Air Pollution's Impact: Vegetation's Slow Death

Air pollution is a pressing global issue, and the greatest environmental hazard to human health. It is of particular concern in urban areas, where elevated pollutant concentrations and potential sufferers converge.

The impact of air pollution on vegetation is complex and depends on the nature of the pollutant and the physiological and biochemical characteristics of the affected plant.

Airborne pollutants can be deposited on the surface of leaves, where they may be absorbed and can have toxic effects. They can also change the pH of the soil, leading to the solubilisation of toxic salts of metals. Particulate matter can cover leaves, reducing light penetration and blocking the opening of stomata, which strongly influences the process of photosynthesis.

Plants can also act as a physical barrier between air pollution and potential sufferers, effectively extending the distance between the source of pollution and the people it affects.

The design and choice of urban vegetation is therefore crucial when using vegetation as an ecosystem service for air quality improvements. The optimal configuration and plant composition of such green infrastructure will depend on the type of pollutant and the specific characteristics of the local environment.

For example, in open-road environments, woody plants may be used to form a continuous barrier, but due consideration must be given to critical parameters, including barrier height, thickness and porosity. In street canyons, only low-level vegetation (shrubs and hedges) should be implemented to facilitate dispersion and deposition.

The selection of plant species is also important, as different species have different effects on air quality. For example, some plants emit biogenic volatile organic compounds (BVOCs) and pollen, which can have negative impacts on air quality. Other plants have leaves with specific micromorphological traits, such as the presence of trichomes and ridges or grooves, which can enhance the deposition of super-micrometre particles.

The choice of vegetation should therefore take into account the scale of the intervention, the context and conditions of the site, and the type of air pollutant.

Air pollution can cause leaf spotting and discolouration

Secondly, air pollutants can be absorbed by leaves through their stomata (microscopic openings). Once inside the leaf, pollutants can interfere with the plant's physiological processes, leading to discolouration and spotting. For example, sulphur dioxide can interfere with the function of the photosynthetic apparatus, leading to chlorophyll degradation and leaf discolouration.

Finally, air pollutants can also have indirect effects on leaf spotting and discolouration. For instance, air pollution can alter the pH of the soil, affecting the availability of nutrients to plants and potentially leading to leaf discolouration.

The specific effects of air pollution on leaf spotting and discolouration will depend on various factors, including the type of pollutant, the concentration, and the physiological characteristics of the plant.

It can lead to a reduction in plant growth and yield

Air pollution can cause a reduction in plant growth and yield in several ways. Firstly, air pollutants can directly affect plants by damaging their leaves and disrupting their physiological processes. For example, pollutants can cause leaf necrosis, chlorosis, and changes in leaf shape and size. These changes can reduce the plant's ability to photosynthesise, which can lead to decreased growth and yield.

Additionally, air pollution can also affect plants indirectly by changing the soil conditions. For instance, pollutants can alter the pH of the soil, making it more acidic or alkaline, which can affect the availability of nutrients and the solubility of toxic salts. This can impact root growth and nutrient uptake, ultimately reducing plant growth and yield.

Furthermore, air pollution can also influence the relationship between plants and insects. Air pollutants can disrupt the chemical communication between plants and insects, making it harder for insects to locate and identify plants. This can have knock-on effects on pollination, plant nutrition, and plant defences, potentially impacting plant growth and yield.

Moreover, the choice of vegetation and its configuration play a crucial role in mitigating air pollution. The optimal structure of a vegetation barrier depends on the local context, including the topography and meteorological conditions. For instance, in open-road environments, tall and dense vegetation barriers can be effective in reducing air pollution levels, while in street canyons, low-level vegetation or green walls are often recommended.

Finally, the selection of plant species is critical. Some species may have both positive and negative effects on air quality. For example, certain plants can emit biogenic volatile organic compounds (bVOCs) and pollen, which can negatively impact air quality and human health. Therefore, careful species selection is necessary to minimise any detrimental effects and maximise the benefits of vegetation in improving air quality.

It can cause leaf necrosis

Air pollution can cause leaf necrosis in several ways.

Particulate matter, such as dust, can cause mechanical harm to plants by reducing light penetration and blocking the opening of stomata, which are the microscopic openings on leaves that allow plants to exchange gases with the atmosphere. This can prevent plants from photosynthesising properly, leading to stunted growth and reduced productivity.

Airborne pollutants can also cause chemical injuries to leaves, such as necrotic lesions, chlorosis (yellowing leaves), reddening, bronzing, and mottling. Ozone, a common air pollutant, damages plants by obstructing stomata and restricting respiration.

Acid rain, which is formed when sulphur dioxide and nitrogen oxides in the atmosphere react with water and other chemicals, can directly damage leaves and make it harder for plants to photosynthesise. It also affects the quality of the soil by washing away essential nutrients and changing its chemistry, making it more difficult for plants to obtain the nutrients they need to survive.

Water pollution can also harm plants by disrupting the delicate balance of water and nutrients that they require to survive.

Soil pollution, caused by the dumping of toxic chemicals or the deposition of airborne pollutants, can strip the soil of its nutritional content and change its chemical properties, preventing plants from obtaining the nutrients they need to thrive.

It can lead to a reduction in biodiversity

Air pollution can lead to a reduction in biodiversity in several ways. Firstly, air pollution can directly harm plants, reducing their ability to photosynthesise and grow. This can lead to a decline in plant populations, which can have a knock-on effect on other organisms in the ecosystem, reducing biodiversity.

Secondly, air pollution can alter the composition of plant communities. For example, some species may be more tolerant of air pollution than others, leading to a change in the types of plants present in an area. This can impact the animals and insects that depend on those plants for food and habitat, reducing biodiversity.

Thirdly, air pollution can also affect the way plants interact with insects. For instance, air pollution can disrupt the chemical signals that plants use to attract pollinators, or it can change the nutritional quality of plants for insects. This can have cascading effects on food webs and ecosystems, ultimately reducing biodiversity.

Finally, air pollution can contribute to climate change, which can lead to shifts in plant communities and alter biodiversity. Warmer temperatures and changes in precipitation patterns can favour some plant species over others, changing the composition of ecosystems.

Overall, air pollution can have direct and indirect effects on plants, which can lead to a reduction in biodiversity.

It can cause soil acidification

Air pollution can cause soil acidification, which can have a detrimental effect on vegetation.

Soil acidification is caused by the deposition of acidic substances, such as sulphur dioxide (SO2), nitrogen oxides (NOx) and ammonia (NH3), which can be released into the atmosphere through the burning of fossil fuels, industrial processes and vehicle emissions. These substances can then be deposited onto the soil through rainfall, causing a decrease in the soil's pH level. This can have a number of negative consequences for vegetation, including:

• A decrease in the availability of nutrients and metal spores
• An increase in the solubility of metals such as aluminium, which can complicate nutrient uptake by plants and lead to forest decay
• Damage to the roots and leaves of plants, which can reduce their growth and productivity
• Changes in the composition of the soil, which can affect the types of plants that are able to grow in the area
• Increased toxicity of the soil, which can be harmful to both plants and animals

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