Greening with pollution-gobbling trees doubles benefits

As India chokes on a cocktail of toxic fumes and particulate matter, researchers are zeroing-on air pollutant-gobbling plant species, that could be marshalled for green belt development along roadsides, thermal power plants, and for curating urban forests to sponge off foul air.

Different plant species respond differently to pollution. While some plants can tolerate fairly high levels of pollution (suspended particulate matter, dust and gases), others are sensitive. The response of plants to air pollution depends upon the type of pollutant present, its concentration, and the length of exposure to it.

In a recent study in Himachal Pradesh, scientists from the CSIR-Institute of Himalayan
Bioresource Technology (IHBT), screened 26 species along the 243 km stretch of National Highway-21, from plains to high altitude passes including the tourist favourite Rohtang Pass. The motivation was to identify plant species that can be incorporated for greening areas around the roadside such that damaging effects of pollutants belched from vehicles are minimised.

“It was basically for prioritising species for green belt development along roadsides. Widening of national highway NH-21 (our study area) is in progress. We therefore initiated work on screening pollution tolerant species that may be planted along the 240 km stretch of road in the Himalayas,” study author Sanjay Uniyal from IHBT told Mongabay India.

Uniyal said that the selected plants should be aligned or planted parallel to road sides at equal distance from each other so that unburnt hydrocarbons and dust could be trapped on the leaf surface of these species. “Native plant species suitable for that particular climate should be planted,” said Uniyal.

India launched the Green Highways (Plantation, Transplantation, Beautification & Maintenance) Policy, 2015 under which one percent of the total project cost of all highways projects will be kept aside for the highway plantation and its maintenance. Early this year the National Green Tribunal issued a show cause notice to the National Highway Authority of India (NHAI) over maintenance of the green cover along national and state highways.

The Central Pollution Control Board (CPCB) suggests guidelines for setting up green belts for containment of air pollution in the human environment, especially in industrial and urban areas. Improvement of aesthetics is a bonus derived through the presence of greenery in these areas.

In course of their work, Uniyal and colleagues sampled vegetation at 29 locations at heights ranging from 500 metres to 3300 metres above mean sea level along the highway. The sampling sites were spread across three pollution gradients highly polluted, moderately polluted and least polluted.

They looked at Air Pollution Tolerance Index (APTI) to classify the species. APTI taps into biochemical characteristics of plants (ascorbic acid, chlorophyll, relative water content, and leaf-extract pH) to identify tolerance levels of different plant species.

For instance, the degree of pollution influences the total chlorophyll content in plant species. Higher chlorophyll in plants strengthens tolerance to environmental pollutants whereas less chlorophyll makes them sensitive. In polluted sites, dust load on leaves and high concentration of noxious gasses have been cited to result in degradation of photosynthetic pigments.

In the assessment, APTI data was integrated with Anticipated Performance Index (API) that incorporates biological and socioeconomic values (plant habit, type of plant, canopy structure, size of the plant, leaf structure, and hardiness) of the species.

Of the 26 plants, silk oak or silver oak (Grevillea robusta), walnut plant ( Juglans regia), Holly oak (Quercus floribunda), fig (Ficus carica) and red cedar (Toona ciliata) were found to be best performers, the study said.

Based on APTI values, silk oak stole a march over the other trees as it scored higher than others in highly polluted, moderately and least polluted sites examined in the study.

Further, the screening pin-pointed plant species such as fig and Himalayan cherry (Prunus cornuta) that had higher dust accumulation potential. “Green belts with prioritized plant species are very effective in such scenarios. Here, plants form a surface capable of absorbing particulate matter, black carbon and dust thereby acting as a sink for pollution. Rough leaves in canopy trap pollutants directly on their surface thus effectively reducing their concentrations in ambient environment,” Uniyal said.

“As per our knowledge, these indices do not yet form a part of policy actions at the centre and state level. Incorporating these would certainly be of importance,” Uniyal said.

Greening the urban sprawl

Descending from the western Himalayas to the Indo-Gangetic Basin, in Varanasi, one of India’s most polluted cities, researchers from Banaras Hindu University (BHU) have figured out which trees are hardy enough to put up with the assault of particulate (total suspended particles, particulate matter 10) and gaseous pollutants (nitrous oxide, sulphur dioxide, ozone) in the city’s urban pockets.

Spread over six successive seasons (summer, monsoon, winter) across a two-year period (January 2013 to December 2014), they mapped out the response of 13 plant species present in residential, traffic and industrial sites in the city exhibiting varying degrees of pollution and sources such as vehicular traffic, biomass burning, resuspension of soil dust from open grounds, transport of PM and gases from the main city and construction activities.

At all the study sites, concentrations of air pollutants, specifically PM (particulate matter) and nitrogen dioxide were above the specified standards, highlighted Arideep Mukherjee of Laboratory of Air Pollution and Global Climate Change at BHU’s Department of Botany.“Plantation of resistant trees can alleviate the local air quality as trees are efficient purifier of air quality,” said Mukherjee.

