Mapping the ability of tea gardens to sequester carbon dioxide

Workers in a tea garden in Assam. Photo by Ishan Jyoti Boraa/Wikimedia Commons.

  • Researchers are mapping the potential of tea plantations in Assam to sequester carbon dioxide and mitigate climate change. Assam is the world’s largest contiguous tea-growing area.
  • Tea agroforestry systems, with tea plants and shade trees, sequester significant amount of carbon dioxide.
  • They can support the implementation of REDD+ programmes and serve as an additional income source on account of their environmental services.

Can tea agroforestry mosaics in Assam, the world’s largest, contiguous tea-growing area in the world pitch in, in the fight against climate change? 

Researchers working on sprawling tea estates to small tea gardens in Assam, in northeast India, are unraveling the carbon sequestration and climate change mitigation potential couched in tea bushes grown under the canopy of shade plants (native trees) in the plantations. The tea industry in Assam is about 172 years old. Assam alone produces more than half of India’s tea.

According to the United Nations, with global demand for tea growing at more than two percent per year, the pressures on land for cultivation is increasing, just as the effects of climate change become ever-more extreme.

“The woody perennial-based land-use systems have relatively high capacities for capturing and storing atmospheric carbon dioxide and imply sustained carbon sequestration in fragile areas,” researcher Rinku Moni Kalita told Mongabay-India. Greenhouse gases (GHGs) in the atmosphere, mainly carbon dioxide, are the principal cause of global warming and climate change.

Kalita said different species of leguminous shade trees are essential components of tea agroforestry system (AFS) providing necessary cover for tea bushes. Some of the common species are silk tree (Albizia chinensis), black siris (Albizia odoratissima), Indigofera teysmanii and cassia (Cassia siamea). These shade trees also help build up biomass carbon stock, improve soil quality through litter-fall and enhanced microbial activities. 

“In our study in Assam, we found that soil component holds maximum proportion (80 percent) of carbon stock followed by shade tree (11 percent), tea bush (five percent) and litter (four percent) components in the system,” said Kalita, assistant professor, department of botany, Biswanath College.

Tea garden with shade plants in Assam’s Sonitpur district. Photo by Amlan Basumatari/Wikimedia Commons.

How plantations are managed drives carbon storage dynamics

Tea AFS with shade trees sequesters a considerable amount of carbon annually, Kalita emphasised. 

“Average carbon sequestration was estimated at 5.98 megagram per hectare per year across the plantations. This means the tea plantations lock down close to 6000 kg per hectare per year of carbon in their biomass,” said Kalita. “If we maintain the plantations under specific conditions, the average annual carbon mitigation potential of tea agroforestry was estimated to be 0.86 Mg (860 kg) per hectare per year,” explained Kalita, adding shade tree was responsible for 57.3 percent of the carbon sequestration in different plantations in northeast India while tea bushes contributed 42.7 percent.

How plantations are managed by growers also drives carbon storage dynamics with factors such as plantation age, shade tree selection, maintenance of biomass stock influencing carbon sequestration rate in this intensively managed system.

“More than 90 percent of the shade trees are native species. The mixing of native species in tea gardens is important for climate shifts, for example, delayed monsoon. The trees have a similar type of phenological calendar and this helps them to adapt to climate extremes,” Kalita said.

Tea growing areas have been witnessing long rainless spells or high-intensity rainfall of short duration in recent years, resulting in waterlogging and soil erosion in tea gardens as per a study conducted by IIT-Guwahati, states a Tea Board of India 2017 seminar note, also emphasising the temperature rise.

Underscoring the role of small tea growers, Kalita said, the tea agroforestry system may be considered as “an interesting compromise” in search of a better “balance between agricultural development, economic gain and fight against deforestation.”

“Well-managed smallholder tea plantations could have implications in compensating for the loss of carbon stocks resulting from land-use conversion and sustainable production of tea providing financial benefits to the tea growers,” he said.

On the emergence of small tea growers, Assam’s Industries and Commerce department states that “the profession has now shifted from the rich to the common man, especially unemployed youths who have taken up tea cultivation as a business venture. Some even cultivate it in their backyards.”

Kalita said even neutral carbon source/sink systems are significant.

For instance, Kalita’s study encompassed sampling sites in the 100-year-old Jalinga Tea Estate, an organic, carbon-neutral tea garden that uses a specific composting method to ensure 75 percent reduction of greenhouse gas emission potential compared to any other tea estate. The composting method ensures a profusion of soil microbes enabling the tea estate to tap into atmospheric nitrogen to fill the estate’s 75 percent of required nitrogen needs. 

The value of carbon sequestration

Another facet of tea gardens that merits mention is their supporting role in implementation of REDD+ (Reduce Emissions from Deforestation and Forest Degradation) under the forest governance framework, the researcher noted.

REDD+ is a financial instrument to incentivise conservation and sustainable management of forests, and thereby achieve a reduction in the GHG emissions resulting from deforestation and forest degradation. It aims at compensating forest owners in developing countries for conserving their forests by putting a value on the forest carbon stocks — one of the many ecosystem services that forests provide, according to TERI.

Sri Lankan researcher Thushari Wijeratne said tea plantations do show good potential to be engaged in payment for ecosystem services and REDD+.

“It is worthwhile to investigate the carbon sequestration capacity of tea plantations and generate the baseline data for proper documentation in REDD+ and build up a greenhouse gas inventory. However, authorities need to be convinced,” said Wijeratne, of Tea Research Institute of Sri Lanka.

“It could also serve as an additional income for environmental services of tea plantations,” Wijeratne added.

In a recent study, Prabhat Pramanik, scientist at Tea Research Association, quantified the potential of tea plantations, along the north and south bank of the Brahmaputra river, to mitigate the greenhouse effect by absorbing atmospheric carbon dioxide.

“We found that tea bushes sequestrated 5,134 kg of carbon dioxide per hectare per year in their biomass. However, this value refers to only 56.2 percent of the total carbon-sequestration potential of tea gardens of northeast India,” said Pramanik.

Pramanik mapped carbon dioxide assimilation potentials of four sections tea bushes that were high-yielding cultivars (TV23 cultivars of ages 11 years and 24) and cultivars that produced high-quality tea and that of shade trees.

High yielding TV23 cultivar assimilated a significantly higher amount of carbon dioxide as compared to quality tea producing cultivars (S3A/3) and mature 25–30 years old tea bushes absorb more carbon dioxide from the atmosphere as compared to younger tea bushes.

In this study, tea bushes sequestrated 52.7 to 61.3 percent of the total carbon dioxide sequestered by the plantations in a tea garden. “This study enabled us to understand that tea bushes play a significant role in mitigating global warming by assimilating and sequestrating atmospheric carbon dioxide,” said Pramanik, adding that the factors that limit carbon sequestration by tea bushes and shade trees are the type of cultivars used, climatic condition, soil types among other factors.

Close-up of tea leaves in a tea garden in Assam. Photo by Bidyut Gogoi/Flickr.

Banner: Tea garden workers in Assam. Photo by Ishaan Jyoti Bora/Wikimedia Commons.

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