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Comparison of carbon storage, carbon sequestration, and air pollution removal by protected and maintained urban forests in Alabama, USA

Comparison of carbon storage, carbon sequestration, and air pollution removal by protected and... International Journal of Biodiversity Science, Ecosystem Services & Management Vol. 8, No. 3, September 2012, 265–272 Comparison of carbon storage, carbon sequestration, and air pollution removal by protected and maintained urban forests in Alabama, USA a a a b Nicholas A. Martin *, Arthur H. Chappelka , Edward F. Loewenstein and Gary J. Keever a b School of Forestry and Wildlife Sciences, Auburn University, 602 Duncan Drive, Auburn, AL 36849, USA; Department of Horticulture, Auburn University, Auburn, AL 36849, USA The Auburn University campus in Auburn, Alabama, USA, was the location for a case study to compare carbon storage, carbon sequestration, and air pollution (CO, O ,NO , PM10, SO ) removal estimates between protected and maintained 3 2 2 urban forests. Results were from a complete tree inventory and i-Tree Eco analysis of the 237 ha maintained and 5.5 ha protected areas of the campus. Trees in the maintained landscapes had an average diameter at breast height of 16.4 cm and 2 2 basal area of 2.24 m /ha when compared with 24.4 cm and 12.04 m /ha for the protected area. The maintained landscapes were estimated to store 6652 kg of carbon per ha and sequester 291 kg/year/ha of carbon. The protected area was estimated to store 41,975 kg of carbon per ha and sequester 1758 kg/year/ha of carbon. Trees in the maintained areas removed 2970 kg/year of air pollution (12.5 kg/year/ha) compared with 560 kg/year for the protected area (102 kg/year/ha), which was 8 times the amount on a unit area basis compared with the maintained landscapes. The results demonstrate differences between maintained and protected forests and how important protected areas are in urban environments in enhancing carbon storage and sequestration and promoting air pollution removal. Keywords: air quality; i-Tree Eco; UFORE model; Urban Forestry; carbon sequestration; urban ecosystem services Introduction (Patterson and Coelho 2009). Most environmental prob- lems found in cities are created locally, and one of the most The urban environment is a dynamic landscape, where effective ways to deal with them is through local ecosystem humans cause changes every day that may be beneficial, services. The services generated also help in increasing the detrimental, short lived, or long lasting. The world’s human quality of life and public health (Bolund and Hunhammar population continues to rise and the migration to cities and 1999). urban areas is increasing (MEA 2005b). In the twentieth Besides determining what ecosystem services are being century, the urban population grew to 2.9 billion, and as of provided, how to value them is also important. Extensive 2005, there were 388 cities worldwide with populations of research has placed values on ecosystem services. Bolund 1 million or more people (MEA 2005b). These trends of and Hunhammar (1999) conducted research on ecosys- constant change and population migration are increasingly tem services in urban areas and observed different urban stressing our urban environments, forests, ecosystems, and ecosystems along with local direct services. They found ecosystem services. that locally generated benefits have a large impact on urban To better understand the changes occurring in our areas and should be addressed in land-use planning. Chee urban areas, we first need to appreciate our environment (2004) investigated how ecosystem service valuation is and what it provides humans. Moll and Petit (1994) defined being developed and how it fits in with economic frame- ecosystem as ‘a set of interacting species and their local, works and found that ecosystem service valuation has the non-biological environment functioning together to sus- potential to affect policies but the techniques still have tain life’. Ecosystem services can therefore be defined as shortcomings. Valuing ecosystem services from market, ‘the benefits human populations derive, directly or indi- price, and cost aspects was focused by Heal (2000), who rectly, from ecosystem functions’ (Costanza et al. 1997); concluded that the role of economics is to help provide more concisely, ‘ecosystem services are the benefits people incentives for conservation of natural systems. obtain from ecosystems’ (MEA 2005a). Research has also been conducted on the effects of Ecosystem services encompass numerous benefits that ecosystem services in urban settings. Nowak and Crane typically vary from region to region and from city to (2002) used field data and model outputs to estimate the city. Urban ecosystem services include air filtering, micro- amounts of carbon storage and sequestration that urban climate regulation, noise reduction, rainwater drainage, trees in the United States provided and their role in reduc- sewage treatment, recreational and cultural values (Bolund ing atmospheric carbon dioxide. Air pollution removal and Hunhammar 1999), carbon storage and sequestration, estimates were determined by Nowak et al. (2006), who energy savings (Nowak et al. 2008), and wildlife habitats *Corresponding author. Email: nmartin@bartlett.com ISSN 2151-3732 print/ISSN 2151-3740 online © 2012 Taylor & Francis http://dx.doi.org/10.1080/21513732.2012.712550 http://www.tandfonline.com 266 N.A. Martin et al. concluded that urban trees in the United States remove to those where maintenance is passive and trees are pro- large amounts of air pollution and, thus, affect air quality. tected, such as parks or arboretums (McDonnell and Pickett Nowak et al. (2006) also estimated a value ($) for the 1990; Welch 1994). It is important to understand how air pollution removal amounts using US median external- these intensity levels of maintenance affect ecosystem ser- ity values for the pollutants. Pandit and Laband (2010) vices, so appropriate management strategies and resources used a large sample of residences in part to evaluate and can be concentrated in areas where they provide the most quantify the effects that shade has on residential energy benefit. The Auburn University (AU) campus, Auburn, AL, consumption and provided energy usage savings according was an ideal location to evaluate these differences, having to different shade amounts. large areas that are intensively maintained, as well as an In addition, techniques and models have been devel- arboretum that is naturalized, protected, and more passively oped to help quantify ecosystem services, such as i-Tree maintained. The information reported here is a part of a Eco and i-Tree Streets (i-Tree 2010a). i-Tree Eco, orig- larger study evaluating the usefulness of i-Tree Eco pro- inally called the Urban Forest Effects (UFORE) model, tocols for a 100% inventory and validating certain i-Tree was developed by the US Department of Agriculture Forest Eco parameters for southern urban forests (Martin et al. Service (USDA FS) (Nowak and Crane 1998). This model 2011). Our goal was to compare the ecosystem services of uses field data and measurements to provide the basis a maintained and protected urban forest, while specifically for ecosystem services’ estimates. Data are input into the evaluating carbon sequestration, storage, and air pollution model that uses allometric equations based on species removal. Using this methodology, a monetary value for the and local climatic data to provide ecosystem services’ amount of each pollutant removed was calculated using estimates (Nowak and Crane 1998; Nowak et al. 2008). median externality values for the United States (i-Tree Benefits of this model are it uses the individual tree mea- 2010c). surements taken in the field as the basis for the resulting ecosystem services that are directly estimated from trunk Methods and crown dimensions and also the fact that it uses locally Study site generated estimates of atmospheric and climatic conditions ◦  ◦ as input for the model. These techniques and models have The study site was the AU campus (32 36 N, 85 30 W) been used in numerous cities in the United States and a few located in Auburn, Alabama (Figure 1). The core campus in other countries (Nowak et al. 2008). encompasses ∼237 ha of maintained landscapes; defined When managing urban forests’ levels of maintenance as those areas under the management of AU Landscape (McDonnell