before the trees are in leaf. In the confined space of a small city garden we may prefer trees to flowers, but we can seldom have both.
The effects of heavy deposits on leaves may be even more important. Evergreen species in heavily polluted areas have been shown to have a rate of transpiration of only about one tenth of normal and the leaves last a much shorter time than they do in pure air. Thus in some conifers the leaves normally live for up to eight years, and contribute to photosynthesis and growth for the whole of their lives. With moderate pollution the leaves may die and fall off in three or four years; heavy pollution may cause annual leaf fall and such trees hardly grow perceptibly and usually die. Some workers have suggested that particles of grime act by bunging up the stomata, but usually it seems that these are left patent and the effects are due to reduced transpiration, and, in some cases, to poisoning from sulphur or other substances. Deciduous trees which lose their leaves each year are often less susceptible to damage from pollution, as the leaves can complete their normal work before they are put, partly or entirely, out of action. Those responsible for planting in public parks in cities and industrial areas are well aware that spruce and firs are less likely to succeed than larch or oak. The exact way in which pollution harms trees is not fully understood.
I have already mentioned that atmospheric pollution by sulphur dioxide is becoming worse rather than better. The air in cities commonly contains 0·1 parts per million, that in rural areas 0·1 parts, but sometimes concentrations as high as 1 part per million may occur locally, under particular weather conditions, at distances from the source. Experiments have shown that most flowering plants show no damage to 0·1 parts per million even with long exposures, but higher concentrations usually cause damage such as leaf blotching and loss of yield. Some of the crop reductions on farms near towns are probably due to this cause, but it seems unlikely that there is much damage to wild life in rural areas. Sometimes this type of pollution may be economically advantageous; the absence from industrial areas of the fungus causing rose mildew is almost certainly due to sulphur in the atmosphere. This suggests that other species of fungi, which are in general much more susceptible to sulphur poisoning than are flowering plants, may be similarly affected. This could be of considerable ecological importance, but there seems little information on the subject. However, as rose mildew soon manifests itself in the suburbs, it would seem likely that other fungi, and susceptible plants of other groups, suffer little damage outside very polluted areas. Nevertheless it would be wrong to be dogmatic about this. Small quantities of sulphur or of other gaseous and solid pollutants which are dispersing through our environment may be more harmful than is generally realised.
It should be noted that although trees may suffer from the effects of pollution, at the same time they do something to alleviate this condition. It has been shown that trees growing in industrial areas may do this in several ways. They filter the air, so the deposits on their leaves are removed from general circulation. They cause turbulence and deviation of the air flow, which may help to ventilate (with less contaminated air) an area of otherwise stagnant pollution. They also remove carbon dioxide and liberate oxygen, an important function on a global scale, but, as mentioned below (see here) even seriously polluted air is almost never deficient in oxygen and dangerous concentrations of carbon dioxide are uncommon. Incidentally, in a highly polluted area where trees are likely to improve conditions, it may be best to plant broad-leaved deciduous species, which are harmed less than evergreen conifers, even though they have less effect in winter when the branches are bare. Conifers will be more efficient, and in some circumstances may be used and considered as “expendable.”
Motor vehicles are responsible for widespread pollution in town and country. The exhaust gases contain a high concentration of carbon monoxide, which is very poisonous to mammals and birds. This gas may reach dangerous levels, particularly to car drivers in traffic blocks in towns, but it is probably dispersed too rapidly in the country to have an appreciable effect. Some three thousand tons of lead are emitted with the exhaust gases of cars in Britain each year. This has been found to accumulate in the vegetation and soil along roadside verges, and although serious damage has been seldom reported up to now, a dangerous concentration could build up locally over a period of years. Lead could possibly enter food chains and have damaging effects far from the source of pollution.
Carbon dioxide is another common constituent of the exhaust from fires, factories and vehicles. It has seldom been found in the high concentrations which are harmful to life, and its presence may even promote plant growth in the way it has been shown to do when CO2-enriched air is pumped into a glasshouse. Thus if the CO2, which is normally only some 0·03-0·04 per cent of the total air, is increased to 0·15 per cent, the rate of photosynthesis in a glasshouse may be more than doubled, and crop yields can be substantially increased. The effects of CO2 from industrial pollution on outdoor crops and on natural vegetation have not yet been thoroughly investigated. It is possible that quite small differences in CO2 may affect the whole pattern of vegetation by stimulating one species of plant more than another. More work on this problem is clearly required.
Recently it has been suggested that CO2 may eventually have a drastic effect on world climate. Coal and other “fossil fuels” are being burned at such a rate that the CO2 content of the whole atmosphere may be raised as much as 25 per cent by the year A.D. 2000, and the level will probably continue to rise. The effects of this are not fully understood but some scientists think the temperature and other properties of the stratosphere may be affected. This could alter the world’s radiation balance, possibly melting the polar ice cap. So far little or nothing has been done to reduce the output of CO2, though some research on ameliorating its possible effects has been suggested. So far most scientists have thought that CO2 pollution was of little importance; it now seems possible that it may cause greater changes to the world than any other man-made factor in our environment. On the other hand, this may be a completely false alarm.
Ozone, the form of oxygen with three atoms in the molecule (O3) instead of the normal two (O2), occurs naturally in tiny quantities, and pollution, particularly from motor vehicles, may increase the amount. As little as one part of ozone in 10,000,000 parts of air has been found, in the U.S.A., to harm many plants and trees, and such ozone poisoning is said to be important in both California and Connecticut, in which state an annual loss of $1,000,000 to vegetable crops is reported. So far, I know of no cases of ozone damage to vegetation in Britain, but with the increasing number of motor vehicles it seems likely to occur either now or in the near future.
Air pollution also affects the soil. Near cities the soil is often considered to be “sour,” because of the sulphur dioxide and other acid-forming substances washed in by the rain. This effect probably does not extend very widely, but many of the chemicals found in rain-water may come from industrial pollution. In the moorland areas of the Pennines we know that the rain brings in substantial quantities of minerals, which contribute to the fertility of the soil. Much of this comes from the ocean, but some from pollution, which here may be having an advantageous effect. The quantities of nutrients are significant, but probably not sufficient to have detrimental effects such as those produced by similar nutrients in much larger amounts in purified sewage, which upset the balance in many rivers (see more).
Botanists have studied the effects of pollution on a wide range of plants, mostly with inconclusive results. They have attempted to find “indicator species” which may be used to measure pollution. Such a species would only grow where pollution was below a certain level. The most successful work has been with lichens. Several species of lichen are absent entirely from the industrial areas of high pollution, and reappear on the outskirts. This problem has been studied in Northern Ireland, near Belfast, and around Newcastle upon Tyne. Fig. 3 shows how the lichen cover of tree trunks increases from the city centre of Belfast to its outskirts. It has been reported that the habit of growth of individual species was affected, so that some seemed barely able to exist where others grew normally. The subject is, however, not an easy one. It is necessary to be competent to recognise individual