They looked at leaf functional traits and tree characteristics together with a statistical modelling approach like general linear modelling (GLM) to assess the combined and individual effects of the major air pollutants.

Leaf functional traits, such antioxidant status, leaf water status, photosynthetic pigments, are adaptations that facilitate plants to grow in a wide range of environmental conditions, Mukherjee said.

“Leaf functional traits are most widely used optimized marker for stress response assessment in plants, which is directly related to plant functioning and tolerance,” said Mukherjee. Among them, maximum effect of air pollutants was observed on non-enzymatic antioxidants followed by photosynthetic pigments and leaf water status. Among the pollutants, PM was the major stress factor followed by ozone, the study said.

“Deciduous trees such as Indian redwood (Caesalpinia sappan), shisham (Dalbergia sissoo) and shirish (Albizia lebbeck) were found to be most tolerant during the present study; followed by semi-deciduous trees such as neem (Azadirachta indica), gulmohar (Delonix regia) and guava (Psidium guajava); whereas, evergreen trees such as cassia (Cassia siamea), banyan (Ficus benghalensis), and eucalyptus (Eucalyptus citriodora) were found to be least tolerant,” the study said.

Death trap of coal-fired power plants

Further east in India’s coal belt in the state of Jharkhand, experts have focused attention on dust scavenging potential of plants and their resistance to pollutants.

In India, where coal-based thermal power plants (TPPs) contribute to nearly 60 percent of the total capacity of electricity generation, plant species grown in the vicinity of thermal power plants (TPP) act as immobile substrates to sink most of the pollutants spewed from their stacks such as fly ash and dust.

Researchers from Department of Environmental Science and Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, estimated the impact of dust load churned out by the Bokaro thermal power station (BTPS), Bokaro, on plants’ dust retention capacity and pollution resistances based on combinations of data on parameters affecting APTI–API and Dust Retention Capacity to develop sustainable green belts.

Their analysis showed that the blackboard tree (Alstonia scholaris), the banyan, Ashoka (Polyalthia longifolia), and mango (Mangifera indica) as the most suitable plant species for green belt formation.

“The deteriorating air quality near pollution sources results in higher atmospheric dust deposition on first available surfaces. Understanding these plant species’ efficiency in filtering and channeling polluted air and their tolerance to polluted environment are important for devising green belts near TPPs,” said study author Suresh Pandian Elumalai.

As per the norms prepared by the Ministry of Environment, Forest and Climate Change (MOEFCC), as much as 33 percent of the project site under thermal power plants is supposed to have green belt coverage.

Green belts incorporating these specific species can long-term existing as well as newly generated dust pollution of nearby industries without affecting the plant health, Elumalai said, adding that experimental study is needed to find a long-term impact on the plant’s photosynthetic rate, transpiration rate, metal accumulation, growth rate, and stomatal index near such industries.

The green paradox

The relationship between floral species and pollution is a tad complicated. Certain species are known to emit airborne chemicals called volatile organic compounds (VOCs) that contribute to higher ozone levels. And while guzzling pollutants plants also face the brunt.

Uniyal emphasises that tolerant trees are least affected as compared to sensitive species. “Some plants may release VOCs in stress conditions. On the other hand, they also help in reducing VOCs. There is a gap between experimental and field studies in this context. However, the potential benefits outweigh the few negatives,” he said.

But to strike a balance in their use and conservation, one should focus on planting a mixture of native trees, shrubs with grasses. “Periodic plantations and maintenance of species plays a significant role in pollution abatement as well as in conservation,” Uniyal said. Arideep Mukherjee from BHU opines that trees that are identified as sensitive can be grown in least polluted environments or can be grown in combination with tolerant tree species under polluted environments.

India lacks a comprehensive green belt policy and such a dedicated policy is the need of the hour, emphasised Chirashree Ghosh, associate professor, department of environmental studies at University of Delhi.

Studies that map and catalogue air pollution tolerant and sensitive trees are crucial to enable effective greening solutions, she said. 

“Instead of a superficial and simplistic approach to plant trees in settings to combat pollution effects, one needs to have an understanding of the species of trees that will have the desired pollution mitigation effect with additional ecosystem services. Otherwise such initiatives will just end up of having aesthetic value and will never give sustainable solution may even detrimental in some ways. Even in compensatory afforestation you need to give due consideration to a plant’s sensitivity index (known as Air Pollution Tolerance Index) and carbon sequestration nature to reduce pollution load of the respective site before planting any sapling,” Ghosh told Mongabay-India.

Banner image: Semi-deciduous trees such as neem (Azadirachta indica) are moderately tolerant of pollutants. Photo by Shashidhara Halady/Wikimedia Commons.

Editor’s Note: We corrected an image that was added earlier, which was that of Peltophorum not C. sappan.