various taxa, and the weighted Simpson index can be calculated. The computation of Simpson indexes for the four Ohio communities revealed little variability, and the communities were rated as reasonably diverse. In contrast, the weighted Simpson indexes revealed areas of concern for each of the four communities. Each of the communities lacked large statured trees, which offer greater environmental benefits at a given age (maturity). For example, Sydnor and Subburayalu (2011) reported a 7.5-fold increase in annual environmental benefits by planting large statured trees in the streets of Brooklyn, OH. Similarly, McPherson et al. (2006) reported an eightfold increase in net annual environmental benefits by planting large statured trees. Planting of medium to small trees is frequently recommended for esthetic reasons, such as maintaining the scale of today’s smaller properties and for ease of management (Ophardt, n.d.). A survey of homeowners whose mature green ash had been condemned because of the presence of EAB revealed that they valued larger trees (Heimlich et al., 2008). None of the homeowners in that survey suggested that their trees were too large. However, we do not recommend the use of large statured trees in locations where size becomes a limitation. For example, large statured trees should not be planted beneath utility lines because utility pruning is expensive, causes conflict, and can interrupt utility service. We suggest only that communities benefit by planting large statured trees where allowed by site conditions. For the city of Toledo, this study revealed overreliance on a single genus (Acer), as indicated by a decline in the weighted Simpson index values at the genus and family levels. Maples comprise 48% of Toledo’s tree population (Sydnor & Subburayalu, 2008b). The recent discovery of ALB in Clermont County, Ohio in June 2011 poses a threat to other Ohio communities with predominant host populations of a single taxon such as maple. The city of Toledo is in danger of losing 8.3 million dollars in annual environmental benefits if ALB
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becomes an established pest in the city (Sydnor & Subburayalu, 2008b), in contrast to the 1.45 million dollar loss in benefits resulting from EAB (Sydnor & Subburayalu, 2008a). These estimated costs do not include tree removal, treatment, or replacement costs. These findings emphasize the need for diversity at not only the species level but also higher taxonomic levels (genus and family), as long ´ & McBride, 2008; Nowak, advocated by forestry scientists (Lacan 2001; Raupp et al., 2006). Diversity, as mentioned above, is a tool rather than an ultimate goal, and does not preclude the role of esthetic considerations. There are many examples of ecosystems that are near-monocultures consisting of only a few species yet have functioned sustainably for hundreds of years, e.g., the coniferous forests of western Washington and Oregon (Burns & Honkala, 1990). In contrast, diversity employed to promote sustainability is a forwardlooking tool that aims to ensure that the loss of single taxon does not disrupt the entire urban forest community in an unsustainable manner. This use of diversity is based on the concept of sharing risks among a number of different taxa. The most useful index is a normalized index whereby a community compares its current rating against the rating that it can maximally expect to attain. Weightings for a variety of desirable traits should be combined or evaluated to develop a more functional and sustainable landscape going forward. Maximizing a single benefit, e.g., environmental benefits which would require the use of only larger growing trees, will result in lower overall diversity, increased maintenance costs, and potentially reduced esthetic appeal. By balancing the risks and benefits of diversity against other benefits such as pest vulnerability, environmental benefit, and taxa adaptability, the weighted Simpson index can assist a community vegetation manager/planner in designing a sustainable and functional landscape in urban communities. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.landurbplan.2012.02.004. References Anonymous. (1979). Guide to rating trees and other plants in Ohio. Ohio Chapter. Columbus, OH: International Society of Arboriculture., 11 pp. Burns, R. M., & Honkala, B. H. (1990). Silvics of North America. Conifers. Agriculture handbook 654 Washington, DC: USDA Forest Service, U.S. Government Printing Office., 675 pp. Council of Tree and Landscape Appraisers. (2000). Guide for plant appraisal (ninth ed.). Champaign, IL. 143 pp. Galvin, M. F. (1999). A methodology for assessing and managing biodiversity in street tree populations: A case study. Journal of Arboriculture, 25, 124–128. Guiasu, R. C., & Guiasu, S. (2003). Conditional and weighted measures of ecological diversity. International Journal of Uncertainty, Fuzziness and Knowledge-Based Systems, 11, 283–300. Guntenspergen, G. (1983). Diversity and stability in a street tree population – Comment. Urban Ecology, 7, 173–176. Heimlich, J. E., Sydnor, T. D., Bumgardner, M., & O’Brien, P. (2008). Attitudes of residents toward street trees on four streets in Toledo, Ohio, U.S. before removal of ash trees (Fraxinus spp.) from Emerald Ash Borer (Agrilus planipennis). Arboriculture and Urban Forestry, 34(1), 47–53. Herms, D. A., McCullough, D. G., & Smitley, D. R. (2004). Under Attack: The current status of the Emerald Ash Borer infestation and the program to eradicate it. American Nurseryman, 200(7), 20–27. ´ Lacan, I., & McBride, J. R. (2008). Pest Vulnerability Matrix (PVM): A graphic model for assessing the interaction between tree species diversity and urban forest susceptibility to insects and diseases. Urban Forestry and Urban Greening, 7, 291–300. Maco, S. E., & Mcpherson, E. G. (2003). A Practical approach to assessing structure, function, and value of street tree populations in small communities. Journal of Arboriculture, 29(2), 84–97. McPherson, E. G, & Rowntree, R. A. (1989). Using structural measures to compare twenty-two U.S. street tree populations. Landscape Journal, 8, 13–23. McPherson, E. G., Simpson, J. R., Peper, P. J., Maco, S. E., Gardner, S. L., Cozad, S. K., et al. (2006). Midwest community tree guide: Benefits, costs, and strategic planting. In USDA Forest Service General Technical Report PSW-GTR-199 (99 pp.).
