A GIS-Based Species Diversity Assessment of Ganesha Park

Jennifer Becker & Nicole McClain

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Abstract Urban-forests provide a potential myriad of ecological and aesthetic benefits. In order for these benefits to be maximized, however, urban-forests must be properly managed and maintained. Of specific importance to the health of the urban forest is species diversity. Our study of Ganesha Park investigates species diversity in terms of richness and evenness to determine if management of the park is adequate for ensuring species diversity. Our results indicate species diversity is substandard, not adhering to the 10-20-30 rule, with additional cause for concern centering around two invasive, non-native trees – Myoporum and Mimosa – which have the potential to rapidly take-over the area if management cannot inhibit their growth. Recommendations for improving the urban-forest structure of Ganesha Park reflect the need for a management plan that begins with immediate removal of the invasive Myoporum and Mimosa trees as well as a focus on incorporating more native species into the landscape.

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Introduction Increasing urbanization and consequent environmental degradation (Kavzoglu and Colkesen 2011; Skärbäck 2007) and deforestation of the urban environment (Cray 2007; Jonnes 2011), combined with the complex interactions of cultural and natural systems, has elevated the importance of urban forestry as a means of improving city conditions through proper maintenance and management of the urban forest (Millward and Sabir 2010; Young 2010). Urban forestry, however, is poorly established in most municipalities (Johnson 1982) and is oftentimes overshadowed by other urban concerns, thus diverting needed resources away from this discipline (Urban Budget Crisis Felling Many Cities' Tree Programs 1992). It is, therefore, necessary to emphasize the importance of the urban forest through the recognition of the multitude of ecological and aesthetic benefits trees provide, while understanding how urban forestry maximizes such benefits. Healthy populations of urban trees are noteworthy for the ecosystem services they provide. Trees mitigate the urban heat island effect through cooling from shading and evapotranspiration (Rosenfeld et al. 1995; Akbari et al. 2001; McPherson and Simpson 2001), while serving as windbreaks in the winter (McPherson et al. 2006). Through their moderation of the climate, trees are a valuable means by which to conserve energy and subsequently reduce costs (McPherson and Simpson 2001). Furthermore, trees reduce air pollution by trapping particulate matter in their canopies, absorbing noxious pollutants through their leaves and producing oxygen through photosynthesis (McPherson et al. 2006). Urban forests are also an important means of controlling stormwater runoff (McPherson et al. 2006; Xiao and McPherson 2003). Trees intercept rainfall in their canopies while their root systems create soil conditions that induce infiltration, thus reducing runoff. Additional benefits resulting from the presence of a healthy urban forest include: reduced noise pollution and increased wildlife habitat (Millward and Sabir 2010). Aside from the myriad of ecological benefits urban trees provide, their aesthetic advantages are also significant and well documented. Urban forests, for example, enhance the appearance of cities and numerous studies have linked their presence with improved physical and mental health of human populations (Dwyer et al. 1992; Seymour 1977; Kuo 2003). Additionally, trees increase the value of surrounding real estate (Tyrvainen and Miettinen 2005; Man et al. 2008),

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consequently increasing the city’s tax revenue and further helping to increase the appeal of the city. Along with the benefits offered by urban forests comes a responsibility for proper maintenance and management. Not only will the ecosystem services provided by improperly cared for trees not reach their full potential, but such trees are potential sources of danger when left unmanaged (Hauer and Johnson 2003). Specifically, proper management of the urban-forest involves a thorough understanding of the species composition in terms of both richness and evenness (Sun 1992). It is necessary to understand the importance of species diversity and the potential, species-specific environmental or human induced stressors that could devastate an urban-forest if species composition is too uniform. While diversity is a highly sought after quality of an urban-forest, it is often obtained through the incorporation of non-native species. Inclusion of non-native species necessitates that the area is well managed because without proper knowledge or maintenance, non-native species can be very invasive, taking over large areas, and possibly bringing in new pests or diseases that could harm native vegetation (Moser et al. 2009). Therefore, ensuring that an urban community obtains the greatest potential benefits from their urban forests, while minimizing possible risks, requires proper management of its species composition. A tree survey conducted in Ganesha Park, the oldest and largest park in the city of Pomona, California, reveals a species composition of relatively low diversity, influenced heavily by invasive, non-native trees. Consequently, the species composition of Ganesha Park reveals the inadequacy of the park’s management and the potential for future risk if improper management continues. Background Species diversity is reflective of two key elements –richness and evenness (Sun 1992). While species richness represents the variety of species present, evenness is a measure of the relative abundance, or frequency, of a species present in an area. Such diversity is a key factor of a sustainable urban-forest (Clark 1997). An urban-forest with low diversity, for example, is more susceptible to human or environmental stressors, such as diseases, that are specific to a certain species. The consequence of low species diversity was exemplified when Dutch Elm