and Pickett 1990; Welch 1994) can affect Services and range from highly visible planting areas to remote parking lots with the level of maintenance cor- the ecosystem services provided because maintenance can responding to visibility and use. The study site included change the urban forest structure, which will in turn affect 237 ha of the maintained campus and the 5.5 ha Davis the services provided. These range from intensively main- Arboretum. tained areas (e.g., street trees and trees near buildings) Figure 1. Aerial photograph of the Auburn University campus and Davis Arboretum. Photograph taken in spring 2008. International Journal of Biodiversity Science, Ecosystem Services & Management 267 The Donald E. Davis Arboretum (Figure 1) (∼2% of scale ranging from excellent (6) to dying/dead (1). A more the size of the maintained campus), established in 1963, detailed description of the sampling methodology used can is maintained by the College of Sciences and Mathematics be obtained by referring to Martin et al. (2011) and i-Tree (Auburn University 2010). Its primary functions are edu- Eco (i-Tree 2010b). Tree locations were recorded with a cation, conservation, and research on ecosystem preser- Global Positioning System (GPS) unit (either a Trimble vation and diversity, which are conducted throughout the GeoXM GeoExplorer 2005 series or a Trimble GeoXT arboretum by letting native plants grow in special habitats GeoExplorer 2008 series, with an external antenna on a that exist in Alabama. The management philosophy of the tripod). arboretum is to encourage native species and habitats and Data were downloaded (daily) from the GPS units to over time the arboretum has evolved from a collection of a desktop computer using the Trimble GPS Pathfinder native trees to an arboretum that is expanding in the num- Office v.4.1 and 4.2 software. The ESRI ArcGIS TM ber of tree species and also native shrubs and herbaceous 9ArcMap v.9.3 software was used for final presenta- plants (Auburn University 2010). tion. Once collected, data were sent to the USDA FS-Urban Forestry South in Athens, Georgia, for analysis. Using this information, carbon storage, carbon sequestration, and Field data air pollution removal for the AU urban forest and Davis Arboretum were compared. Field data were collected during a complete tree inven- tory of the AU campus from summer 2009 to spring 2010 during full leaf conditions (Martin et al. 2011) i-Tree Eco analysis following i-Tree Eco procedures (i-Tree 2010b, 2010c), which resulted in a complete population sample of both Estimates provided by i-Tree Eco included carbon stor- age, carbon sequestration, and air pollution removal (i-Tree the AU main campus and Davis Arboretum. There were 2010c). Carbon storage is the amount of carbon stored in 16 attributes measured for each tree including tree species, the tree as biomass. Carbon sequestration is an estimated diameter at breast height (dbh) (1.37 m above the ground), rate for a given tree of the amount of carbon removed tree height, average crown width, dieback, and a relative from the air and stored in the tree annually. Carbon stor- tree condition rating modified from Webster (1978) and age and carbon sequestration occur when trees fix carbon Council of Tree and Landscape Appraisers (2000). Total number of stems per tree was recorded and dbh during photosynthesis and then store the excess carbon was measured using a logger’s diameter tape. Minimum as biomass, thus removing atmospheric carbon dioxide tree dbh to be included in the inventory was 2.54 cm, (CO ), a dominant greenhouse gas (Nowak and Crane and for multi-stem trees, up to the six largest stems were 2002). i-Tree Eco uses the field data collected for the recorded. Any tree that could not be measured at dbh was trees in combination with a series of calculations and allo- measured at 0.3 m from the ground-line following i-Tree metric equations to estimate the carbon storage. The Eco Eco protocol (i-Tree 2010b, 2010c). Total tree and bole model then uses tree diameter in combination with tree height were evaluated using either an MDL LaserAce growth models and equations to estimate the annual carbon TM hypsometer or a Laser Technology, Inc. TruPulse 360B sequestration rate for the trees (i-Tree 2010c). rangefinder. Total tree height was determined by measur- i-Tree Eco provides removal estimates of certain air ing from the alive or dead top of the tree down to the pollutants, specifically carbon monoxide (CO), ozone ground-line. Bole height was recorded as the height from (O ), nitrogen dioxide (NO ), particulate matter <10 µm 3 2 the ground-line to the bottom of the foliage of the low- (PM10), and sulfur dioxide (SO ), which is why the term est branch of significance. Crown width was the average ‘air pollution’ is used and not greenhouse gases (i-Tree of two measurements taken from the crown edges at 90 2010a, 2010c). For this case study, the air pollutants were angles (i-Tree 2010b; Martin et al. 2011). not evaluated separately because total air pollution removal Percent dieback and percent crown missing were also was the focus. The model uses a combination of field data, tree cover data, US Environmental Protection Agency determined for each tree. Dieback was evaluated by (EPA) pollution concentration monitoring data, and hourly observing all sides of the tree and assigning an overall National Climatic Data Center (NCDC) weather data from estimate of the percent dieback. Ranges of <1%, 1–10%, the local area as input. The model then uses the input 11–25%, 26–50%, 51–75%, 76–99%, and 100% dieback along with a series of equations to estimate the amount of were used to assign tree conditions of excellent, good, fair, poor, critical, dying, and dead, respectively. Percent crown air pollution removed. The model then estimates a mone- missing was estimated similarly to percent dieback, by tary value ($) using US median externality values for each viewing all sides of a tree and estimating the overall per- pollutant (i-Tree 2010c). cent missing in 5% increments. Directional pruning and branch loss from damage (ice, wind, etc.) are examples Carbon sequestration comparison that could attribute to missing crowns. The relative condi- tion rating accounted for visible damage such as dieback, To compare carbon sequestration for the maintained land- missing crown, presence of insects or disease, visible root scapes of the AU campus and protected Davis Arboretum, damage, and proximity of infrastructure and used a rating gross carbon sequestration amounts, as estimated by i-Tree 268 N.A. Martin et al. Eco, were divided by the total area to obtain a carbon Lagerstroemia spp. was the most common species in sequestration value on a unit area basis. Regression equa- the maintained landscapes of the main campus, whereas tions were developed for the campus and the arboretum, Pinus palustris, Liquidambar styraciflua, and Quercus using carbon sequestration as the dependent variable and nigra were the most common in the arboretum (Table 2). dbh as the independent variable. Intercepts and slopes were The five most abundant species comprised ∼49% of compared (α = 0.05) to determine differences in carbon the total population for the maintained campus com- sequestration for the two areas. pared with 18% for the Davis Arboretum, indicating much more diversity in the arboretum, with 160 tree species present compared with 139 for the maintained Results campus. Tree characteristics for the AU campus and Davis Arboretum are described in Table 1. The average dbh for Carbon storage, carbon sequestration, and air pollution the AU campus was 16.4 cm and for the arboretum was removal 24.4 cm (standard deviation: 19.6 and 19.4, respectively). The AU campus and the arboretum differed drastically The carbon storage estimate for the arboretum was ∼15% (16% and 62%, respectively) in canopy cover. The trees of the total for the main campus and carbon sequestration in the arboretum exhibited larger mean total height, crown was ∼14% (Table 3). However, when estimated on a per ha width, and basal area, whereas the AU campus contains basis, the arboretum stored and sequestered over 6 times only about 12% of the total number of trees in the main- more carbon than the main campus. There were no large tained landscapes (Table 1). differences in the estimated average amount of carbon sequestration per tree by diameter class between the AU Table 1. Overall tree characteristics for maintained areas of