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Nowak, D. J. (1994). Air pollution removal by Chicago’s Urban Forest. In E. G. McPherson, D. J. Nowak, & R. A. Rowntree (Eds.), Chicago’s Urban forest ecosystem: Results of the Chicago Urban forest Climate Project. USDA, Forest Service, Gen. Tech. Rep. NE-186 (pp. 63–81). Nowak, D. J. (2001). Potential effect of Anoplophora glabripennis (Coleoptera: Cerambycidae) on urban trees in the United States. Journal of Economic Entomology, 94, 116–122. Nowak, D. J., Stein, S. M., Randler, P. B., Greenfield, E. J., Comas, S. J., Carr, M. A., et al. (2010). Sustaining America’s urban trees and Forests: A forests on the edge report. Gen. Tech. Rep. NRS-62. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station., 27 pp. Ophardt, M. C. (n.d.). Small trees for home landscapes. Retrieved from http://www.treesforyou.org/Selection/Articles/sml-tre.htm Poland, T. M., & McCullough, D. G. (2006). Emerald Ash Borer: Invasion of the urban forest and the threat to north America’s ash resource. Journal of Forestry, 104, 118–124. Raupp, M. J., Cumming, A. B., & Raupp, E. C. (2006). Street tree diversity in Eastern North America and its potential for tree loss to exotic borers. Arboriculture and Urban Forestry, 32, 297–304. Richards, N. A. (1983). Diversity and stability in a street tree population. Urban Ecology, 7, 159–171. Sanders, R. A. (1981). Diversity in the street trees of Syracuse, New York. Urban Ecology, 5, 33. Santamour, F. S., Jr. (1990). Trees for urban planting: Diversity, uniformity and common sense. In Proceedings of the 7th conference of the metropolitan tree improvement alliance. Vol. 7 (pp. 57–65). Seigert, N. W., & McCoullough, D. C. (2006). Potential production of Emerald Ash Borer adults: Tree, site and landscape level applications. Annapolis, MD: Poster, XVII USDA Interagency Research Forum on Gypsy Moth and Other Invasive Species.
Shannon, C. E. (1948). A mathematical theory of communication. Bell System Technical Journal, 27(379–423), 623–656. Simpson, E. H. (1949). Measurement of diversity. Nature, 163, 688. Sun, W. (1992). Quantifying species diversity of streetside trees in our cities. Journal of Arboriculture, 18, 91–93. Sydnor, T. D., Bumgardner, M., & Todd, A. (2007). The potential economic impacts of Emerald Ash Borer (Agrilus planipennis) on Ohio, U.S., communities. Arboriculture and Urban Forestry, 33(1), 48–54. Sydnor, T. D., Gooding, R. F., & Bishop, B. L. (2007). Guide to plant appraisal of trees and other plants in Ohio (seventh ed.). Columbus, OH: International Society of Arboriculture. Ohio Chapter, 40 pp. Sydnor, T. D., & Subburayalu, S. (2008). Losses of annual environmental benefits from EAB in Toledo, OH, USA. In Proceedings of the society of American foresters 2008 national conference Reno, Nevada. Sydnor, T. D., & Subburayalu, S. (2008b). An analysis of tree benefits for Toledo, Ohio. Retrieved from http://senr.osu.edu/urbanforestry/pdfs/Toledo.pdf Sydnor, T. D., & Subburayalu, S. (2009). An analysis of street tree benefits for Dublin, OH. Retrieved from http://senr.osu.edu/urbanforestry/pdfs/Dublin 3 23 09.pdf Sydnor, T. D., Subburayalu, S., Chatfield, J., Draper, E., Stone, A., Smith, K., et al. (2010). Street tree evaluation project: Forty years of street tree evaluation in five communities. Ohio State University Extension Bulletin #877. Columbus OH. 100 pp. Sydnor, T. D., & Subburayalu, S. (2011). Should we consider expected environmental benefits when planting larger or smaller tree species. Arboriculture and Urban Forestry, 37, 167–171. USDA Forest Service. (2006). i-Tree: Tools for managing and assessing community forests. Retrieved from http://www.itreetools.org
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Landscape and Urban Planning journal homepage: www.elsevier.com/locate/landurbplan
Assessing street tree diversity in four Ohio communities using the weighted Simpson index S. Subburayalu ∗ , T.D. Sydnor 1 School of Environment and Natural Resources, The Ohio State University, 2021 Coffey Road, Columbus, OH 43210-1085, United States
a r t i c l e
i n f o
Article history: Received 15 April 2011 Received in revised form 25 January 2012 Accepted 5 February 2012 Available online 3 March 2012 Keywords: Environmental benefit Pest resistance Diversity Urban forestry Species adaptability Simpson index
