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Designing the ecological city.

Ecology lessons in the city? The idea wouldn't seem so far-fetched if cities were designed - and trees planted - with urban environments in mind.

In 1869 Ernest Haeckel, a German scientist known for his ability to popularize science, introduced the world to the concept of ecology. Derived from the Greek work oikos, meaning house, the term "ecology" was used by Haeckel to explain the interactions of organisms with their environment.

Since the, the concept of ecology has expanded to include a wide variety of natural phenomena. Paleontologist and writer Stephen Jay Gould muses, "Ecology [has become] a label for anything good that happens far from cities or anything that does not have synthetic chemicals in it."

Our track record in building cities, as Gould suggest, has very little to do with ecology. The people who design and build cities have largely isolated themselves from the natural sciences. Cities could be considered the "black holes" of the landscape. Darwin, Haeckel, and Gould would likely have little interest in cities. A city is not the place to discover new species, view a wealth of non-human diversity, or marvel over nature's complex balance. It is rather a place where nature is subdued, where plants and animals struggle to survive.

But in fact, the average city has a 30 percent tree cover and some species of thriving wildlife. Birds and squirrels have adapted well in cities where trees are abundant, while rats and mice dominate those with little or no vegetation. In communities like Portland, Oregon; Chicago, Illinois; and Washington, DC - where natural greenways connect city with rural countryside - wildlife can be diverse and abundant. A Sunday drive near a major greenway may include glimpses of deer, fox, woodpeckers, and other wildlife.

In cities plant communities struggle through various stages of succession, mostly early stages. Cities may not be a place to study the wealth of plant diversity, but they offer an opportunity to create some.

The American Forestry Association believes cities need to discover their naturalness and incorporate it into future development. Allowing cities to develop in total disregard for naturalness is the root, as well as the perpetuating force, of many environmental problems. Building cities and suburbs with little room to grows trees lowers the quality of life and is costly to the pocketbook, heart, and mind.

Our understanding of the urban landscape is limited, but the opportunities for improving it are considerable. Change starts with understanding, so we have assembled some new information about ecology in the city.

Although city and countryside are worlds apart, a developing school o thought is dedicated to designing cities with nature in mind. Innovative land planners and designers like Ian McCarg, Mike Hough, and J.T. Lyle address and evaluate the natural landscape as part of the design process and use existing natural conditions in a positive way. They suggest that urban design elements which short-circuit a natural cycle or reduce diversity are negative factors. Working with natural systems creates a solid foundation for building an ecological landscape, a landscape that fits the physical environment and is able not only to survive but to thrive.

A story about Tucson, Arizona, may demonstrate this point.

Tucson is a desert city but is definitely not deserted. Rapid population growth over the last 20 years has put tremendous stress on the water resource there. As a result, city leaders have taken some extraordinary steps to be encourage water conservation.

Planners compared water-meter readings in summer and winter and found summer use to be much higher. They determined that the landscape was to thirsty, accounting for over half of the total water used by the city. New policies encouraged residents to tear out grass, trees, and shrubs and replace them with rocks, sand, and other non-living features. The policies became landscape ordinances, and an ad campaign helped reinforce the so-called xeriscape philosophy. (Xeric means "using little or no water," so a xeriscape is a landscape that requires only small amounts of water.)

That's when Greg McPherson and Joanne Gallaher came along. Both landscape architects with a background in urban forestry, they were convinced that city officials had shot themselves in the foot. Joanne, who works for a private firm, and Greg, with the University of Arizona, recognized the need to conserve water, but felt planners had been misled by statistics that showed only one dimension of the problem.

Gallaher and McPherson saw a much different set of facts. Though removing some of the landscaping in town would result in water savings, other kinds of vegetative landscaping used small amounts and in the long run helped conserve water. Replacing landscape plants with rocks was exactly the wrong thing to do. Rocks soak up heat and cause water loss though evaporation. Greg and Joanne proposed that the city take a lesson from nature and promote an ecological landscape.

An ecological landscape is one that mimics nature and makes use of natural processes and plants. Vegetation needs to be selected to fit the climate, soils, and moisture limits of a particular site. For Tucson, this directive meant selecting trees that would grow and thrive in desert conditions - and use little water. The water used by one mulberry, for example, is equal to that needed by eight palo verde, a tree native to Tucson's desert environment.

Modern engineering practices compound the water problem by dumping valuable stormwater into elaborate piping systems that short-circuit the natural cycle, and rapidly remove water from city streets. The water needs over much of the landscape could be met if this stormwater could be utilized. There are opportunities to slow rainwater movement and make use of it. Such water "harvesting" can be accomplished by making minor changes in curbs and gutters.

Other water-conservation methods include drip irrigation, and the use of mulch and gray water. By replacing grass with mulch, water is saved and the list of appropriate landscape plants is increased.