the campus and the arboretum (Table 4). Statistical analyses Auburn University campus and the protected Davis Arboretum (data not shown) indicated that there was no significant using i-Tree Eco inventory procedures. difference in slope (p-value =−0.0994) between the AU campus and Davis Arboretum. There was a significant Auburn difference in intercept (p-value < 0.0001) between the University campus Davis Arboretum campus and arboretum with the campus having the larger intercept coefficient, indicating that the smaller diameter Area sampled (ha) 237 5.5 trees on the campus were larger in diameter than those in Number of trees 7345 891 the arboretum and were in better condition. Number of species 139 160 Average dbh (cm) 16.4 24.4 On average, the maintained landscapes on the campus Average tree height (m) 8.5 12.7 were estimated to remove 12.5 kg/year/ha of air pollution Average tree crown 6.7 7.6 ($67/ha). The Davis Arboretum was estimated to remove width (m) 2 a Basal area (m /ha) 2.24 (0.001–1.9) 12.04 (0.001–1.13) Estimated canopy 16 62 Table 3. Carbon storage and sequestration rates for the Auburn cover (%) University campus and Davis Arboretum as of 2009–2010. Estimated 10,757,390 1,316,806 compensatory value Auburn University ($) Campus Davis Arboretum Notes: represents the range for all trees. Carbon storage (kg) 1,576,469.88 230,864.84 Estimated canopy cover determined by dividing the total canopy- (6,652/ha) (41,975/ha) projected ground area calculated by the model by the total area Gross carbon 69,063.88 9,670.94 inventoried. sequestration (291/ha/year) (1,758/ha/year) Estimated compensatory value calculated by i-Tree Eco is based on (kg/year) the Council of Tree and Landscape Appraisers (CTLA) method (i-Tree 2010b). Table 2. The five most common species for the Auburn University campus and Davis Arboretum with total number of trees and the percent of the total population (Pop.). Auburn University Campus Davis Arboretum Species # of trees % Pop. Species # of trees % Pop. Lagerstroemia spp. 1639 22 Pinus palustris 37 4 Quercus phellos 596 8 Liquidambar styraciflua 34 4 Pinus taeda 565 8 Quercus nigra 33 4 Magnolia grandiflora 464 6 Quercus alba 27 3 Quercus lyrata 363 5 Quercus stellata 26 3 Total 3,627 49 157 18 International Journal of Biodiversity Science, Ecosystem Services & Management 269 Table 4. Average carbon sequestration per tree (kg/year) by other urban areas. Those results could then be used to diameter class (cm) for the Auburn University campus and Davis aid in development and planning strategies to optimize Arboretum. ecosystem services. However, when evaluating air pollu- tion removal, it is important to remember that pollution Average carbon sequestration per tree (kg/year) concentrations can vary site to site based on local envi- Dbh (cm) Auburn University campus Davis Arboretum ronments and conditions and this should be taken into consideration. 1–15 3 3 Comparing the results from this study site to other 16–30 8 8 31–45 15 16 study sites in the south-eastern United States is crucial for 46–60 22 25 evaluation. The area where this case study was done is 61–76 32 35 relatively small in comparison with other study sites that 77+ 54 59 have been established in the south-eastern United States. It is important to remember that this case study was for a relatively small area (compared with large cities, suburbs, Table 5. Air pollution removal rates and removal values for etc.) and that within the case study itself, the arboretum the Auburn University campus and Davis Arboretum as of is small in comparison with the maintained campus. For 2009–2010. example, the City of Auburn was estimated to have an aver- Removal amount age pollution removal value of $0.29/tree/year in 2008 (kg/year) Removal value ($) (Huyler et al. 2010) compared with the estimated aver- age removal value of $2.29/tree/year for the maintained Auburn University 2,969.1 (12.5/ha) 15,880.27 (67/ha) landscapes of the AU campus and the Davis Arboretum Campus Davis Arboretum 560.2 (101.9/ha) 3,013.10 (548/ha) combined and $3.38/tree/year for the arboretum alone. Ozone (O ) and PM10 were the air pollutants estimated to have the highest removal amounts for both study sites (Huyler et al. 2010). The City of Auburn was estimated 102 kg/year/ha of air pollution ($548/ha), or ∼8 times to store an average of 1.8 kg carbon/tree (Huyler et al. more on a per ha basis (Table 5). 2010), and the maintained landscapes of the AU campus and Davis Arboretum combined were estimated to store an average of 219 kg carbon/tree and 259 kg carbon/tree Tree condition for the arboretum alone. The differences between the sites Differences in tree condition between the AU maintained could be attributed to 81.9% of the trees in Auburn having a landscapes and the Davis Arboretum were evaluated. Over dbh of <15.24 cm (Huyler et al. 2010), compared with only 60% of the trees on the maintained portion of the AU 43% for the AU campus and Davis Arboretum combined campus were rated as being in excellent or good con- and for the campus and arboretum alone. This indicates dition and about 3% in very poor or dying/dead con- that areas with larger trees will provide more ecosystem dition (Figure 2(a)). Approximately 71% of the trees in services (Escobedo et al. 2009a, 2009b). the Davis Arboretum were rated as being in excellent or good condition and about 1% in very poor or dying/dead condition (Figure 2(b)). Across species, for trees with Carbon sequestration comparison a dbh of ≥21 cm, approximately 28% and 17% of all Results for carbon sequestration from the AU campus trees in the arboretum and on the main campus, respec- and Davis Arboretum inventory were compared to carbon tively, were rated in good or excellent condition. For trees sequestration results from Gainesville, Florida (Escobedo with a dbh of ≥31 cm, approximately 18% and 10% of et al. 2009a). Estimated average per tree sequestration the respective populations fell into these categories. The rates by diameter class (1–15 cm, 16–30 cm, 31–45 cm, remaining trees were in fair, poor, very poor, or dying/dead 46–60 cm, 61–76 cm, and 77+ cm) for Gainesville were condition. 2, 9, 17, 9, 33, and 111 kg/year, respectively. Using the same diameter distribution classes, the estimated seques- tration rates for the AU campus and arboretum combined Discussion were 3, 8, 16, 23, 32, and 54 kg/year and 2, 8, 16, 25, Carbon storage, carbon sequestration, and air pollution 36, and 62 kg/year for the arboretum alone. Major differ- removal ences in carbon sequestration were in the 46–60 cm and Estimating the carbon storage, carbon sequestration, and 77+ cm diameter classes, with the latter having the largest air pollution removal provided by the maintained land- differences. These differences in the larger diameter classes scapes on the AU campus and the Davis Arboretum was could be the product of several factors, such as the small the main objective of this case study. To estimate the number of trees with large diameters on the campus and in full value of the urban forest, which would include recre- the arboretum, and differences in species composition and ation and other ecosystem services, the direct benefits that tree condition (Escobedo et al. 2009a, 2009c; Martin et al. they provide must be quantified and also compared with 2011). 