|During this century, the form and character of
cities and towns have changed dramatically. Fifty to one hundred years ago
urban areas were different; cities were smaller and skylines rarely rose above
eight to ten stories. Automobiles were not the major method of personal
transportation, and the ratio of open space to buildings was dramatically
different. Even the lifestyles of urban residents have changed.
Cities are dynamic, with ever-changing land uses. This is particularly true today. In planning the urban forest, it must be assumed that cities and towns will continue to change. The likelihood of changes and the character of potential environments should be carefully considered. Tree selection must assume that the urban forest will be dynamic, if it is to respond to the changing conditions of the community.
Space for Trees
The stereotypical image of the "city" as a place of continuous tall buildings and intensive, pedestrian -oriented streets is accurate for only a small percentage of urban areas. Most urban environments consist of automobile oriented, low density land uses and recently constructed buildings.
Cities are actually more spread out than the stereotype suggests, with a higher proportion of open space to buildings. However, the presence of open space does not necessarily imply more room for the urban forest. The majority of open space is devoted to transportation corridors and car storage. Space is almost always a limiting factor in some way. It could be a conflict between vehicles, pedestrians and trees for space at ground level. Above ground, the canopies of trees interfere with overhead utilities and views. Beneath the ground, tree roots compete with utilities and paving.
In urban environments, natural areas with continuous vegetation are replaced by discontinuous and fragmented vegetation. The space available for trees occurs in small pieces rather than large ones. In some areas, such as downtown cores, fragmentation of planting sites is extreme. Greater continuity can be found in residential neighborhoods, stream corridors, steep slopes or around other extensive land forms.
There appears to be a trend toward increasing density in many new residential and commercial developments. This threatens a corresponding reduction in the space available for trees.
Environmental conditions tend to be more varied in urban areas than in less developed areas. For example, asphalt surfaces surrounding one site make it hot and dry. But around the corner, buildings cast shadows that cut temperatures and minimize evaporation of soil moisture. Light, wind, temperature, and soil conditions can change abruptly from one spot to another.
Urban conditions are frequently more severe for the growth of plants than in natural environments. On the other hand, conditions could also be better due to irrigation or lack of competition.
Seasonal variation may also be extreme compared to more natural areas. A site may be hotter in the summer and colder in the winter. Or, it may be drier in the summer and wetter in the winter due to soil compaction and the presence of paved surfaces. However, the city as a whole generates heat that usually keeps temperatures warmer in all seasons than more natural areas. This is called the "urban heat island" effect.
While it's not possible to generalize about the severity of urban conditions, one thing is certain. Conditions vary considerably, and it's wise to assess each site individually. The best way to identify site conditions is to tour on foot. Be sure to consider changes that are planned in the vicinity, even if plans are not yet final. The construction of streets and buildings, rechanneling surface water, and routing utility corridors can all affect the ability of a site to provide for the needs of trees. While it's best to anticipate urban evolution, it's safe to assume that plants and people will always have to adapt to unforeseen changes.
One goal of urban forest management is to create and maintain the maximum amount of visual and biological diversity. Preserving the uniqueness of different areas involves selecting trees that compliment the activities occurring there. This may require drawing on a broad range of plant species to create a distinct character. The urban landscape can be divided into four broad land use categories: natural areas, parklands and campuses, residential property, and fragmented spaces. These categories are based primarily on activities that take place in them and the mood created by those activities.
Natural areas are relatively undisturbed. The typical urban resident also considers them to be "natural," because of their size and location. These areas tend to occupy land considered unbuildable due to poor accessibility, rugged topography, poor soil, or inappropriate hydrology. They are frequently linear, such as ravines, stream corridors and steep slopes. Other types of natural area in this category may include: "greenbelts," flood plains, wetlands, ponds, waste areas, abandoned land and "buffer zones" around undesirable land uses, such as land fills.
Activities occurring in natural areas range from casual recreation to no activity at all. Increasingly, these areas are seen as valuable components of the urban environment, because of their aesthetic qualities, their symbolic value as reminders of the natural environment and their value as critical natural habitat.
