Vehicular exhaust is a significant cause of air pollution in India, resulting in the rise of many fatal respiratory illnesses. With 249 million registered vehicles that contribute to growing levels of air pollution, India faces a hazy future. A solution to this problem seems to be ‘urban greening’ or planned plantation of trees within cities, as evidenced by a study of 55 cities in the United States, which helped remove 7 lakh metric tonnes of air pollutants annually.
Urban greening could involve open park areas or tree covers lining the roads. However, each of these approaches requires specific tree species that can survive the levels of pollution within their surroundings. In a recent study, a team of researchers from the Forest Research Institute in Dehradun has tested the hardiness and adaptability of mango and silver oak trees for high levels of pollution. The study was published in the journal PLOS One.
“Selecting tree species that are more tolerant and efficient to combat air pollution could be useful for plantation along the roadside. However, there is little information available to select the appropriate trees”, say the researchers.
Hence, for their study, they looked at the mango and silver oak trees planted along the NH-72 highway, which sees about a thousand vehicles moving between busy tourist destinations like Haridwar and Dehradun per day. The researchers also compared the stress response of these trees to the mango and silver oak trees planted in the pollution-free air of the reserved forest.
On a hot summer day, the shade under a tree is inviting because the canopy not only blocks sunlight but also cools the immediate surroundings. Special pores on the leaf surface, called stomata, release water vapour into the atmosphere, resulting in a drop in temperature in the immediate surroundings.
In the current study, the researchers found that mango trees along the NH-72 had a 120% increase in their ability to cool the environment, whereas silver oak showed an increase of 51%. They also observed that both the tree species were better at reducing the levels of dust from the microenvironment by a fantastic 191% than the same tree species planted in the reserved forest.
Trees also absorb heavy metals from the vehicular exhaust and utilise them for their growth. Higher accumulation capacity of a tree means that it is better at cleaning the environment from heavy metals. The authors found a 45-50% increase in the levels of the heavy metal copper within the leaves of trees along NH-72.
Although trees benefit us by reducing pollution levels, increased vehicular traffic induces physical changes in them, like the thickening of leaves, to guard them against pollution. The study shows that on average, trees along the highway had 43% thicker leaves than their reserved forest counterparts. Thicker leaves restrict the stomatal pores and reduce the gaseous exchange between the environment. The study showed that the release of water vapour through the leaf surface was reduced by 43% for the silver oak trees and 33% for the mango trees planted along the road.
The ability of the trees to absorb atmospheric carbon dioxide, a greenhouse gas, was also compromised under the stress of pollution. The presence of pollutants reduced the ability of the trees to capture environmental carbon dioxide by 15% for mango trees and 26% for the silver oak. The flow of gases across the stomata reduced by 43% in the trees along the highway, as compared to those planted in a reserve forest.
Interestingly, the highway trees were surprisingly better at using water optimally. These trees were able to generate ample organic raw material with a limited water supply available to them. The efficiency of water use in silver oak trees increased by 30% along the highway. Trees along the road retained 8% more water within their leaves than those in the reserved forest.
Air pollutants also cause many stress-induced chemical reactions within the plant. These result in the formation of reactive oxygen species. These chemicals, containing oxygen and an imbalance of positive or negative charge, attack proteins and other chemicals, leading to oxidative damage. As a response to this stress, the trees planted along the highway had increased levels of proline- a chemical that scavenges reactive oxygen species. The higher the level of proline, the better the ability of the tree to adapt to pollution. Silver oaks had a 35% increase, whereas mango trees had a 48% increase in the levels of proline when exposed to air pollution.
“The trees may have different responses to the pollutants located in different geographical areas. Thus it becomes important to investigate responses in varying diverse physiographic locations to select trees that could be used to combat air pollution in a given geographical area,” suggest the researchers.
Although the trees were defensive towards air pollutants, they were sensitive to its devastating effects. The leaves of trees planted along the highway had an acidic pH that rendered them vulnerable to contaminants. The researchers also report a reduction in the total chlorophyll content of these leaves. Chlorophyll is the green-blue pigment in the leaves that absorbs sunlight and helps in photosynthesis. The cause of reduced chlorophyll might be the increased vehicular exhaust or penetration of dust within the leaves.
Presently, India is facing a boom in the urban population with almost thirty-five cities housing a million-plus people. The increase in urban population will be accompanied by an increase in the number of vehicles and thus an increase in pollution. Although more trees can mitigate pollution, we lack research on what species to choose.
“The urban green space, when planted with selected trees having adequate potential to combat air pollution, could be one of the effective ways to reduce ambient air pollution of the city,” say the authors, stressing on the need for urban greening the right way. The roadside plantations act as our first line of defence line against vehicular pollution. “The investigation traces the tolerance capacity of the selected trees against air pollutants and could be used to select other tree species that have the potential to combat air pollution,” they conclude.