What is an urban forest?

An urban forest is a collection of trees and other woody vegetation found in and around human developments. An urban forest can be thought of as a gradient of trees extending from the street trees of densely packed urban cores, past the landscaped suburban plots, and all the way out to the remnant forests of exurban (or edges of urban) lands. It includes all the woody vegetation found in urban parks, industrial landscapes, residential properties, wetlands, riparian corridors, coastal ecosystems, greenways, and nature preserves, regardless of ownership (Figure 1).

Tree Canopy Coverage

Urban forest managers use a range of measurements to describe and value the urban forest and its benefits. One measurement that can be made over large areas of land with relative ease is the quantification of tree canopy coverage. Tree canopy cover is the percent of a given land area (e.g., city, national forest, etc.) covered by leaves and branches when viewed from above. Canopy coverage assessments are important tools that allow a community to estimate current canopy coverage, understand the extent of the urban forest, and track potential changes over time. Canopy coverage can be measured in the field with specialized equipment or by analysis of aerial and satellite imagery.

Florida has 29 metropolitan and micropolitan census-designated areas, representing 51 of the 67 counties and over 98% of the state's population (US Census Bureau 2019). These census-designated areas represent geographical regions with at least one densely populated urban area and related economic ties. Metropolitan areas must have one city or town with at least 50,000 people, while micropolitan areas must have a city or town with a population between 10,000 and 50,000 people (US Census Bureau 2019).

To assess the urban forest throughout the state, we analyzed canopy coverage and its associated benefits in each of these census-designated areas. Tree canopy coverage was estimated using a point-based sampling approach. This method generates random points within a designated boundary on high-resolution aerial imagery. The random points are then assessed by a photo interpreter and classified as "Tree/Shrub" or "No-Tree." The classified points are tallied and divided by the total number of points to reach an overall canopy coverage percentage.

Tree canopy coverage ranged from 18.6% in the Okeechobee micropolitan area to 74.4% in the Crestview-Fort Walton-Destin metropolitan area (Table 1). In general, canopy coverage tended to decrease from north to south and west to east across the state (Figure 2).

Urban Forest Benefits

Urban forest ecosystems provide a variety of economic and environmental benefits (Livesley et al. 2016), including shading homes to create energy savings, intercepting rain to reduce stormwater, improving air quality by filtering pollutants, and sequestering carbon to offset emissions associated with climate change. Many urban forest benefits are influenced by the combined surface area of all the leaves in a tree's canopy (Peper and McPherson 2003). Researchers use leaf area measurements to estimate the benefits provided by individual trees in an urban forest (Figure 3).

Researchers have developed ecosystem services models that use urban forest data to calculate the total economic value of all trees in a designated area, typically at the city or county level. Prior urban forest ecosystem service assessments for Gainesville and Tampa, Florida can be found at https://edis.ifas.ufl.edu/fr265 (Tampa) and https://edis.ifas.ufl.edu/fr414 (Gainesville). Evaluation of these benefits allows city managers and citizens to gauge the importance of the urban forest compared to other key infrastructure elements and to budget for the appropriate management of this natural resource.

Currently these models are able to estimate only some of the more tangible benefits of the urban forest, like the ones mentioned above. There are many other important benefits, such as wildlife habitat, recreational value, and human psychological effects. Researchers are working to apply economic values to these less tangible but important services. While all of these models are based on the best available science at the time, the data they produce are still just estimations.

For this study, the total acreage of each metropolitan and micropolitan area was calculated in a geographic information system (ArcGIS v10.5, ESRI). Acreage of tree canopy was estimated by multiplying the total area of each census-designated boundary by the canopy coverage percentage obtained during the aerial imagery interpretation process. We used the estimation of "canopy area" (Table 1) in each metropolitan and micropolitan area to calculate the value of benefits received from their corresponding urban forest. Benefit production rates (e.g., tons of air pollution removed per acre) and the monetary values for air pollution, avoided runoff, carbon sequestration, and carbon storage were based on data obtained from the i-Tree Canopy software v7.0 (https://canopy.itreetools.org/benefits/).

Air Pollution Removal

Toxic air pollutants such as carbon monoxide (CO), nitrogen dioxide (NO₂), ground level ozone (O₃), sulfur dioxide (SO₂), and particulate matter (PM₁₀ and PM_2.5) can cause adverse effects to human health, disrupt ecosystem processes, and reduce visibility in cities (EPA 2019). Carbon monoxide, sulfur dioxide, and nitrogen dioxide gas are released into the atmosphere mainly through the burning of fossil fuels in power plants, industrial facilities, and automobiles. Ground-level ozone is created by chemical reactions between air pollutants and sunlight (EPA 2019). Particulate matter can be released directly from a source, such as unpaved roads, fields, and smokestacks, or created in the atmosphere through complex chemical reactions.

Air pollutants have been shown to affect cardiovascular and respiratory health, with long-term exposure potentially leading to the development of serious diseases (Stieb et al. 2009). In addition to the human health effects, air pollutants negatively affect the environment by contributing to pollution of coastal waters, smog production, and the formation of acid rain (Manisalidis et al. 2020).

Tree leaves primarily remove air pollutants by directly absorbing them or indirectly capturing them on their surfaces (Grote et al. 2016; Nowak et al. 2006). Altogether, the trees in Florida's 29 census-designated areas remove over 600,000 tons of combined air pollution each year, saving Florida residents an estimated $605 million in annual air-pollution-related health care costs (Figure 4; Table 2). Estimated removal amounts for each air pollutant are listed by micropolitan and metropolitan area in Table 3.

