THE AIR IN OUR CITIES that smarts our eyes and chokes our lungs also damages—and sometimes kills—our shrubs and trees. Runoff of salt spread on streets to melt snow and ice in winter harms lawns and other growing things. Plants are needed that will survive, even thrive, amid smoke, grime, fumes, chemicals.
Plants absorb carbon dioxide and supply us with oxygen in the process of photosynthesis. At the same time, they reduce pollutants in water and soil. They also remove significant amounts of gaseous pollutants and particles from the air. The microscopic plants in soil also reduce air pollutants and degrade many toxic chemicals that enter the soil.
Plants hold topsoil in place. Thus, they reduce sediment and excess nutrients which pollute water. Plants also make effective sound barriers, and so reduce noise pollution.
In the United States, ozone is the major pollutant that affects vegetation.
Other air pollutants of concern to plant scientists are peroxyacetyl nitrate (PAN), sulfur dioxide, and fluorides. Nitrogen dioxide and ethylene are not as likely to cause acute injury, but they may stunt the growth of plants and cause their premature old age.
Ozone and PAN are photochemical oxidants formed by sunlight acting on products of fuel combustion, particularly the nitrogen dioxide and hydrocarbons that come from motor vehicle exhausts.
Ethylene is also a product of fuel combustion, and to a very minor extent it is produced by vegetation.
Sulfur dioxide results from smelting ores and from burning fuels containing sulfur—such as coal and crude oil.
Fluorides are emitted in the production of aluminum, steel, ceramics, and phosphorus fertilizers.
Some of the injuries caused by air pollutants are given next.
Ozone causes many small irregular lesions, called fleck or stipple, on the upper leaf surface of broad-leaved plants. Injury can develop on both leaf surfaces on upright growing species like grain or grass. Veins of the leaves tend to remain green unless general yellowing (chlorosis) occurs. Chlorotic lesions occur also on pine needles, along with tip dieback. Injury occurs primarily on lower leaves of plants.
Peroxyacetyl nitrate (PAN) causes collapse of tissue and silvering, glazing, or bronzing usually of the lower leaf surface. The injury may appear as transverse bands. PAN affects younger leaves than are affected by ozone.
Sulfur dioxide causes irregular blotches between the veins of leaves. These blotches show on both leaf surfaces. Injured tissue is white, gray, or ivory with larger veins remaining green. On grasses and similar plants with parallel veins, injury appears as streaks and general blight on leaf tips.
Nitrogen dioxide suppresses plant growth without marking the leaves when concentrations are low. High concentrations may produce leaf markings resembling sulfur dioxide injury.
Fluoride causes necrosis (death) of leaf margins and tips. On some plants—for example, citrus, poplar, and corn—chlorotic patterns on the leaves may be the principal symptoms.
Ethylene causes wilting of blossoms and drooping of the younger leaves, followed by premature yellowing and defoliation.
Pollution in towns and cities is seldom from a single pollutant. Usually there are many pollutants and their total effect is often much greater than you would expect, knowing their individual effects. There are also symptoms that mimic air pollution injury. They may be caused by insects and diseases or by poor nutrition, soil compaction, drought, cold, and high salt content in soil. Injury from air pollutants may make plants more susceptible to injury from some diseases and insects.
Premature aging of leaves caused by air pollutants is often confused with natural aging. The best way to know the full effects of photochemical oxidant air pollutants on plants is to grow the same plants in greenhouses in both unfiltered air and clean filtered air. Scientists have done this, with remarkable results.
For example, studies near Los Angeles with citrus showed that yields of fruit were only about half as much in unfiltered air as in carbon-filtered air, even though the leaves were almost free of injury in the unfiltered air. Ozone was considered the primary cause of the reduced yield. It and other oxidants are effectively removed by activated carbon filters. Special filters are required to remove pollutants such as fluoride and ethylene.
Other studies, at Beltsville, Md., have shown that many plants benefit from air filtered through carbon to remove oxidants. Certain varieties of potato, onion, radish, and beans almost doubled their growth in greenhouses with carbon-filtered air. And they were free of the injuries observed in unfiltered air.