a b s t r a c t Diversity is a key factor which should be considered during planning street replacements and involves assessment of existing street population for various factors especially resistance of trees to pests. Street tree replacement histories in the United States reveal extensive replacement of American elms killed by Dutch elm disease (DED) with maples, or ashes. In turn, these trees are now under attack by Asian longhorn beetle (ALB) and emerald ash borer (EAB). Two commonly used evaluation measure of diversity namely the 10:20:30 rule and the Simpson index takes into account only the number and evenness of the taxa in a street tree population. However these indexes fail to consider the differential functional values of trees which are critical in building a sustainable urban forest while attempting to increase diversity. The proposed weighted Simpson index which weights the absolute abundance of tree taxa based on some functional variable could be used as a guide to identify areas of focus for a community. A case study involving four communities in Ohio namely Toledo, Westerville, Dublin and Yellow Springs was conducted to examine the usefulness of the weighted Simpson Index. Three functional variables including environmental benefits, pest vulnerability and taxon adaptability at three different taxonomic levels, family, genus and species, were used to arrive at the weighted Simpson indexes for the four communities. Results revealed the usefulness of weighted Simpson Index in exposing areas of concern for each community. Generally all the four communities were lacking larger statured trees with greater environmental benefits. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The need for diversity in street trees was reinforced by the recent invasion of the emerald ash borer (EAB), Agrilus planipennis Fairmaire, in the urban forests of Michigan and subsequently in more than eight other states in the United States and two provinces in Canada. Researchers in urban forestry have recommended that diversification of community street tree populations be considered before planting and replacement (e.g., Poland & McCullough, 2006). Biologically diverse tree plantings and replacements must be incorporated into community forests to reduce potential damage from pests. For example, after the loss of American elm, Ulmus americana, which was caused by Dutch elm disease (DED), Ophiostoma spp. Buism., many elms were replaced by either maple or ash trees, which are now under attack by the Asian longhorn beetle (ALB), Anoplophora glabripennis Motschulsky, or EAB, respectively. To further minimize risk of pests, researchers have also recommended
∗ Corresponding author. Tel.: +1 614 247 8035; fax: +1 614 292 7432. E-mail addresses: [email protected] (S. Subburayalu), [email protected] (T.D. Sydnor). 1 Tel.: +1 614 292 3865. 0169-2046/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.landurbplan.2012.02.004
diversifying community forests at higher taxonomic levels, as pests ´ & McBride, generally operate at the genus and family levels (Lacan 2008; Nowak, 2001; Raupp, Cumming & Raupp, 2006; Seigert & McCoullough, 2006). Achieving diversity in street tree plantings must involve an initial evaluation of diversity in the existing populations. A number of methods to evaluate diversity have been applied in urban forestry ´ & McBride, 2008; McPherson & Rowntree, (Galvin, 1999; Lacan 1989; Raupp et al., 2006; Sanders, 1981; Santamour, 1990; Sun, 1992). Two commonly used methods for assessing street tree diversity are (1) the target-based 10:20:30 heuristic guideline (Santamour, 1990) and (2) non-target-based mathematically computed indexes, such as the Simpson and Shannon-Weiner indexes (Sanders, 1981; Sun, 1992). According to the 10:20:30 guideline, the street tree population of a community should consist of no more than 10% of a single tree species, 20% of a single genus and 30% of a single family. This guideline was proposed to protect urban forests from serious pest outbreaks. Raupp et al. (2006) noted an important limitation of this target-based guideline, i.e., the guideline does not consider the fact that most pests attack more than one tree species, genus or family. These authors therefore recommended diversification at the genus, family and perhaps ordinal levels. Similarly, Richards (1983) argued
S. Subburayalu, T.D. Sydnor / Landscape and Urban Planning 106 (2012) 44–50
that this guideline lacks a scientific basis and that following the guideline without considering the adaptability of various taxa can result in a less stable population. A similar limitation of non-target-based indexes, such as the Shannon-Weiner index (Shannon, 1948), Simpson index (Simpson, 1949) or the inverse of Simpson index (Sun, 1992), is that they are based entirely on the number and relative abundance of all the taxa being evaluated (species, genus or family). Although the use of these indexes has a scientific basis, their use as an evaluation tool in the plant selection process is limited to the number and evenness of the taxonomic unit being evaluated. Richards (1983), in outlining reasonable guidelines for street tree diversity, correctly noted that such diversity should relate to the set of conditions and objectives for a given community. Urban foresters and municipal arborists should consider several factors or limitations that are specific to a location and/or tree species when planning plantings or replacements. These factors include, but are not limited to, pest vulnerability, environmental benefits, and adaptability.
