A tree grows in India

Written by
Pooja Makhijani, Princeton Institute for International and Regional Studies
Dec. 14, 2022

Cities consume 78 percent of the world’s energy and produce more than 60 percent of greenhouse gas emissions, according to UN-Habitat. To mitigate the effects of greenhouse gas emissions on climate change, scientists around the world are advancing new technologies for carbon capture and storage. Fortunately, the best carbon capture technology already exists: trees. Trees reduce carbon in the atmosphere by sequestering it in new growth every year; as a tree grows, it stores more carbon by holding it in its accumulated biomass.

An ongoing research project in the Urban Nexus Lab attempts to quantify the carbon mitigation potential of urban greenery in India. Since October 2021, the lab has collaborated with the Indian cities of Delhi, Pune and Siliguri to advance non-destructive field methods for the development of local urban tree data and to better understand the potential of trees in Indian cities to offset emissions from other infrastructure provisioning sectors, toward net-zero emissions.

Assessing the carbon sequestration potential of trees in any given city requires an understanding of the size-diameter relationship, commonly called the “allometric relationship,” and the growth rate of local trees. A reliable scaling methodology is also needed to use relatively small samples of field data to generate citywide carbon stock and sequestration estimates. However, these three components — the allometric relationships, growth rates and scaling to citywide estimates — are existing knowledge gaps in India. Allometric relationships and growth rates of Indian urban trees have never been investigated, and no consistent methods exist in India for reliably converting small scale urban tree inventories to citywide carbon estimates.

“Small-scale studies in the U.S have shown tree allometry is different for urban trees relative to rural trees of the same species in the same eco-region,” says Joshua Eastman ’22, who worked in the Urban Nexus Lab as part of his independent studies in civil and environmental engineering as an undergraduate and as an intern post-graduation. “If we want to produce estimates of urban biomass in India, we need to determine what that ratio is for India.”

The researchers championed LiDAR technology, a method for determining variable distance by targeting an object with a laser and measuring the time for the reflected light to return to the receiver. LiDAR has been used successfully in forestry to map individual trees in forests and to predict forest volume and biomass — but never before in India.

The Urban Nexus Lab operates under the aegis of Anu Ramaswami, the Sanjay Swani ’87 Professor of India Studies, professor of civil and environmental engineering, PIIRS and the High Meadows Environmental Institute, and director of the M.S. Chadha Center for Global India. This project is supported with funds from the Chadha Center’s India Data Project, which, along with the Urban Nexus Lab, aims to advance new methods for developing novel datasets on people, nature and infrastructure in cities across the U.S. and India. Bhartendu Pandey, lead urban data scientist and postdoctoral researcher in the Department of Civil and Environmental Engineering, is concurrently developing data science approaches to derive first-order estimates of carbon storage and sequestration in trees across all urban areas in India using field measurements.

Monalisa Sen is program coordinator in biodiversity at ICLEI - Local Governments for Sustainability South Asia, the regional outpost of a global network of more than 2,500 local and regional governments. She coordinates the Urban Nexus Lab’s entire on-the-ground research operation; her team conducted over 300 LiDAR scans and 160 increment cores — a process by which an instrument called an increment borer extracts a small, pencil-sized piece of wood from the trunk of a tree — of urban trees in these three cities. Researchers in India faced early challenges: For example, the equipment malfunctioned unexpectedly in India’s climate, and the data collectors struggled at first with producing high-resolution output scans.

But, Sen says, despite early hiccups, this methodology has broad appeal and application. The Indian Forest Service calculates carbon sequestration calculations annually, but only in forests: Such data does not exist for urban spaces and urban trees, she explains. “Cities in India are interested in understanding how much emissions are being reduced or how much carbon is being sequestered by the trees that they have and which trees should they plant or which trees they should not plant,” she says. “Carbon sequestration analysis for trees is very complicated. We hope that this methodology will be a much simpler exercise for cities to undertake.”