Benefits of Trees

While the physical reactions that create air pollution are complex, a major element in the formation of pollution is air temperature. Temperature contributes to the production of smog, and ozone in particular. Trees help reduce the production of ozone by reducing air temperature through shading and evapotranspiration (American Forests, April 1996).

Additionally, trees reduce air pollution by intercepting airborne particulates and by absorbing gaseous pollutants. The U.S. Forest Service puts a $3.8 billion value on the air pollution annually removed by urban trees. According to a Nature Conservancy study, “most of the cooling and filtering effects created by trees are fairly localized, so densely populated cities—as well as those with higher overall pollution levels—tend to see the highest overall return on investment (ROI) from tree plantings.”

Urban forestry can greatly contribute to the increase and conservation of local biodiversity. Strategically placed trees combined with shrubs, grasses and other food sources provide habitats for a diverse population of wildlife. Fruit (seeds, nuts, etc.) from trees provide food, and the entire tree serves to support diversity of wildlife, that in turn improves the plant diversity through pollination and dispersal of seeds. 

Trees and forests provide shelter and habitat. A mature oak tree can support over 500 types of pollinators through the seasons, in particular during their larvae and caterpillar stages. Live or fallen trees, leaves, branches, and trunks create habitat for insects, bugs, and amphibians. In particular for pollinators, we often think of seasonal flowers, but trees provide year round habitat and food. Trees also flower supporting thousands of blooms. 

Increasing plant diversity also improves wildlife diversity. In cities, we have wildlife beyond birds, butterflies, and squirrels; we can observe coyotes, opossums, bats, and snakes in urban areas. All are beneficial and play a role in our urban ecosystem. 

The buildings, roads, and other “impervious” surfaces in a city create “heat islands” — areas where temperatures can be up to 22 degrees higher than in the rural areas around it. According to the EPA, heat islands create greater need for air conditioning and energy consumption, air and water pollution, and higher mortality. 

Heat waves are believed to cause more deaths in the United States than all other natural disasters combined. Trees properly placed can lower temperatures as much as 5 degrees F (American Forests, 1995a) in these heat islands. Tree-lined streets and greenspace with abundant trees not only have an aesthetic value, but the tree canopy provides cool comfort for pedestrians and cyclists on hot days. 

Trees in urban areas reduce the amount of carbon in the atmosphere in two ways. First, as trees grow, they take in carbon from the air and store it in wood, plant matter, and in the soil, making them what scientists call “carbon sinks.” Second, they reduce the energy needed for urban heating and cooling, which in turn reduces the amount of carbon dioxide produced by fossil-fuel power plants. 

During one year, a mature tree will absorb more than 48 pounds of carbon dioxide from the atmosphere and release oxygen in exchange. (Arbor Day Foundation) The amount of carbon a tree is able to sequester is influenced by many factors, including species, age, size, and location.

In a study of the City of Chicago, trees annually sequestered the equivalent amount of carbon emitted from all forms of transportation in one week (Nowak et al. 1994). With some additional tree planting, it was estimated that this could be increased to the amount of carbon emitted by two months of transportation use in Chicago. 

“Forests play a major role in the pledges made by countries towards meeting the targets set by the Paris Climate Agreement, meeting up to a quarter of greenhouse gas emissions reductions up to 2030. This is a huge contribution considering they only contribute 10% of emissions, while fossil fuels contribute 90%.” 

“There is no doubt forests have enormous potential to mitigate against climate change, primarily through reducing deforestation, planting new forests and managing existing forests,” says a report in Nature Climate Journal.

Money may grow on trees. The concept of “ecosystem services” calculates an economic value for natural resources. Trees have defined benefits that functions as a natural green infrastructure that reduces or replaces the need for constructed infrastructure solutions. The value of these services can vary depending upon the location, conditions, and tree species and age. A recent study calculated urban trees save us up to $12 billion a year in the U.S. when measured for just three factors: reducing mortality and illness and lowering electricity use for air conditioning. A series of international third-party studies have shown that trees increase property prices by between 5% to 18% (CABE Space, 2005 and Morales et al, 1983). The USDA Forest Service Pacific Southwest Research Station’s (PSW) scientists have found that for every $1 spent on planting and caring for a tree, the benefits that it provides are two to five times that investment.  

Trees are hard at work, year-round and year over year: perennial giving trees, if you will. We invest heavily in the public infrastructure of our cities: utilities, roads, police, and fire services, for example. Likewise, trees also serve us as part of a natural green infrastructure that improve the performance of and offset some of the damage caused by the built environment.

By producing shade in warm climates, trees help conserve energy as they intercept radiant heat. This “canopy cooling effect” helps to offset higher temperatures created by urban heat islands. Trees planted as windbreaks can save energy in colder climates reducing heat dissipation. Increasing tree cover by 10% (which is equivalent to adding three trees in optimal locations per building) could reduce total energy use for heating and cooling by 5 to 10% (Nowak et. al., 1994). 