Coherent management policies for these types of areas have rarely been developed. They typically receive little or no maintenance, and are left to their own devices. They have almost always been disturbed by human activities at some point in their history. In most cases, these areas are in some stage of plant succession as wild plants and animals re-establish themselves. For these areas, select native trees that enhance or maintain the feeling of a "natural" environment by preserving an "uncontrolled" appearance.
Parklands and campuses
include traditional parks that host recreational activities, community open space in planned residential developments, freeway interchanges, as well as business and industrial parks with a campus-like atmosphere. These developments are normally large, with a high percentage of open space to paved or built-up areas. They typically contain large lawns and clearly defined beds of flowers or shrubs. Trees are planted as individuals, groves or small woodlands. Although they usually receive relatively high levels of maintenance, some people consider their character to be "natural." This feeling may grow from traditional landscape maintenance techniques and "naturalistic" plant arrangements.
These areas are intensively used. Even as a campus of industrial or commercial buildings, the landscape may simply be an area viewed by workers and visitors-but rarely entered. If it is only looking out an office window, these people are using the outdoor areas and benefitting from the presence of well-maintained trees and lawns. So, observation represents a high level of "use."
Attitudes toward these areas may be changing, prompting a change of management. Measures are sometimes taken to reduce maintenance costs, encourage wildlife habitat or allow for a wider variety of uses to occur within these campus areas. The long term implications of these trends must be considered when trees are selected.
| Residential land
comprises a very large portion of most urban environments. In these areas small parcels are typically under the private ownership of different people. Several issues grow out of this pattern of ownership.
Buildings divide the open space into small, tight fragments. The visual character and the level of maintenance varies dramatically from properties with a relatively "natural" appearance to those that are manicured and open. The intensity of uses include passive visual appreciation, active recreation and even storage of personal possessions. There may not be a consistent pattern of open space. In some cases, buildings and paved surfaces occupy most of a lot, while on others, they cover only a small portion of the total area.
| Because of the diversity found in residential
areas, planting guidelines should be general rather than specific.
Streets, plazas, and fragmented planting areas have the greatest variety of physical conditions. They range from the narrow, canyon-like streets of the urban core to wide open areas around suburban streets. Between these extremes are streets and boulevards in residential, industrial or commercial areas. Fragmented open space includes parking lot islands and narrow planting strips around parking lots or next to buildings. Fragmented areas and streets fall into the same category, because plant selection criteria and environmental conditions are similar.
In these areas, available space is restricted and the land is divided into small pieces. Urban rooftop gardens where plants must be grown in containers offers an extreme example.
When selecting plants for this component of the urban forest, remember that it is essentially a symbolic forest it is also the most visible part of the urban forest, and the largest part in public ownership. Rather than designing a natural forest, the urban tree manager chooses plants that will bear the burden of this symbolism under the most stressful of the urban environments.
Tree Selection Considerations
One of the Most important aspects of urban forestry is selecting and acquiring trees. As stewards of the urban forest, today's tree planters have an obligation to contribute the best possible trees to future generations.
Tree selection and acquisition are also among the most challenging activities of urban foresters. Thousands of species are available, many of them genetic hybrids that have not been tested by time. Few people know all of the species. Objective information on long-term maintenance requirements and other potential problems is not readily available. In the absence of hard data, some tree managers assume that maintenance needs correlate directly to the rate of growth. In other words, a fast growing species will require more long-term maintenance than species with moderate or slow rates of growth. It may also be reasonable to assume that larger trees require greater expense for maintenance and replacement. With proper care, a tree could live for centuries. It makes little sense to ignore known shortcomings, simply because the problems will become apparent only when the tree matures.
