The Value of Trees
Trees provide ecological services that include 1) reduced air pollution, 2) storm-water control, 3) carbon storage, 4) improved water quality, and 5) reduced energy consumption.
Trees reduce air pollution by trapping particulate matter in their leafy canopies and by absorbing noxious pollution into their leaves. The particulate matter is eventually washed away with rain. Absorbed pollutants are incorporated into the soil after leaf fall where they are broken down by microbes. These actions reduce human health problems related to air pollution. Tree canopies also intercept large amounts of rain, reducing the amount of runoff that is discharged into streams and rivers and extending the time that a watershed has to absorb rainfall. This reduces flooding and erosion. As trees grow they accumulate biomass that absorbs carbon and nutrients, locking them into a biological cycle that keeps them out of the atmosphere and hydrosphere. The storage of carbon reduces the greenhouse effect that is linked to problems of global climate change. Absorbed nutrients stay out of water bodies where they would otherwise harm fish and other aquatic species.
In summer, trees ameliorate climate by transpiring water from their leaves, which has a cooling effect on the atmosphere. At night, when the earth radiates heat back into space, temperatures often drop to the cooling or dew point, when water vapor, some of which is produced by trees during the daytime, condenses. This releases latent heat back into the atmosphere. When groups of trees intercept sunlight and use it for photosynthesis, they shade roads, buildings, and other structures, and they help reduce energy consumption.
Benefits to society are harder to quantify, but that does not mean they are less important than the ecological services that trees provide. Societal benefits include increased job satisfaction, faster recovery time for hospital patients, and improved child development. For example, hospital patients who have a view of trees out of their window recovered more quickly than patients who did not (Ulrich 1984). Similarly, employees who could look out their office windows and see trees and nature were happier at work (Miller 1997). Both of these have dollar values, like lower health-care costs and increased worker productivity, but it is harder to assign an exact dollar amount to them. Properly placed and maintained trees have even been shown to reduce crime (Kuo et al. 1998) and enhance cognitive development in children (Wells 2000).
Many outdoor recreation activities, such as picnicking, hiking, or even just sitting on a back porch are more enjoyable in and around trees. Trees provide homes and are an important component of habitat for many wildlife species. Observing wildlife in community nature parks is one of the fastest growing forms of outdoor recreation in the United States.
The aesthetic value of landscape trees can be measured by determining how property values increase for yards that have trees in them. Property values can increase as much as 20 percent when trees are present on the land. The monetary value of an individual tree can be determined by an experienced appraiser. Tree appraisal considers a variety of factors such as the species, size, condition of the tree, and its location in a landscape.
Excerpted from “Value, Benefits, and Costs of Urban Trees” by Brian Kane and Jeff Kirwan.
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