Cooling our baking cities.

July. We expect it to be hot, especially in the city. But do you Easterners remember last February? It was warm too, fooling the forsythias into behaving as if it were April. Rising like a golden sunrise out of the pavement near Logan Circle in Washington, DC, the flowers added some color to the winter cityscape, but I got the feeling we would pay for this oddity later on, in the summer. Winter temperatures were a little more moderate in the suburbs, mild but not warm enough to pull one over on the bloomin' plants. The forsythia bloom doesn't tell us much about global conditions, but it does help us focus on a related issue-the warming of cities. The 80s have brought us five of the warmest years on record, as well as an awareness of an atmospheric phenomenon called global warming (see "ReLeaf for Global Warming" in the November/December 1988 issue). In a way, what's happening in cities depicts global warming in miniature. Many of the same factors are involved-fewer trees, more pavement and manmade structures, carbon dioxide and pollutants trapped within the atmosphere.

Researchers at the Lawrence Berkeley Laboratory (LBL) in California report that average temperatures in the city can five to nine degrees Fahrenheit) higher than those in the surrounding suburbs. Imagine yourself as a satellite looking down on the globe and zeroing in on a region of the U. S. with your infrared scanner as an eyeball. Whatever region of the country you choose, you'll note urban hotspots that usually (depending on the type of cameras used) appear blue while cooler vegetation zones show up red. The hotspots are often called "heat islands."

But what makes a city form a climate so much warmer than the surrounding countryside? The fundamental laws of science producing heat islands are easy to demonstrate. Did you ever walk barefoot across an asphalt road on a warm summer day? If the road had a white line on it, you were lucky: maybe it saved you from burning your feet. Proceeding toward a grassy spot along the side of the road and then stopping under the shade of a tree, you experienced a gradation of climates" within a limited area. Dark surfaces soak up more heat than light ones, so naturally they get hotter. And while living plants absorb heat, they also transpire water, which keeps them cool. The city is an artificial environment low in green plant material and water, and high in manmade materials that soak up heat, stagnate the air, and reduce moisture. City centers can be especially hot because buildings, asphalt, and concrete comprise 70 to 90 percent of the area. As grass and trees infiltrate the city space, the temperature cools.

Because urban heat islands affect our health, our economy, and our environment, they have become an increasingly important facet of scientific study. Researchers Hashem Akbari and Art Rosenfeld at LBL have used computer models to estimate the cost of rising temperatures in urban areas. Their research shows that with each degree of increase in temperature within a heat island, Americans spend $1 million per hour in cooling expenses. The hotter the temperature, the more cooling units operate and the less efficient they become. Trees operate as nature's air conditioners (evaporative coolers), but as we will see later, a tree here and there within a heat island can easily be overwhelmed.

There has been an occasional news story suggesting that trees contribute to-rather than offset-pollution. Intuitively, most of us know that trees don't pollute like cars and factories do, and research is proving it. The Akbari and Rosenfeld studies indicate that 20 to 30 percent of air pollution in cities may be the direct result of warm temperatures. The heated air acts like a bubbling crucible that aids the chemical mixing of unwanted pollutants.

Most cities not meeting EPA clean-air standards get into trouble when temperatures are as high as the 90s, but no violations have occurred below 74 degrees. U.S. Forest Service scientists William Smith and Leon Dochinger have shown that trees can remove polluting particulates from the air as well as nitrogen oxides, airborne ammonia, and sulfur dioxide. Forest Service researcher Rowan Rowntree theorizes that carbon dioxide, one of the major gases contributing to the greenhouse effect and global warming, is concentrated in the heat island dome, adding to the problem.

The science of measuring and evaluating the heat-island effect is relatively new, but opportunities for dousing the urban sizzle and making cities more livable do exist. Akbari and Rosenfeld suggest two of the most economical ways of cooling the hotspots are to add trees and to increase the reflectivity of city surfaces. Simply stated, increase the tree cover and change some of the dark surfaces to light. The energy models these researchers have produced show savings of 30 to 50 percent in cooling demands where trees and lighter surfaces have been added. In California's Sacramento Valley, more tree cover could replace the need for building two conventional power plants. That's a big savings for energy users and a positive step toward cleaner air.

A good example of a heat island that was cooled by vegetation exists in Palm Springs, California. For many years, temperatures there were rising. In the early 1970s they peaked, then started cooling down. In recent years, Palm Springs' average temperature has dropped two to three degrees below that of the surrounding desert. The reversal, according to Robert Balling, director of the Arizona State Laboratory of Climatology, can be linked to a sudden upsurge in golf-course construction that occurred at the same time. By tipping the ratio of concrete to trees in favor of trees, Palm Springs was able to turn from heat island to cool oasis.