Sycamore seedlings in the carbon-filtered air were 25 percent taller than those in unfiltered air.
Levels of the photochemical oxidants at Beltsville and along the East Coast are only about a third of those in the Los Angeles basin. But plants grown in the East, with its higher soil and air moisture content, are much more sensitive to pollutants than plants grown in the arid West.
Losses from air pollutants can be serious or minor, depending on the variety of crop planted. One variety of potato, Norland, showed severe leaf injury and marked reduction in yield of tubers in unfiltered air, whereas another variety, Kennebec, did not.
Eventually, there will be increased demand from the public for plants that tolerate air pollution. The greatest need will be for plants tolerating photochemical oxidants. Levels of these pollutants are increasing, and their distribution is widespread.
Scientists know that genetic variation in resistance to pollutants occurs in many species of plants. One plant survives; another does not. So, they identify and save seed from the one that does on the theory that the plant has a natural tolerance to air pollution. The widespread use of tolerant varieties will do much to reduce losses.
Losses from air pollutants may be further reduced by breeding plants to increase tolerance. This has been done successfully for cigar-wrapper tobacco in the Connecticut Valley.
To some extent, the breeders of crop and horticultural plants unknowingly have developed pollution-tolerant plants when selecting plants most free of the leaf injury. For example, the alfalfa variety Team developed at Beltsville has greater tolerance to ozone than varieties developed in other parts of the country with less air pollution. The cotton variety Acala SJ-1, developed and used in California, has more tolerance to ozone than varieties from the Southeastern United States, where levels of these pollutants are much lower.
Plants are good air pollution detectives, and their use for this purpose will increase. West Germany requires planting of forest species around certain industries as a check on emission of toxicants. Sensitive plants may show visible effects of pollution long before their effects can be observed on animals or materials.
Plants are cheaper than specialized instrumentation as pollution detectives. They respond to several pollutants—effects can be additive—and they indicate whether pollutants of biological significance are present.
Of course, we also need instrumentation including, for some pollutants such as photochemical oxidants, a national grid of devices with the information summarized by computers and made available immediately to the public. The monitoring is primarily needed from June through September when oxidant levels are highest and vegetation is making most of its growth. In the Los Angeles basin the need is almost year round.
When plants are injured by pollutants we know they are, at the same time, removing some pollutants. In the case of fluoride pollution, leaves of tolerant species may contain several hundred parts per million of fluoride without visible injury. Leaves also remove pollutants, such as ozone and dust particles, just by contact with leaf surfaces. But more of the gaseous pollutants are removed when stomata or microscopic pores are open. There are several thousand of these pores on each square inch of leaf surface. Normally they are open in the day and closed at night.
Lower forms of plant life also remove pollutants in air, soil, and water. For example, some of the micro-organisms in soil remove carbon monoxide and hydrocarbons, such as ethylene, when the air above the soil surface mixes with air in the soil. Other microorganisms degrade toxic chemicals so residues do not build up.
Chemical reactions seem to be primarily involved in soil removal of other pollutants such as sulfur dioxide and nitrogen dioxide.
Maintaining an abundance of vegetation is essential for pollution control. Top soil should be kept in place, and we need to propagate more plants than we destroy by our activities.
Trees and pollution could almost make a separate chapter. Although trees generally live a long time, air pollution can kill them.
For more than 95 years the sulfur dioxide spewed out by smelters in the United States and Europe has been killing trees, mostly the conifers, like pines, spruces, and firs. Extreme damage to conifers by sulfur dioxide was found in timbered areas around smelters, but losses also occurred in urban industrial centers where sulfur dioxide was the major pollutant.
In 1924, after a long struggle with conifer culture, the Royal Botanic Gardens decided to concentrate its future conifer plantings in rural Kent rather than at Kew, near London.