45
of tree species in stressful urban sites are also key factors that must be considered to ensure the long-term survivorship and health of trees. 1.4. Weighted Simpson index
The losses caused by DED and EAB demonstrate the importance of diversity in street tree populations. An early study showed that the landscape and nursery industry in Michigan suffered about $11 million in damages from EAB, primarily to Fraxinus pennsylvanica Marsh (green ash) and Fraxinus americana (white ash) (Herms, McCullough, & Smitley, 2004), while losses in urban areas were much higher. A more recent study indicated that the losses caused by EAB in Ohio were between $1.8 and 7.5 billion (Sydnor, Bumgardner, & Todd, 2007). Understanding the current and probable future risks from pests is crucial in making planting and ´ and McBride (2008) outlined a matrix replacement decisions. Lacan for evaluating the effects of insects and diseases on an urban forest at a citywide scale that would help in developing lists of tree species for planting.
As discussed above, a variety of factors must be considered when determining appropriate street tree diversity. Focusing on the evenness or abundance of one taxon, e.g., species, and ignoring other factors can prove detrimental to the stability and functionality of a community’s urban forest. For example, if the nearly 20,000 urban forest trees in Dublin, OH consisted of 20 species of Acer, the area might be considered to be diverse using the Simpson index; however, the population would be susceptible to a pest such as ALB (Sydnor & Subburayalu, 2009). Similarly, if factors such as environmental benefits, adaptability, and management ease are emphasized in the process of considered for tree selection, a community may end up with a limited number of species, leaving a large proportion of the population susceptible to biotic events such as a DED or an EAB infestation (Guntenspergen, 1983). Guiasu and Guiasu (2003) outlined the use of a modified or weighted Simpson index in assessing the diversity of crayfish in 27 lakes, from Georgian Bay region, in the province of Ontario, Canada. The weighted Simpson index is essentially the same as the Simpson index except that it uses qualitative weights that reflect additional information about the taxon, e.g., its economic significance, ecological importance or biotic sensitivities. The aim of the present study was to explore the application and usefulness of the weighted Simpson index in evaluating diversity in street tree populations in four Ohio communities. Tree taxa in these communities were weighted according to pest vulnerability, environmental benefits, and taxon adaptability to provide a more robust means to identify areas of concerns for each community and to determine appropriate street tree diversity for planning additional plantings and replacement.
1.2. Environmental benefit
2. Materials and methods
In addition to their esthetic value, urban forests provide several critical ecosystem services to the more than 220 million people who live in the urban areas of the United States, a value which is often overlooked (Nowak et al., 2010). Innovative tools are available to quantify the ecosystem services provided by street trees, including air pollution removal, carbon sequestration, energy conservation, and storm water management. One such tool is “i-Tree streets” in the i-Tree software suite (Maco & Mcpherson, 2003; USDA Forest Service, 2006), a peer-reviewed suite of urban forestry tools. The ecosystem benefits offered by trees vary not only by tree species but also by size. For example, large trees (>30 in. DBH) remove an amount of air pollutants that is approximately 60–70 times greater than that removed by small trees (<3 in. DBH) (Nowak, 1994). Therefore, species that are larger at maturity will typically produce greater environmental benefits than do smaller trees. To achieve a sustainable urban forest, tree planting, and replacement decisions need to consider the community’s desire for the increase of associated ecosystem services offered by the trees.
Weighted Simpson indexes were calculated for the communities of Dublin, Toledo, Westerville, and Yellow Springs, Ohio in two steps. The first step assigned weights to the various taxa for the factors of environmental benefits, pest vulnerability, and taxon adaptability. The second step computed the weighted Simpson index values at three taxonomic levels (species, genus, and family) as described below.
1.1. Pest vulnerability
1.3. Taxon adaptability Richards (1983) emphasized the importance of tree adaptability and concluded that taxonomic diversity should relate to the diversity of site conditions and the functional requirements of the community’s streets. The Council of Tree and Landscape Appraisers (CTLA, 2000) presented guidelines for rating species adaptability as one of three factors to be used in calculating values for landscape trees in a particular state or region. The adaptability and longevity
2.1. Computation of weights 2.1.1. Environmental benefits The total environmental benefits at maturity (energy, storm water, carbon sequestration, air quality, and esthetic benefits in dollars) for 229 different tree species commonly found in Ohio were calculated using the i-Streets software. The benefits were expressed as hundredths of a dollar per tree at maturity. The size at maturity for each species was an estimate on the basis of expert knowledge (Anonymous, 1979; Sydnor et al., 2010). To standardize the weights to range between 0 and 1 (Appendix 1), the computed dollar benefits were then transformed using the following formula:
Wi =
Bi − Bmin Bmax − Bmin
where Wi is the weighting for the ith taxon, Bi is the computed benefit of the ith taxon, and Bmin and Bmax are the lowest and the highest benefit values computed for the commonly found species in Ohio.