270 N.A. Martin et al. (a) Good/excellent Fair Poor Very poor Dying/dead Tree dbh (cm) (b) Good/excellent Fair Poor Very poor 100 Dying/dead Dbh (cm) Figure 2. Tree condition by diameter class determined by the overall condition rating for the Auburn University campus (a) and Davis Arboretum (b). Note that the Auburn campus has ∼8 times the number of trees as the Davis Arboretum. Protected versus maintained urban forests of the maintained AU campus. McPherson et al. (1997) reported that 60–70% more air pollution could be removed Differences between protected and maintained urban by large, healthy trees as opposed to small trees, indicat- forests are important in understanding how to maximize ing that these trees are vital in increasing air pollution ecosystem services if it is the desired outcome. The most removal. When examining tree condition by diameter class, effective way to demonstrate differences in ecosystem ser- the arboretum, in general, appears to have higher tree con- vices provided by the main campus and arboretum was dition ratings, especially for larger diameter trees. Reasons to express our findings on a unit area basis. Results from for this could be because these trees are in a protected this case study indicated that the arboretum was esti- area with limited disturbances from construction or cam- mated to remove more than 8 times the amount of total pus maintenance (roads, power lines, water lines, etc.). air pollution per ha as the campus, which resulted in a Tree condition could also be a factor in why the inter- removal value that is $481 more per ha/year than cam- cepts for the AU campus and Davis Arboretum differed. pus. Air pollution removal is estimated to increase from The average condition of the trees planted on the AU 2970 kg/year to 24,144 kg/year if the maintained land- campus may be higher where larger, nursery-grown speci- scapes of the AU campus had a forest structure similar to mens are planted, compared with the arboretum where the that of arboretum, with the removal value increasing from smaller, younger trees are more likely regenerated naturally $15,880 to $129,837. However, a forest structure similar and may be under competition. It is important to remem- that of the arboretum may not be practical for the campus ber that the management goals and philosophies of the because of the infrastructure demands such as buildings, campus and arboretum are not the same with the campus roads, sidewalks, and utilities. focusing on maintained landscapes and the arboretum on Tree condition and size may play a role in the differ- encouraging more native or natural settings. Naturally, the ences in ecosystem services. In general, trees in the Davis benefits provided by each would not be the same; however, Arboretum were larger and in better condition than those 2.5–7.9 8–12.9 13–20.9 21–30.9 31–40.9 41–50.9 51–60.9 61–70.9 71–80.9 81–99.9 100+ 2.5–7.9 8–12.9 13–20.9 21–30.9 31–40.9 41–50.9 51–60.9 61–70.9 71–80.9 81–99.9 100+ Number of trees Number of trees International Journal of Biodiversity Science, Ecosystem Services & Management 271 it is important to remember what can be provided by natu- roads, and buildings). Even if infrastructure could be built ral settings and then try to guide our maintained landscapes in a natural setting without disturbing the area, disservices and urban areas in that direction. such as maintenance and damage to the infrastructure When evaluating canopy cover of urban and protected by the trees (heaving of sidewalks) would be greatly areas, it is important to discuss the urban heat island effect. increased. Appropriate planning can address some of these This phenomenon occurs when there are higher air and sur- issues. Because of development, not all natural areas can, face temperatures because of large areas of heat-absorbing or should, be saved; however, the most beneficial areas can surfaces in urban areas with higher amounts of energy be determined and then protected to help offset the loss of usage (Bolund and Hunhammar 1999; Sokecki et al. 2005). ecosystem services when sites are cleared for construction. This is important in estimating ecosystem services such as New construction sites are almost always landscaped when carbon sequestration and air pollution removal values since finished and this helps to offset the loss of vegetation, but factors such as temperature affect tree growth, productivity, the benefits provided by the new, almost always smaller and uptake of pollutants. Natural areas with more vegeta- plantings, does not come close to the benefits being tive cover can help mitigate this effect because they do not provided by well-established natural areas. The end result have either as many or as much heat-absorbing surfaces of the urban setting needs to be determined first so that as open urban areas, or because these areas shade the sur- infrastructure and green spaces can be balanced to provide faces from the sun causing less heat to be absorbed. More the most benefits possible. vegetative cover results in more evaporative cooling, which With urban environments come different levels of in turn lowers air temperature (Bolund and Hunhammar maintenance, depending on where you are and what type 1999; Sokecki et al. 2005). If canopy cover were to be of urban vegetation is present, among other factors. Areas increased on the AU campus, the urban heat island effect that are more protected, not maintained as intensively, could be reduced, possibly leading to larger, healthier trees. and are allowed to grow in more of a natural state pro- The potentially healthier trees could then possibly provide vide more ecosystem services at a lower cost, so more more ecosystem services. work should be done to leave natural areas in our urban Cost of tree maintenance and loss in tree value due environments because of their increased value in ecosys- to construction damage can also differ for protected and tem services. These increased services can be attributed maintained urban forests. The City of Gainesville, Florida, to the fact that protected areas contain larger trees that spent $1,559,932 (∼$10.57/tree) on care for the public are typically in better condition. Urban designers should urban forests in 2007 (Escobedo and Seitz 2009). Modesto, use the information and findings presented in this arti- California, had expenditures of $2,686,516 ($29.46/tree) cle to become better informed and possibly re-think urban for its urban forest from 1997 to 1998 (McPherson et al. designing in general so that the environment can be taken 1999). Natural areas, with less intensive management, into account, not only to help preserve it but also to incor- have much lower costs of maintenance, making their net porate it into the designs so that more benefits can be worth higher. Hauer et al. (1994) projected that the City provided. As more and more people become aware and pro- of Milwaukee, Wisconsin, has a loss in street tree value tective of our environments and the benefits they provide, of $792,100/year due to construction damage. Ecosystem more emphasis will be placed on designs where potential disservices, or costs, also have to be considered (Escobedo ecosystem services are the main focus of the design. In the et al. 2011; Pataki et al. 2011). Disservices (pollutants future, we need to focus on preserving areas of the urban from power equipment such as vehicles, saws, and mow- forest that provide more ecosystem services, specifically ers) include the cost of maintenance, increase in allergens, the protected areas where our mature trees are in better and attraction of wildlife. When examining differences condition so that ecosystem services can be optimized. between protected and maintained urban forests, ecosys- However, the entire urban forest needs to be considered tem disservices have to be estimated along with ecosys- and evaluated during the developmental stages so that the tem services to fully understand net benefits (McDonnell appropriate balance of developed areas and green spaces and Pickett 1990; Escobedo et al. 2011; Pataki et al. can be sustained. 2011). The trade-off between ecosystem services and disservices is very important in development planning Acknowledgements (Escobedo and Seitz 2009; Escobedo et al. 2011). The authors thank Dudley Hartel and Eric Kuehler of the USDA An understanding of the interactions between built and Forest Service-Urban Forestry South office for their assistance natural areas (urban–rural gradient) is also important and guidance during the project. They also thank Jonathon (McDonnell and Pickett 1990). The urban environment Bartlett, Mark Caldwell, Andrew Parker, Elliot Glass, Ann Huyler, and Efrem Robbins for their assistance with data col- needs both infrastructure and green spaces; however, lection; James Ransom and Daniel Mullenix for their technical they have to be balanced to address the needs of the assistance; Dr. Greg Somers for statistical guidance and Charlie urban population. As stated earlier, if the entire urban Crawford, Superintendent of AU Landscape Services, for his environment had a forest structure like that of a natural assistance throughout the project. This project was funded in part area, there would be no room for the infrastructure that is by Auburn University and the USDA Forest Service Co-operation Agreement FS-SRS-09-CA-11330150-053. necessary to sustain life in an urban setting (e.g., houses, 272 N.A. Martin et al. References [cited 2010 Oct 15]. Available from: http://www.itreetools. org/resources/manuals/i-Tree%20Eco%20Users%20Manual. Auburn University [Internet]. 2010. Auburn (AL): Donald E. pdf. Davis Arboretum, Auburn University; [cited 2010 Nov 20]. i-Tree-Tools for Assessing and Managing Community Forests Available from: http://www.auburn.edu/arboretum. [Internet]. 2010c. Washington (DC): USDA Forest Service; Bolund P, Hunhammar S. 1999. Ecosystem services in urban [cited 2010 Oct 15]. Available from: http://www.itreetools. areas. 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Comparison of carbon storage, carbon sequestration, and air pollution removal by protected and maintained urban forests in Alabama, USA