The case for ecological landscaping was strengthened last summer when soaring temperatures drew worldwide attention to a phenomenon called the greenhouse effect. As part of its Global ReLeaf program, (see "ReLeaf for Global Warming" in the November/December 1988 issue), the American Forestry Association is calling national attention to the vital role trees play in cooling communities and reducing greenhouse gases in the air. Shading Tucson with trees and other vegetation will lower temperatures, save energy, and reduce water use. The cooler it is, the less water evaporates, the less we perspire, and the less the plants transpire.

The city of Tucson is changing the way it looks at trees, thanks to the efforts of Greg and Joanne and other groups, both governmental and private. One private group, the Southern Arizona Water Resource Association (SAWARA), has been instrumental in educating the public to the problem and ecological solutions. The use of native plants to create an ecological landscape in Tucson offers a lesson other arid communities might follow.

In most cities, including Tucson, the number of native trees that can survive in town is limited by the physical environment. The urban climate is generally much hotter and drier than the surrounding countryside. If you live near a city circled by an express-way or beltway, you probably hear about these differences from the weatherman. It's always hotter inside the beltway and is center city than in the suburbs. This is why climatologists refer to cities as heat islands, where temperature can be nine to 12 degrees hotter than in the countryside. Some native trees won't stand up to these seemingly subtle differences; some non-native, southern, or drought-tolerant species are more appropriate in a city's warmer environment.

Soil in cites also differs greatly, and is a factor that is generally not considered in choosing species to plant there. Most soil scientist have little to say about urban soils, which reveal more about construction and compaction than about the structure of the standard soils of a given area. Most trees require good organic matter and air space, and urban soils are low in both.

The trees that survive best in cities are tough species that can tolerate drought, poor soils, and periodic abuse. These plants are rarely native to the site, and are often the result of some evolutionary adaptation. Swamp trees, for example, have developed unique root systems that survive flooding by growing very near the surface and somehow extracting air from drenched soils. This ability allows swamp trees to have the best survival record in cities.

In the East and Midwest, the factor that most limits tree selection is compact soils lacking the air needed for root growth. In Tucson and throughout most of the Southwest, the limiting factors are heat and water. Native and adapted trees offer the best options because they are generally able to withstand both drought and heat. But what makes sense in the Southwest does not apply to other regions. The buildings and rocks in Tucson act much like the surrounding desert, but cities in the East, Northwest, and Midwest become less and like their native environment as they grow.

A city's environment is literally molded by the hand of man. From an ecological point of view cities are spartan, and the challenge is making the environment more suitable for things to grow. Urban foresters have had to use both imagination and skill to develop and maintain the tree cover, but statistics indicate we are losing the battle. Scientists can improve tree health, but they aren't magicians. We must go beyond the trees and see the city as a living landscape/ecosystem.

Geneticists ave probed the evolutionary development of specific trees in an effort to find tough ones to cope with the city's abuse, heat, and compacted soil. Another innovation involves planting smaller trees where there are above-ground restrictions like powerlines, signs, and buildings. Smaller trees lower maintenance costs, demand less of the site, and cost less to remove when they die. But small trees may be an indicator of declining ecological quality in cities.

Improving the ecology of cities calls for larger trees, more trees, and better spaces for trees to grow. The trend toward smaller trees may reduce the friction between city structures and the plant kingdom, but it reduces the ability of trees to buffer the environment. If trees are going to cool the pavement and shade buildings, they will have to be tall enough to supply shade. If they are going to reduce the impacts of the harsh winter wind, they have to be big enough to affect air flow. If they are going to affect stormwater flows and improve air quality, the space they occupy will have to increase. If the movement toward smaller trees continues, city streets will soon be lined with potted plants that have little effect on the urban ecology.

The average city today has a tree canopy covering about one-third of its area; the health of the trees forming this green umbrella is declining. Growing space is probably the most significant element limiting the urban forest. Preliminary data shows that the closer we go to the city center, the shorter the life of the average tree. A tree's lifespan and size are directly related to the size and quality of the space where it grows. The city of the 21st century needs to double its tree cover and increase the lifespan of the average tree from 32 to 60 years.

The Fourth Urban Forestry Conference, to be held in October of 1989, has the theme, Making Cities Safe for Trees. Making the changes needed to create a healthy urban forest requires action on political, social, and technical fronts. Citizens need to speak out for larger trees, for more trees. Political leaders needed to develop policies that make the city more receptive to trees. This may mean changing the structure of the pertinent government agency or finding skilled urban foresters to supply more leadership during community development and redevelopment. It may mean changing the specifications for planting city trees and following the advice of landscape experts who know the construction details needed to keep trees alive.

In December 1988, AFA's National Urban Forest council gathered experts from around the country to address the technical issues of making space for trees in the city. The finding of that meeting will be covered in this column in the next American Forests.
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Author:Moll, Gary
Publication:American Forests
Date:Mar 1, 1989
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