Although it may be tempting to choose species that appear frequently in the landscape, popularity should not determine suitability for urban uses. Many desirable trees that require minimal maintenance seldom find their way into the urban landscape and may not be available from nurseries. Finding the best species requires research. Nursery catalogs are one source of information. The book, Plants that Merit Attention, Volume 1, Trees, may be useful for identifying nursery sources for a selected group of species. It was prepared by the Horticulture Committee of the Garden Clubs of America, edited by Janet M. Poor and published in 1984 by Timber Press.
|Picking the best tree species for a particular site is similar to separating gold ore from the surrounding rock. Beginning with an extensive list of trees, the forester must sift out unwanted material until only the right choice remains. To be selected, trees must pass through three levels of selection similar to a series of sieves. These considerations can be grouped into three categories: site, design, and maintenance.|
|Site Considerations - The environment
that will support trees encompasses space, moisture, soil, and other physical
conditions. As mentioned earlier, the environment is affected greatly by the
land use of a particular area.
Space could limit growth now or in the future. Consider the way that a particular tree will interact with the surroundings. Remember that trees occupy space above the ground, at ground level, and below ground.
The tree canopy offers the most immediate and visible concern. Most people would like to see a tree fill the space available. However, canopies that grow too large will require heavy pruning and possibly removal. Crowns in streetscapes could interfere with adjacent buildings, overhead utility lines, and views. Trees could also shade devices designed to capture solar energy or gardens that need ample sunlight.
Ground level refers to the space that begins at the soil surface and extends up to fourteen feet above the ground. Some areas require no clearance at ground level; sidewalks need ten feet for pedestrians; and streets need fourteen feet. Selecting trees with these requirements in mind can prevent unnecessary pruning later.
The space available below ground could be restricted by utility lines, building easements, vertical retaining walls or heavily compacted soils. Considering the way that trees fill space helps to choose a tree that will fit the site when mature.
Climate may vary from one site to another. Sunlight, wind, moisture, air quality, and temperature extremes all influence the choice of tree species. The conditions in an urban park are similar to those in undeveloped areas. But cities also have concrete canyons with low levels of sunlight, and the urban equivalent of deserts-large paved areas with intense sunlight, high temperatures, and little moisture.
Soil conditions must be assessed to identify fertility, aeration and drainage. Matching these site factors to the needs of potential tree materials is critical to the success of any planting.
|Design Considerations - Various elements in the landscape relate to each other on an aesthetic level as we] I as a physical level. Trees can be selected for the way their size, shape and color blends-or contrasts-with other elements. Trees may be selected for their texture or fragrance. The timing of leaf set and drop or the schedule of flowering may be important. The type of shade or the density of the crown could also affect the choice.||
Overview of Eugene, Oregon, 1988.
|A tree's above-ground branching system-or
crown-determines the overall shape. Some tree shapes are more suitable than
others for particular sites. And, shape affects the amount of volume that the
tree will occupy. Common tree shapes include: oval, round, columnar, pyramidal,
Oval trees fit nicely into most street plantings, because they grow upward and don't interfere too much with adjacent vehicular traffic. Because of their somewhat upright growth habit, oval trees can generally be placed closer to buildings than more spreading trees. But, oval shaped trees may reach a lofty height, making them unsuitable for planting underneath power lines.
Round trees have a spreading growth habit, resulting from more than one terminal leader. Lateral branches growing from the leaders may ascend, extend horizontally or sweep downward. Trees with an ascending branch structure are more compatible with city streets where there are buildings nearby or large trucks that must pass underneath. Trees with a descending branch pattern may require more pruning while the tree is small to maintain visual or vehicular clearance. However, once they grow larger, downward sweeping branches are valued for the sense of enclosure they suggest. A horizontal branching structure demands a greater distance between adjacent trees or buildings, if it is to attain a natural form and require minimal pruning.
Columnar trees usually have tightly ascending branches with narrow branch angles and short branches. Trees with this shape are valued for their narrow width that enables them to be planted in tight spaces where there is not enough room for a tree with a spreading branch structure.
Pyramidal trees often start off as columnar. With age, the lower branches get bigger and droop under their own weight. Many trees in this category have a single dominant branch from which the side branches grow. While conifer trees, such as pines, best illustrate the pyramidal shape, there are also many broadleaf trees in this category.
Vase shaped trees may be the most appropriate for planting on city street trees, because they grow up and out. They also arch over streets and sidewalks to form a shady canopy, while maintaining visual clearance and vehicular access.