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2.1.2. Pest vulnerability Biotic and abiotic pest vulnerability weights for each of the 229 species were based on nearly 60 years of experience (Anonymous, 1979; Sydnor et al., 2010). Ratings ranged from 0 to 1 (Appendix 1), with 0 representing certain attack and certain failure and 1 representing no probability of attack or impact from biotic or abiotic factors. These pest vulnerability weightings were based on factors present in Ohio. If this approach were to be extended to other regions of the country, multidisciplinary panels of experts would be needed to establish vulnerability weights based on the dominant plant communities. In this sense, Ohio is remarkably homogeneous when compared with California and many other states.
adaptability). To aid in comparing indexes and because the Simpson index can produce varying ranges of values, indexes were normalized to range between 0 and 1 using the following formula: dw =
Dw max Dw
and Maximum Dw (max Dw ) is computed as follows: 1 4 m
Wi −
i=0
(1 − (m/2))
m
i=0
2
(1/Wi )
3. Results and discussion 2.1.3. Taxon adaptability Species weights were developed using the accumulated knowledge of a panel of experts. Species adaptability ratings for more than 200 tree species have been published in a valuation guide for the state of Ohio (Sydnor, Gooding, & Bishop, 2007). The ratings are composites of opinion on species performance from a group of professionals, including arborists, nursery producers, landscape managers, and urban foresters. The ratings range from 0 to 100% and are generally presented as a 20 percent range, e.g., 80–100% for Quercus alba (white oak). Larger ranges are also seen in the ratings for certain trees, such as Acer platanoides (Norway maple), with a range of 40–80% recorded for this species of which the performance and functionality vary in differing situations in the state. Norway maple becomes invasive in natural settings in the northeastern part of Ohio. To calculate the species weight, the percentage was expressed as a decimal fraction (Appendix 1), and the mid-point of the range was used (e.g., 0.9 for white oak and 0.6 for Norway maple in the examples above). 2.1.4. Weighting at the genus and family levels for all factors Individual weights at the species level computed for various factors as outlined in Sections 2.1.1–2.1.3 were weighted according to the number of trees in each species within a particular genus or family in a community to compute weights at the genus and/or family levels using the following formula:
m
i=0
Wj =
Wi × Ni m
where Wj is the weight for the jth genus or family, Wi is the weight at the species level computed for the various factors, i = 1, 2, 3, . . ., m is the number of different species within each genus or family, Ni is the number of trees in the ith species and m is the total number of species in the jth genus or family. 2.2. Calculation of indexes The Simpson index (D) is calculated as follows: D=1−
m
p2i
i=0
where m is the number of species and pi is the proportion of the ith species in the given population. The weighted Simpson index (Dw ) is calculated or weighted as follows: Dw =
m
Wi pi [(1 − pi )]
i=0
where Wi is the weight of the ith species, m is the number of species and pi is the proportion of the ith species in the given population (Guiasu & Guiasu, 2003). A weighted Simpson index was produced for each factor (pest vulnerability, environmental benefits, and taxa
3.1. Yellow Springs The Yellow Springs street tree inventory was conducted by Ohio State University’s Greene County Extension with the assistance of the Yellow Springs tree committee and citizen volunteers. A total of 1646 trees were inventoried. The tree inventory was conducted only to the genus level, as that was the plant identification level with which the volunteers felt confident. There were 1646 trees in 40 genera and 26 families. The computed indexes are presented in Table 1. The Simpson index values at the genus and family levels were 0.89 and 0.85, respectively. The weighted indexes ranged between 0.17 and 0.61 across the taxonomic levels. The lowest value (0.17) was computed at the genus level weighted by environmental benefit, with a corresponding normalized index value of 0.07 (a comparison of the community against itself). As indicated by this value, although the Simpson index values suggest a reasonable diversity in street tree populations, the diversity from the point of view of environmental benefits was very low. Yellow Springs has a large number of small statured trees, such as Malus Mill. (crabapple), Picea A. Dieter (blue spruce), Cercis L. (eastern redbud), Prunus L. (cherry) and Amelanchier Medik. (serviceberry). Such small statured trees comprised approximately 25% of the population. Future plantings in Yellow Springs could increase the environmental benefit by planting larger statured trees, such as Gleditsia L. (honeylocust), Catalpa Scop. (catalpa), Liquidambar L. (sweetgum), and Celtis L. (northern hackberry), which are part of the existing inventory. There was a similar reduction in the diversity index value when it was weighted by pest vulnerability (0.61) and by taxa adaptability (0.55) at the genus level. This result is due partly to the presence in the population of 5.2% Fraxinus L. (ash), which is declining because of EAB. The corresponding normalized indexes of 0.68 and 0.62, respectively, indicate that Yellow Springs would benefit by modifying the proportions of the existing inventory to increase taxa adaptability and reduce pest vulnerability. 3.2. Dublin A total of 18,662 public trees were inventoried in Dublin and were identified as 58 species, 33 genera, and 18 families. The Simpson index values ranged from 0.86 at the family level to 0.95 at the species level (Table 2). Increased diversity at the species level is expected because the number of species was greater than the number of genera, which was in turn greater than the number of families. The weighted diversity measures showed a decline across different factors and levels, with the lowest value being 0.30 at the genus level for environmental benefits with a corresponding normalized value of 0.12. This result indicates that Dublin has yet to utilize their existing inventory of trees to maximize the environmental benefits. Small statured trees, such as Malus, Crateagus L. (hawthorn), Syringa L. (lilac), and Amelanchier, comprise 7.7% of the total population but deliver only 4% of the air pollution benefits and 1.2% of the esthetic benefits (Sydnor & Subburayalu, 2011).
