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Thursday, June 24, 2021

Explained: Mumbai’s tree-fall incidents and why the BMC appealed to plant only native species

Here's the difference in indigenous and non-indigenous species, and a look at whether planting of more local varieties of trees would reside tree-fall incidents in Mumbai.

Written by Sanjana Bhalerao , Edited by Explained Desk | Mumbai |
Updated: May 26, 2021 8:07:57 am
MumbaiA fallen tree due to a storm triggered by Cyclone Tauktae, in Mumbai. (Express Photo by Ganesh Shirsekar)

Over the past few days, Mumbai has witnessed a high number of tree-fall incidents, leading to suggestions from activists as well as Mumbai’s mayor that the Brihanmumbai Municipal Corporation should plant only indigenous species in the city. The Indian Express explains the difference in indigenous and non-indigenous species, and whether planting of more local varieties of trees would reduce tree-fall incidents in Mumbai.

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How many tree-fall incidents did Mumbai witness last week?

As many as 2,364 branches and trees were damaged in the metropolis in the three days – from May 16 to 18 – when the Extremely Severe Cyclone Tauktae brushed past the Mumbai Coast, with wind speed reaching up to 114 km/hour. Of the total, 812 trees were uprooted, while 1,552 trees lost their branches. The total number of trees that fell during the three days was substantially higher than the total tree-fall complaints that the BMC deals with during the four-month monsoon season. On average, BMC receives 600 tree-fall complaints a month between June-September.

How many of the trees that fell were non-indigenous?

The BMC in its post-cyclone inspection revealed that 70 per cent of 812 trees were non-native species, including Gulmohar, rain tree and royal palm among others. In the aftermath, BMC appealed to the citizens and private organisations to plant only native tree species during plantation drives.

What are the native or indigenous tree species in Mumbai?

The BMC warned that native species should not be confused with old trees or trees that are widely present. Over at least three decades, the government has imported exotic species, and new tree species were planted across the city for beautification. For example, Gulmohar or Rain trees are widely found in Mumbai, however, they are not native species.

By definition, a “native” plant is living, growing, and reproducing naturally in a particular region. After studying the local agro-climatic conditions, including soil quality, humid weather, the BMC prepared a list of 41 native trees that can be planted in Mumbai and are part of the trees in the Konkan belt. These are Wad, Pimpal, Umber, Kanchan, Kadamba, Gunj, Palas, Nim, Mahogany, Moh, Bahawa, Sag, Arjun, Ain, Kinjal, Sita Ashok, Undal, Nagkeshar, Champa, Shivan, Shirish, Karanj, Bakul, Bell, Taman, Hirda, Behda, Coconut, Amla, Khair, Tetu, Mango, Putranjiva, Wild Almond, Bibba, Parijatak, Rita, Sandalwood, Phanas and Chafa.

Why Botanists and BMC feel that non-indigenous species of trees are more prone to falling?

Native species of Mumbai can cope up with excess water/humid conditions of the city and can withstand heavy rainfall and wind. Experts said that it is not that non-indigenous species won’t survive, however it will require more maintenance, attention and care and even after that the trees may not acclimatise. The roots of the non-indigenous trees are unable to hold on to the ground, are fragile and give away in heavy rain or strong winds. Botanists warned that new species may also compete with native species for land, water, and food. Foreign species may also carry diseases that can harm the native species.

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Is planting foreign varieties of trees the only reason why Mumbai is seeing an increase in tree-fall incidents?

Environmentalists and urban conservationists said that old trees and non-native species cannot be solely blamed for the damage to the green cover in the city. It is not the dead trees, but also the healthy ones that get uprooted. Environmentalists blamed the rapid concretisation for the increase in the number of trees getting uprooted.

A 2014 survey of 1965 “Rain Trees” by Vanashakti, had found up to 4-5ft of concrete, tar and other construction material surrounding over half of the trees’ base without much soil. Network of underground cables, lack of water percolation into the soil because of concretisation around the tree bases is weakening the trees leading to tree fall during heavy rains, especially on footpaths. Around 308 trees out of the 812 that got uprooted were roadside i.e., on sidewalks and footpaths.

According to Maharashtra (Urban Areas), Preservation and Prevention of Tree Act, 1975, 1-metre space should be left around tree trunks. The National Green Tribunal has also directed that there should be a 1-metre space around tree trunks for better growth to preserve and protect trees.

The BMC doesn’t plant trees on the roadside or footpaths. However, during building and footpath construction, road-relaying, underground pipeline, the 1-meter space around the trees needs to be left around the tree bases/trunks.

What is the reason for the high number of branches falling off in Mumbai?

Activists say this is happening due to Unscientific Tree Pruning being conducted by the BMC. As per the 2019 audit, there are over 1.75 lakh roadside trees. The BMC claims that ahead of monsoon, 60k-70k trees are pruned. However, the pruning exercise undertaken by the BMC is criticised to be “unscientific”. In 2019, a citizens-led audit of trees in Colaba revealed that 17 of the 100 trees surveyed on JD Somani Marg and Captain Prakash Pethe Marg face the risk of falling.

Experts said that the idea behind a pruning exercise is to maintain balance the weight of the tree, it should not be dangerously tilting to one side. Mumbai Mayor Kishori Pednekar said that she will write to the Chief Minister to amend the Tree Authority Act to allow the BMC to trim more and bigger branches. However, experts warned against arbitrary tree trimming.

There is no set rule about the number of branches to be cut or spread of the canopy. Experts said it differs with each tree. If the winds lash a tree, it should pass through it. Otherwise, air- turbulence is created within a tree (big or small bushy ones) that can pull it down. As per the arboriculture technical norms, not more than 20-30 per cent of trees’ canopy should be pruned in a year. However, haphazard tree trimming continues in the city.

As per the tender condition of the contractors appointed for tree pruning exercise has included a special condition to have a horticulturist or arborist on board, however, the condition is not mandatory.

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