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Disease wiped out the American Elms once lining streets and standing prominently in city parks. The overreliance on this species for landscaping proved to be devastating as city after city was wiped out of their Elms, with more than 80 million being eliminated since the 1930s. Since this time, much greater emphasis has been placed on the importance of species diversity in urban settings, thus revealing the need for careful management and planning of urban-forests to ensure that proper diversity is met. Proper species diversity can be judged against several standards, with some of the more conservative measures recommending that no more than 5% of an area consist of the same species (Burcham 2009). Although such conservative measures are concerned with creating a diverse population that will not fall victim to species-specific stressors, such stringent guidelines can also be difficult to meet if the conditions of the urban environment are not conducive to the growth of many plants. Thus, a commonly used benchmark has been developed that is less stringent, but research still indicates that it is a good safeguard against species-specific stressors (Galvin 1999). The 10-20-30 rule, as it is commonly referred to, states that devastation of an urban-forest can be avoided if no more than 10% of the trees are the same species, no more than 20% of the trees are from the same genus and no more than 30% of the trees are from the same family (Santamour 1990; Galvin 1999). For management of the urban-forest, this is the guideline that is oftentimes sought after and used as a standard to indicate if species diversity is acceptable or inadequate and a possible cause for concern (Galvin 1999). Following such, increased species diversity in urban-forests has commonly been associated with moderate urbanization and the introduction of non-native ornamental trees (McKinney 2008). Although the incorporation of non-native trees can potentially increase diversity, their inclusion can also lead to the downfall of urban forests if not well managed (Kovaks 2011). Whether they were purposely planted for ornamental reasons, washed into the area via storm drains or even dumped as compost piles, if not taken care of, they can quickly become invasive and take over valuable space native vegetation needs to survive. This overcrowding can lead to problems such as: canopy deformation, shade-evoked branch dieback, turfgrass loss, soil erosion and an overall decay of the area’s vegetative health.

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The non-native trees that thrive here in Southern California tend to come from sub-tropical climates. Since Southern California has a Mediterranean climate, there tends to be seasons where precipitation is almost void. Due to this fact, when precipitation does fall, non-native trees engulf most of the valuable water that would usually be sparsely shared amongst the native plants. Additionally, non-native plants have been known to change the flow of water, the quality of soil and all together change habitats, thus impacting native plants. Furthermore, non-native trees tend to bring diseases and parasites that the native plants have no immunity to (Kovaks 2011). Admittedly, to ensure the health of the urban-forest and maximize both ecological and aesthetic benefits, non-native trees need to be well managed and well maintained. Non-native trees are usually more costly and time consuming to manage (Pimental 2000), and if inadequately cared for, their presence can turn into more of nuisance than a way to increase species richness to ensure proper diversity. Study Area Our study area is Ganesha Park – a forested urban park located in Pomona, California. Established in the late 1800s, Ganesha Park is the oldest and largest (spanning approximately 60 acres) park in the city of Pomona (population of approximately 150,000). The park incorporates hillsides of vegetative cover – portions of which contain very steep, and often, unstable terrain. It is the forested hillsides that are of primary interest to this study. Methodology In order to examine the species composition of the park to assess the adequacy of the park management, an extensive tree survey has been conducted. The survey incorporated 206 trees surrounding “Inspiration Point” – a central and easily recognizable location within the park (Pg. 11, Figure 1). Boundaries for our data collection were marked by the presence of two small roads (Evergreen Dr. and Tonner Dr.) which defined the extent of our area surrounding “Inspiration Point”. This area, which incorporates approximately 7,000 square meters, was deemed representative of the vegetative cover of the hillsides of Ganesha Park and was thus our major determinant for choosing this location to conduct our tree survey.

