Analysis: Well-chosen and well-placed vegetation in an urban environment can provide significant benefits
In Ireland's mission for cleaner air, native plants emerge as unsung heroes. Their pivotal role includes creating appealing verdant landscapes, combating pollution, and exposing environmental health. Understanding the role of local vegetation and integrating its potential could unlock a greener, healthier future where communities can breathe more easily.
Does air pollution impact vegetation or vice versa?
Historically, Ireland has had episodes of particularly poor air quality. In 1982, Dublin experienced the start of severe smog events following excessive bituminous (smoky) coal burning. The smoke and ashes became inescapable, especially on cold and windless days. The persistent smoke caused politicians to question the use of coal, its benefits, and its environmental protection responsibilities. These events resulted in numerous hospital admissions and deaths due to respiratory diseases.
This led to the introduction of the first Air Pollution Act in 1987, followed by the Marketing, Sale and Distribution of Fuels Regulation in 1990, which banned the sale of bituminous coal in Dublin. Soon, the regulations were extended to other parts of Ireland. Today, Ireland is actively monitoring air quality and reviewing clean air regulations. However, currently, 1,300 premature deaths per year in Ireland are attributed to air pollution, primarily to particulate matter (PM).
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Ireland is renowned worldwide for its green landscapes. Nevertheless, due to its small size and lack of ecological and geological variation, Ireland has a limited variety of plant species compared to other European countries.
Ireland has 28 native Irish tree and shrub species, including common varieties such as Hazel, Birch and Willow. These plants generally provide a peaceful and visually appealing environment and offer practical cooling and shading benefits. Additionally, plants are essential in balancing the environment's oxygen and carbon dioxide levels and serve as a food source for animals and humans.
However, many plant species are becoming or at risk of extinction, which has multiple knock-on effects, including loss of biodiversity, impact on food security and medicinal resources, loss of economic opportunities, such as forestry, agriculture and ecotourism, and decline in ecosystem services like air and water purification.
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Recently, there has been increasing research analysing the effects of vegetation in urban environments, primarily for its essential qualities like improving human well-being. However, there is a lack of studies investigating how plants can be bioindicators to determine air quality.
Using plants as indicators of ambient air quality
Some plant species exhibit biophysical traits that can aid in air pollution mitigation. Well-chosen and well-placed vegetation in an urban environment can provide significant economic and social benefits, such as well-being, and a beneficial impact on mitigating the effects of traffic or industry emissions by filtering and absorbing air, thus improving air quality. When the vegetation is placed correctly, e.g., near a high-trafficked road, it can shield people from some of the pollution sources by lowering exposure to exhaust emissions such as gases and road dust and surface and air temperatures, by providing shade and cooling through evapotranspiration.
Native Irish plant species such as whitebeam (Sorbus aria), aspen (Populus tremula), mountain ash (Sorbus aucuparia), juniper (Juniperus communis) and European white birch (Betula pendula) have proven tolerance in heavily polluted areas and facilitate natural plant recovery. Other species, such as Scots pine (Pinus Sylvestris), have shown the ability to improve under various meteorological factors and air pollution.
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Plants can detect excessive pollutant concentrations and often provide the first evidence that the air is polluted. This can be visible damage to leaf surfaces, as tree leaves take up many airborne pollutants. The severity of plant damage caused by air pollution may vary with the time of day and meteorological and environmental conditions such as temperature, wind, sunlight, precipitation, and soil type. Plants can also adapt to these conditions by developing abnormal symptoms or growth.
Certain air pollutants, notably ozone and nitrogen dioxide, can adversely impact plant health and growth and affect their reproductive potential, morphology, and physiological processes like water and nutrient uptake and the photosynthetic activity of chlorophyll. Some native plants, such as Irish native ash (Fraxinus excelsior), sweet cherry (Prunus avium), and willow (Salix caprea), have been observed to be sensitive to air pollution, often exhibiting visible damage on their leaves. Moreover, air pollution poses risks to crop yields by adversely impacting growth and productivity. Such impacts are particularly relevant in Ireland's case due to its economic reliance on the agricultural sector.
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Researchers often assess physiological changes such as stomatal changes, leaf damage (i.e. discolouration) and dust or metal accumulation or chlorophyll content by strategically placing plants in areas of interest (biomonitoring stations) to collect airborne pollutants, or by performing a controlled biochemical analysis to assess responses to different air pollutant concentrations. Additionally, plants are inexpensive and easy-to-use bioindicators to determine spatial and temporal distribution patterns.
Lichens are commonly employed in research to monitor air quality as they are highly prevalent in terrestrial ecosystems, can tolerate a wide range of environmental conditions, and do not have roots, stems, leaves or vascular tissue; instead, they directly absorb water, nutrients and pollutants from the air and rain through their surface. When wetted, pollutants are deposited to their surfaces as gases, vapours, or fine particles dissolve and are absorbed. Depending on diverse lichen species, most prefer to grow in clean-air areas (e.g. bushy/fruticose lichens, such as usnea lichens), and their sensitivity to air pollution makes them significant air quality indicators as they will not grow in areas that have high-levels of air pollution.
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In contrast, others can tolerate polluted air - crustier (crustose) lichens, such as golden shield lichens, usually come in vibrant colours and are commonly found where nitrogen dioxide and ammonia levels are high, especially near farmlands, traffic-heavy roads or sea cliffs. Too much ambient air pollution may make lichens toxic in short time frames.
Additionally, too much pollution can result in the complete disappearance of lichens. About 1,250 lichen species are found in Ireland, all capable of telling us about the air quality we breathe due to susceptibility to air pollution. This can be measured by observing lichen physiological damage or changes (such as discolouration (bleaching) and reduced photosynthesis) or measuring the amounts of airborne substances accumulated in lichens.
While we often associate greenery with serenity, a clear mind, and wellness, we also forget that it can truly help us in more ways than just making us feel good. Certain vegetation types can be easily damaged by ambient air pollution, yet some are resilient to its effects and can help mitigate air quality. Careful and smart planning strategies to implement native species in our cities and towns that can reduce pollution could be an answer to our battle to preserve healthy air.
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The views expressed here are those of the author and do not represent or reflect the views of RTÉ
