Air Pollutants: Responses of Plant Communities

Air Pollutants: Responses of Plant Communities


Responses of plant communities to air pollutants have been investigated much less than those of individual plants or single plant species. From studies comparing individuals of different species, it is clear that the sensitivity varies considerably among species. However, because of interactions between species, such as competition and facilitation, individual species responses do not represent the responses of the same individuals growing in communities. From the existing knowledge, it is difficult to draw a generalized picture of community responses. Rather, different communities tend to show individualistic responses.


Atmospheric pollutants can roughly be divided into three groups:

1) phytotoxic compounds that cause only adverse or toxic effects, such as ozone (O3); 

2) essential macro and micronutrients that can act as fertilizers at low deposition rates but may have adverse effects at high deposition rates, such as sulphur dioxide (SO2) or nitrogen compounds (NOx, NHL); 

3) pollutants that in most cases represent an essential resource, such as carbon dioxide (CO2). 

In the case of pollutants causing adverse or toxic effects, it can be expected that a species that exhibits high sensitivity to the air pollutants when grown in monoculture will suffer even more when grown in competition with a less sensitive species. In the case of air pollutants that act as potential macro- and micronutrients, the situation is more complex because both the beneficial effects, including growth promotion due to the additional nutrient supply, and the adverse effects, such as lowered stress resistance, must be taken into account on top of any plant-plant interactions.


The current knowledge of the responses of plant communities to changes in atmospheric quality is based on a much smaller database than knowledge of the responses of single species grown in isolation. Roughly less than 1% of the experimental studies with air pollutants, such as O3, SO2, NOx, NHL, or elevated CO2, have involved plant communities. The vast majority of experiments have used single plants or monocultures of a species grown under conditions that are not representative of their natural environment. This is mainly because field experimentation with plant communities is demanding and requires exposure experiments with large plot sizes, replicate numbers, and long durations to account for the biological variation found in natural habitats. 

The sensitivity and responsiveness of plants to air pollutants differ considerably among species, and a sensitive species may be even more affected when grown together with a less responsive species because of the competitive advantages of the latter. Besides competition, other forms of interactions exist between different species, leading to a disadvantage, an advantage (e.g., facilitation), or no effect on either of the partners. Any alteration of the environmental conditions, such as a change in atmospheric quality, may affect these interactions and, hence, the specific responses to the change. Therefore, community reactions cannot easily be predicted from the results of exposure experiments with single species. 

Some studies with communities have shown surprising results, e.g., the success of one particular species, which could never be expected from exposures with the same species grown in isolation. Hence, pollutant effects on plants are modified by the environment and by the presence of other species. Nevertheless, some basic response patterns have emerged, and examples of illustrative studies are given below.

Tropospheric Ozone Phytotoxicity Overestimated?

Tropospheric ozone is regarded as the most important and most widespread gaseous air pollutant in many industrialized regions of the world. Its background concentrations have at least doubled over the past century. Recent approaches to defining thresholds above which exposures do not harm sensitive vegetation have been based on the Critical Levels concept, which uses the exposure index AOT 40, i.e., the sum of all hourly ozone concentrations exceeding a baseline of 40 ppb during hours when global radiation levels exceed 50 W m2. For instance, an AOT 40 of 3000 ppb calculated over a three-month growing the period is currently assumed to protect agricultural crops from the negative effects of O3 on yield and to prevent damage to natural vegetation.

Changes in Plant Communities Exposed to Sulphur Dioxide Responses of plant communities to SO2 are known either from field studies around point sources of emission or from fumigation experiments. The general picture emerging from these studies is that of a strong impact of SO2 leading to different zones of vegetation corresponding to the severity of the pollutant exposure. Acute concentrations may inhibit plant growth completely and even lead to soil erosion because of the high load of acidity and the complete absence of vegetation.

An example from a temperate forest region showed that further away from the emission source a zone existed with a low-density cover formed by resistant grasses, herbs, and dwarf shrubs. With increasing distance and decreasing exposure, the herb cover increased and tree species of poor habitats could survive, thus providing the picture of a tree-line ‘‘ecotone’’ (the so-called ‘‘Kampfzone’’). Symptoms of forest decline could be observed over longer distances from the emission source, and only at exposure levels close to the background concentrations did the upper tree canopy, which is most prone to adverse SO2 effects, regain its natural shape and performance.

Nitrogen Deposition—An Unwanted Fertilizer

Nitrogen (N), like sulphur (S), is a macronutrient. However, the demand for vegetation for nitrogen is much higher than for sulphur. Typically, N:S ratios in plant tissues approximate 10:1 to 15:1. Therefore, sulphur becomes toxic at much lower rates of deposition compared with nitrogen, and current loads of nitrogen often lead to enhanced growth and productivity rather than to injury. However, eutrophication of ecosystems due to excess nitrogen inputs may also have adverse effects: Nutrient-poor stands that carry some of the most diverse plant communities may be lost, and ecosystems may become more susceptible to different forms of additional stresses. 