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Abstract

International Journal of Biodiversity Science, Ecosystem Services & Management Vol. 8, No. 3, September 2012, 265–272 Comparison of carbon storage, carbon sequestration, and air pollution removal by protected and maintained urban forests in Alabama, USA a a a b Nicholas A. Martin *, Arthur H. Chappelka , Edward F. Loewenstein and Gary J. Keever a b School of Forestry and Wildlife Sciences, Auburn University, 602 Duncan Drive, Auburn, AL 36849, USA; Department of Horticulture, Auburn University, Auburn, AL 36849, USA The Auburn University campus in Auburn, Alabama, USA, was the location for a case study to compare carbon storage, carbon sequestration, and air pollution (CO, O ,NO , PM10, SO ) removal estimates between protected and maintained 3 2 2 urban forests. Results were from a complete tree inventory and i-Tree Eco analysis of the 237 ha maintained and 5.5 ha protected areas of the campus. Trees in the maintained landscapes had an average diameter at breast height of 16.4 cm and 2 2 basal area of 2.24 m /ha when compared with 24.4 cm and 12.04 m /ha for the protected area. The maintained landscapes were estimated to store 6652 kg of carbon per ha and sequester 291 kg/year/ha of carbon. The protected area was estimated to store 41,975 kg of carbon per ha and sequester 1758 kg/year/ha of carbon. Trees in the maintained areas removed 2970 kg/year of air pollution (12.5 kg/year/ha) compared with 560 kg/year for the protected area (102 kg/year/ha), which was 8 times the amount on a unit area basis compared with the maintained landscapes. The results demonstrate differences between maintained and protected forests and how important protected areas are in urban environments in enhancing carbon storage and sequestration and promoting air pollution removal. Keywords: air quality; i-Tree Eco; UFORE model; Urban Forestry; carbon sequestration; urban ecosystem services Introduction (Patterson and Coelho 2009). Most environmental prob- lems found in cities are created locally, and one of the most The urban environment is a dynamic landscape, where effective ways to deal with them is through local ecosystem humans cause changes every day that may be beneficial, services. The services generated also help in increasing the detrimental, short lived, or long lasting. The world’s human quality of life and public health (Bolund and Hunhammar population continues to rise and the migration to cities and 1999). urban areas is increasing (MEA 2005b). In the twentieth Besides determining what ecosystem services are being century, the urban population grew to 2.9 billion, and as of provided, how to value them is also important. Extensive 2005, there were 388 cities worldwide with populations of research has placed values on ecosystem services. Bolund 1 million or more people (MEA 2005b). These trends of and Hunhammar (1999) conducted research on ecosys- constant change and population migration are increasingly tem services in urban areas and observed different urban stressing our urban environments, forests, ecosystems, and ecosystems along with local direct services. They found ecosystem services. that locally generated benefits have a large impact on urban To better understand the changes occurring in our areas and should be addressed in land-use planning. Chee urban areas, we first need to appreciate our environment (2004) investigated how ecosystem service valuation is and what it provides humans. Moll and Petit (1994) defined being developed and how it fits in with economic frame- ecosystem as ‘a set of interacting species and their local, works and found that ecosystem service valuation has the non-biological environment functioning together to sus- potential to affect policies but the techniques still have tain life’. Ecosystem services can therefore be defined as shortcomings. Valuing ecosystem services from market, ‘the benefits human populations derive, directly or indi- price, and cost aspects was focused by Heal (2000), who rectly, from ecosystem functions’ (Costanza et al. 1997); concluded that the role of economics is to help provide more concisely, ‘ecosystem services are the benefits people incentives for conservation of natural systems. obtain from ecosystems’ (MEA 2005a). Research has also been conducted on the effects of Ecosystem services encompass numerous benefits that ecosystem services in urban settings. Nowak and Crane typically vary from region to region and from city to (2002) used field data and model outputs to estimate the city. Urban ecosystem services include air filtering, micro- amounts of carbon storage and sequestration that urban climate regulation, noise reduction, rainwater drainage, trees in the United States provided and their role in reduc- sewage treatment, recreational and cultural values (Bolund ing atmospheric carbon dioxide. Air pollution removal and Hunhammar 1999), carbon storage and sequestration, estimates were determined by Nowak et al. (2006), who energy savings (Nowak et al. 2008), and wildlife habitats *Corresponding author. Email: nmartin@bartlett.com ISSN 2151-3732 print/ISSN 2151-3740 online © 2012 Taylor & Francis http://dx.doi.org/10.1080/21513732.2012.712550 http://www.tandfonline.com 266 N.A. Martin et al. concluded that urban trees in the United States remove to those where maintenance is passive and trees are pro- large amounts of air pollution and, thus, affect air quality. tected, such as parks or arboretums (McDonnell and Pickett Nowak et al. (2006) also estimated a value ($) for the 1990; Welch 1994). It is important to understand how air pollution removal amounts using US median external- these intensity levels of maintenance affect ecosystem ser- ity values for the pollutants. Pandit and Laband (2010) vices, so appropriate management strategies and resources used a large sample of residences in part to evaluate and can be concentrated in areas where they provide the most quantify the effects that shade has on residential energy benefit. The Auburn University (AU) campus, Auburn, AL, consumption and provided energy usage savings according was an ideal location to evaluate these differences, having to different shade amounts. large areas that are intensively maintained, as well as an In addition, techniques and models have been devel- arboretum that is naturalized, protected, and more passively oped to help quantify ecosystem services, such as i-Tree maintained. The information reported here is a part of a Eco and i-Tree Streets (i-Tree 2010a). i-Tree Eco, orig- larger study evaluating the usefulness of i-Tree Eco pro- inally called the Urban Forest Effects (UFORE) model, tocols for a 100% inventory and validating certain i-Tree was developed by the US Department of Agriculture Forest Eco parameters for southern urban forests (Martin et al. Service (USDA FS) (Nowak and Crane 1998). This model 2011). Our goal was to compare the ecosystem services of uses field data and measurements to provide the basis a maintained and protected urban forest, while specifically for ecosystem services’ estimates. Data are input into the evaluating carbon sequestration, storage, and air pollution model that uses allometric equations based on species removal. Using this methodology, a monetary value for the and local climatic data to provide ecosystem services’ amount of each pollutant removed was calculated using estimates (Nowak and Crane 1998; Nowak et al. 2008). median externality values for the United States (i-Tree Benefits of this model are it uses the individual tree mea- 2010c). surements taken in the field as the basis for the resulting ecosystem services that are directly estimated from trunk Methods and crown dimensions and also the fact that it uses locally Study site generated estimates of atmospheric and climatic conditions ◦  ◦ as input for the model. These techniques and models have The study site was the AU campus (32 36 N, 85 30 W) been used in