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Table 1 Simpson index and weighted Simpson index values computed for public trees in Yellow Springs, Ohio at three taxonomic levels. Yellow Springs
Simpson index
Species Genus Family
NA 0.89 0.85
Environmental benefit
Pest vulnerability
Weighted
Normalized
Weighted
Normalized
Taxa adaptability Weighted
Normalized
NA 0.17 0.36
NA 0.07 0.13
NA 0.61 0.59
NA 0.68 0.69
NA 0.55 0.53
NA 0.62 0.69
Table 2 Simpson index and weighted Simpson index values computed for public trees in Dublin, Ohio at three taxonomic levels. Dublin
Species Genus Family
Simpson index
0.95 0.87 0.86
Environmental benefit
Pest vulnerability
Weighted
Normalized
Weighted
Normalized
Weighted
Normalized
0.33 0.30 0.63
0.10 0.12 0.43
0.61 0.56 0.56
0.59 0.68 0.76
0.64 0.59 0.58
0.54 0.65 0.74
These examples demonstrate the value of larger statured trees, such as Gleditsia, Catalpa, and Quercus L. (oak), if the environmental and political situations of an area permit larger statured trees. The decline in the diversity measures weighted by pest vulnerability and species adaptability rating can be explained in part by the higher percentages of green ash (11.8%) and white ash populations (4.6%) in Dublin, the sensitivity of these native ashes to EAB, and their low weightings for pest vulnerability. 3.3. Westerville A total of 12,176 public trees were inventoried, representing 102 different species in 54 genera and 30 families. The Simpson index values ranged from 0.81 at the family level to 0.92 at the species level (Table 3). The higher species diversity ratings resulted from the higher numbers of species in Westerville. Again, however, the weighted diversity measures showed a decline across different factors and taxonomic levels, with the lowest value being 0.25 at the genus level for environmental benefits, with a corresponding normalized value of 0.08. Smaller statured trees, such as Amelanchier, Cornus L., Crataegus, Maackia Rupr, Malus, Syringa, Cercis, Picea, Cercidiphyllum Siebold & Zucc., Eucommia Engl., and Ostrya Scop., comprise 22.7% of the total population. The decline in potential environmental benefits could therefore be offset by planting larger statured species, genera, or families. The decline in the diversity measures weighted by pest vulnerability and taxa adaptability resulted from the presence of 9.9% ashes in the population and the lower ratings of ashes for these two characteristics. 3.4. Toledo A total of 84,782 public trees were inventoried by the Toledo Division of Parks and Forestry and identified as 170 species, 73 genera, and 34 families. The computed indexes are presented in Table 4. The high Simpson index value of 0.91 recorded for Toledo at the species level is due to the presence of 170 different species in the population. The sharp declines in the Simpson index value from 0.91 at the species level to 0.75 at the genus level and 0.74 at the family level are attributed to the 51% of the street trees that belong to a small family, Aceraceae Juss., which is represented in Ohio by the single genus Acer L. (maple). The community of Toledo is advised to limit any future plantings of maples, regardless of size, because of maple’s sensitivity to ALB, as demonstrated elsewhere in Ohio. The weighted diversity measures show a decline across different factors and levels when compared with the unweighted Simpson index. The lowest value was recorded at the genus level (0.24) when weighted by environmental benefits with a corresponding normalized value of 0.06, demonstrating the need to plant larger statured
Taxa adaptability
trees. A substantial decline at the genus level was also observed in the diversity measures when weighted by pest vulnerability (0.50) and taxa adaptability (0.49), again indicating the overreliance on a single genus and family and the sensitivity of those taxa to ALB. 3.5. Comparing the communities When making comparisons between the communities, it is more appropriate to use the weighted Simpson indexes as opposed to the normalized Simpson index because the use of latter can be misleading since the absolute number of taxa can differ between the communities (Guiasu & Guiasu, 2003). The Simpson index can be considered to be the probability that the next plant encountered will belong to a different taxon at the level being investigated (species, genus, or family). No differences in the Simpson index values were found for the four communities, except for the decline noted for Toledo at the genus and family levels, resulting primarily from the street tree population that consists of more than 50% of maples (Fig. 1). When the Simpson index is weighted for environmental benefits or taxa adaptability, Dublin clearly displays greater diversity across all levels (Figs. 2 and 4, respectively). When the Simpson index is weighted by pest vulnerability, Yellow Springs displayed the highest pest resistance ratings at both the genus and species levels (Fig. 3). 3.6. How to use weighted indexes Weighted indexes should be used as a means to identify areas of focus and not as a final result. Maximizing one particular weighted index may lead to weaknesses in other areas. Preferably, communities should assess their performance in more than one functional
Fig. 1. Simpson index values for four different communities in Ohio (Yellow Springs, Dublin, Westerville, Toledo) at three taxonomic levels.
