Values of Urban Trees
The basis for the value of an urban tree could be emotional, aesthetic, or it could be strictly utilitarian. However, people seldom perceive value as strictly aesthetic or monetary. There is often substantial overlap that makes "value" difficulty to classify. In many communities, public spending on tree care and management reflects an approximate value of trees. Spending patterns that go unchallenged, especially among an informed public, indicate the value people associate with trees.

The following categories describe different values that people place on trees. They are arranged primarily by their measurability. The least measurable values are discussed first.

Psychological and Aesthetic Values
Although difficult to gauge, uplifted spirits is one important benefit of trees. Some of the difficulty in measuring these benefits may grow out of society's decision to exclude tree values from the marketplace. Other emotion-based commodities, such as flowers, perfume, view property, prestige automobiles, and entertainment, are readily assigned monetary values. But with proper treatment, researchers can tie monetary values to the emotional benefits of trees.

The pleasure and good feelings we associate with trees may be far more practical than generally believed. Data on the connection between vegetation and human health are beginning to accumulate. For example, surgery patients who could see a grove of deciduous trees recuperated faster and required less pain-killing medicine than matched patients who viewed only brick walls. And, prisoners with cells overlooking green landscapes used prison health facilities significantly less than prisoners whose cells provided views of other prison facilities. The vaguely expressed "enjoyment" people associate with trees may be partly a subconscious sign of substantial health benefits.

Social Values
In Oakland, California, a neighborhood tree planting program generated community identity, cooperation, and benefits similar to those reported for urban gardening. After coming together to plant trees, Oakland residents continued working together with "paint-up-fix-up parties", neighborhood protective societies, and community gardens.

Historic Values
Trees provide important symbolic links with the past. If a living tree is associated with important events, the tree takes on historical values unrelated to aesthetics or usefulness. For example, a community would normally value a tree that shaded the deliberations of the community's founders. A tree would also be valuable if planted by George Washington or some other important figure in history. Aside from specific events, old trees may be regarded as important simply because they have lived through eras with which we have few other connections.

As for emotional and aesthetic values, historic values of trees depend primarily on community attitudes. If historic trees are threatened by changes, such as new buildings and street widening, the issue will usually be settled by public pressure not by market forces.
Photo Credit - Lane County Museum, Eugene, Oregon, First Christian Church, 1922.
Photo Credit - Lane County Museum, Eugene, Oregon, First Christian Church, 1922.
Environmental Values
People value both the aesthetic and physical quality of our environment. Trees contribute to this quality by modifying local climates, reducing noise and air pollution, and by protecting soil and water.

Climate control is one important service trees provide naturally in the landscape, but the urban landscape is far from natural. Streets, parking lots and buildings have changed the climate of urban areas by absorbing solar radiation. Water that once percolated into the soil and later evapotranspired from soil and plants now drains away or dries on the hard surfaces. These changes have increased the temperatures of cities. Compared to the surrounding rural areas, the urban "heat islands" are five to nine degrees Fahrenheit warmer (three to five degrees Celsius).

Trees help moderate the "heat island" effect. They also greatly increase human comfort: indoors or outdoors. On hot days, trees pump hundreds of gallons of water through their foliage. This water evaporates, keeping the tree and its immediate surroundings cool.

While groves of trees reduce local air temperatures, individual trees increase human comfort primarily by controlling solar radiation, not air temperature. (Radiation is the movement of heat from a warmer body, the Sun, to a cooler body, the Earth.) Trees and other vegetation shield people from direct sunlight. Trees also shade soil, pavement, buildings, and other surfaces that would absorb solar energy and then radiate that heat back to the surroundings. Without the protection of trees, city dwellers are literally surrounded by radiant heat.

At night, radiation moves heat in the opposite direction: from the relatively warm earth to the relatively cool sky. Again, tree cover steps in by blocking radiant heat loss from homes and people. Icy mornings provide evidence of this process, lawns otherwise white with frost often have green circles under the trees.

Indoor air temperatures are also affected by trees growing around buildings. During hot weather, trees reduce cooling costs by buffering high air temperatures and blocking unwanted solar energy. But during winter months, solar gain is desirable, because it cuts heating costs. To get the best balance, on the south and west sides of buildings plant deciduous trees that have thin, open branches to allow winter sun penetrate into the building. In addition, the schedule of leaf growth and leaf drop should coincide with the need for heating and cooling. Few, if any, species will meet these requirements perfectly, but it's wise to select species that give the best possible match.

