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THE EFFECTS OF ATMOSPHERIC POLLUTION ON VEGETATION
Jean-Pierre GARRECCForest Research Centre of Nancy,
Air Pollution Laboratory
54280 CHAMPENOUX
Content:
- The manner in which atmnospheric pollutants affect plants
- Sensitivity of plants to atmospheric pollution
- Utitization of the effects of atmospheric pollution on plants
- Conclusion
The effects of atmospheric pollution on vegetation have evolved continuously with time in both their nature and intensity, as well as location. In France, it is generally admitted that atmospheric pollution began to be considered as a real nuisance for vegetation about the 1910's.
Since that date, three great characteristic periods can be schematically outlined:
The first period was situated approximately between the years 1910 and 1980.
Three great pollutants dominated during this period: SO2, HF, and NOx. The concentration of these pollutants was continuously increasing until about 1975, the time when maximum atmospheric pollution was recorded in France. The pollutants from this period possessed 4 principal characteristics: they were primary pollutants, highly concentrated, quite typical, and most of all, they caused local pollution. At the vegetation level, they had a very strong impact within a small distance (several dozens of kilometres), causing obvious and characteristic necrosis of the foliage, and a more or less important death the polluted trees.
The second period lasted from the 1980's until nowadays.
During this period, three great pollutant families were present: the photooxydants, of which ozone was the most abundant and most dangerous, the dry and wet acid deposits, and the nitrogen deposits, consisting of NOx and NH3. The majority of the pollutants from this period could be characterised in the following manner: these were secondary pollutants; generally, their concentration was small and they often appeared far from the well-known pollution sources; they weren't very typical and they persisted over large areas (several hundreds of kilometres), causing regional and transboundary pollution.
At the vegetation level, they are at the origin of badly defined injuries, such as chlorosis and declining of the trees with loss of vitality but quite low mortality.
The third period began in the middle of the 90's.
Pollution in the next years will be characterised by the continuous presence of secondary pollutants (photooxydant pollution and acid pollution), to which two great atmospheric modifications were added: the continuous increase of CO2, and to a lesser extent, the increase of the UV-B flux, resulting from the partial destruction of the ozone protective layer in the stratosphere.
These pollutants of the future possess the following characteristics: they are life-lasting, they cause global pollution at the planetary level, they cause an additional green-house effect, and some of them, like CO2 and the nitrogen compositions (NOx, NH3) can also have some fertilising effect on the plants.
The direct effects of these pollutants on vegetation are still being studied but it is already known that the increase of CO2 risks to notably change plant and tree physiology.
Indirectly, because of the rise of temperature, caused by these pollutants, they will also have some other influence on vegetation with possibility of draughts, fires, violent winds, and rise of the sea level.
THE MANNER IN WHICH ATMOSPHERIC POLLUTANTS AFFECT PLANTS
The reaction of a plant, exposed to an atmospheric pollutant can be considered as the result of a succession of biochemical and physiological events, starting with the absorption of the pollutant and eventually ending in some damages. Generally, this process can be divided into 4 steps:
- Absorption of the pollutant
The main route through which atmospheric pollutants penetrate into plants are the stomata of the leaves.
- Perturbations
These are the first phytotoxical effects of the pollutants, resulting from the change of structure and/or functions of the cells in the leaf interior, and the effects at the level of cellular metabolism, following them.
- Homeostasis
After a perturbation, live systems try to restore their normal metabolism through reparation mechanisms (elimination of all malfunctions) or compensation mechanisms (the perturbation and its physiological consequences persist but mechanisms counterbalance their harmful effect).
Reparation and compensation can be partial or complete: the damages vary from one plant to another. Resistant plants possess great capacity to recover cellular damages.
- Damages, at the physiological and metabolic level
They result from the inability to repair or compensate for the malfunctions and the metabolic changes, resulting from the absorption of the pollutant.
The sum of the pollutant's effect on each cell results in change of metabolism of the tissues and organs. This is manifested first by hidden injuries (reduction of growth, loss of vitality, and, for the cultivated plants, reduction of yield and quality), and then, by visible injuries, generally at leaf level, (necrosis, chlorotic spots, premature ageing). It should be noted that this leaf necrosis could be typical of a certain pollutant: apical and marginal necrosis for fluoride, internerve necrosis for SO2, and puncti-form necrosis for ozone.
Remarks
While the effects of a pollutant on a single plant can be divided, as it was just shown, into hidden and visible injuries, in characterising the effects of a pollutant on vegetation communities (agricultural crops, forests etc.), we speak of injuries and damages.