numerous cities in the United States and a few located in Auburn, Alabama (Figure 1). The core campus in other countries (Nowak et al. 2008). encompasses ∼237 ha of maintained landscapes; defined When managing urban forests’ levels of maintenance as those areas under the management of AU Landscape (McDonnell and Pickett 1990; Welch 1994) can affect Services and range from highly visible planting areas to remote parking lots with the level of maintenance cor- the ecosystem services provided because maintenance can responding to visibility and use. The study site included change the urban forest structure, which will in turn affect 237 ha of the maintained campus and the 5.5 ha Davis the services provided. These range from intensively main- Arboretum. tained areas (e.g., street trees and trees near buildings) Figure 1. Aerial photograph of the Auburn University campus and Davis Arboretum. Photograph taken in spring 2008. International Journal of Biodiversity Science, Ecosystem Services & Management 267 The Donald E. Davis Arboretum (Figure 1) (∼2% of scale ranging from excellent (6) to dying/dead (1). A more the size of the maintained campus), established in 1963, detailed description of the sampling methodology used can is maintained by the College of Sciences and Mathematics be obtained by referring to Martin et al. (2011) and i-Tree (Auburn University 2010). Its primary functions are edu- Eco (i-Tree 2010b). Tree locations were recorded with a cation, conservation, and research on ecosystem preser- Global Positioning System (GPS) unit (either a Trimble vation and diversity, which are conducted throughout the GeoXM GeoExplorer 2005 series or a Trimble GeoXT arboretum by letting native plants grow in special habitats GeoExplorer 2008 series, with an external antenna on a that exist in Alabama. The management philosophy of the tripod). arboretum is to encourage native species and habitats and Data were downloaded (daily) from the GPS units to over time the arboretum has evolved from a collection of a desktop computer using the Trimble GPS Pathfinder native trees to an arboretum that is expanding in the num- Office v.4.1 and 4.2 software. The ESRI ArcGIS TM ber of tree species and also native shrubs and herbaceous 9ArcMap v.9.3 software was used for final presenta- plants (Auburn University 2010). tion. Once collected, data were sent to the USDA FS-Urban Forestry South in Athens, Georgia, for analysis. Using this information, carbon storage, carbon sequestration, and Field data air pollution removal for the AU urban forest and Davis Arboretum were compared. Field data were collected during a complete tree inven- tory of the AU campus from summer 2009 to spring 2010 during full leaf conditions (Martin et al. 2011) i-Tree Eco analysis following i-Tree Eco procedures (i-Tree 2010b, 2010c), which resulted in a complete population sample of both Estimates provided by i-Tree Eco included carbon stor- age, carbon sequestration, and air pollution removal (i-Tree the AU main campus and Davis Arboretum. There were 2010c). Carbon storage is the amount of carbon stored in 16 attributes measured for each tree including tree species, the tree as biomass. Carbon sequestration is an estimated diameter at breast height (dbh) (1.37 m above the ground), rate for a given tree of the amount of carbon removed tree height, average crown width, dieback, and a relative from the air and stored in the tree annually. Carbon stor- tree condition rating modified from Webster (1978) and age and carbon sequestration occur when trees fix carbon Council of Tree and Landscape Appraisers (2000). Total number of stems per tree was recorded and dbh during photosynthesis and then store the excess carbon was measured using a logger’s diameter tape. Minimum as biomass, thus removing atmospheric carbon dioxide tree dbh to be included in the inventory was 2.54 cm, (CO ), a dominant greenhouse gas (Nowak and Crane and for multi-stem trees, up to the six largest stems were 2002). i-Tree Eco uses the field data collected for the recorded. Any tree that could not be measured at dbh was trees in combination with a series of calculations and allo- measured at 0.3 m from the ground-line following i-Tree metric equations to estimate the carbon storage. The Eco Eco protocol (i-Tree 2010b, 2010c). Total tree and bole model then uses tree diameter in combination with tree height were evaluated using either an MDL LaserAce growth models and equations to estimate the annual carbon TM hypsometer or a Laser Technology, Inc. TruPulse 360B sequestration rate for the trees (i-Tree 2010c). rangefinder. Total tree height was determined by measur- i-Tree Eco provides removal estimates of certain air ing from the alive or dead top of the tree down to the pollutants, specifically carbon monoxide (CO), ozone ground-line. Bole height was recorded as the height from (O ), nitrogen dioxide (NO ), particulate matter <10 µm 3 2 the ground-line to the bottom of the foliage of the low- (PM10), and sulfur dioxide (SO ), which is why the term est branch of significance. Crown width was the average ‘air pollution’ is used and not greenhouse gases (i-Tree of two measurements taken from the crown edges at 90 2010a, 2010c). For this case study, the air pollutants were angles (i-Tree 2010b; Martin et al. 2011). not evaluated separately because total air pollution removal Percent dieback and percent crown missing were also was the focus. The model uses a combination of field data, tree cover data, US Environmental Protection Agency determined for each tree. Dieback was evaluated by (EPA) pollution concentration monitoring data, and hourly observing all sides of the tree and assigning an overall National Climatic Data Center (NCDC) weather data from estimate of the percent dieback. Ranges of <1%, 1–10%, the local area as input. The model then uses the input 11–25%, 26–50%, 51–75%, 76–99%, and 100% dieback along with a series of equations to estimate the amount of were used to assign tree conditions of excellent, good, fair, poor, critical, dying, and dead, respectively. Percent crown air pollution removed. The model then estimates a mone- missing was estimated similarly to percent dieback, by tary value ($) using US median externality values for each viewing all sides of a tree and estimating the overall per- pollutant (i-Tree 2010c). cent missing in 5% increments. Directional pruning and branch loss from damage (ice, wind, etc.) are examples Carbon sequestration comparison that could attribute to missing crowns. The relative condi- tion rating accounted for visible damage such as dieback, To compare carbon sequestration for the maintained land- missing crown, presence of insects or disease, visible root scapes of the AU campus and protected Davis Arboretum, damage, and proximity of infrastructure and used a rating gross carbon sequestration amounts, as estimated by i-Tree 268 N.A. Martin et al. Eco, were divided by the total area to obtain a carbon Lagerstroemia spp. was the most common species in sequestration value on a unit area basis. Regression equa- the maintained landscapes of the main campus, whereas tions were developed for the campus and the arboretum, Pinus palustris, Liquidambar styraciflua, and Quercus using carbon sequestration as the dependent variable and nigra were the most common in the arboretum (Table 2). dbh as the independent variable. Intercepts and slopes were The five most abundant species comprised ∼49% of compared (α = 0.05) to determine differences in carbon the total population for the maintained campus com- sequestration for the two areas. pared with 18% for the Davis Arboretum, indicating much more diversity in the arboretum, with 160 tree species present compared with 139 for the maintained Results campus. Tree characteristics for the AU campus and Davis Arboretum are described in Table 1. The average dbh for Carbon storage, carbon sequestration, and air pollution the AU campus was 16.4 cm and for the arboretum was removal 24.4 cm (standard deviation: 19.6 and 19.4, respectively). The AU campus and the arboretum differed drastically The carbon storage estimate for the arboretum was ∼15% (16% and 62%, respectively) in canopy cover. The trees of the total for the main campus and carbon sequestration in the arboretum exhibited larger mean total height, crown was ∼14% (Table 3). However, when estimated on a per ha width, and basal area, whereas the AU campus contains basis, the arboretum stored and sequestered over 6 times only