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Dhaka Tribune

How trees and waterbodies help reduce temperatures

  • Trees capture carbon, release oxygen
  • Almost all types of trees purify air
  • Phytoplankton releases more oxygen than plants
Update : 30 May 2024, 10:19 AM

Since mid-April, the entire country has been tormented by a mild to severe heatwave. However, some areas within a densely populated Dhaka city and other regions are experiencing lower temperatures than their surrounding areas, thanks to the presence of wider canopy coverage and water bodies like ponds and lakes.

The unbearable heat has triggered frustration and anger among the people, causing them to take to social media to decry tree felling for construction works, the filling up of water bodies, and other activities, including the rising number of fuel-run vehicles and air conditioners, which contribute to increasing temperatures. Many aggrieved netizens and green groups have conducted campaigns in favour of tree planting and against tree felling.

Going with the tide, many people also took up tree plantation initiatives in April, when the heatwave lingered for an unusual period after decades. May, too, has been hotter than last year.

But April and May are not the perfect months for planting saplings, experts say. They also stress the need for choosing the right place and the right varieties.

Thanks to the mainstream media supporting the cause, some initiatives to cut down trees had to be shelved upon outrage expressed by locals. For example, in the face of protests, the Dinajpur Zilla Parishad suspended cutting down trees beside the Sadar-Setabganj Road, which eventually saved 78 age-old trees. However, a road-widening project had already resulted in the removal of at least 340 trees.

Local protests halted plans to cut down trees in the CRB area of Chittagong for a flyover project and delayed the construction of some buildings at Jahangirnagar University.

Photo: Probir Kumar Sarker/Dhaka Tribune

Trees that are natural air coolers

Prof Saleh Ahmed Khan teaches botany at Jahangirnagar University, a green campus located on the outskirts of the capital in Savar that is home to a number of lakes and ponds.

“Here, the average temperature remains 2-3 degrees Celsius lower than in Dhaka because of the trees, ponds, and lakes,” he told Dhaka Tribune.

Prof Khan said the JU campus has 917 species of plants, and the authorities should take responsibility for proper preservation. There should be inter-disciplinary research on its ecosystem.

He also justified the students' opposition to the construction of buildings by cutting down trees and their demand for a master plan. 

He added: "Building new infrastructure won't harm trees when there's a master plan."

“Trees with greener leaves and greater canopy coverage essentially interact more with their environment. Because of these trees, the sun's heat does not directly reach the ground or surface. Thus, trees help keep the temperature down.”

Such trees can be native or foreign species, he said. Among the native plants are Java Cassia or Sonalu, White Siris or Koroi, Jackfruit, Banyan Tree, Arjuna, Terminalia Chebula or Haritaki, Beleric or Bahera, Neem, False Ashoka or Debdaru, Chapalish or Chaplash, Blackboard Tree or Chhatim, and Aphanamixis Polystachya or Pitraj.

Of the exotic varieties, mahogany, and Royal Poinciana or Krishnachura can help reduce temperatures.

On the other hand, plants that are not very helpful are Acacia or Akasmoni, Eucalyptus, and the silk cotton tree or Shimul, Prof Saleh added.

What to know before planting trees

Prof Md Nuhu Alam of the JU Botany Department said: “We have to remember that trees help us prevent air pollution and maintain balance.

“The proportion of trees in an area should be correct, so that there is at least 25% of greenery. Trees basically reduce pollution, sequester carbon, and supply oxygen to the environment. Almost all types of plants help us reduce pollution in the vicinity. In that case, we need to pay more attention to the proportion of trees in a certain area than the species or type of tree.”

Prof Saleh Ahmed Khan said there are certain times and patterns for planting these trees.

"It is best to plant trees at the end of the monsoon because the soil contains moisture. The saplings should be planted in a line on the landscape, parallel or side by side, keeping in mind the type of trees and their canopy coverage.

Maintaining distance is crucial for trees to receive sufficient canopy coverage and thrive. If the land is plain, each tree should be planted at a distance of 6 to 10 feet from others.”

Prof Nuhu Alam echoed Dr Khan, saying that time is of the essence in planting trees.

“If you plant a tree during this summer, the tree will not survive. In that case, it should be applied when the soil moisture is sufficient.”

Photo: Probir Kumar Sarker/Dhaka Tribune

Role of water reservoirs

With regard to the presence of water bodies, Prof Saleh Khan said the aquatic plants in the water also absorb heat. “If there is a balance between the number of water bodies and plants in an area, it will maintain the humidity.”

Prof Nuhu Alam emphasized the importance of water bodies alongside trees.

“Aquatic plants and plankton take up more carbon and supply oxygen to the environment. As reservoirs hold water, the humidity in the air remains tolerable. Both plants and water bodies are necessary to control the temperature in an area," he said.

Is water hyacinth useful?

People treat water hyacinths as aquatic weeds that grow in ponds and other water bodies and serve as mosquito breeding grounds.

Professor Saleh Ahmed Khan said aquatic weeds like water hyacinths do not play much role in an aquatic ecosystem. 

“It is not a plankton. An aquatic ecosystem may have it, but it often prevents our native aquatic plants and microorganisms from growing properly,” he explained.

Professor Nuhu Alam also said that water hyacinth is not a phytoplankton, which plays a bigger role in ecosystems.

“Phytoplankton releases ten times more oxygen into the air than terrestrial plants. It plays a very important role in producing energy at the primary level and developing into the secondary and tertiary levels of our food chain.”

He added that water hyacinth is treated like a weed, but it cannot be said that it does not have any benefits. Water hyacinths retain water. An aquatic ecosystem may contain it to a certain degree, but not necessarily in excess.

Prof Nuhu Alam also rejected the perception that ponds with water hyacinths are mosquito breeding grounds. Water is essential for mosquito larvae to survive, however, it can exist in any stagnant water.

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