Ozone and sulfur dioxide cause chlorotic dwarf disease and other ailments of white pine in the Eastern United States. Some of the affected white pines are in rural areas far removed from industry and urban centers. However, during periods of air stagnation the blanket of polluted air may cover a whole region from Maryland to Massachusetts.
But these are trees of the forest. In the East or the West, or anywhere in between, we might plant a Douglas-fir, a ponderosa pine, or an eastern white pine as an ornamental to grace our home grounds or to landscape a factory site, but they are not the usual trees of urban areas. What about the shade trees—elms, oaks, maples, planes; what about our ornamental trees—cherries, magnolias, flowering crabapples? We do not know the extent that these trees, which make life so much more livable, are suffering because of a polluted environment. Both acute and chronic injury are known to occur on some of these species because of pollutants.
Especially in our cities, trees are of inestimable value as they reduce noise, produce shade, filter out dust particles, and perhaps most importantly, provide an esthetic link between urban man and his wilderness heritage.
Reducing air pollution at the source will help to maintain the trees, but pollution probably cannot be eliminated. Therefore, we must find ways for man and trees to live better under urban conditions.
Any reduction in vitality of a tree makes it a more likely host for insect and disease attack and lowers its resistance to other environmental stresses such as drought. What can be done to improve the resistance of trees to pollution?
As with horticultural and crop plants, trees must be found that are sufficiently tolerant of pollutants so as to be free of acute injury. Also, they must maintain satisfactory vigor when exposed to existing pollution levels. Even if pollution is reduced significantly at the source, tolerant trees will still be needed to thrive at the reduced pollution levels.
How can we select pollution-resistant trees? One obvious technique is to survey urban trees to determine which species have endured on our city streets over the years.
Two species immediately come to mind. These are the ginkgo and the Chinese tree-of-heaven (Ailanthus), the tree that grows even in Brooklyn. The ginkgo is an acceptable shade tree, especially the male trees that do not produce the characteristic odoriferous fruit. Ailanthus, however, cannot be considered at all desirable, except perhaps in the most desperate situations.
Reports of “natural” resistance vary from one region to another, and a tree species deemed resistant in Houston may be quite susceptible in Buffalo.
But town and city streets are not an ideal laboratory. Growing conditions vary tremendously, even from one side of a street to the other, and may influence a tree’s response to gaseous air pollutants. Furthermore, since urban air pollution—especially photochemical smog—is a rather recent manifestation of civilization, there has not been sufficient time for any significant degree of natural selection for pollution tolerance to have taken place.
We can select for resistance by subjecting young tree seedlings or detached plant parts to measured amounts of pollutant gases under controlled conditions. Special fumigation chambers are being used to study the effects of various gases, alone or in combination, on trees as well as other plants. Even with the limited facilities available at the present, some progress is being made.
What has been found? Certain relationships between species have been established. For instance, European linden is more tolerant of ozone than is white ash. On the other hand, linden is more susceptible to salt in the soil. But the most important finding of the fumigation studies, exceeding even the differences between species, is the significant differences in resistance among individual plants within a species.
Urban tree culture is rapidly turning from dependence on particular species to the use of selected clones. All the members of a clone are propagated vegetatively from an individual tree (by grafting, budding, or rooted cuttings) and have the same genetic constitution, like identical twins.
It is in individual selection that our greatest hope lies. The selection of pollution-tolerant trees, and the combination of air pollution resistance with other desirable characteristics such as disease resistance and tolerance to drought and salts, is possible by selective breeding. Trees could be developed also for efficiency in removing pollutants from the atmosphere.
Trees live a long time; tree breeding takes a long time. But the improved trees resulting from today’s research will last a long time—to enhance our towns, suburbs, and cities for the good of the people.
Salts occur naturally in soils and waters and may be considered pollutants only when man introduces extraneous salts. This obviously occurs when salt is used to de-ice city streets and highways.
Salts supply plants with mineral nutrients essential for their growth. When present in excess, however, salts are injurious.
