Mahogany trees cut effect of greenhouse gases: Study

Indian researchers have discovered that the mahogany tree releases a sulphur compound that can reduce atmospheric warming caused by greenhouse gases. In a first-of-its-kind study, a seven-member team from the Indian Institute of Science Education and Research (IISER), Mohali, found that the big-leaf mahogany tree (Swietenia macrophylla King) known for its high-quality timber emits significant amounts of the compound dimethyl sulphide (DMS) that aids cloud formation and cooling of the earth’s surface, impacting regional air quality and climate.

The evergreen forest tree is native to the Americas but is also cultivated in India and Southeast Asia.

The study ‘Significant emissions of dimethyl sulfide and monoterpenes by big-leaf mahogany trees: discovery of a missing dimethyl sulfide source to the atmospheric environment’ was published on January 13 in Atmospheric Chemistry and Physics, an open-access peer-reviewed scientific journal published by the European Geosciences Union (EGU). The Union also bestowed an outstanding student presentation award to the study’s lead author from IISER Mohali.

“Sulphate particles formed from the oxidation of DMS gas in the air backscatter the incoming solar radiation into space. As a result, radiation doesn’t reach the earth’s surface, making it cool,” said professor Vinayak Sinha, corresponding author and head of earth and environmental sciences, IISER-Mohali. He explained that greenhouse gases such as carbon dioxide and methane, on the other hand, absorb infra-red radiation coming from the earth’s surface, making the planet warmer.

Till now, studies have found that marine phytoplankton is a major source of DMS. “No one had considered DMS to be a significant emission from a terrestrial tree. Our work identified the mahogany tree as one of the missing natural sources of ambient DMS over the Amazon rainforest,” said Sinha.

The big-leaf mahogany tree occupies more than 2.4 million sqkm and is listed under Appendix II of the Convention on International Trade in Endangered Species, which places control on its exploitation. In India, the tree is cultivated in Kerala and some parts of north India. However, there is no accurate inventory on the locations of the trees in other parts of India, said researchers.

“The study findings are surprising because the ocean is considered to be the main source of DMS and thus, the authors appear to be able to resolve part of the puzzle as to why high DMS fluxes appear over tropical forests,” said Dr Ben Poulter, research scientist in the Biospheric Sciences Laboratory, NASA Goddard Space Flight Centre, who was not involved in the study. “Such field and laboratory studies are very helpful to understand how and why plants produce biogenic volatile organic compounds (BVOCs) and provide an opportunity to understand sources of air pollution, how the chemistry of the atmosphere behaves, and what opportunities there are to minimise harmful pollutants,” Poulter said.

Essentially, all trees and plants emit BVOCs that play a vital role in biosphere-atmosphere interactions. “Sulphate particles, which are hygroscopic [they love water] in nature, are very receptive as cloud condensation nuclei and can, therefore, play a significant role in the formation of clouds. They can also affect the cloud life cycle in terms of precipitation,” said Sinha.

The team provided the first estimates for global DMS. “In the future, it will be interesting to expand on this work to see how DMS production is affected from seedling to adult stages of mahogany growth; whether DMS is produced in many tropical trees species, and to incorporate these processes into global chemistry models to reduce uncertainties and improve our understanding on ecosystem and atmosphere interactions,” said Poulter.

The findings, researchers said, will aid in assessing the impact of changes in land-use/land-cover (LULC) on atmospheric chemistry and air quality in regions with mahogany plantations. “At present, LULC representations for India in the global models have a lot of inaccuracies,” said Sinha. “There is an urgent need to get an accurate representation of region-specific vegetation and information on the quantum of emissions with respect to the existing climatic conditions.”

While mahogany is the only tree identified to produce significant DMS emissions, recent studies have reported low DMS emissions from shrubs such as hibiscus and some tropical tree species. “Other trees could also be emitting it and we just don’t know because such screening studies have not been carried out on them,” said Sinha.