Pollution

Trees face a lot of threats. Some of these are natural, others are brought about directly or indirectly by ourselves. Some problems are more important than others. A few FAQs are explained here.

ACID RAIN

Man's activities have resulted in many changes to the environment - some subtle, others more evident - and a number of these may affect trees. Acid rain is one but others exist - ozone, nitrogen enrichment and the greenhouse effect are all major problems in their own right.

What is acid rain?

Many people use the blanket term "acid rain" for all kinds of air pollution. Strictly speaking that is wrong. Acid rain should only be used when referring to precipitation that is more acidic than it should be.

Rainfall is naturally mildly acid. Much of the acidity comes from carbon dioxide dissolved in rainwater and other sources such as volcanoes and the ocean. Acid rain occurs when certain types of pollutant are dissolved in the rain making it more acid than normal.

The proportion of manmade and natural acidity in rainfall is variable throughout Britain. In the west of Scotland, experts think about half the acidity is man-made. As you come eastwards, the proportion steadily increases; on the east coast, perhaps 90% of the acidity may be from pollution rather than nature.

Where does it come from?

Burning fossil fuels like coal and oil is the biggest source of man-made acidity in the atmosphere. The acidity comes from two gases produced during combustion; sulphur dioxide and nitric oxide (which quickly turns to nitrogen dioxide). These act with other substances in the air to form sulphuric acid and nitric acid respectively, the most important man-made acids in acid rain. Most of the sulphur produced in Britain comes from power stations. Car exhausts are a major source of the oxides of nitrogen.

Sulphur dioxide is derived mainly from burning fossil fuels and its' adverse effects have been known about for years. Legislation has led to major reductions in the amount of SO2 but too much is still released annually. Only recently have sulphur dioxide levels dropped sufficiently to grow some species of trees successfully on some of the more polluted parts of Britain such as the South Pennines.

Does all the acidity fall as rain?

Not all of it does. Gases and particles released during combustion can fall to the ground in dry form (called dried deposition) where they may then result in an increase in soil acidity.

Has acid rain occurred in the past?

Acid rain is not new in the UK. It first raised its head over 200 years ago in areas like the South Pennines, associated with industrial towns such as Manchester, Stoke-on-Trent and Sheffield. Acid rain and other forms of air pollution became steadily worse until the mid 20th century.
After the acid fogs or "smogs" of the 1950's that killed thousands of people in London, legislation was introduced to reduce pollution in towns and cities. Smokeless zones cut down some pollutants and tall chimneys spread the rest far more widely. In the UK, rainfall is now becoming less acidic because less sulphur is released into the air.

What effect is acid rain having in Britain?

We know that the increase in acidity of some fresh waters is linked to acid rain. Areas worst affected are those where the rocks are already naturally acidic and the soils are thin. Where the rocks are calcium rich such as chalk and limestone, the acidity is neutralised as it enters and passes through the soil.

Some forest soils have become more acidic. Partly that is due to the dead needles falling down and partly because the trees trap atmospheric moisture - and with it its acidic component - which works its way down to the forest floor. It appears that conifer forests can act as giant sieves, filtering pollution out of the air and transferring it to the soil so that streams in forests growing on thin, acidic soils might become more acidic than similar water courses in grasslands.

So far trees in Britain have largely been unaffected by the very acidic rain which has denuded many forests in Central Europe.

OZONE AND OTHER GASES

The history of ozone holes and the greenhouse effect is not described in detail on our website but there are many others dedicated to this topic. Similarly the role of tropical forests in greenhouse effects is beyond our immediate brief.

WILL THE GREENHOUSE EFFECT HARM OUR UK TREES?

Most people are now convinced that global warming is a reality although the experts agree to disagree on whether our climate will become warmer or colder.

Any change in temperature could have major effects on our trees. They may also be linked to whether the climate gets wetter or drier and if storms become more frequent and violent.

We are unsure what effects increasing carbon dioxide levels may have on trees. Slight rises in concentration may promote growth in trees but we are not really 100% sure.

It may be that some of the species growing here at the moment - particularly if they are already near the limits of their natural distribution - would be particularly badly affected. Recent work has suggested that beech may disappear on drier sites if our rainfall becomes less or more erratic.

The Forestry Commission monitors tree health in the UK but we still have a lot to learn about the effects of pollutants on trees here. At the moment, we can only best-guess what the long-term implications of global warming and pollutants may be.

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