Design requirements usually grow from site characteristics. For example, street trees typically need clear trunks. They grow as individual specimens, widely spaced in streets, without competition from adjacent trees until nearly mature. By contrast, trees for parks often grow in large groups. Early in their lives, they protect each other. But long before reaching their mature size, they begin to compete.
Nurseries now offer a wide selection of crown shapes, especially from clones. For example, the red maple clones (Acer rubrum) range from the narrow upright Armstrong to the widespreading, rounded Schlesinger. Similar variation can be found in the Callery pear ( Pyrus calleryana ) and Norway maple( Acer platanoides ) groups. This diversity of crown forms allows for more precise matching of trees to site conditions.
While clones are genetically predisposed to uniform growth, they do not guarantee uniform performance in the landscape. Site conditions vary considerably over relatively small distances. A series of clonal trees placed along a street could encounter a variety of site conditions that could affect tree performance and appearance.
Shoot development, or crown configuration, also affects the success of urban tree plantings. The development of a multi-trunked character, the height of the first branches and the presence of poor branch attachments may adversely impact the vigor of a tree, or require more intensive management to make the tree compatible with adjacent land uses.
Figure 3. The "Unfolding Urban Landscape" with trees of different shapes: oval, round, columnar, pyramidal, and vase-shaped. Illustration by Ann Christensen.
Maintenance considerations - Maintenance will re required of all tree species, but some will require far more than others. Given limited budgets, it makes sense to give maintenance requirements a relatively high priority among selection criteria. Pruning needs, fertilizer, water and pest management, and root control should all be anticipated.
Cities have long faced the problem of roots breaking sidewalks. Many people have searched for "deeprooted" species. However, the depth of rooting is more a function of site conditions, than an inherent characteristic of a species. Actually, most species are regarded as having shallow roots at maturity. Even so-called "taprooted" trees possess their vertically oriented roots for a short period of their lives. To reduce the importance of sidewalk damage as a criterion, use appropriately sized trees for the given space and planting techniques, such as large pits, root barriers, or deep planting.
Tolerance of stressful conditions is an important criterion for tree selection, and every site offers some type of stress. Generally, successful street tree species tolerate poor soil conditions, including fine texture and flooding. Tolerance to polluted soil may be needed. If salt is commonly used to clear the icy sidewalks, trees with a tolerance of salt should be selected. if the site has been disturbed, the pH level may influence the choice of plants.
Naturally, a species' susceptibility to harmful insects and disease should be considered, along with the cost of controlling them. For example, autumnal is flowering cherry (Prunus subhirtella) occurs widely, even though this genus is vulnerable to one of the most common diseases in the area: brown rot. Controlling brown rot requires an expensive multi-spray program. So, planting this species brings with it the burden of an intensive pest management effort.
Poor branch attachments, such as those with embedded bark, narrow angles of divergence or poorly spaced branches, become a major maintenance problem as trees mature. Some of these problems can be corrected at planting. Others can be eliminated by rejecting poorly structured trees from the nursery at planting.
Not every tree or every planting situation lasts for hundreds of years. Being dynamic, cities have spaces that are available for relatively short periods, making them suitable for short-lived trees or those that begin exhibiting undesirable traits as they mature. For example, big leaf maple (Acer macrophyllum) grows rapidly for 40-60 years. Then, branches begin to break out of the crown, and pruning requirements increase. In a similar manner, western white pine (Pinus monticola) reaches 60 to 80 feet in height after only 40 or 50 years. At this age and size, the trees become susceptible to white pine blister rust, which is always fatal. In situations where a tree is only needed for fifty years, both species would be excellent choices. While they develop problems as they grow older, in youth they offer a number of positive ornamental and management features.
Depending on the site, species' ability to withstand drought, freezing, wind, and air pollution should also be considered.
Urban areas offer a mosaic of environments and design possibilities, so there is tremendous opportunity to use a large range of trees in the urban forest. With such a great number of environments and design considerations, an equally large number of plants may be appropriate for use. This diversity reinforces the need for thorough site analysis and careful design for every project.