The injuries result from a short exposure to high concentrations of the pollutant, with visible effects, mainly at leaf level.
In most cases, these injuries do not influence yield or reproduction.
The damages correspond to the invisible effects of a pollutant, but with consequences for the plants' economic production, genetic resources, and cultural values.
As a general rule, the appearance of leaf injuries is a reaction at some peak of pollution, while the loss of yield is the result of continuous exposure.
SENSITIVITY OF PLANTS TO ATMOSPHERIC POLLUTION
Different factors are determining for the appearance of damages, caused by the pollutants to the plants:
Nature of the pollutants
Depending on their nature, pollutants can be more or less toxic for vegetation. Thus, for example, if we take as reference HF, a strongly phytotoxic pollutant, the major atmospheric pollutants will be approximately: O3 - 80 times, SO2 - 120 times, and NO2 - 1000 times less toxic for the higher plants.
Pollutant dose
The product of the concentration by the period of exposure defines the notion of dose. This is not the dose in the surrounding air which is determinative of the exercised effect, but this is the dose in the plant's interior. It is something quite difficult to determine.
For a certain dose, the impact varies depending on concentration, length and sequence of exposure. Generally, it has been ascertained that the same dose of pollutant has greater impact if applied within a short period of time than within a long period of time. The increase of leaf damages and decrease of yield are rather a function of the increase of concentration than the length of exposure, the direct conclusion from this being that concentration peaks are a particular risk to vegetation.
Actually, we use more and more the notion of critical levels (for gaseous pollutants, inug/m3) or critical loads (for deposits in kg/ha/an), i.e. the "quantitative estimation of the exposure to a pollutant, below which the significant negative effects will not affect the sensitive elements of the environment, to the present state of our knowledge".
Abiotic factors
The environmental factors that influence plant physiology, and in particular, stomata functioning, play an important role in the absorption of the pollutant, and hence, in the plant's response.
The plants, and the stomata in particular, are sensitive to the climatic factors (wind velocity, light, temperature, relative humidity, concentration of CO2), and the edaphic factors. The variation of these factors can cause great distortions between the supplied dose, the dose effectively absorbed, and the expected effects.
Biotic factors
Leaf age and phenological stages
The sensitivity of a plant towards a certain pollutant depends on leaf age and its stage of development. As a general rule, it seems that the greatest sensitivity towards pollutants is registered with middle-aged leaves at the stage of rapid growth.
Genetic variability
Depending on their genus, type and variety, plants can be more or less sensitive to pollutants.
The relative sensitivity of plants to pollutants is actually always known aposteriori.
Plants are generally classified arbitrarily into 3 or 4 great classes of sensitivity towards pollutants: very sensitive plants, middle sensitive plants, little sensitive plants, and resistant plants (if they exist).
As a result of the numerous field observations in the surrounding of pollution sources, and also as a result of laboratory fumigation, the lists of plants' relative sensitivity to different pollutants were established. However, it should be pointed out that whether resulting from the field or from the laboratory, the established sensitivity lists provide reference, depending not only on the plant but also on the environmental conditions under which the observations were performed, as it was just said. For this reason, one should be careful when using the numerous sensitivity lists available in scientific literature.
Illnesses and parasites
It has been established that illnesses and parasite-caused injuries could affect the intensity of pollution effects and vice versa.
UTILIZATION OF THE EFFECTS OF ATMOSPHERIC POLLUTION ON PLANTS
PLANT BIOINDICATIONPlant bioindication of atmospheric pollution is a relatively simple and flexible technique for observation and analysis of the reactions of vegetation exposed to polluted environments under natural conditions.
In practice, plant bioindication is applied in two forms:
- passive bioindication, i.e. utilisation of endemic vegetation or
- active bioindication, i.e. utilisation of selected plants cultivated in pots.
Whether bioindication be passive or active, this technique always applies to two types of plants:
- bioindicators (plants, sensitive to pollutants) as an object of observation of necrosis and/or biochemical analysis or
- bioaccumulators (plants, resistant to pollutants) as an object of analysis of the pollutants' internal concentrations.
In the utilization of these different categories of plants, the essential goal of plant bioindication is to detect the presence of a pollutant, to identify its zone of impact, and to have an idea of its concentration in the atmosphere (sentinel plants).
Because of these advantages (integration of the pollutants' biological effect, easy following, quick detection and obtaining of results, low cost), plant bioindication as an additional facility to the physico-chemical apparatus networks, turns to be an increasingly useful source for characterisation or following of air quality.
