The urban forest provides a community numerous benefits and is composed of a mix of native and non-native species. The mix of tree sizes and conditions, as well as the distribution of trees is determined by climate, urbanization patterns, and human preferences. To better understand Pensacola and southern Escambia County's urban forest and its social, economic, and environmental benefits, we developed this publication to help assess: 1) composition and structure, 2) canopy cover, 3) carbon sequestration and storage, 4) air pollution removal, and 5) energy effects on residential buildings due to urban forests. We then compare Pensacola and southern Escambia County's urban forest with forests in other cities in the state of Florida. The information in this publication can provide useful benchmarks and information to urban foresters, residents, and planners so they can better manage this resource (Escobedo et al. 2007).
Data was collected by sampling 79 random 0.04 hectares (ha) (0.10-acre) plots during 2008 over an area of 2,289 ha (5,654 acres) in the greater Pensacola areas or, more specifically, southern Escambia County, Florida (Figure 1). Hereafter, this publication will refer to the study area as southern Escambia County. In these plots we measured tree diameter at breast height (DBH) (e.g., woody species with DBH greater than 2.5 cm regardless of growth habit), species, height, crown characteristics, location, and distance and direction relative to residential buildings. We also collected information on tree canopy cover, land use conditions, and shrub and surface cover. The data were analyzed using USDA Forest Service's Urban Forest Effects (UFORE) model (http://www.ufore.org). Key parameters estimated by the model include leaf area, which is the sum of all tree leaf surfaces; carbon storage, which in our model is the proportion of woody biomass held in the tree's stem and branches over its lifetime; and carbon sequestration, which is the estimated amount of annual carbon removed by trees through their growth.
To estimate carbon storage, the model uses the relationship between a tree's size and its dry weight biomass (Escobedo et al. 2009c). Approximately 50% of a tree's dry weight biomass is carbon. The average annual growth for specific types of trees, as well as their size and condition, were accounted for in estimating carbon sequestration rates (Nowak and Crane 2002). Since carbon sequestered by trees is often exchanged in markets in units of carbon dioxide, carbon estimates were converted to carbon dioxide (CO2) equivalents by multiplying by 3.67. Values were multiplied by $4 per metric ton of CO2 equivalents ($4/mtCO2) based on 2008 market values of the now discontinued Chicago Climate Exchange.
The amount of air pollution removal by trees in southern Escambia County was estimated using tree cover and leaf area data as well as available hourly pollution and weather data for 2000. The amount of pollution removal was calculated for ozone (O3), sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), and particulate matter less than ten microns (PM10). Finally, estimates of urban tree effects on residential buildings energy use (e.g., heating and cooling) were based on field measurements of the distance and direction of trees greater than 20 feet tall relative to space-conditioned residential buildings less than 2 stories high. The UFORE model also incorporated tree type (e.g., evergreen or deciduous), building type and age, regional climate characteristics, and common carbon dioxide emissions from the generation of electricity in the southeastern United States (McPherson and Simpson 1999, Nowak and others 2006).
1. Urban Forest Structure and Composition
Southern Escambia County's urban forest was composed of a relatively diverse number of species (Escobedo et al. 2009a). A total of 616 trees were measured and 65 different species were identified. Approximately 13 percent of all trees sampled were non-native to the state of Florida. Increased tree diversity has the potential to minimize the overall impacts of a species-specific insect or disease by reducing the number of host individuals harmed or killed by the pest. An increase in the number of exotic-invasive plants can pose a risk to native plants if these out-compete and displace native plants.
The 10 most common species accounted for 82 percent of all trees. The three most common species in the city were laurel oak (Quercus laurifolia), swamp cyrilla (Cyrilla racemiflora), and loblolly pine (Pinus taeda), at 38, 10, and 8 percent of the total tree population, respectively (Figure 2). Tree composition varied by land use. Chinese tallow tree (Triadica sebifera) dominated commercial lands, crape myrtle dominated industrial lands, and laurel oaks dominated residential areas, as well as forest and vacant lands. Chinese tallow is a prohibited species in Florida and is listed as a Category I invasive plant by the Florida Exotic Pest Plant Council (http://bugwoodcloud.org/CDN/fleppc/plantlists/2019/2019_Plant_List_ABSOLUTE_FINAL.pdf). Therefore, the dominance of Chinese tallow trees in commercial areas raises concerns for the management of urban landscapes and nearby natural areas.
The study area had an estimated 720,720 trees. Trees with diameters at breast height between 2.5 and 13 cm (1 and 6 inches) account for 80 percent of southern Escambia County's total tree population. This is not uncommon for urban forests (Escobedo et al. 2009a). The highest tree density occurs on forest lands with 1,705 trees per ha (690 trees/acre) followed by vacant lands with an average of 890 trees per ha (360 trees/acre), followed by residential areas with 141 trees/ha (57 trees/acre) and then by commercial and industrial lands both with 49 trees/ha (20 trees/acre) (Figure 3). The average tree density in southern Escambia County, taking into account all of its land use, is 315 trees/ha (127 trees/acre), which is greater than many other cities in the United States, which average 14 to 119 trees/acre (Nowak and others 2006).