Most plant species from natural habitats are adapted to nutrient-poor conditions and, because they have a low competitive ability against nitrophilous species, they can only compete successfully in systems with low nitrogen input. Consequently, changes in species composition caused by high nitrogen loads, often associated with a loss of biodiversity, have been observed in several ecosystems. Over the past decades, the forest floor vegetation at many locations in western, central, and northern Europe has seen an increase in nitrophilous species. In heathlands, which represent seminatural ecosystems in most of their area of distribution in western Europe, a transition to grasslands has occurred. 

Apparently, the equilibrium between nutrient output by periodic removal due to grazing and sod removal and nutrient input by mineralization in these heathlands has been disturbed. This is due to not only the absence of management (e.g., less or no sheep grazing, abandonment of sod removal) but also excess nitrogen deposition from the atmosphere. Somewhat similar effects have been observed in nutrient-poor grasslands, in particular in calcareous grasslands. Because of the soil conditions or as a result of management leading to the removal of nutrients by grazing or haymaking, these grasslands remain nutrient-poor and carry a high species diversity with many endangered plant and animal species present, and, therefore, they have been set aside as nature reserves. 

Excess nitrogen deposition to calcareous grasslands strongly stimulates the growth of Brachypodium pinnatum, leading to the formation of a dense cover that reduces the light quantity and quality in the lower parts of the canopy, which, in turn, causes a drastic reduction in species diversity. Besides these effects, nitrogen deposition has been shown to reduce stress resistance. Coniferous trees and heathland shrubs such as Calluna vulgaris exhibited lower tolerance to drought and frost after moderate exposure to ammonia (NH3), and the attack on C. Vulgaris by an insect herbivore, the heather beetle (Lochmaea suturalis), was more severe under conditions of excess nitrogen.

CO2 Enrichment: The Planet Might Get Greener, But Is This All?

In general, CO2 enrichment promotes vegetation growth and productivity and makes plants more efficient in their use of resources. Irrespective of the type of the CO2 fixation pathway (C3, C4, or CAM) of a particular species, higher plants profit from CO2 enrichment by increased water-use efficiency. In addition, plants with C3-photosynthesis show higher nitrogen-use efficiency when grown at elevated CO2 levels.

At present, the ambient atmospheric CO2 concentration limits plant growth in species involving the C3 fixation pathway. In contrast, C4 plants possess an efficient CO2 pre-fixation mechanism, and their photosynthesis is CO2-saturated at the present atmospheric CO2 concentration. Therefore, C3 plants are expected to profit more from CO2 enrichment than C4 plants in terms of growth and productivity. This expectation has been confirmed in some, but not all, experimental field studies with plant communities. In a salt marsh under long-term CO2 enrichment, the C3 sedge gained competitive advantages over the C4 grasses. 

In a tallgrass prairie ecosystem composed of tall warm-season perennial C4 grasses and smaller cool-season perennial C3 grasses plus C3 forbs and C3 Cyperaceae members, eight years of exposure to CO2 enrichment had little effect on the C4 grasses but caused a decline in cool-season C3 grasses and an increase in C3 forbs and C3 Cyperaceae.

 In a calcareous grassland composed of C3 species belonging to different functional groups (graminoids, nonleguminous forbs, and legumes), the legumes were most responsive to CO2 enrichment in one year, and the forbs in the following year. At the species level, the biomass response ranged from a decrease in several Trifolium species to an increase of 271% in Lotus corniculatus and 249% in Carex flacca. These examples illustrate the current difficulty in predicting any general responses to CO2 enrichment across different plant communities.


Responses of plant communities to air pollutants may be much more subtle and therefore more difficult to detect than effects on single species or individual plants. Unfortunately, knowledge is restricted to a limited number of experimental exposures with artificial mixtures of species and to an even more limited number of field exposures involving natural plant communities. 

Thus, at this stage, it is difficult to draw general conclusions from limited data. Some general patterns emerge from studies either of the effects of SO2 pollution, with climax tree species being most susceptible, or of excess nitrogen deposition, e.g., loss of species adapted to nitrogen-poor habitats, whereas no general picture is available concerning effects of O3 or elevated CO2. This is due to the lack of data and the fact that individual communities show specific responses. Furthermore, hardly any knowledge exists about the resilience of plant communities in response to air pollutants.

From long-term studies on SO2 effects, it can be concluded that communities and ecosystems can regain their structure and function upon the termination of the air pollution stress. It may well be possible that complex plant communities can react flexibly to air pollutants, and, consequently, effects are less than expected. 

Conversely, it seems possible that any pollution level above the natural background could have subtle adverse effects—e.g., in terms of losses in biodiversity, stability, or other ecosystem services—and thus should not be tolerated. Furthermore, it should be noted that the effects of combinations of air pollutants on plant communities have rarely been investigated. Therefore, the current approach to protecting vegetation and ecosystems from adverse effects of pollutants utilizing the Critical Levels and Critical Loads concept must be reevaluated regularly and critically as new information on air pollution effects at the community level becomes available.

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