In humid regions, rain readily leaches salts out of the soil, and salinity is not normally a problem. In subhumid and arid regions, plants must be watered, and salts present in irrigation waters are the main source of salt accumulation in the soil. Many irrigated areas in modern as well as in ancient times have been “salted out” or severely damaged as a result of salt accumulation.
When you water your plants they absorb water, but leave behind in the soil most of the salts that were in the water. The only way to remove these salts is to use more water than that which evaporates and is used by the plants. If the excess water can drain away below the roots, it will carry with it the excess, unwanted salts. This is called leaching.
Leaching with a 6-inch depth of water, for example, will reduce the salinity of the top foot of soil by 50 percent, and a 12-inch depth of water will reduce it by 80 percent. Chemical amendments, such as gypsum, are needed to reclaim sodium-affected soils. Subsoil drainage must be provided if it is not naturally adequate.
What are the symptoms of salt injury? As the level of salinity increases, leaves, stems, flowers, and fruits are generally smaller. Stunting and, in extreme cases, death of plants are usually the only observable effects on most non-woody plants. Woody plants—that is, trees, shrubs, and vines—are damaged by accumulations of sodium or chloride in the leaves. Characteristic tip or marginal leaf burns develop. Burned leaves often drop off the plants, and this may be followed by dieback of stems and eventual death of the plants.
Plant species exhibit a wide range of salt tolerance. The most tolerant economic species, such as Bermudagrass, are able to tolerate salt concentrations about 10 times as great as those tolerated by the most sensitive species—African-violet, rose, and strawberry for example.
Some idea of the salt levels tolerated by plants can be given in terms of the total salt concentration in soil water bathing the roots. Sensitive species are affected when the soil water contains more than 0.2 percent total salts in solution. Moderately tolerant plants are affected above 0.5 percent, and tolerant plants above 1 percent. For comparison, sea water has a salt content of 3.5 percent, and saturated brine a concentration of 35 percent.
Most nonwoody flower crops are moderately salt tolerant. Shrubs vary widely in salt tolerance. Natal plum, bougainvillea, oleander, dodonea, bottle brush, and the ground cover, rosemary, are quite salt tolerant. Most shrubs are moderately tolerant. The most sensitive shrub species include Algerian ivy (a groundcover), Burford holly, pineapple guava, and rose.
Trees from normally saline habitats, like the tamarisks and mangroves, are highly tolerant. Other tolerant species include the black locust and honey locust. Coniferous trees, such as the blue spruce, white pine, and Douglas-fir, are relatively sensitive, but ponderosa pine and eastern red cedar are moderately tolerant as are also white oak, red oak, spreading juniper and arborvitae.
Plants normally absorb salts through their roots, but many will take up salts directly through their leaves if the foliage is wetted by sprinkling. Fruit trees, such as plum and other stone fruits, and citrus absorb salts so readily through their leaves that sprinkler systems usually must be designed to avoid wetting the foliage of these trees. Uptake of salt by the leaves of shrubs has not been studied, but salt-spray damage has been observed in coastal areas of Florida and Australia where sea spray wets the foliage. Leaf damage from salt-water spray has also been noted for trees and shrubs planted along highways that were de-iced with salt.
The extensive salt damage to trees, shrubs, and grass along streets and highways de-iced by salt is harder to control by plant selection. Salt concentrations in water draining off the highway can be so high that no plant adapted to northern conditions may be able to survive. Further, the loss of magnificent roadside trees may not be acceptable, even if some humble salt-tolerant replacement species is available.
It would be better to install drains to carry the brine solutions away from the highway without damaging roadside trees. Design of new highways and roadside plantings should take into account the effects of de-icing by salt.
Land will be increasingly used for the disposal of liquid and solid wastes to reduce contamination of our waterways and to take advantage of the exceptional capacity of soil to remove and decompose many waste materials. Tolerance of plants to salinity, as well as other pollutants, will have to be taken into account.
Salinity, like air pollution, cannot be completely eliminated. However, if adequately salt tolerant plant species are readily available, the salinity may not be damaging.