Tree crown condition also varies by land use. Overall, 78 percent of the trees were classified as being in good and excellent condition, and 14 percent were classified as being in poor condition, declining, or dead. Industrial land use had the greatest percentage of excellent and good trees, whereas forest land use had the highest percentage of trees with poor or worse condition most likely due to lack of active tree maintenance, past hurricanes impacts, and removals.
In summary, a large percentage of southern Escambia County's trees are smaller, which, in most cases, indicates a younger urban forest. Many different native trees can be found throughout the city. More than a half of all trees are found on forested lands. Land use change and hurricane impacts could have affected the urban forest structure assessed in this publication.
2. Canopy Cover, Ground Cover, and Leaf Area
Most ecosystem services from trees are linked directly to the amount of healthy urban forest structure and often measured by tree canopy cover (Escobedo et al. 2008b). Urban forest cover is dynamic and changes over time due to factors such as urban development, hurricanes, removals, and growth. The amount of a city's canopy cover depends on its land use, climate, and people's preferences. This section examines how tree composition and location influence urban forest canopy and leaf area, and how tree and ground surface covers vary across southern Escambia County.
Results obtained from the UFORE model and field data indicate tree cover in southern Escambia County was 14 percent while shrub cover, often present under trees, was 24 percent. Herbaceous surface cover (e.g., lawns, gardens, pastures) was 41 percent, impervious surface cover (e.g. concrete, roads, tar) was 22 percent, and buildings covered 11 percent of southern Escambia County (Figure 4). The amount of urban forest and impervious cover is often used as an indicator or standard by planners to establish future goals and targets. Over half of all trees were found in residential, industrial, and vacant areas. As expected, transportation areas were dominated by impervious surface. Building cover was greatest in industrial areas and also occurred as a measurable component of the land cover type in residential and commercial areas.
While all tree species contribute to the community's overall urban forest cover, some species contribute more than others because of their size (e.g., crape myrtle versus a live oak). Approximately 57 percent of southern Escambia County's tree cover is evergreen (evergreen trees maintain their leaves year round and provide year-round functions). In southern Escambia County, trees that dominate in terms of leaf area are laurel oak (Quercus laurifolia), live oak (Quercus virginiana), and swamp cyrilla (Cyrilla racemiflora). Tree species that dominate in terms of actual numbers are laurel oak, swamp cypress, and loblolly pine (Pinus taeda).
Figure 5 shows a comparison between the top ten tree species contributing to the canopy cover in southern Escambia County as defined by leaf area relative to their total numbers. For example, even though Carolina laurel cherry (Prunus caroliniana) and Chinese tallowtree are common in southern Escambia County, their overall leaf area contributes less to the area's canopy than their abundance numbers would indicate. Live oaks, on the other hand, comprise only 5 percent of all trees in southern Escambia County, yet they contribute to 28 percent of the area's total leaf area.
It is important to realize that urban forest cover can change over time because of urban development, windstorms, tree growth, and land use. Many tree benefits are linked directly to the amount of healthy leaf surface area (Escobedo et al. 2008b). By planning and managing tree canopy cover and the extent by tree species, the urban forest manager can develop comprehensive management goals and objectives to improve ecosystem services.
3. Carbon Sequestration and Storage
Climate change is an issue of global concern. Urban trees can help reduce concentrations of atmospheric carbon dioxide through their growth and by reducing energy use in buildings through shading and modifying winds. This reduction in building energy use can reduce carbon dioxide emissions produced at fossil-fuel based power plants as part of the process of generating electricity. By estimating the amount of carbon dioxide removed by trees and their shading and windbreak effects on buildings, we can determine the role of urban forests in mitigating climate change and also assign an economic value to the amount of carbon sequestered by an urban forest (Escobedo et al. 2011).
Young trees with a small DBH sequester little carbon due to the limited growth and size. Eventually if they continue to stay healthy and grow they will accumulate more carbon as their biomass increases. Large trees in southern Escambia County greater than 77 inches in DBH continue to sequester the most carbon (Table 1). Live oaks, laurel oaks and slash pines store 53, 27, and 4 percent of all carbon respectively. Laurel oaks, live oaks, and slash pine sequester 36, 32, and 3 percent of all carbon, respectively.
Healthier and larger trees sequester the greatest amount of carbon annually (Escobedo et al. 2008c). As trees grow, they store more carbon by assimilating it in their woody tissue. As trees die and decay, they release much of the stored carbon back into the atmosphere. Southern Escambia County's trees sequestered 10,189 mtCO2 per year with an economic value of $56,411. This is value based on the 2008 Chicago Carbon Exchange Estimates. Values today although differing, are likely increased. Figure 6 depicts a comparison of the economic value and net carbon dioxide sequestered by trees located in areas dedicated to different land uses. Trees located on forest lands sequester more CO2 than residential due to greater tree density in forest versus residential land uses.