Stormwater Runoff

Stormwater runoff is the rainwater that flows over the ground after a rain event. Impervious surfaces, such as roads, parking lots, and rooftops, do not allow water to infiltrate into the soil. Instead, these impervious surfaces swiftly direct large volumes of water into nearby stormwater drains that typically discharge into neighboring waterbodies. In urban areas with increased impervious surfaces, stormwater runoff can be a significant source of pollution to local waterbodies. As water flows over impervious surfaces, it can pick up many different pollutants (e.g., antifreeze, grease, pesticides, bacteria, etc.) that are present on these paved surfaces.

Trees help combat the negative effects of stormwater runoff by capturing rainfall on their leaves and bark, thereby reducing the amount of water hitting impervious surfaces. In addition, tree roots and old fallen leaves can promote soil conditions that allow more water to enter the soil during a rain event. Collectively, the urban forests in the 29 metropolitan and micropolitan areas intercept an estimated 50 billion gallons of water a year, resulting in savings of over $451 million in avoided annual stormwater treatment costs (Table 4). To put this volume of water in context, that is enough to fill approximately 75,000 Olympic-sized swimming pools each year (Figure 5).

Carbon Sequestration and Storage

Carbon dioxide (CO₂) is a major greenhouse gas that plays a significant role in global climate change. Carbon dioxide is mainly released to the atmosphere through the burning of fossil fuels (EPA 2019). Trees can help combat climate change by taking in carbon dioxide from the atmosphere. During photosynthesis, trees take in atmospheric carbon dioxide and store it as carbon in their trunks, branches, and roots. A tree will continue to sequester and store carbon until it dies.

Carbon sequestration and storage rates are often presented as "carbon dioxide equivalents" as a way of measuring carbon footprints. Carbon dioxide equivalents report a single number to represent the amount of carbon dioxide that would create the same impact as all of the greenhouse gases combined (e.g., carbon dioxide, methane, nitrous oxide, and ozone). For example, because methane is a more powerful greenhouse gas, one ton of methane is equivalent to 25 tons of carbon dioxide (EPA 2019).

Equivalent calculators can be used to express these extremely large emission values in terms that are easier to digest and understand (Figure 6). Florida's urban forests sequester (i.e., capture through active growth) 65 million tons of carbon dioxide equivalent a year, which translates to an estimated $3 billion in annual benefits (Table 5). Florida's urban forests store (in their wood) a total of one billion tons of carbon dioxide equivalent, worth an estimated $76 billion in services (Table 6).

Valuable Natural Resource

Florida's urban forests are an extremely valuable natural resource that provides an estimated $4.1 billion in annual benefits for the state's citizens and visitors (Table 7). In addition, these urban forests will provide an estimated $76 billion in climate change benefits over their lifespan as trees continue to grow, storing more carbon in their tissues. It is important to remember that the benefit numbers and monetary values presented in this report are estimations obtained from scientific models. While these numbers may not be absolute, they are based on the best available science and are important for estimating the value of urban forests and the services they provide. In addition, this valuation of Florida's urban forest only includes some of the more tangible benefits, and we did not assess every county in the state. Many of the benefits presented in this report are influenced by the health and size of an individual tree's canopy. Preservation and management of the urban forest is critical to ensure that citizens receive the maximum benefits that urban trees can provide.

Acknowledgments

The authors would like to acknowledge the following people for their indispensable help conducting the canopy assessments: Brooke Anderson, Saige Middleton, and Hunter Thorn.

Literature Cited

Grote, R., R. Samson, R. Alonso, J. Amorim, P. Cariñanos, G. Churkina, S. Fares, D. Le Thiec, Ü. Niinemets, T. N. Mikkelsen, E. Paoletti, A. Tiwary, and C. Calfapietra. 2016. "Functional Traits of Urban Trees: Air Pollution Mitigation Potential." Frontiers in Ecology and the Environment 14 (10): 543–550.

Livesley, S. J., E. G. McPherson, and C. Calfapietra. 2016. "The Urban Forest and Ecosystem Services: Impacts on Urban Water, Heat, and Pollution Cycles at the Tree, Street, and City Scale." Journal of Environmental Quality 45:119–124.

Manisalidis, I, E. Stavropoulou, A. Stravropoulos, and E. Bezirtzoglou. 2020. "Environmental and Health Impacts of Air Pollution: A Review." Frontiers in Public Health 8:14. https://doi.org/10.3389/fpubh.2020.00014.

Nowak, D. J., D. E. Crane, and J. C. Stevens. 2006. "Air Pollution Removal by Urban Trees and Shrubs in the United States." Urban Forest & Urban Greening 4 (3–4): 115–123.

Peper, P. J., and E. G. McPherson. 2003. "Evaluation of Four Methods for Estimating Leaf Area of Isolated Trees." 2 (1): 19–29.

Stieb, D. M., M. Szyszkowicz, B. H. Rowe, and J. A. Leech. 2009. "Air Pollution and Emergency Department Visits for Cardiac and Respiratory Conditions: A Multi-city Time-Series Analysis." Environmental Health 8 (25).

United States Census Bureau. 2019. Florida Counties by Population. Florida Demographics by Cubit. Accessed May 2019. https://www.florida-demographics.com/counties_by_population

United States Environmental Protection Agency. 2019. "Criteria Air Pollutants." Accessed May 2019. https://www.epa.gov/criteria-air-pollutants

Resources

Greenhouse Gases Equivalencies Calculator. United States Environmental Protection Agency. Web. Accessed 5/20. https://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator

i-Tree Canopy. i-Tree Software Suite v7.0. Web. Accessed 4/20. https://canopy.itreetools.org/

NAIP imagery. United States Department of Agriculture-Natural Resources Conservation Services. Web. Accessed 3/20. https://datagateway.nrcs.usda.gov/GDGHome_DirectDownLoad.aspx

Tables