Owing to the biological information on the impact of pollutants on live organisms given by the presence of leaf necrosis on plant bioindicators, the latter are now used to visually make city population aware of the air pollution problem .
THE EFFECT OF OZONE ON PLANT PRODUCTION
In France, actually all rural or forest areas are now affected by more or less important ozone concentrations. It should be reminded that ozone is a pollutant originating mainly from automobiles, which is formed from the hydrocarbones and nitrogen oxides contained in the exhaust gases under the influence of solar radiation and heat.
A number of works have been devoted to the effect of ozone on crop yield. In USA, it is believed that ozone levels are responsible for 90% of the loss of yield of cereal crops, resulting from atmospheric pollution. The studies performed in this country during the '80-ies revealed, for example, that the available ozone levels caused an average wheat yield loss of 12,7%. Later, in Europe, other studies on the response of beans, exposed to ozone influence, revealed that yield loss could reach up to 25%.
Using different works of American and European authors, it has been established that, in the beginning of the 90'ies, the loss of yield of winter wheat in France, due to ozone, varied between 10 and 25%, depending on the considered area (the area to the south-west of Paris being more affected than the area to the east).
For forest production, it has been admitted that, in the Western European regions not very polluted, the local ozone concentrations are responsible for a loss of yield of the order of 5 to 10%. However, these shortages of forest production are actually masked by the positive effect exerted on the forests by the increase of CO2 and the nitrogen deposits.
It should be reminded that, according to Directive 92/72 of EC, the established vegetation protection threshold for ozone is 65ug/m3 (averaged diurnal value) and 200ug/m3 (averaged hourly value).
CONCLUSION
At the vegetation level, it is interesting to point out that, by changing of scale (transition from local pollution to global pollution), atmospheric pollution is no longer an attribute of the urbanised or industrialised areas, and that nowadays, actually all rural or forest areas that were once upon a time famous for their "pure" air, are equally affected.
While, in the years to come, the problems related with the increase of ozone in the atmosphere will be the most troublesome for the plants of our regions, it should not be forgotten that a number of other regions of the world will continue to be concerned with very high local pollution such as, for example, the SO2 pollution (countries from Eastern Europe, developing countries), with all dramatic consequences the latter might have for local vegetation.
It should not be forgotten also that pollutants like CO2 or nitrogen deposits (NOx, NH3) which have a favourable effect on vegetation in a first step are increasing in a similar manner.
Dr Jean-Pierre Garrecc is the director of the Air Pollution Laboratory (LPA) which is a part of the Forest Ecophysiology Research Unit in the National Institute of Agronomic Research (INRA).
The two general scientific purposes of the group are:
- To analyse the ecological and physiological effects of environmental stresses and long term changes on trees.
- To develop new plant bio-monitoring methods of air pollution.
A scientist (D. Le Thiec), two research engineers (B. Richard and C. Rose) and two technicians (G. Nourrisson and F. Radnai) are working together with Dr Garrec within the following research areas:
- Ecophysiology of the main forest species:
- Physiological analysis of the tree-atmosphere "interface" (cuticle, waxes, stomata, bark) and impact of environmental stresses (interactions).
- Analysis of forest decline process and ageing: role of ozone and acid fogs; development of biophysical tests for decline analysis.
- Effects of increasing atmospheric CO2, O3, UV-B and surfactants on tree growth and phenology, water use efficiency, drought resistance, physicochemical properties of cuticles, composition of waxes and ion exchange related to stomatal movements.
- Assessment of the effects of drought on gas exchange (transpiration and photosyntesis) and water relations of the trees.
- Dysfunction in mineral nutrition induced by deficiencies: histological X ray analysis and consequences on stomata functioning.
- Role of cuticle and phylloplan in the plant-insect relationships.
- Effect of dry deposit on leaf surface.
- Effect of ozone on structure and element content of pollen - relation with allergicity.
- Biological assessment of air pollution:
- Research of specific bio-indicator and bio-accumulator plants of the new air pollutants like O3, dust, nitrogen deposit, organic micro-pollutants (pesticides, benzene, HPA, formaldehyde, etc...)
- Characteristics of the accumulation of pollutants by plants: effects of physiological and environmental parameters.
- Mapping special and temporal distribution of air pollutants by plant bio-indicators in polluted areas: urban, rural or industrial areas, highways, etc...
- Use of plant bio-indicators for educational and didactical purposes.
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