How to fortify trees in our concrete jungles to save environment

Last week, hundreds of trees in Delhi were uprooted when a sudden squall with wind speeds of 100 km per hour lashed the national capital. It is a common sight in Indian cities in the wake of heavy rains or storms. However, other than natural causes, human errors can also contribute to such felling.

According to environmentalist Pradip Krishen, who has authored Trees of Delhi: A Field Guide and Jungle Trees of India: A Field Guide for Tree Spotters, when a ferocious storm strikes it may damage trees that are weak, crowded or lacking in sufficient anchorage.

The force of nature is enough to sort out forests, including felling trees away from the wary gaze of city dwellers and human mismanagement. Yet, it is important to understand the nature of trees to limit their damage in city spaces.

One of the factors behind storms damaging trees is the degree to which the wind is res­isted, says Krishen. “If a canopy is particularly thick, such as a neem tree that has a fairly short bole, it offers more resistance to the wind. Think of it like a sail catching the wind. Trees that have a thick canopy are likely to blow off more than ones that have a more porous canopy. So, this is also something city managers take into account when they are planting trees,” he adds.

Krishen shares that in one particularly severe storm in Delhi a decade ago, he discovered that neem trees were affected to a much greater extent than others. He also noticed that the neem trees had lost their heartwood — the inner, central layers of the wood of a tree — rendering them similar to empty cylinders.

He explains some trees are characterised by their taproots (the main root that grows downwards) and others by their surface roots (such as fig trees that are anchored only at the surface). Also, the spread of a tree is roughly equivalent to the spread of its canopy. The neem is a tree with a strong taproot. He observed that the fallen trees had no taproots left.

“My surmise is these trees grew at a time when the water table was higher in Delhi. When the water table had dropped below a certain level, their taproots also shrivelled and were no longer able to access water. This may have caused the heartwood to rot,” Krishen says, adding that it has therefore become all the more critical for cities like Delhi to plant trees that survive naturally without additional watering.

To put it another way, it is important to plant trees that are native. Native doesn’t necessarily mean trees that are nat­ive to India, Krishen explains, but those that have adapted to a particular kind of soil. A city can be segregated into multiple microhabitats, with specific tr­ees becoming “native” to each.

Another factor that decides the fate of trees is brittleness. Some, including the ornamental gulmohar, is notoriously bri­t­tle and lacks elasticity. They are among the likeliest candid­a­tes of casualties in a sto­rm. Ci­t­ies are advised to assess the el­asticity of a trunk before cho­osing to plant a species.

Following the windstorm in Delhi last week, media reports also cited officials saying that concretisation around the fallen trees weakened them perilously. Experts have also pointed to a National Green Tribunal order in 2013 against any concrete work within a radius of one metre of the tree.

Paving trees right up to the trunk, says Krishen, causes the surface roots that anchor them to wither away. In many cities abroad, he adds, metal grates around the tree extend up to 6-8 feet on either side. Mulch or bark is also placed in the radius of the grates to help water enter the soil and into the tree’s roots.

In British India, when Delhi was part of Punjab prior to 1911, the provincial government req­uired the arboriculture department to plant trees and submit a report every five years about which trees were surviving well, says Krishen. “We don’t have such systems in place now in horticulture departments, so everybody is learning afresh.”

Safe and Strong

• Trees with a thick canopy are likely to blow off more than ones with a porous canopy, says environ­mentalist Pradip Krishen
• As water table shrinks in cities, it has become critical to plant trees that survive naturally
• It is important to plant native trees — ones that have adapted to a particular kind of soil
• Brittle trees that lack elasticity, such as gulmohar, are easy casualty
• Paving trees with concrete up to the trunk causes their surface roots to wither away