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Table 3 Simpson index and weighted Simpson index values computed for public trees in Westerville, Ohio at three taxonomic levels. Westerville
Species Genus Family
Simpson index
0.92 0.87 0.81
Environmental benefit
Pest vulnerability
Weighted
Normalized
Weighted
Normalized
Taxa adaptability Weighted
Normalized
0.28 0.25 0.53
0.05 0.08 0.14
0.59 0.56 0.52
0.44 0.55 0.67
0.59 0.55 0.51
0.36 0.51 0.64
Table 4 Simpson index and weighted Simpson index values computed for public trees in Toledo, Ohio at three taxonomic levels. Toledo
Species Genus Family
Simpson index
0.91 0.75 0.74
Environmental benefit
Pest vulnerability
Weighted
Normalized
Weighted
Normalized
Weighted
Normalized
0.30 0.24 0.51
0.04 0.06 0.13
0.60 0.50 0.49
0.32 0.47 0.58
0.61 0.49 0.48
0.25 0.40 0.57
Fig. 2. Simpson index value weighted by environmental benefit for four different communities in Ohio (Yellow Springs, Dublin, Westerville, and Toledo) at three taxonomic levels.
Fig. 3. Simpson index weighted by pest vulnerability for four different communities in Ohio (Yellow Springs, Dublin, Westerville, and Toledo) at three taxonomic levels.
Fig. 4. Simpson index weighted by taxa adaptability rating for four different communities in Ohio (Yellow Springs, Dublin, Westerville, and Toledo) at three taxonomic levels.
Taxa adaptability
area, such as storm water, air pollution, energy benefits, pest vulnerability, and adaptability. Weighted ratings could and perhaps should be established by one or more experts in a particular field, e.g., entomology, if they are to be applied on an area or regional basis. Alternatively, if a community forester feels sufficiently confident in making judgments for his/her community, he/she could weigh various characteristics on the basis of his/her own experience. We recommend the use of composite ratings across disciplines for a characteristic such as pest vulnerability, including at least the aspects of plant pathology, entomology, and plant physiology. The normalized weighted index is useful as a tool for evaluating the performance of a community against itself and as a guide in selecting priorities when planning tree replacement. A smaller normalized index suggests that there is room for improvement using plants that are already in the community’s plant list. Using additional taxa would also add diversity; however, because a normalized index is a comparison against itself, it uses only plants currently in the inventory. The weightings used for pest vulnerability and adaptability in the present study are subjective and can change quickly in situations such as the introduction of a new exotic pest to a region, e.g., ALB in Ohio. It is preferable to incorporate the collective wisdom of a group of multidisciplinary evaluators in the decision-making process, as difference evaluators tend to view the problem from differing perspectives. Community personnel could determine their own indexes if they were sufficiently knowledgeable/experienced, or regional weightings could be used. Another issue in the use of weighted indexes to increase diversity is defining the spatial scale to be used. Heimlich, Sydnor, Bumgardner, and O’Brien (2008) found that the citizens surveyed in Toledo did not value uniformity in their street tree planting; however, most city planners and landscape architects design single-species plantings along streets or street segments to impart a sense of unity. Community foresters normally control only public plantings within their jurisdiction, although they may offer suggested lists for private consideration. The communities in the present study range in population and size from Toledo (with a human population of 287,201, a street tree population of 87,782 trees, and an area of 224.6 km2 divided into 63 separate management units) to Yellow Springs (with a human population of 3487, a street tree population of 1676, and an area of 4.9 km2 in one management unit). Using political units as in this study, single-species plantings along a street or street segment can be performed. We therefore suggest that either an entire political unit or a management unit as defined by a larger community are appropriate units for the evaluation of diversity, assuming that the larger unit wishes to perform additional analyses to support its position for an initiative.
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Weighted Simpson indexes can also be used to weigh desirable characteristics or functions for consideration in an existing tree inventory or as a guide to identify additional trees that could be used to enhance the selected functionality with a program such as i-Tree Species. In most instances, a community does not have a number of diversity indexes from similar-sized communities within a state or region that can be used to make comparisons. Thus, for many communities, the most useful index will be the normalized index in which the community compares its current rating against the rating that it can maximally expect to attain. Weightings for a variety of desirable traits can be combined and evaluated to develop a more sustainable landscape over time. Any single rating is based on historical experience and is thus backward looking. The goal is a sustainable landscape or ecosystem over time, which requires reduced inputs such as labor, pesticides, and fertilizers to remain viable in the future.
