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Evanston has enjoyed designation as a Tree City USA for 25 years. This recognition by the Arbor Day Foundation reflects the community’s appreciation of its urban forest and its commitment to maintaining this valuable resource.
Climate changes that are predicted for this region, however – warming temperatures, more variable precipitation, more frequent episodes of intense rainfall and high winds – will create stressful conditions for Evanston’s trees. Moreover, a vulnerable urban forest will be less able to help shield the community from the impacts of climate change.
Several things are at stake, and the community of Evanston reaps many benefits from its trees.
Trees play an important role in curbing global warming by removing carbon dioxide (CO2) from the air. As they grow, trees store carbon in their stems, leaves, branches, and root systems while releasing oxygen into the air.
How much can Evanston count on its trees to absorb its CO2 emissions? The answer: Not much. In an analysis conducted for the Forestry, Native Plants and Water Task Force of Citizens for a Greener Evanston, a Northwestern University student estimated that Evanston’s trees absorb – at most – an amount that represents about 2 percent of the community’s annual CO2 emissions.
This imbalance between the carbon storage capacity of Evanston’s trees and the community’s CO2 emissions illustrates the extent to which human-caused emissions have overwhelmed the natural carbon cycle. Carbon storage, however, is just one of many benefits the community derives from its trees.
Trees help shrink Evanston’s carbon footprint in another way, by reducing the demand for air conditioning and heating and thus the greenhouse gas emissions associated with power production. Strategically placed trees reduce heat gain in buildings by shading them from the summer sun. And in winter, properly selected and placed trees shield buildings from the wind, reducing air infiltration and heat loss.
On a larger scale, the cooling benefits of trees can be felt in Evanston’s wooded areas and leafy neighborhoods. According to the U.S. Department of Energy, shading and evapotranspiration (the process by which water moves through a plant and is released as vapor into the air) can reduce surrounding air temperatures as much as by 9°F.
Air Pollution Control
Air pollution poses a serious threat to human health, damages trees and other vegetation, and degrades metals, paint, and stone. Trees improve air quality and reduce the formation of ozone by absorbing gaseous pollutants and intercepting small airborne particles. They also curb the formation of ozone by shading streets and parking lots, thereby lowering fuel tank temperatures of cars parked there. This results in fewer emissions of hydrocarbons that evaporate out of leaky fuel tanks and worn hoses.
The resulting economic benefits are significant. Indeed, the student analysis mentioned above found that Evanston’s trees save the community an estimated $500,000 annually in avoided costs — for medical care, lost productivity due to illness, increased structural maintenance, and pollution abatement.
Trees reduce stormwater runoff by holding rainfall temporarily on their leaves and branches. Some of this intercepted water evaporates while the remainder drips to the ground. In addition, tree roots and leaf litter create soil conditions that promote the infiltration of rainwater into the soil. Trees thus reduce the volume of runoff and slow the flow of stormwater into the sewer system.
By controlling rainwater where it falls, trees help reduce the amount of pollution that washes into the drainage system. Stormwater runoff picks up pollutants – oil, grease, lawn chemicals, bacteria, sediment – as it flows across yards, driveways, and city streets.
Reducing runoff is of particular importance to communities like Evanston that have combined sewer systems in which stormwater, domestic sewage, and other waste waters are collected in the same pipes and conveyed to a treatment plant. There the rainwater is treated to the same extent and at the same cost as raw sewage. During heavy rainfalls in this area, when the flow exceeds the storage and treatment capacity of the system, the combined waste stream is allowed to flow untreated into Lake Michigan.
Trees provide important habitat for birds and other wildlife. They supply food, protection from severe weather, and safe resting, roosting and hiding areas. Urban trees are critical to many migratory birds that otherwise would face long stretches with no resting or feeding places.
Trees add beauty to individual properties, neighborhoods and the community as a whole. Other benefits cited in a recent U.S. Department of Agriculture report* include increased property values, safer and more sociable neighborhood environments, reduced stress, better health outcomes, and noise reduction.
The ability of Evanston’s trees to continue to provide the benefits outlined above is threatened by the expected impacts of climate change. Protecting this important community resource is a vital component of Evanston’s climate action plan.
* Lower Midwest Community Tree Guide: http://www.fs.fed.us/psw/publications/documents/psw_gtr219/psw_gtr219.pdf