The Value of Urban Trees
Maryland is home to about 4.8 million people. It is ranked 14th in the nation in population, and it has the 6th highest population density with an average of 489 people per square mile statewide. Nearly 80% of Marylanders live in the greater metropolitan areas of Baltimore and Washington. The population density in this metropolitan area is 1,336 people per square mile. The forests in these metropolitan areas are important assets to Marylanders.
Urban forests include people, vegetation of all types, water, soil, wildlife, and an environment largely built by humans. The environment includes houses, factories, offices and other buildings, sidewalks, sewers, sediment ponds, roads, railways, subways, and utility lines. Plants include vegetable gardens, shrubs, introduced species, turf, and vines, as well as trees. A variety of animals live in urban forests including dogs and cats, white-tailed deer, squirrels, chipmunks, birds, pheasants, raccoons, and rats and other rodents.
Often, streams and rivers are piped underground or channelized. Streets and highways are frequently built in flood plains. Generally, soils are disturbed, graded, and compacted, with low levels of oxygen and high levels of salt. As a result, trees and forests in the urban environment are subject to stresses and limitations not commonly found in rural areas. Therefore, plans to establish or maintain urban forests must be adapted to suit the urban environment.
Urban forests are typically categorized into four zones. The zones are the suburban fringe, the suburbs, the city residential, and the city center. The suburban fringe is the area between rural areas and established residential communities. Here, new subdivisions are found and new development is expected. In the suburbs much of the natural forest has been removed for housing subdivisions. Scattered patches of trees remain.
The city residential zone features individual homes and townhouses on small lots. In this zone the space allotted to trees is about half that in the suburbs. The city center has limited space for trees. Trees grow in pots, holes in sidewalks, vacant lots, greenways, parks, and on buildings.
Urban forests are increasing in size at about the same rate as development. Between 1985 and 1990, 10,000 acres of forest in addition to 10,000 acres of agricultural lands were developed each year. Trees and forests remaining after development occurs contribute to the urban forest.
The Value of Urban Trees
Urban forests make important contributions to society. They have environmental, social, aesthetic, and economic values. In general, the health and quality of urban forests and street trees indicates the health of neighborhoods and other urban areas.
Forests and trees reduce air pollution by absorbing gaseous pollutants and filtering dust, ash, and smoke. A dense grove of trees about 50 feet wide reduces apparent loudness of noise by as much as 50%. Forests and trees buffer glare caused by lights and the sun, provide wind protection, and cool the air. They provide habitat for wildlife and improve the quality of our lives. Some of the savings related to trees are shown below.
National Averages show that a single tree reduces costs of air conditioning, erosion control, pollution control, and wildlife habitat.
The aesthetic value of urban forests and street trees is difficult to measure. But trees have been found to increase the value of property. The value of a lot with trees averages 5-7% higher than a lot without trees. The increase in value can be as much as 20%, and lots with trees often sell faster than lots without them.
Runoff and erosion from storms is reduced because leaves slow water allowing it to soak into the soil. This reduces runoff by about 7% and reduces the need for erosion control structures. Smaller drainage pipes may be sufficient, thus saving money on materials, installation, and maintenance. Additionally, less sediment and pollution collects in stream bottoms and flows into the Chesapeake Bay.
When street trees are removed because they are dangerous, diseased, or dead, they are often recycled. Many communities are chipping branches and small trees for mulch for residents. Other communities provide residents with firewood from tree removals. However, urban trees provide most of their benefits while they are alive.
Healthy Urban Forests
The health of urban forests depends on many factors. Stresses influencing the health of urban forests include lack of space for roots to grow, too little or too much water, nutrient deficiencies, insects and diseases, and low levels of oxygen in the soil. Additional stress is added by improper pruning and other treatments, wounds from machines and people, artificial lights, altered waterways, compacted soil, and soil contaminated with salt, pesticides and fertilizers. Nationwide, the average life span of a tree in a downtown area is less than 10 years. Proper planning and species selection decrease the impacts of these biotic and human-caused stresses.
Trees in rights-of-way and other publicly owned trees make up about 5% of the urban forest. Maintenance crews for municipalities and counties are generally the caretakers of these publicly owned trees. Generally, tree care budgets designate about 75% of the funds for planting and about 25% for tree removals. Maintenance of trees, such as corrective pruning, watering, mulch, pest control and monitoring, and leaf removal, are often not budgeted. Frequently, these treatments are not implemented when budgets are tight.