Vegetation on the golf courses helped to lower the temperature naturally, through evaporation and transpiration (evapotranspiration) of water during photosynthesis. Trees never get as hot as asphalt because evaporation helps to moderate their temperature, much as a person perspires to keep cool. When plants wilt, the leaves are demanding more water than is available; if the demand continues, the plant may die. Thus, trees play an important role in improving the city climate, but also require a moderated climate for their own survival.

At the University of Washington in Seattle, researchers Jim Clark and Roger Kjelgern are studying vegetation and its effects on microclimates. They measured three different types of urban forest-defining them as Park, Plaza, and Canyon-to demonstrate how trees interact with local climate to become building blocks for improving the microclimate. This study has far-reaching implications for better understanding how trees react or cope with heat island effects.

In Parks, described as large assemblages of plants, there may be one or more layers of plant material, including trees, shrubs, and grass. Plazas are areas where there is more concrete than trees. Standard city scenes that may be categorized as plaza-type "forests" are parking lots and streetside plantings. But many streetside plantings, especially those in center city, are better defined as Canyons because they are partially shaded by tall buildings. Both the park and plaza types of urban forest have full exposure to the sun, which provides plenty of energy for photosynthesis. Plaza and canyon plantings have limited soil resources for root activity because they are surrounded by concrete.

So trees in parks have the best of both worlds-full sunlight and adequate space for root growth. Not surprisingly, then, park trees in the Seattle study were able to maintain the growth process under most climatic conditions. The rate of growth for park trees was about twice that of trees in plazas.

Plaza trees are much more likely to turn the growth process on and off as weather conditions fluctuate, because limited soil space is available to support growth. If the soil can't hold enough moisture to support the rapid transportation of water as the day heats up, the abundance of heat and light can easily overwhelm the energy conversion factories within the leaves. As summer conditions linger, the shut-down mechanism of the tree often proves inadequate to maintain the tree even in this dormant condition. The end result: trees die. Plaza-type microclimates are common in cities because streets and parking lots dominate the city scene. These types of trees actually receive more sunlight than trees in an open field, because they receive additional rays reflected off buildings, sidewalks, and streets. Combine this with the overall warmer temperatures found in cities, and it is easy to see why trees have a tough time within urban heat islands.

Planting drought-resistant species can help survivability of trees in Plaza areas, but even the toughest of the arboreal breed need water. Changes in soil and moisture conditions provide a better solution.

The Seattle study indicated that Canyon trees receive only about 50 percent as much sunlight as Park trees. But growth of the same tree species in these two locations was essentially the same. How could that be? The answer was apparent after close inspection of horticultural adaptations. Canyon trees adjusted to reduced sunlight by developing more branches than the Park trees and by orienting their leaves to catch the maximum amount of sun. Leaves were larger, thinner, and oriented perpendicular to the ground. Conversely, on the Park trees, where light was abundant, leaves were oriented parallel to the ground. So Canyon trees captured enough sun to match the available moisture, and growth kept pace with that of the Park trees.

According to this study, then, the Plaza-type trees in our cities need our help before they can help us cut the sizzle. Improving soil and moisture conditions has already been mentioned as part of the solution. Another way to increase the survival of Plaza trees is to focus on planting (or transplanting) bigger trees to these areas. Large trees can stand up better to heat and reduced moisture, because their roots draw moisture from a larger area. Big trees then have a greater impact in reducing the effects of the urban heat island.

Planting groups of trees rather than forcing trees to fend for themselves is another good idea, particularly in Plaza locations. When trees and shrubs grow in groups, they are better protected from sun and wind and are able to survive conditions that would wipe out the single tree. That is because these tree islands mimic the natural forest, to which trees are much better adapted than the settings where we often force them to live. Even the climate within these layered tree groupings can be noticeably cooler than the surrounding cityscape. The change is called climate amelioration, and it can help urban heat islands to at least have some islands of relief (or ReLeaf) within them.

Clark and Kjelgern, by studying the heat and light effects on trees in different types of city environments, were able to find niches for certain types of trees, shrubs, and plant groupings. This might have been easier to do in the moist Seattle climate than in other urban areas, but it points to the need for similar studies nationwide.

The research into city microclimates has just scratched the surface of a complex and important scientific endeavor making space for trees and making good use of that space. Plants are very effective climate-control devices, but they are sensitive to climatic factors and can easily fall victim to extreme conditions of heat, water, and light. As we tackle the enormous problem of global warming, we should also attempt to cool the hotspots in our cities -hotspots that contribute to global warming and perhaps depict the problem in miniature. Trees can help us do the job if we let them do their job.