Air pollution kills Dhaka’s trees too

As air pollution increases in Dhaka city, the trees which fight back against pollution are decreasing in number.

Scientists say the greenery is at risk, due to the emissions from the city’s vehicles and particulate in the air.

The roadside trees of the capital city are the hardest hit by pollution. Their chances of survival have dropped by 30 per cent. These facts were published in a joint study conducted by Dhaka University’s departments of chemistry and botany.

The research article was published on 12 October in the international publication Springer Nature Applied Sciences journal. The researchers said this was the first such sensitivity study of plants conducted in the country. The research showed that the city’s debdaru, mahogany and jackfruit trees were being harmed by nitrogen dioxide emissions from the vehicles and particulate matter.

It was said that the leaf surfaces absorbed the particulate matter, the sulphur dioxide, nitrogen dioxide, carbon dioxide and other pollutants in the air.

Member of the research team and professor of botany at Dhaka University, Mohammed Jasim Uddin, said that if one uses a black umbrella for long, it gradually becomes discoloured.

Similarly, plants gradually lose their tolerance to pollution. Their chlorophyll content decreases and the plants lose their capacity to manufacture food.

The dust particles block the pores of the leaves and photosynthesis is hampered. Sulphuric acid is produced when sulphur dioxide mixes with water, which can remove the magnesium in the chlorophyll. The leaves lose colour and their shape changes too.  The trees’ capacity to produce flowers and fruit decreases too. Eventually the trees die as a result of long-term pollution.

According to the global report, ‘The State of Global Air 2019’, 123,000 persons died due to diseases related to air pollution in Bangladesh in 2017. This report prepared jointly by the US- based Institute of Health Metrics and Evaluation and the Health Effect Institute, was published on 3 April this year. Bangladesh stands fifth in deaths caused by air pollution, it said.

The US-based AirVisual’s World Air Quality Report 2018 ranked Dhaka second among the world’s most polluted cities. The particulate matter in Dhaka city’s air is 10 times higher than the permitted rate of World Health Organisation  (WHO).

Trees struggle to survive

The Dhaka University research team collected leaves of the debdaru, mahogany and jackfruit trees from three types of areas in Dhaka city during the summer (March-May), rainy season (June-September) and winter (November-February) in 2017-18. The areas included the busy roadsides of Farmgate and Gulshan, Uttara and Dhanmandi residential areas, and the Botanical Garden.

Anything below 12 in the Air Pollution Tolerance Index (APTI) means the trees’ sensitivity is at risk. The research showed that in summer and in the rainy season, the trees’ APTI was 7.34 and 8.10 respectively. That means the trees’ survival capacity was relatively less in these seasons. But this fell even more in winter (6.69). Trees by the roadside were worse affected than those in controlled areas like the botanical garden and in residential areas.

Leader of the research team and professor of chemistry at Dhaka University, Abdus Salam, told Prothom Alo there were less green spaces and water bodies in Dhaka to tackle pollution. Unplanned urbanisation over the past three years has caused a decrease in the city’s surface water, water bodies and green trees. And lack of proper management of construction, repairs and digging in the city has stepped up pollution further. These tasks are being carried out in an uncoordinated manner by various agencies. The problems are known and so are the solutions, but no effective measures are being taken to control the pollution.

Green replaced by pollution

Three researchers of Japan’s Kyoto and Hokkaido University assessed the green area of Dhaka city and the result of their study was published in January in the US-based Institute of Civil Engineers journal. Member of the research team, RAJUK’s assistant planner and researcher at Hokkaido University, Mustafizur Rahman, told Prothom Alo that in 1995 the green space covered 12 per cent of the city. In 2015 this had decreased to 8 per cent. Now it will not exceed 6 to 7 per cent.

Water bodies play a significant role in reducing pollution, but the unplanned urbanisation of Dhaka city is seeing its water bodies, low-lying land, canals and rivers being filled. According to the Bangladesh Institute of Planners, in keeping with the Dhaka Area Plan (DAP) of 2010, 22 per cent of the water bodies have been filled by January 2019.

According to the environment department’s report published in March this year, 58 per cent of the city pollution was caused by brick kilns. Another 26 per cent of the pollution was caused by dust and vehicle emissions.

Chief executive of Bangladesh Environmental Lawyers Association (BELA), Rizwana Hassan, said that Dhaka and other big cities were being stripped of their trees in the name of development.

If the remaining plants and trees also lose their capacity to survive, it will be difficult to live in this city, she warned.

To overcome this situation, the local government institutions must take the responsibility of extensive tree plantation in the city, Rizwana suggested adding if not, neither trees nor people will survive in Dhaka city.

• This report, originally published in Prothom Alo online edition, has been rewritten in English by Ayesha Kabir.