Providing Space For Trees In New Construction
Space for trees is readily available in open spaces around homes, in public parks, and school grounds. But in parking lots and along streets, adequate space for trees may only be available if it was specifically included during the design of the overall development.
During hot weather, parking lots can be as forbidding as great deserts. With proper planning, trees can minimize the heat. Spaces for trees should be scattered throughout the paved area, rather than only along the perimeter. These planting strips should be large enough to accommodate trees that will eventually grow large canopies and shade the parked cars. Areas surrounded with pavement should allow a square area at least six feet on a side. And, an area eight feet square would do much to prevent tree roots from eventually becoming a problem. Governmental bodies that regulate shopping center developments can require that the parking lots contain ample space for trees as well as for cars.
Street trees comprise about 85 percent of publicly-owned urban forests. They occupy an important and often overlooked-public land base. These unpaved ribbons along streets are commonly known as planting strips, parkways, or treelawns. Minimum widths identified by municipal codes are often inadequate for supporting mature trees, and some codes fail to mention trees at all. Since the size of existing street rights-of-way have remained the same for centuries, only future rights-of-way offer hope for adequate treelawns.
There are two practical ways to design treelawns of more appropriate location and width. The first approach combines the sidewalk and the curb as a single unit (monolithic design), so the treelawn occurs between the sidewalk and the right-of-way boundary. Called a boundary treelawn, it has been used widely on the West Coast since the middle of this century. The tree can be located off-center near the right-of-way boundary relatively far away from the sidewalk.
|The more traditional arrangement,
called a curbside treelawn, is located between the curb and the
sidewalk. Besides using this space for street trees, curbside treelawns serve
as storage for snow plowed from the street. Usually wide street rights-of-way
make extra-wide curbside treelawns possible.
No standards exist for the width of treelawns. Prudence suggests that treelawns should be made extra-wide to accommodate large trees that will form a canopy over the streets. Wide treelawns also reduces the chance that roots will damage sidewalks. Preventing treelawns from cutting into the amount of land available for building is simply a matter of shortening the building setback.
Figure 4. Street trees may be planted within a curbside treelawn, median, behind the curbside sidewalk in the boundary treelawn, or outside the right-of-way, on private property.
|Boundary treelawns make good use of space. For
example, a tree planted one-half foot from the right-of-way boundary would be
three and one-half feet from the sidewalk. While greater distance between trees
and sidewalks may be better, three and one-half feet is reasonable. Since trees
customarily grow midway between curb and sidewalk, achieving the same amount of
space between trees and concrete in a curbside treelawn would require seven
To implement any of the these concepts, a local government may have to amend its regulations concerning location of the treelawn, width of the right-of-way, and distance of the building setback.
Designing for Root Control
Tree roots are notorious for buckling the pavement of sidewalks and streets. Uneven surfaces may pose a hazard to pedestrians and motorists. Researchers are looking into several methods of preventing, or at least delaying, this type of damage. Planting time offers the best opportunity to prevent conflict between trees and pavement. Here are some techniques that may minimize the damage:
Select species that best fit the available space (Fig. 5).
Obtain trees produced on rootstocks with inherently deeper growth.
Choose trees with columnar root balls.
Install planting hole liners, deflection barriers (Fig. 6), pervious barriers, and chemicals barriers, to inhibit shallow root development.
|Deflection barriers may be plastic
sheeting that diverts roots away from a paved surface. Properly designed
underground planters or planting hole liners can also block lateral growth, and
force roots deeper. Pervious barriers could be fashioned from loosely woven
plastic fabric or geotextile situated in the soil to prevent roots from
penetrating into areas where they would cause damage. Chemical barriers, such
as copper screen or geotextile impregnated with herbicide, inhibit growth in
the tips of roots as they grow near the barrier.
Figure 6. Deflection barriers direct the lateral root growth.
|Researchers are testing these methods individually and in various combinations. Until proven strategies emerge, it is prudent to simply allow ample space for root growth, and where that is not possible, plant smaller trees.||
Figure 5. Careful attention must be given to site conditions, either existing or proposed, when selecting the trees to be planted.