The concept of air pollution is defined by the act on the protection of atmospheric air of 21.04.1966 r.
According to this definition, it is the introduction of solid substances into the atmosphere, liquid or gaseous, which may result in exceeding the permissible concentrations of substances in the atmospheric air. It needs to be highlighted that, that these are standards set by sanitary authorities and relate to human organisms. Overall, Plants exposed to the constant influence of polluted air are damaged by pollutants in much lower concentrations. At the same time, the ordinance of the Council of Ministers of 13.09.1966 r. and dated 27.05.1972 r. they define the permissible concentration 17 substances for specially protected areas and protected areas.
Cities are considered protected areas and the permissible sulfur dioxide concentrations are on average daily 0,35 mg / m3 and one time during the 20 minutes 0,9 mg / m3. Similarly for nitrogen oxides (expressed as nitrogen pentoxide) the permissible concentrations are 0,3 i 0,6 mg / m3. Permissible dust fall is 250 t / km2 / year.
In most cities, vegetation is usually exposed to the harmful effects of air pollution. Generally speaking, among the sources of air pollution in the city, three basic sectors of the economy can be listed as the main sources, namely industry, communication, heating. The share of pollutants from these sources in the overall air pollution in individual cities or even districts may vary. Individual branches of the economy pollute the air in their characteristic way. Heating mainly emits dust, Sulphur dioxide, nitrogen oxides, fluor. Characteristic air pollutants emitted by the means of transport are mainly nitrogen oxides, hydrocarbons, acetylene, aldehydy, soot, dust, heavy metals (mainly lead) Carbon monoxide, carbon dioxide, ozone in the inside.
Depending on the type of industrial plants in the city (district) the 'mix' of air pollutants is changing. Among the pollutants caused by industry, over 200 different ingredients.
So far, the mechanism of plant damage caused by the most harmful substances has only been partially known, like sulfur dioxide, fluor. These are complex processes, and the effects of these substances on plants are manifold. Sulfur dioxide affects plants directly and indirectly. Direct influence of high concentrations of 0.1-0.4 ppm of sulfur dioxide (0,29-1,16 mg / m3) leads to the so-called. acute damage, i.e.. clear necrosis of the tissue between the leaf veins as a result of the rapid death of entire parts of the leaf crumb. The high concentration of SO2 most often causes plasmolysis of cells. Lower concentrations, below 0,2 ppm of sulfur dioxide ( = 0,58 mg / m3), cause the so-called. chronic damage, most often in the form of chlorotic red spots on the leaves. Single cells are found in mesophyll (or small groups) dead or at least showing degradation of the chloroplasts. The exact "chain of damage" of cells and disturbances in metabolism is not yet known. However, the following plant reactions have already been found: inhibition of photosynthesis, effects on growth substances and enzymes, water management and the movement of materials in tissues as a result of the accumulation of air pollutants in the soil. SO2 also causes soil acidification and reduction of its biological activity.
The sensitivity of plants to air pollution is relative. It depends on many factors: Seasons, period of impact, the type and concentration of the pollutant, species, and even plant varieties, century (plant development stage), from the habitat, and from disease and pest control.
Deterioration of ecological conditions and any disturbances caused by human activity usually increase the sensitivity of plants and reduce their ability to regenerate.. At the same time, the influence of phytotoxic gases may intensify leaf damage. Phytotoxic concentrations of some of the most important pollutants have been determined many times, both under experimental conditions, and off-road. However, the results of these studies are indicative only in the data, specific conditions. Sulfur dioxide can be phytotoxic even in concentrations 0,02 ppm (0,058 mg / m3). Increasing the concentration causes a disproportionate increase in damage; if e.g.. concentration was 2 mg SO2 / m3, approx 2% leaf surface, if it was 12 mg SO2 / m3 - aż 77% surface. In urban conditions, it is very difficult to determine the concentrations that cause damage to trees, because there are many harmful factors at the same time.
External symptoms of plant damage caused by air pollution are not very characteristic both on the macroscopic scale, how micro- and ultra-microscopic, so that you can distinguish the effects of the various factors. The generally recognized external symptoms of SO2 damage to leaves include brown discoloration of the tissue between the leaf veins, starting in its marginal parts. Likewise, the discoloration begins at the end of the needle. As a result of fluoride damage, the needles usually change color along their entire length.
Gaseous air pollutants are accumulated by plants mainly in the leaves. Sulfur content, fluoride, Heavy metals and other harmful components in the leaves testify to the general condition and danger of the plant. Sulfur content greater than 0,18% dry matter of needles proves high air pollution with sulfur dioxide.
The normal content of fluoride in tree leaves ranges from 0.2-0.5 mg%. However, examination of the leaves after the occurrence of damage shows an increase in the fluoride content even up to 60 mg%.
In addition to the concentration of harmful substances, the amount of damage to the plant is also influenced by the period of impact of the pollution. Extending this period causes a disproportionate increase in damage. Breaks in the occurrence of air pollution create opportunities for the plant to "rest" - neutralize or expel the absorbed poisonous gases. There is often almost constant air pollution in the city. The emission of pollutants during the night during the growing season is also important.