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Trees present within our boundaries were tagged, numbered and recorded. Measurements taken for each tree include: GPS location, elevation, species (genus) identification, diameter at breast height (DBH), clinometer readings and the surveyor’s distance from the tree when the clinometer readings were taken. This data could then later be used to calculate the height and estimated age for each tree to help evaluate diversity. Tree conditions were also assessed based on the recommendations and guidelines of USDA Forest Service and categorized as good, fair, poor or dead (USDA Forest Service 2000). Tree conditions were assessed to help identify unhealthy trees or areas of concern in later analyses. To further assess the park’s species diversity, all data was entered into a GIS. Analyses were carried out with ArcGIS 10.0. Data Limitations Measurements were taken for every tree within our survey site with only a few exceptions due to data limitations. In all, 12 trees were omitted from the survey due to inaccessibility or danger. Trees completely surrounded by dense vegetative cover, unable to be reached by the surveyors, and for which accurate measurements were not obtainable were omitted from the survey (Pg. 12, Figure 2). Trees situated on very unstable terrain, where obtaining measurements would have put the surveyors at high risk for injury, were also omitted from the survey (Pg. 12, Figure 3). Measurements obtained during filed work were recorded with a Garmin Etrex, a unit with an accuracy of plus or minus 16 feet. Under dense vegetative cover and on sloping terrain GPS accuracy decreases, introducing possible errors into the dataset for x and y coordinates. With very closely spaced trees, the GPS coordinates were oftentimes so similar that the GPS would not always pick up a difference in the coordinates from one tree to another. Although multiple points in a location may exist, the maps created may appear that only one point exists, when in fact, a cluster of points is truly present in a given location.

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Our identification of tree species was further limited to identification at the genus level due to the increased difficulty of identifying trees at the species level. Results Compilation of our survey data revealed that twelve tree genera comprise our study area (Pg. 13, Figure 4). Of these twelve genera, their relative abundance was central to this investigation. Notably, the composition of species was dominated by only four genera – Myoporum (34%), Mimosa (20%), Eucalyptus (19%) and Quercus (15%) – accounting for 88% of the total species abundance (Pg. 14-15, Figures 5-9). Using the 10-20-30 rule as a baseline to begin our assessment of species composition, we have found solid evidence that the species diversity is substandard. While both Myoporum and Mimosa are at, or exceeding, the accepted limit, stating that no more than 20% of a single genus should constitute an area, Eucalyptus and Quercus are also quite close to the threshold. The relative lack of species diversity can, therefore, be considered a threat to the sustainability of Ganesha Park and reflective of poor park management. Our tree condition assessment, for example, further reveals the dangers of an urban-forest structure with inadequate species diversity. Of the 38 Eucalyptus trees present in our study area, 89% were noted in fair to poor condition and an additional 5% were classified as dead as based on the USDA guidelines. Largely, similar signs of wear are exhibited on each of the Eucalyptus trees in the population including: infestation with the Red Gum Lerp Psyllid (Pg. 16, Figure 10), splitting and peeling bark (Pg. 16, Figure 11) and decay in the trunk zone (Pg. 16, Figure 12). The similarity of tree conditions could be reflective of local environmental and human induced factors specific to Eucalyptus trees. Flanking the edges of the study area and serving as a windbreak, the Eucalyptus tree is a valuable asset to the urban-forest structure; however, the overwhelmingly poor condition of the trees and the relative abundance of the species prompts concern for the overall health of the urban-forest and possible devastation of the species if the conditions are not improved. Aside from the comparison of species frequency against the benchmark, consideration must be given to species richness. It is through our assessment of