Trees could reduce summer energy consumption by up to 20% and winter consumption by up to 30%. In Dade County, Florida, over half of the residential energy consumed is for air conditioning. Trees currently provide direct energy savings of 3.3%. Adding one mature tree in the right location at each home would give an additional 6.4% energy savings (American Forests, April 1996).

Trees, combined with shrubs and grasses are very important in soil stabilization. While groundcover holds the topsoil in place, the roots of trees secure large blocks of soil, which is particularly important on steep slopes (USDA Forest Service, 1995). This could be extremely important in stabilizing slopes in urban areas prone to landslides. 

Streamside buffers, comprised of trees, shrubs, and grasses, filter out surface and shallow subsurface pollutants before they enter watercourses. These filter strips also help control bank erosion, protect and enhance aquatic environments, provide wildlife habitat and recreational sites, and increase biodiversity. These systems can reduce both the sediment and the pollution load of streams which run through urban areas. Trees, by increasing water infiltration into the ground, decrease stormwater runoff loads on urban drainage systems, thus reducing costs for maintenance. At a South Miami Residential study site, a 21% existing tree canopy reduces stormwater runoff by 15% (American Forests, April 1996).

Trees produce edible fruits (including nuts) and is a valuable source of food for both humans and wildlife. Fruit trees are a perennial food source that can be less resource intensive than annual crops. Fruit trees grown in yards, public areas, and in managed “food forests” can help to address food insecurity. Some communities have organizations that promote and guide foraging fruit trees on public and private land (with permission) channeling fresh foods to food banks as well as personal consumption. Trees can create environments ideal for fungi (mushrooms), many which are edible. 

Trees in urban areas are planted and preserved as living environmental assets, unlike trees grown for intentional harvest and used in commercial products from building lumber and paper to nail polish and grated cheese. 

In the urban forest, branches, leaves and fruits can be beneficial by-products, becoming wildlife habitat, compost, mulch, and food for humans as well as wildlife. Large trees felled in cities are being harvested as urban wood and become art or furniture with unique local histories. Wood debris from natural disasters or reclaimed wood from thoughtful demolitions are increasing popular materials for home decor and art. 

Trees are beautiful, and for that alone they have tremendous qualitative value. Science is also providing quantifiable evidence confirming how trees improve quality of life in many ways. There is measurable healing and wellness effects of trees on our physical and mental health: from lowering stress to improving academic outcomes. Hospitals that provide views of trees and access to nature have shorter patient recovery times, and residents of neighborhoods with higher tree canopy seem to take fewer prescription antidepressants. 

Ancient activities like “forest bathing” are shown to enhance our physical as well as psychological health. Living within walking distance of trees can lower the risk of developing psychological distress and lead to better overall health; this effect is specific to trees and not just a greenspace of open lawn. Furthermore, trees show “very strong association” with lower crime rates that may be related to more people outside thus providing more community engagement. 

Trees and shrubs can be effective buffer in screening-out urban noises, especially when planted in contiguous rows in widths of 16 feet or more. The physical bulk of trees dull or soften sound waves that attempt to pass through them and further dampen these sounds by adding noises of their own, a phenomenon called “masking.” A row of trees can cut the ambient noise level approximately in half (Arbor Day Foundation).

Research at the Universities of Iowa and Washington indicate that trees and their symbionts can breakdown pesticides and carcinogenic groundwater contaminants, such as atrazine and trichloreoethelene, into harmless compounds (Black,1995). Trees are now being planted on landfill caps to help reduce pollutants while improving the environment. A major study of Chicago estimated that trees in that city annually removed 15 metric tons of carbon monoxide, 84 tons of sulfur dioxide, 89 tons of nitrogen dioxide, 191 tons of ozone, and 212 tons of small particulates. The estimated value of this pollution removal was $1 million for trees in the city itself and $9.2 million for the entire Chicago area (Nowak et. al.,1994). 

Urban forestry can play an important role in securing water supplies for growing urban populations. There is a great need to protect and reforest watersheds to maintain an adequate and consistent flow of potable water. Without an adequately forested watershed for sources of city water, large quantities of water flush and disappear downstream when it rains. Pervious surfaces are needed to allow water to infiltrate into the soil and provide a reserve and a consistent supply of water during the dry season. 

Trees can lessen the burden on sewer infrastructure by mitigating the volume of stormwater runoff. They also reduce runoff pollutants by increasing ground water infiltration. This process helps purify the water, and recharge aquifers, which results in a more consistent flow and quantity of potable water. The state of New York’s $2 billion investment in watershed with trees saved the state $10 to $100 billion by avoiding the construction and annual operating costs of new water filtration stations.