Air pollution control is another way that trees improve the urban environment. The reductions in air pollution are modest, and air pollution poses some risk to the trees themselves.

Trees are fairly effective at removing both solid and gaseous particulates from the air. In one study, stands of trees reduced particulates by 9 to 13 percent, and the amount of dust reaching the ground was 27 to 42 percent less under a stand of trees than in an open area. Among gaseous pollutants, ozone, chlorine, fluorine, sulphur dioxide and PAN (peroxyacetylnitrate, a photochemical component of smog) are all absorbed by trees. In most cases, these gases also damage the trees. Unfortunately, trees remove little, if any, carbon monoxide which amounts to roughly one-half the total weight of air pollutants in this country.

Increasingly, carbon dioxide is being recognized as a "greenhouse gas" pollutant with potentially devastating consequences, such as global warming, dramatic changes in rainfall patterns, and rising sea levels that threaten flooding in coastal cities Since photosynthesis in green plants consumes carbon dioxide, plants could help to counteract the increase of this gas in the atmosphere. Rosenfeld, Martin, and Rainer report that planting urban trees could reduce heating and cooling demands enough to significantly cut fossil fuel consumption. They suggest that urban trees could be about 10 times as effective as forest trees for lowering carbon dioxide in cities.

Noise pollution from highways and other sources can be reduced with trees. Used alone, trees must be planted in belts 35 to 100 feet wide to create noticeable reductions. However, earth berms can cut traffic noise by up to half, if they are tall enough to hide the source of noise and are planted with trees, shrubs, and grasses. Where this kind of adjustment to the topography is not possible, a row of trees and a solid wall reaching up to the base of the crowns will provide a similar reduction.
First Christian Church, Eugene, Oregon, 1986. Although these trees are protected under the Historic Tree Ordinance, it is a tree that is no longer recommended for planting by the current administration because of thorns.
First Christian Church, Eugene, Oregon, 1986. Although these trees are protected under the Historic Tree Ordinance, it is a tree that is no longer recommended for planting by the current administration because of thorns.
Soil and water quality are protected by trees. In urban settings, large areas are covered by buildings, pavement, and other impervious surfaces. Instead of percolating into the soil, rainwater and snowmelt are concentrated and accelerated, increasing soil erosion and silt accumulation in streams. Trees and other vegetation protect the soil from erosion. Along watercourses, roots and fallen leaves help hold the soil together and shield it against the cutting forces of surface water. Vegetation also absorbs some of the force of failing rain, so soil particles are not dislodged. And, the leaf litter that accumulates under trees creates an environment for earthworms and other organisms that help maintain soil porosity.

In studies at Pennsylvania State University, tracts of trees in municipal watersheds were used to purify partly treated sewage and protect surface waters. By adjusting sewage water application rates researchers prevented the ground water from becoming contaminated with nitrates. Ninety percent of the water applied went into recharging the underlying aquifer. Heavy metals, a worrisome component of municipal sewage, did not become a problem.

Monetary Values
Urban trees often have substantial monetary values. A number of studies have shown that real estate agents and home buyers assign between 10 and 23 percent of the value of a residence to the trees on the property. Local governments capture some of this monetary value because enhanced property values increase assessed values and the tax base.

Appraisal methods have been developed for landscape plants, including trees. The standard for estimating the monetary value of landscape vegetation, usually accepted by insurance companies, courts and public agencies, is Valuation of Landscape Trees, Shrubs, and Other Plants: A Guide to the Methods and Procedures for Appraising Amenity Plants. This guide was prepared by the Council of Tree and Landscape Appraisers and published by the International Society of Arboriculture, P.O. Box 71, Urbana, Illinois 61801. It is now in its seventh edition. The guide describes two methods to estimate tree value.

The first method simply identifies the amount of money needed to replace the tree. Replacement value can be used for trees less than or equal to eight inches in diameter and commercially available in appropriate sizes. Adjust the cost of the replacement tree for condition and location. Then add the cost of planting and establishment, including labor, equipment, materials and maintenance. Costs of guarantees and a reasonable profit margin could also be added. If the purpose of the estimate is to settle a claim for the death or damage of a tree, removal and cleanup costs for the dead or damaged tree are also included.

Technical competence and experience are required for accurate tree valuations.
Technical competence and experience are required for accurate tree valuations.
The second method uses a formula that multiplies the cross-sectional area of the trunk by a value (currently $27) per square inch. It then adjusts this value for species, condition, and location. Trees of different sizes are measured at various distances from the ground. Tree diameter determines where the cross-sectional area is measured. tree diameter - cross sectional area table
Factors affectnig tree value - Table
Regardless of the method, determining the adjustment factors for species, condition, and location requires technical competence and experience. A tree valuation is only as credible as the arborist who makes the determination.