4. Conclusion One of the commonly used measures of diversity in street tree population is the Simpson index. This index considers only the proportions of various taxa. However, when planning future street tree replacements for enhancement of long-term functionality, several other factors must also be considered, including the environmental benefits from trees, pest vulnerability, and taxa adaptability. Communities face differing problems over time. For example, another agency might identify an area of concern and request that the community step up its efforts to enhance benefits in an area such as reduction of storm water runoff. The taxa of street trees can be identified and weighted to assist in reducing runoff or reducing pest vulnerability. These functional values can be attached as weights to the various taxa, and the weighted Simpson index can be calculated. The computation of Simpson indexes for the four Ohio communities revealed little variability, and the communities were rated as reasonably diverse. In contrast, the weighted Simpson indexes revealed areas of concern for each of the four communities. Each of the communities lacked large statured trees, which offer greater environmental benefits at a given age (maturity). For example, Sydnor and Subburayalu (2011) reported a 7.5-fold increase in annual environmental benefits by planting large statured trees in the streets of Brooklyn, OH. Similarly, McPherson et al. (2006) reported an eightfold increase in net annual environmental benefits by planting large statured trees. Planting of medium to small trees is frequently recommended for esthetic reasons, such as maintaining the scale of today’s smaller properties and for ease of management (Ophardt, n.d.). A survey of homeowners whose mature green ash had been condemned because of the presence of EAB revealed that they valued larger trees (Heimlich et al., 2008). None of the homeowners in that survey suggested that their trees were too large. However, we do not recommend the use of large statured trees in locations where size becomes a limitation. For example, large statured trees should not be planted beneath utility lines because utility pruning is expensive, causes conflict, and can interrupt utility service. We suggest only that communities benefit by planting large statured trees where allowed by site conditions. For the city of Toledo, this study revealed overreliance on a single genus (Acer), as indicated by a decline in the weighted Simpson index values at the genus and family levels. Maples comprise 48% of Toledo’s tree population (Sydnor & Subburayalu, 2008b). The recent discovery of ALB in Clermont County, Ohio in June 2011 poses a threat to other Ohio communities with predominant host populations of a single taxon such as maple. The city of Toledo is in danger of losing 8.3 million dollars in annual environmental benefits if ALB
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becomes an established pest in the city (Sydnor & Subburayalu, 2008b), in contrast to the 1.45 million dollar loss in benefits resulting from EAB (Sydnor & Subburayalu, 2008a). These estimated costs do not include tree removal, treatment, or replacement costs. These findings emphasize the need for diversity at not only the species level but also higher taxonomic levels (genus and family), as long ´ & McBride, 2008; Nowak, advocated by forestry scientists (Lacan 2001; Raupp et al., 2006). Diversity, as mentioned above, is a tool rather than an ultimate goal, and does not preclude the role of esthetic considerations. There are many examples of ecosystems that are near-monocultures consisting of only a few species yet have functioned sustainably for hundreds of years, e.g., the coniferous forests of western Washington and Oregon (Burns & Honkala, 1990). In contrast, diversity employed to promote sustainability is a forwardlooking tool that aims to ensure that the loss of single taxon does not disrupt the entire urban forest community in an unsustainable manner. This use of diversity is based on the concept of sharing risks among a number of different taxa. The most useful index is a normalized index whereby a community compares its current rating against the rating that it can maximally expect to attain. Weightings for a variety of desirable traits should be combined or evaluated to develop a more functional and sustainable landscape going forward. Maximizing a single benefit, e.g., environmental benefits which would require the use of only larger growing trees, will result in lower overall diversity, increased maintenance costs, and potentially reduced esthetic appeal. By balancing the risks and benefits of diversity against other benefits such as pest vulnerability, environmental benefit, and taxa adaptability, the weighted Simpson index can assist a community vegetation manager/planner in designing a sustainable and functional landscape in urban communities. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.landurbplan.2012.02.004. References Anonymous. (1979). Guide to rating trees and other plants in Ohio. Ohio Chapter. Columbus, OH: International Society of Arboriculture., 11 pp. Burns, R. M., & Honkala, B. H. (1990). Silvics of North America. Conifers. Agriculture handbook 654 Washington, DC: USDA Forest Service, U.S. Government Printing Office., 675 pp. Council of Tree and Landscape Appraisers. (2000). Guide for plant appraisal (ninth ed.). Champaign, IL. 143 pp. Galvin, M. F. (1999). A methodology for assessing and managing biodiversity in street tree populations: A case study. Journal of Arboriculture, 25, 124–128. Guiasu, R. C., & Guiasu, S. (2003). Conditional and weighted measures of ecological diversity. International Journal of Uncertainty, Fuzziness and Knowledge-Based Systems, 11, 283–300. Guntenspergen, G. (1983). Diversity and stability in a street tree population – Comment. Urban Ecology, 7, 173–176. Heimlich, J. E., Sydnor, T. D., Bumgardner, M., & O’Brien, P. (2008). Attitudes of residents toward street trees on four streets in Toledo, Ohio, U.S. before removal of ash trees (Fraxinus spp.) from Emerald Ash Borer (Agrilus planipennis). Arboriculture and Urban Forestry, 34(1), 47–53. Herms, D. A., McCullough, D. G., & Smitley, D. R. (2004). Under Attack: The current status of the Emerald Ash Borer infestation and the program to eradicate it. American Nurseryman, 200(7), 20–27. ´ Lacan, I., & McBride, J. R. (2008). Pest Vulnerability Matrix (PVM): A graphic model for assessing the interaction between tree species diversity and urban forest susceptibility to insects and diseases. Urban Forestry and Urban Greening, 7, 291–300. Maco, S. E., & Mcpherson, E. G. (2003). A Practical approach to assessing structure, function, and value of street tree populations in small communities. Journal of Arboriculture, 29(2), 84–97. McPherson, E. G, & Rowntree, R. A. (1989). Using structural measures to compare twenty-two U.S. street tree populations. Landscape Journal, 8, 13–23. McPherson, E. G., Simpson, J. R., Peper, P. J., Maco, S. E., Gardner, S. L., Cozad, S. K., et al. (2006). Midwest community tree guide: Benefits, costs, and strategic planting. In USDA Forest Service General Technical Report PSW-GTR-199 (99 pp.).
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