Red Earth Environment

Scientists are studying the effects of climate change by measuring tree growth

Researchers measure tree trunk respiration with a gas monitor

Researchers measure tree trunk respiration with a gas monitor  

This is part of an ambitious pan-India project, partially Central government-funded, conducted by researchers affiliated with Bengaluru’s National Centre for Biological Sciences

It is just after sunrise but the forest canopies in North Karnataka’s Sirsi are already a cacophony of activity. Woodpeckers drum on tree trunks. Sunbirds, small enough to fit into the palm of your hand, flit through vines with their distinct high-pitched trills.

But 25 metres below, on terra firma, and too busy to pay attention to the medley in the canopy, field researcher H.V. Raghavendra quietly goes about his work. To anyone watching, much of Raghavendra’s work would appear boring — maybe even bizarre.

Armed with a measuring tape, he jots down the girth of giant trees and the height of barely-noticeable seedlings unfurling their first leaves on the wet forest floor. He digs out coin-like ‘i-buttons’ buried in the wet soil to make sure they are doing their jobs: recording soil temperature. Small nets suspended from tree trunks catch falling leaves. Raghavendra will collect the leaf litter every month and then dry, powder and weigh each batch at his field station a few kilometres away before sending them to Bengaluru.

These peculiar exercises are aimed at deciphering answers to some of the most pertinent questions confronting the world today: how do changing weather patterns affect forests? What are the larger implications for India’s forests?

Tapes and measures

This is part of an ambitious pan-India project, partially Central government-funded, conducted by 10 researchers affiliated with Bengaluru’s National Centre for Biological Sciences (NCBS).

Led by senior scientists Mahesh Sankaran and Jayashree Ratnam, the team is collecting and analysing vegetation, soil and climate data from seven one-hectare plots across India, to capture changes across varying habitats: from the evergreen forests of Karnataka and the Andamans, to the dry deciduous tracts of Tamil Nadu and Andhra Pradesh. “The aim is to study the effects of climate change in India by studying regional carbon cycles,” says Ratnam.

The carbon cycle is the fascinating story of how carbon, one of the most abundant elements on earth, circulates in and out of the environment. Carbon gets fixed in plants and trees when they breathe in carbon dioxide, an atmospheric greenhouse gas that causes global warming. Forests, some of the biggest mechanisms of sequestering carbon, or carbon ‘sinks’, are often crucial to counter the effects of climate change.

But, according to the U.S.’s Energy Information Administration, emission of human-caused carbon dioxide is set to increase to 33.8 billion metric tonnes per year by 2020, up from 29.7 billion metric tonnes in 2007. Fallouts include a rise in temperature and altered rainfall patterns. In 2014, a study showed that more intense wet spells and more frequent dry spells characterised the South Asian summer monsoon between 1981-2011 as compared to previous years (1951-1980).

“The Indian monsoon has changed over the decade and is more unpredictable now,” Ratnam says. “This could also change forest dynamics and their future functioning. We hope to capture the responses of forests to changing climates by tracking them in real time.”

Though seemingly simple, tracking climate change — by studying regional carbon cycles — involves piecing together information from multiple sources.

Their measurements of seasonal and annual growth and rainfall will reveal not only yearly monsoon and rainfall trends but also the resulting tree growth patterns. With this database, predicting how forests in India will respond to climatic changes in the Indian subcontinent can become far easier and more robust.

The plots also reveal other vital, but less obvious, pieces of information. For instance, the proportion of various elements such as nitrogen, carbon and phosphorus in green vegetation, leaf litter (forms of carbon that plants harbour and later lose) and soil samples, can help quantify organic matter and in turn, carbon values. These, as well as the tree-growth data, are plugged into estimates of carbon sequestration by forests on a seasonal and yearly basis.

The team is also tracking seeds on the forest floor to see how many seeds grow into saplings and of those, how many into trees. The numbers of seeds produced and the timing of flowering and fruiting can change due to climatic variations, says scientist Karthik Teegalapalli, who coordinates the project.

Respiration collars fixed on tree trunks and the ground help calculate the amount of carbon dioxide that tree trunks and soils release. They also help estimate photosynthesis levels and give clues about how efficiently plants produce energy. Natural processes such as tree deaths and rates at which plants decompose are also crucial to understand the bigger picture.

Prediction model

Currently, the team monitors 11,726 trees (of nearly 400 species). And the data has begun revealing interesting nuances in tree growth, says Sankaran. “We are learning that in dry forests, tree girths can shrink substantially in the dry season, sometimes more than they grow in the wet season,” he says.

And preliminary results from the Andaman rainforests suggest that during years of normal rainfall, the forests act as carbon sinks, drawing carbon dioxide from the atmosphere. “However, during drought years, they switch to being sources, releasing more carbon dioxide than they absorb,” adds Sankaran. “This can have important implications for carbon cycling in the future, especially if droughts become more frequent, as has been predicted.”

These are exactly the kind of results the team hopes to generate in the coming years. The team’s data on local plant physiology and weather can also be used to refine existing climate prediction models that are now based on estimates from other parts of the world.

“But watching trees grow is slow business,” says Ratnam. The natural processes span decades; the first detailed results will start pouring in four years from now. Each plot involves intensive work and challenges include securing long-term permits to conduct their research in forested areas.

But then, studying vegetation is not considered as ‘interesting’ as studying animals. “Also, people often don’t see a direct societal impact, so it is difficult to convince them about the value of this data,” says Ratnam. “But with the interesting results we have been getting, we hope this will change this soon.”

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Printable version | Jul 5, 2020 5:05:10 PM | https://www.thehindu.com/sci-tech/energy-and-environment/scientists-are-studying-the-effects-of-climate-change-by-measuring-tree-growth/article22999809.ece

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