The value of a particular species or variety can be looked up in an established tree listings or judged by a local arborist. Value is often affected by local usage.

Physical condition is also a factor' regardless of the kind or size of the plant. Parts deformed by crowding, storm damage, fire, insect damage, disease or other cause are primary concerns when judging condition. Careful inspection should reveal these and other factors.

Plant location also affects value, especially when it is near buildings, utility lines or other plants. Plants used in planned landscapes often have higher appraised values than those growing wild.

Appraising a landscape plant requires considerable time on the site for looking at the tree, its environment, measuring and examining its parts, collecting samples, taking photo graphs, and asking questions of people living nearby. In addition, the process requires decisions based on specialized knowledge of plant pathology, aesthetics, ornamental horticulture, and tree physiology. Appraisers often call on other experts for consultation in some of these areas.

Monetary value can also be assigned to some of the benefits trees bring to the urban environment.

As mentioned earlier, sewage treatment costs could be cut by applying treated effluent to trees and other landscape plantings. Irrigating with recycled waste water reduces the need for costly evaporation ponds and sewage lagoons, as well as the dumping of municipal wastes into rivers, lakes and oceans. And, water-loving species could serve as natural water pumps for areas with high water tables.

Trees can cut energy costs, too. Trees shade buildings and pavement, which reduces the temperatures in and around buildings. By cutting air-conditioning costs, trees indirectly reduce carbon dioxide emissions equivalent to fifteen times the amount the tree alone could absorb. According to Akbari, et al., planting trees to reduce peak-load energy demand by one kilowatt-hour costs about one cent.

Three well placed trees can cut air-conditioning costs by ten to fifteen percent. For every ton of new wood that grows, about 1.8 tons of carbon dioxide is removed from the air and 1 .3 tons of oxygen is produced. Reducing carbon dioxide through tree planting costs about 0.3 to 1.3 cents per pound.

Akbari, et al., estimate that there are 100 million available tree planting spaces around American homes and businesses. Planting trees in those spaces could reduce atmospheric carbon dioxide by an estimated 18 million tons per year, and save consumers $4 billion each year.

Economics and Decision Making
In simpler times, municipal tree budgets were often increased routinely based on inflation and additional trees without close scrutiny for efficiency. increasingly, however, city budgets are strained, and tree managers must justify their funding requests.

This native vegetation was preserved to serve as a valuable buffer between buildings and parking areas.
This native vegetation was preserved to serve as a valuable buffer between buildings and parking areas.
Sound records on the costs of tree management operations help urban foresters justify budgets. However, many cities do not keep adequate records or the cost of retrieving the information manually has been too high. Fortunately, the cost of microcomputers has plummeted while their capabilities have soared. The hardware and software capable of maintaining records for thirty thousand or more trees can now be purchased for between $3,000 and $5,000.

A number of cities have computerized data systems for their trees, and increasingly good tree management software is coming on the market at reasonable prices. These systems allow tree managers to improve their organization in four ways.

First, computerized records allow managers to increase efficiency. They can identify the costs of planting and maintaining various species. Some species are much more costly to maintain than others, but differences may not be obvious without good records. Operational costs can also be tracked, and adjustments made to increase efficiency. For example, scheduled maintenance of all trees on a block costs much less per tree than requested maintenance of single trees. These cost differences may persuade decision-makers to provide the funding needed to permit more scheduled maintenance.

Second, workload projections permit jobs to be scheduled efficiently, coordinating the appropriate personnel and equipment.

Third, the value of a community's trees can be calculated. When maintenance is shown as a percentage of value, the cost of tree management should compare favorably with the costs of maintaining streets, sewers, and other assets. Over time, records should show that trees grow in value, while most other urban assets decline in value.

Fourth, budget needs can be projected and supported with local data. Most urban tree managers excel in the care and treatment of individual trees. Many have mastered the administrative and political aspects of local government. it is now time for them to use modern tools to track costs and increase efficiency.

Urban forest managers must become adept at communicating the value of urban trees, if their requests are to compete for an appropriate share of local budgets. Powerful computers that retrieve and summarize information can help them generate and communicate this information (Fig. 1).

Figure 1. One example of the king of report that can easily be prepared when work records and financial expeditures are traced on a computer.
Figure 1. One example of the king of report that can easily be prepared when work records and financial expenditures are traced on a computer.
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