4. Air Pollution Removal
On average, 1 square meter of tree cover removes 7 grams of air pollutants in southern Escambia County. Total pollution removal was greatest for particulate matter less than ten microns, followed by ozone, carbon monoxide, nitrogen dioxide, and sulfur dioxide. Figure 7 compares the pollution removed and the resulting economic health benefits to society at large. It is estimated that annually trees in the study area removed 25 tons of air pollution (CO, NO2, O3, PM10, SO2). This is approximately $146,000 US dollars in health benefits per year.
5. Energy Effects on Residential Buildings
Trees affect energy use by shading buildings, reducing temperatures by providing evaporative cooling, and blocking winter winds. Trees tend to reduce building air conditioning use in the summer months and can either increase or decrease building energy use in the winter months depending on the location of trees relative to a building. Based on the size of a building and the surrounding trees, we can place an economic value on the effects on energy use in residential buildings (Escobedo et al. 2008d).
Based on the 2007 average retail price of electricity in Florida (EIA 2007), trees in southern Escambia County are estimated to provide about $306,000 in savings because of reduced air conditioning and heating use. However, trees can also increase heating use in winter by approximately $32,000 dollars annually due to the shading of the sun which results in increased heating. Table 2 provides a breakdown of the air conditioning and heating use and price savings as well as heat emissions costs by residential trees.
Trees clustered together near a building can create a microclimate cooling system via evapotranspiration (the evaporation of water from plant surfaces and bodies of water) and shade. Finally, trees properly positioned around a building can direct wind air flow to the building to help cool it down in warmer months or away from the building to diminish cooling effects during cooler months (Meerow and Black 2003).
The placement of trees around a building can influence the amount of energy required to maintain acceptable temperatures inside the building. Trees planted on the west side block the increase of solar heat in the afternoon during summer, and trees on the east and south sides of the house will block the solar heat in the summer. In more northerly areas of Florida, the same trees will increase the heating requirement for the structure in the winter if they are not deciduous trees (McPherson et al. 1999). This negative shading effect is caused by evergreen trees blocking solar heat from reaching a structure to warm it during north Florida's colder months. Relying on the principle of "the right tree in the right place," if deciduous trees which lose their leaves in the fall, are planted on the south and east sides, the sun's heat will reach the structure.. Ultimately, homeowners determine how cool or warm they prefer the inside of their homes to be and tree placement effects may vary from person to person and home to home.
By influencing energy production in power plants, trees can also affect emissions of carbon dioxide (CO2) and other greenhouse gases. In doing so trees can indirectly lower (or increase) CO2 emissions by power plants, and this offset of avoided emissions can result in economic savings to the community (McPherson et al. 1999). Using the average price of CO2 on the Chicago climate exchange of $4 per ton of CO2 emissions avoided (August 2008), the effect of trees on residential building energy use can result in $13,770 in benefits and $3,377 in costs: a benefit-cost ratio of 4:1. Table 3 provides a breakdown of the energy savings and costs due to southern Escambia County's urban forests.
In summary, it is important for homeowners to plant trees in the right place to maximize cooling benefits in the summer and solar heat gain in the winter. See Meerow and Black (2003) for more specific landscaping suggestions. It is important to consider that energy savings are also affected by the occupant's use of the air conditioning and heating systems as well as the energy use efficiency characteristics of buildings and heating-cooling units (Escobedo et al. 2008d). It is important for homeowners and landscape architects to carefully consider placement of trees around structures to maximize energy benefits.
6. Comparing Southern Escambia County's Urban Forest with Others in Florida
It can be difficult to determine which tree species are the most important contributors to an urban forest. This is because certain species are numerous in an urban forest yet they have low leaf area and vice versa. But in general there are some methods to determine the overall role of a particular tree species in the urban forest. Ecologists overcome this uncertainty by calculating the Importance Value (IV) for each species based on its relative frequency (% of population) and relative leaf area. When these values are summed the IV can be used to standardize tree species and rank and compare the importance of tree species (Figure 8). Laurel and live oak are particularly important species in southern Escambia County relative to other tree species found in other urban areas in Florida. Certain tree species are found in high numbers only in southern Escambia County, while others are found only in other Florida cities.
Urban forests in southern Escambia County have a greater tree density (number of trees per hectare) in comparison to Gainesville, Miami-Dade County, and Tampa (Table 4). Tree cover numbers in southern Escambia County are likely a result of the study area encompassing coastal, highly urbanized portions of the county. Larger trees in Gainesville might explain that city's larger amounts of carbon storage compared to other Florida cities.
The information presented in this document can be used to establish baselines and provide an insight into existing urban forest structure and its ecosystem services (Escobedo et al. 2011). It can be used to formulate management strategies and goals that maximize benefits and minimize safety risks to citizens. The information is especially useful for developing and establishing medium and long-term management goals and objectives (Escobedo et al. 2007). Understanding a community's urban forest structure, community perceptions, and available resources can be used to maximize the benefits of an urban forest.
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Comparison of estimated average carbon stored and sequestered per tree in one year by diameter at breast height (DBH) size classes in southern Escambia County.
The benefits and costs based on energy use effects due to tree shading, windbreak, and climate effects near residential buildings in southern Escambia County.
Annual energy savings and costs due to tree location around residential buildings in southern Escambia County.
Urban forest cover, composition, and carbon storage for four cities within the state of Florida.