Urban Forests and Their Influence on Pollution Levels Across Urban Landscapes

To better understand the effects of urban forests on major cities and statewide urban areas in the United States, the USDA Forest Service’s Urban Forest Inventory and Analysis (FIA) program combines field measurements with social and climatic data to provide estimates of ecosystem services. Historically, ecosystem services provided by urban trees were only calculated at the city level, but methodological advances now allow for these services (removal of carbon monoxide (CO), nitrous oxide (NO 2 ), sulfur dioxide (SO 2 ), ozone (O 3 ), 2.5 µm particulate matter (PM 2.5 ), and associated human health benefits) to be modeled for urban areas across the nation. The economic value and health impacts of the trees are based on estimates from i-Tree Eco and the Environmental Protection Agency’s Benefits Mapping and Analysis program (BenMAP-CE) which incorporates population, baseline healthcare costs, and local air pollution measurements. Here we analyze how modeling ecosystem services for individual trees improves our understanding of synergies between air pollution and urban systems. Based on urban FIA data from the cities of Madison and Milwaukee, WI and the urban areas across Wisconsin, we found that, on average, the statewide urban areas had higher canopy cover (22%) than the two cities, and that the urban forests of WI remove15,000 tons of air pollution annually. However, on a per tree-cover basis, the trees in Milwaukee had 4 times the air pollution removal value of Madison ($930 per acre tree cover vs $233 per acre tree cover), and 2 times the value of air pollution removal for urban areas in general ($449 per acre tree cover). These results highlight the presence of regional variation in air pollution removal by trees across urban systems and show that by shifting from a city-level ecosystem service assessment to individual tree estimates, we are able to obtain more localized and regionally representative estimates of ecosystem services provided by urban forests.