During the growing season, the sensitivity of plants to harmful substances changes. In deciduous trees, buds and young, leaves not fully developed, they are less sensitive, the greatest sensitivity is found in fully developed leaves during the most intense assimilation. Old leaves, at the end of the growing season they are again less sensitive. Changes in the sensitivity of conifers have a different course than in deciduous trees.
Conifers show the greatest sensitivity in early spring and during the growing season. Older needles are most easily damaged in early spring, created in previous years. Developing new needles show the greatest sensitivity once they reach their final size, i.e.. in late spring and early summer. Also in autumn and - if photosynthesis takes place - also in winter, conifers are exposed to damage (these plants can assimilate in winter, when it is mild and as soon as they have water). All meteorological factors, which enhance photosynthesis and trigger the opening of leaf stomata, like for example. light, high relative humidity, optimal temperature, they also enhance SO2 uptake.
Turned out, that fertilization, especially nitrogen, can increase plant resistance to SO2. Apart from the sensitivity of the assimilating organs to air pollution, an important feature is the tree's ability to regenerate after damage has occurred. This ability depends on the anatomical and physiological properties and the overall health of the plant, which in turn depends on the overall ecological conditions.
Deciduous trees have greater regeneration potential than conifers. Annual leaf renewal, greater content of reserve materials and greater number of sleeping buds promote better regeneration after damage by any factor. Under unfavorable environmental conditions, the regenerative capacity decreases, as they improve, it grows.
The usefulness of individual tree species for cities can be proved by the so-called. ability to withstand given specific conditions. This concept can be defined as a set of all species properties, needed to overcome all disturbances caused by the urban environment. The so-called. forest tree resistance ranks. We have much less data on the strength of other tree and shrub species planted in cities. The table below shows the "resistance series" of young trees determined under laboratory conditions (in the test cabins) and supplemented on the basis of field observations, in which the assessment criterion was leaf damage caused by SO2. Sensitivity was rated on a scale from 4 (the most sensitive) do 20 (the least sensitive).
Trees and shrubs are very sensitive:
1. Wild pine 4,3
2. Hypericum calyx (4,5)?
3. Pinus rigidity 4,7
4. Deciduous larch 4,8
5. Salix purpurea 4,8
6. A heavy pine tree 5,0
7. A tall pine tree 5.1
Sensitive trees and shrubs:
8. A fragile willow 5,2
9. Salix pentandra 5,2
10. Amelanchier floribunda 6,2
11. Abies concolor 6,3
12. Pinus griffithii 6,5
13. Hearted linden trees 6,5
14. Picea omorika 6,7
15. Pinus Jeffreyi 6,7
16. Pinus montana 6,7
17. The female willow 6,7
18. Potentilla fruticosa 6,8
19. Salix alba, S. fragilis 6,8
20. Corylus colurna 7,0
21. Pinus 7,0
22. Larix leptolepis 7,1
The least sensitive trees and shrubs
87. Quercus petraea 18,0
88. Aesculus parviflora 18,3
89. Juniperus virginiana 18,5
90. Gleditsia triacanthos 19,3
91. Prunus virginiana 19,3
92. Thuja orientalis 19,3
93. Chamaecyparis pisifera 19,9
94. Sycamore acerifolia 20,0
95. Sophora japonica 20,0
Larchs are distinguished by a high sensitivity of leaves, nevertheless, thanks to their high regeneration capacity, they are suitable for industrial areas. Moreover, black alder shows a great ability to regenerate, poplar, field maple. On the other hand, spruce has a low regeneration capacity, pine, beech and hazel.
Soil contamination
Trees growing in the streets are additionally exposed to the harmful effects of salt used to prevent icing of the road surface, and often - despite the ban - sidewalks. Most often sodium chloride is used for this purpose (salt). In recent years, calcium chloride has been added to table salt in a ratio depending on the temperature and the state of icing of the street. In the climate of Poland, striving to ensure the safety of road traffic, the permissible norm is often exceeded, which is 2 kg of salt per 1 m2 of road.
The normal concentration of soil solutions is 2.2-18.2 mg of salt in 100 g of soil. W 1973 r. the salt content in the soils of street greenery in Warsaw reached 265 mg w 100 g of soil, which is 0,265%. As a result of such salinity, the suction power of the root cells is reduced (too little difference between the concentration of the cell solution and the soil solution), which in turn causes the phenomenon of physiological drought. Moreover, the Na2CO3 produced in the soil has a toxic effect on plants and worsens many properties of soils.
The immediate and most dangerous effect of soil salinity is the accumulation of chlorine ions for plants (Cl- ) in the leaves. Chlorine ions are taken from the soil along with water. After exceeding the upper limit of chlorine concentration in the leaves, they die. The process can be slow and starts with yellowing, browning and dying off of the marginal parts of the leaf. It is accepted, that the limit values are for horse chestnut 0,6; poplar and linden and clones 0.8-1.0 Cl in the dry matter of leaves, for the plane tree, around 1,0%.