about 12% of the total number of trees in the main- more carbon than the main campus. There were no large tained landscapes (Table 1). differences in the estimated average amount of carbon sequestration per tree by diameter class between the AU Table 1. Overall tree characteristics for maintained areas of the campus and the arboretum (Table 4). Statistical analyses Auburn University campus and the protected Davis Arboretum (data not shown) indicated that there was no significant using i-Tree Eco inventory procedures. difference in slope (p-value =−0.0994) between the AU campus and Davis Arboretum. There was a significant Auburn difference in intercept (p-value < 0.0001) between the University campus Davis Arboretum campus and arboretum with the campus having the larger intercept coefficient, indicating that the smaller diameter Area sampled (ha) 237 5.5 trees on the campus were larger in diameter than those in Number of trees 7345 891 the arboretum and were in better condition. Number of species 139 160 Average dbh (cm) 16.4 24.4 On average, the maintained landscapes on the campus Average tree height (m) 8.5 12.7 were estimated to remove 12.5 kg/year/ha of air pollution Average tree crown 6.7 7.6 ($67/ha). The Davis Arboretum was estimated to remove width (m) 2 a Basal area (m /ha) 2.24 (0.001–1.9) 12.04 (0.001–1.13) Estimated canopy 16 62 Table 3. Carbon storage and sequestration rates for the Auburn cover (%) University campus and Davis Arboretum as of 2009–2010. Estimated 10,757,390 1,316,806 compensatory value Auburn University ($) Campus Davis Arboretum Notes: represents the range for all trees. Carbon storage (kg) 1,576,469.88 230,864.84 Estimated canopy cover determined by dividing the total canopy- (6,652/ha) (41,975/ha) projected ground area calculated by the model by the total area Gross carbon 69,063.88 9,670.94 inventoried. sequestration (291/ha/year) (1,758/ha/year) Estimated compensatory value calculated by i-Tree Eco is based on (kg/year) the Council of Tree and Landscape Appraisers (CTLA) method (i-Tree 2010b). Table 2. The five most common species for the Auburn University campus and Davis Arboretum with total number of trees and the percent of the total population (Pop.). Auburn University Campus Davis Arboretum Species # of trees % Pop. Species # of trees % Pop. Lagerstroemia spp. 1639 22 Pinus palustris 37 4 Quercus phellos 596 8 Liquidambar styraciflua 34 4 Pinus taeda 565 8 Quercus nigra 33 4 Magnolia grandiflora 464 6 Quercus alba 27 3 Quercus lyrata 363 5 Quercus stellata 26 3 Total 3,627 49 157 18 International Journal of Biodiversity Science, Ecosystem Services & Management 269 Table 4. Average carbon sequestration per tree (kg/year) by other urban areas. Those results could then be used to diameter class (cm) for the Auburn University campus and Davis aid in development and planning strategies to optimize Arboretum. ecosystem services. However, when evaluating air pollu- tion removal, it is important to remember that pollution Average carbon sequestration per tree (kg/year) concentrations can vary site to site based on local envi- Dbh (cm) Auburn University campus Davis Arboretum ronments and conditions and this should be taken into consideration. 1–15 3 3 Comparing the results from this study site to other 16–30 8 8 31–45 15 16 study sites in the south-eastern United States is crucial for 46–60 22 25 evaluation. The area where this case study was done is 61–76 32 35 relatively small in comparison with other study sites that 77+ 54 59 have been established in the south-eastern United States. It is important to remember that this case study was for a relatively small area (compared with large cities, suburbs, Table 5. Air pollution removal rates and removal values for etc.) and that within the case study itself, the arboretum the Auburn University campus and Davis Arboretum as of is small in comparison with the maintained campus. For 2009–2010. example, the City of Auburn was estimated to have an aver- Removal amount age pollution removal value of $0.29/tree/year in 2008 (kg/year) Removal value ($) (Huyler et al. 2010) compared with the estimated aver- age removal value of $2.29/tree/year for the maintained Auburn University 2,969.1 (12.5/ha) 15,880.27 (67/ha) landscapes of the AU campus and the Davis Arboretum Campus Davis Arboretum 560.2 (101.9/ha) 3,013.10 (548/ha) combined and $3.38/tree/year for the arboretum alone. Ozone (O ) and PM10 were the air pollutants estimated to have the highest removal amounts for both study sites (Huyler et al. 2010). The City of Auburn was estimated 102 kg/year/ha of air pollution ($548/ha), or ∼8 times to store an average of 1.8 kg carbon/tree (Huyler et al. more on a per ha basis (Table 5). 2010), and the maintained landscapes of the AU campus and Davis Arboretum combined were estimated to store an average of 219 kg carbon/tree and 259 kg carbon/tree Tree condition for the arboretum alone. The differences between the sites Differences in tree condition between the AU maintained could be attributed to 81.9% of the trees in Auburn having a landscapes and the Davis Arboretum were evaluated. Over dbh of <15.24 cm (Huyler et al. 2010), compared with only 60% of the trees on the maintained portion of the AU 43% for the AU campus and Davis Arboretum combined campus were rated as being in excellent or good con- and for the campus and arboretum alone. This indicates dition and about 3% in very poor or dying/dead con- that areas with larger trees will provide more ecosystem dition (Figure 2(a)). Approximately 71% of the trees in services (Escobedo et al. 2009a, 2009b). the Davis Arboretum were rated as being in excellent or good condition and about 1% in very poor or dying/dead condition (Figure 2(b)). Across species, for trees with Carbon sequestration comparison a dbh of ≥21 cm, approximately 28% and 17% of all Results for carbon sequestration from the AU campus trees in the arboretum and on the main campus, respec- and Davis Arboretum inventory were compared to carbon tively, were rated in good or excellent condition. For trees sequestration results from Gainesville, Florida (Escobedo with a dbh of ≥31 cm, approximately 18% and 10% of et al. 2009a). Estimated average per tree sequestration the respective populations fell into these categories. The rates by diameter class (1–15 cm, 16–30 cm, 31–45 cm, remaining trees were in fair, poor, very poor, or dying/dead 46–60 cm, 61–76 cm, and 77+ cm) for Gainesville were condition. 2, 9, 17, 9, 33, and 111 kg/year, respectively. Using the same diameter distribution classes, the estimated seques- tration rates for the AU campus and arboretum combined Discussion were 3, 8, 16, 23, 32, and 54 kg/year and 2, 8, 16, 25, Carbon storage, carbon sequestration, and air pollution 36, and 62 kg/year for the arboretum alone. Major differ- removal ences in carbon sequestration were in the 46–60 cm and Estimating the carbon storage, carbon sequestration, and 77+ cm diameter classes, with the latter having the largest air pollution removal provided by the maintained land- differences. These differences in the larger diameter classes scapes on the AU campus and the Davis Arboretum was could be the product of several factors, such as the small the main objective of this case study. To estimate the number of trees with large diameters on the campus and in full value of the urban forest, which would include recre- the arboretum, and differences in species composition and ation and other ecosystem services, the direct benefits that tree condition (Escobedo et al. 2009a, 2009c; Martin et al. they provide must be quantified and also compared with 2011). 