7. Retell the text using the words and expressions from exercise 3 and the phrases given below

due to…

the main reason for…

generally speaking…

as a result of…

in turn…

although…

so on and so forth…

for instance…

UNIT 2

Many Causes of Forest Decline

1. Skimming

Read quickly and get the gist of a passage. Do not try to understand every word; just try to get a general idea. Choose a new title for the passage, from this list:

(a) air pollutants responsible for forest decline

(b) nutrients imbalance

(c) nutrients shortages

(d) reasons of forest damage

(f) measures taken to improve trees’ health

(g) assessments of tree damage

In the natural world the plants and animals are continually being subjected to various kinds of stress. And so it is with the trees. Besides shortages of water and nutrients, competition from other individuals and attack by other species are among the forms of natural stress to which they may be exposed.

Tree damage is thus nothing new. But air pollutants constitute an additional factor, coming on top of the natural ones. Trees will therefore be more sensitive, in a polluted environment, to high natural stress, such as occurs for instance under extreme weather conditions with long periods of drought. Stressed trees will moreover be less resistant to attacks from fungi and insects. Extensive pest attacks may often be a successor phenomenon, where the precedent is to be found in air pollution.

It is impossible to assign responsibility for forest damage to any one air pollutant. Three outstanding general causes are here given, the effects of which may vary from place to place, depending on the accompanying stress factors:

- Direct damage from gases and acid substances coming into direct contact with leaves and needles. The most widespread damage of this kind is probably that which comes from ground-level ozone.

- Soil acidified through atmospheric deposition. Nutrients are leached out and/or bound more firmly in the soil. The more concentrations of metals in the soil are, the more damage is given to the root. The processes of decomposition in the soils low down, and mycorrhizal development is hindered.

- Nutrients in imbalance from additions of airborne nitrogen. The more nitrogen additions are made, the faster tree grow. But there may then arise shortages of other nutrients, especially if the soil is acidified. The greater volume of foliage resulting from the extra nitrogen is thought, too, to increase the risk of desiccation. Large additions of nitrogen make the trees more sensitive moreover to frost, and a further disturbing effect of nitrogen deposition is the diminishing of mycorrhizal fungi.

Extensive forest damage has been recognized in Europe since the late 1950s, especially in the mountain region of central Europe where the boundaries of Poland, Germany, and the Czech Republic converge. But it was not until the beginning of the 1980s that the connection between air pollutants and forest decline was taken seriously. From 1986 onwards there has been an all-European program for continuous assessment of the damage. This is confined to measuring the loss of foliage (leaves and needles), sample trees being assessed in a five-degree scale, in which up to a 25-per-cent loss is considered to be within natural variation. The survey for the year 2000 reported that about one out of four trees showed a defoliation of more than 25 per cent, i.e. were classified as damaged.

Scientists disagree as to whether the degree of defoliation is an altogether reliable measure of the state of the trees' health. The effects of air pollutants may perhaps be better read off from the balance of nutrients in the leaves or needles, although that is more difficult to measure.