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not only the abundance of species, but the specific species present in our study area, that management of the park is deemed inadequate and has thus prompted additional concern for the health of the urban-forest. Our investigation of Inspiration Point showed that 78% of tree species were non-native, with two specific genera – Myoporum and Mimosa – causing the greatest concern because of their invasive properties (Pg. 17, Figure 13). Comprising 34% of the total tree population, Myoporum (which is native to Australia and New Zealand), is the most invasive tree at Inspiration Point due to its remarkable lateral growth of up to 30 feet that essentially takes away space and resources that are needed for the growth of native trees. Although commonly found serving as a property divider or a natural fence, Myoporum must be regularly maintained due to the rapid take-over that happens. Our spatial analysis of the Myoporum distribution reveals significant clustering of the species indicative of its growth pattern and rapid establishment (Pg. 18, Figure 14). High clustering (as analyzed with ArcGIS Nearest Neighbor Spatial Analyst) (Pg. 19, Figure 15), with observations revealing no significant reason for species placement, combined with the visible lack of Myoporum maintenance (i.e. falling branches, overgrown vegetation) confirms that the take-over of this species was not planned, but is rather the result of management that is ill-prepared to deal with the invasive properties of this tree. Using size as a proxy for age, our results also show that many young Myoporum exist within our study area, revealing the continuing potential for an increased take-over by this species if removal of this younger generation is uninitiated. We have further identified the clusters and located a region within our study area that encompasses major clusters and therefore serves as a boundary for a problematic part of the urban-forest (Pg. 20, Figure 16). Additionally, Mimosa (native to China), is extremely invasive, accounting for 20% of the plant species, and an immediate cause for concern. Once widely used as an ornamental tree in key, well managed locations, it is now often considered undesirable and recommended for removal because of its invasive and harmful properties. During its growth in adolescence, the Mimosa can grow up to 50 feet tall, possibly blocking sunlight that is needed for the photosynthesis of surrounding, native species. Along with its invasive properties, Mimosas are found to have high levels of toxicity in their wood, which causes nausea to people

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who work with it (Totemeier 1985). Additionally, the seeds of the Mimosa are known to have high levels of active neurotoxins that upon ingestion can cause seizures and death to humans or animals (Totemeier 1985). It is a tree that in a very small percentage may be acceptable, but its large relative abundance, reveals that immediate attention must be given to the management of this species. As with the Myoporum, our spatial analysis of Mimosa distribution indicates highly clustered patterns (Pg. 21, Figure 17), reflective of its invasive growth properties. Our Average Nearest Neighbor report ran through ArcGIS, returned a Z-Value of -12.249586 – a highly significant result illustrating clustering. Many seedlings and newly established Mimosa trees were found throughout the study area and particularly amongst the clusters, revealing the continuing potential for this species to take-over more of the urban-forest if proper steps are not taken to curb the growth of this species. With ArcGIS spatial analysis, we have identified the greatest concentrations of Mimosa and located a region on a map that encompasses these hot spots to help ensure that the most problematic areas are identified (Pg. 21, Figure 17). Conclusion Our tree survey indicates that the species diversity within Ganesha Park does not meet the accepted standards and is therefore reflective of inadequate park management. Particularly troublesome is the apparent invasion of two non-native species – Myoporum and Mimosa – which together account for 54% of the tree species within the study area. The high density clustering and notable young plants and seedlings of these two invasive plants suggest that they will continue to take-over the park if left unattended. Informed management techniques must be implemented immediately to ensure that proper species diversity is present in the park to safeguard it against catastrophic loss initiated by human or environmental stressors as well as the rapid take-over of invasive vegetation. We recommend that the first steps towards ensuring diversity are through the removal of the Myoporum and Mimosa trees – specifically in the area of high-density clusters highlighted in our spatial analysis. Their removal will help inhibit future spread of the species through the sprouting of their seeds.

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We recommend that management of the urban-forest center around the incorporation of more native species as opposed to non-native species. As native species are generally a better fit for the environment, they can require less maintenance than non-native species, thus requiring less management and saving costs, which in the harsh economic times we are facing today, constitutes an invaluable savings. In order for the city of Pomona to enjoy the greatest ecological and aesthetic benefits from their urban-forest, proper management of the species, and therefore diversity, is necessary.

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Figure 1

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Figure 2

Figure 3

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Genus Abundance

Arborvitae 1

Ceratonia (Carob) 2

Douglas-Fir 2

Eucalyptus 38

Ficus 9

Heteromeles (Toyon) 2

Jacaranda 2

Magnolia 1

Mimosa 41

Myoporum 68

Pinus (Pine) 5

Quercus (Oak) 30

Total: 201

Figure 4

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Figure 5

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Figure 6: Myoporum

Figure 7: Mimosa

Figure 8: Oak Figure 9: Eucalyptus

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Figure 10: Red Lerp Psyllid

Figure 11: Peeling / Scarred Bark

Figure 12: Trunk Decay

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Figure 13

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Figure 14

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Average Nearest Neighbor Summary

Average Nearest Neighbor Summary

Observed Mean Distance: 0.000020

Expected Mean Distance: 5.072997

Nearest Neighbor Ratio: 0.000004

z-score: -15.775516

p-value: 0.000000

Figure 15

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Figure 16

21

Figure 17

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Average

Figure 18

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