Plants are harmed by a variety of air pollutants, either directly or indirectly. Pollutants and issues of particular concern include ground-level ozone, a major component of smog, acid rain, and sulphur dioxide (SO2).

The impact of poor air quality on plants can be acute (rapid and dramatic), chronic (long term), or hidden. Often the symptoms of air pollution are visually similar to the natural aging of leaves in the fall season; however, air pollution can make these conditions appear much sooner than they would otherwise.

In general, plants grow more slowly and can become more vulnerable to disease, pests and difficult environmental conditions such as drought and cold. One possible exception to this is ecosystems that are nitrogen deficient and can experience a period of greater growth when exposed to nitrogen oxides (NOx) and its acid rain derivatives. In bodies of water, this nutrient enrichment situation is known as eutrophication. More often, however, plant growth is limited by elements such as phosphorous, as opposed to nitrogen.

Some plant species are more sensitive to air pollution than others. This is of particular concern for agriculture and forestry where any reduction in crop growth, productivity or survival can have significant and detrimental impacts on the economic viability of these industries. Understanding these differential plant sensitivities allows us to monitor the impact of air pollution throughout the environment. This is the basis of the Ecological Monitoring and Assessment Network (EMAN) community lichen monitoring protocols.

How ground-level ozone harms vegetation

Plants are very sensitive to increased concentrations of ground-level ozone. Microscopic openings on the surface of each leaf, called stomata, are important in the natural exchange of gases, including the uptake of carbon dioxide. When ozone enters through the stomata, it can interfere with a variety of biochemical and physiological processes, including photosynthesis.

Plant leaf cells become injured and even die as a result of ground-level ozone. This can be seen as small black or brown spots on broad-leafed plants, or yellow (chlorotic) spots on conifer needles.

Although plants may respond to increased levels of ground-level ozone by closing each stomata with specialized cells called guard cells, this response also closes the stomata to carbon dioxide, which is essential for plant photosynthesis and growth. This is of particular concern during the day, when plants need carbon dioxide the most, yet ozone concentrations are highest.

How acid rain harms vegetation

Acid precipitation can harm plants in a variety of ways. Increased soil acidity can decrease the amount of nutrients and essential elements available to plants. Acidity can also hinder the ability of root systems to effectively uptake nutrients. Alternatively, acid precipitation can damage the surfaces of leaves and needles.

How sulphur dioxide harms vegetation

Similar to both acid rain and ground-level ozone, sulphur dioxide can enter through the stomata of plant leaves. Once there, it can dissolve in the water found within the leaf and form an acid that can interfere with biological processes and injure the plant.

Symptoms of damage caused from high concentrations of sulphur dioxide are the yellowing, or chlorosis, of the tissue between the veins of broad leaves, or the browning of the tip of conifer needles.

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