270 N.A. Martin et al. (a) Good/excellent Fair Poor Very poor Dying/dead Tree dbh (cm) (b) Good/excellent Fair Poor Very poor 100 Dying/dead Dbh (cm) Figure 2. Tree condition by diameter class determined by the overall condition rating for the Auburn University campus (a) and Davis Arboretum (b). Note that the Auburn campus has ∼8 times the number of trees as the Davis Arboretum. Protected versus maintained urban forests of the maintained AU campus. McPherson et al. (1997) reported that 60–70% more air pollution could be removed Differences between protected and maintained urban by large, healthy trees as opposed to small trees, indicat- forests are important in understanding how to maximize ing that these trees are vital in increasing air pollution ecosystem services if it is the desired outcome. The most removal. When examining tree condition by diameter class, effective way to demonstrate differences in ecosystem ser- the arboretum, in general, appears to have higher tree con- vices provided by the main campus and arboretum was dition ratings, especially for larger diameter trees. Reasons to express our findings on a unit area basis. Results from for this could be because these trees are in a protected this case study indicated that the arboretum was esti- area with limited disturbances from construction or cam- mated to remove more than 8 times the amount of total pus maintenance (roads, power lines, water lines, etc.). air pollution per ha as the campus, which resulted in a Tree condition could also be a factor in why the inter- removal value that is $481 more per ha/year than cam- cepts for the AU campus and Davis Arboretum differed. pus. Air pollution removal is estimated to increase from The average condition of the trees planted on the AU 2970 kg/year to 24,144 kg/year if the maintained land- campus may be higher where larger, nursery-grown speci- scapes of the AU campus had a forest structure similar to mens are planted, compared with the arboretum where the that of arboretum, with the removal value increasing from smaller, younger trees are more likely regenerated naturally $15,880 to $129,837. However, a forest structure similar and may be under competition. It is important to remem- that of the arboretum may not be practical for the campus ber that the management goals and philosophies of the because of the infrastructure demands such as buildings, campus and arboretum are not the same with the campus roads, sidewalks, and utilities. focusing on maintained landscapes and the arboretum on Tree condition and size may play a role in the differ- encouraging more native or natural settings. Naturally, the ences in ecosystem services. In general, trees in the Davis benefits provided by each would not be the same; however, Arboretum were larger and in better condition than those 2.5–7.9 8–12.9 13–20.9 21–30.9 31–40.9 41–50.9 51–60.9 61–70.9 71–80.9 81–99.9 100+ 2.5–7.9 8–12.9 13–20.9 21–30.9 31–40.9 41–50.9 51–60.9 61–70.9 71–80.9 81–99.9 100+ Number of trees Number of trees International Journal of Biodiversity Science, Ecosystem Services & Management 271 it is important to remember what can be provided by natu- roads, and buildings). Even if infrastructure could be built ral settings and then try to guide our maintained landscapes in a natural setting without disturbing the area, disservices and urban areas in that direction. such as maintenance and damage to the infrastructure When evaluating canopy cover of urban and protected by the trees (heaving of sidewalks) would be greatly areas, it is important to discuss the urban heat island effect. increased. Appropriate planning can address some of these This phenomenon occurs when there are higher air and sur- issues. Because of development, not all natural areas can, face temperatures because of large areas of heat-absorbing or should, be saved; however, the most beneficial areas can surfaces in urban areas with higher amounts of energy be determined and then protected to help offset the loss of usage (Bolund and Hunhammar 1999; Sokecki et al. 2005). ecosystem services when sites are cleared for construction. This is important in estimating ecosystem services such as New construction sites are almost always landscaped when carbon sequestration and air pollution removal values since finished and this helps to offset the loss of vegetation, but factors such as temperature affect tree growth, productivity, the benefits provided by the new, almost always smaller and uptake of pollutants. Natural areas with more vegeta- plantings, does not come close to the benefits being tive cover can help mitigate this effect because they do not provided by well-established natural areas. The end result have either as many or as much heat-absorbing surfaces of the urban setting needs to be determined first so that as open urban areas, or because these areas shade the sur- infrastructure and green spaces can be balanced to provide faces from the sun causing less heat to be absorbed. More the most benefits possible. vegetative cover results in more evaporative cooling, which With urban environments come different levels of in turn lowers air temperature (Bolund and Hunhammar maintenance, depending on where you are and what type 1999; Sokecki et al. 2005). If canopy cover were to be of urban vegetation is present, among other factors. Areas increased on the AU campus, the urban heat island effect that are more protected, not maintained as intensively, could be reduced, possibly leading to larger, healthier trees. and are allowed to grow in more of a natural state pro- The potentially healthier trees could then possibly provide vide more ecosystem services at a lower cost, so more more ecosystem services. work should be done to leave natural areas in our urban Cost of tree maintenance and loss in tree value due environments because of their increased value in ecosys- to construction damage can also differ for protected and tem services. These increased services can be attributed maintained urban forests. The City of Gainesville, Florida, to the fact that protected areas contain larger trees that spent $1,559,932 (∼$10.57/tree) on care for the public are typically in better condition. Urban designers should urban forests in 2007 (Escobedo and Seitz 2009). Modesto, use the information and findings presented in this arti- California, had expenditures of $2,686,516 ($29.46/tree) cle to become better informed and possibly re-think urban for its urban forest from 1997 to 1998 (McPherson et al. designing in general so that the environment can be taken 1999). Natural areas, with less intensive management, into account, not only to help preserve it but also to incor- have much lower costs of maintenance, making their net porate it into the designs so that more benefits can be worth higher. Hauer et al. (1994) projected that the City provided. As more and more people become aware and pro- of Milwaukee, Wisconsin, has a loss in street tree value tective of our environments and the benefits they provide, of $792,100/year due to construction damage. Ecosystem more emphasis will be placed on designs where potential disservices, or costs, also have to be considered (Escobedo ecosystem services are the main focus of the design. In the et al. 2011; Pataki et al. 2011). Disservices (pollutants future, we need to focus on preserving areas of the urban from power equipment such as vehicles, saws, and mow- forest that provide more ecosystem services, specifically ers) include the cost of maintenance, increase in allergens, the protected areas where our mature trees are in better and attraction of wildlife. When examining differences condition so that ecosystem services can be optimized. between protected and maintained urban forests, ecosys- However, the entire urban forest needs to be considered tem disservices have to be estimated along with ecosys- and evaluated during the developmental stages so that the tem services to fully understand net benefits (McDonnell appropriate balance of developed areas and green spaces and Pickett 1990; Escobedo et al. 2011; Pataki et al. can be sustained. 2011). The trade-off between ecosystem services and disservices is very important in development planning Acknowledgements (Escobedo and Seitz 2009; Escobedo et al. 2011). The authors thank Dudley Hartel and Eric Kuehler of the USDA An understanding of the interactions between built and Forest Service-Urban Forestry South office for their assistance natural areas (urban–rural gradient) is also important and guidance during the project. They also thank Jonathon (McDonnell and Pickett 1990). The urban environment Bartlett, Mark Caldwell, Andrew Parker, Elliot Glass, Ann Huyler, and Efrem Robbins for their assistance with data col- needs both infrastructure and green spaces; however, lection; James Ransom and Daniel Mullenix for their technical they have to be balanced to address the needs of the assistance; Dr. Greg Somers for statistical guidance and Charlie urban population. 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Journal

International Journal of Biodiversity Science, Ecosystem Services & Management – Taylor & Francis

Published: Sep 1, 2012

Keywords: air quality; i-Tree Eco; UFORE model; Urban Forestry; carbon sequestration; urban ecosystem services

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