World Forest Day 2024: Indian forests are losing their ability to absorb carbon dioxide due to climate change

Indian forests are facing a silent crisis. Rising temperatures, erratic rainfall and deforestation are weakening their ability to absorb carbon dioxide (CO2), a vital role in combating global warming. This poses a major threat to the country’s ambitious climate goal of creating “an additional carbon sink of 2.5-3 billion tonnes of CO2 equivalent through additional forest and tree cover by 2030”, as per its Nationally Determined Contributions, updated in August 2022. 

Currently, forests in the country store 26,145 billion tonnes  of CO2. The heart of the issue lies in photosynthesis, the process by which plants convert sunlight and CO2 into energy and oxygen. While increasing CO2 levels can initially boost photosynthesis, the warming sun acts like a cruel twist, hindering enzymes crucial for the process. 

Water stress from hotter temperatures and changing rainfall patterns further disrupts this vital function. Recent research by teams headed by Subimal Ghosh at the Indian Institute of Technology, Bombay, and Rajiv Kumar Chaturvedi at the Birla Institute of Technology and Science, Goa, reveals a surprising truth: In key forested regions of Northeast, Peninsular India and the Western Ghats, despite an increase in green cover, there has been a decrease in CO2 absorption over the past two decades (2001-2019).  

The scientists studied CO2 absorption by forests by looking at leaf area index (LAI), a measure of the total green cover of a region, and net primary productivity (NPP), the total CO2 absorbed by plants after accounting for respiration. The study, published in the journal Nature on December 3, 2023, says that India — the second highest contributor to the post-2000 global greening—has recorded an 18.51 per cent increase in lai during 2001-2019, but it “fails to translate into increased carbon uptake due to warming constraints”. 

“The studied forest areas recorded lai increase of 6.75 per cent between 2001 and 2019. Still, their npp decreased by 6.19 per cent in the last two decades,” says Ghosh, the lead author of the Assessment Report 6 Working Group I report published by the UN Intergovernmental Panel on Climate Change (IPCC) in 2021. 

The studied forests contribute to more than 31 per cent of the npp of India and are also warming hotspots, says the report. The findings only give a broad picture as they are based on satellite data alone. The next step should be to carry out ground observations. 

“While calculating LAI the species type should be taken into account — whether it is evergreen or deciduous and for how long there is seasonal shedding. The type of canopy (the uppermost layer of the foliage of leaves in a forest) has to be taken into account as well. Canopies with multiple layers are better at photosynthesis than single-canopy forests because of the increase in photosynthetic space,” says MD Subash Chandran, professor at the Centre for Ecological Sciences, Indian Institute of Science, Bengaluru. 

Such factors can only be known if extensive ground-level data is available for each type of forest at the canopy level. The research also does not take into account carbon fertilisation, the artificial enrichment of the atmosphere of greenhouses with carbon dioxide. “So, we do not know the direct impact of CO2 levels on forests, we only have an indication of the indirect impacts of warming on photosynthesis,” says Ghosh. “We however know, through other studies, that at extremely high levels of warming, photosynthesis would be severely affected despite an increase in CO2 levels.” 

Warming is a more crucial factor than rise in CO2 levels but in the case of rising temperatures, making generalisations as in the case of CO2 levels is not a good idea. The impact of warming has to be studied at an individual species level,” says K Sreejith, scientist at the Kerala Forest Research Institute at Thissur. 

Role reversal In recent years, the country has seen a series of short-term experiments with ground-level data to understand the impact of CO2 rise on forests and their vegetation. The studies are scattered but broadly hint at the adverse impact of the global warming on forests. One such study by the Indian Institute of Tropical Meteorology, Pune, found that the Kaziranga National Park in Assam is already a net carbon emitter, a situation that is set to worsen with further warming. 

A research paper published in Nature on August 23, 2023, states that the photosynthetic ability of tropical forest trees generally starts to fail when the leaf temperature reaches 46.7oC. It says that an average atmospheric temperature rise of 4oC could mean that tropical trees stop producing primary nutrition for themselves or other species. 

Sreejith’s team studied the impacts of rising temperatures on mangrove forests in Kerala and found them to be the most tolerant to increases in temperatures in any ecosystem across the world. They also identified six mangrove species that are drought-tolerant. Chaturvedi, in an ongoing study with the Indian Space Research Organisation (ISRO), is using specialised digital cameras called phenocams to study the impact of weather changes on flowering, fruiting and leafing patterns in the Netravali forest area of Goa. “Ground observation is more difficult, when compared to modelling, because it requires more financial and human resources,” says Chaturvedi. 

A research paper published in Trees, Forests and People in June 2022 studied the phenological responses of 28 tree species, 58 shrub species and 72 herb species across four semi-deciduous and evergreen forests at an altitude of 680-1860 m above sea level in Uttarakhand between 2018 and 2020. The study found that the higher temperature (0.49°C) in the winter period, more rainfall (by 48.5 mm) and greater relative humidity (9 per cent) in 2019 as compared to 2018 led to the early appearance of buds and leaves by two weeks.

During the winter of 2019-20 the scientists also observed a delay in the process of leaf senescence (process of deterioration with age resulting in the development of seeds and fruits) as compared to the winter of 2018-19. “This shift may have certain structural and functional implications in these forests that need to be investigated for a longer period to understand the impact of climate change,” says the study. 

The phenological changes are not limited to the plant world and happen in birds and animals as well. And the phenological changes in one species can trigger phenological changes in others, leading to a cascade and eventual collapse of ecosystems, especially when they belong to the same ecosystem and climatic changes are affecting these interdependent processes across the world. For example, “spring arrival times of 117 European migratory bird species over five decades show increasing levels of mismatch to other spring events, such as leaf out and insect flight, which are important for bird survival. 

Such mismatches are likely to have contributed to population decline in some migrant species, particularly those wintering in sub-Saharan Africa”, according to the “Status of Global Climate in 2022” report published by the World Meteorological Organization in April 2023. “Not all species in an ecosystem respond to the same climate influences or at the same rates,” says the report. 

Limited data

The combined impact of rising CO2 levels, increasing warming, changes in rainfall patterns, decreasing soil moisture and carbon fertilisation on forests can be studied properly only with the help of observational studies that are supported by satellite observations or vice versa. Such elaborative studies, which need to be done over long periods of time, are in formative stages in India. 

“One way is to look at vegetation change data from permanent sample plots with respect to increasing temperatures and elevated CO2 levels. These have to be at least 30-year-long datasets for the observations to be statistically valid. The longest dataset in India is 20 years,” says Sreejith. He is currently comparing this dataset with global datasets in collaboration with Oxford University and Leeds University to formulate a common platform shared across the world to build a better understanding. 

He has permanent plots to make these observations at 40 locations along the Western Ghats, but admits that these are not enough. Moreover, drawing correlations between the experimental data and the anecdotal experiences of people living near the forested areas is a challenging task. “Forests are extremely complex systems and therefore we cannot conclude that an impact is occurring due to warming and other changes just on the basis of experiments or observations, but that is the best we can do,” he says. 

Even ISRO is undertaking several long-term studies under its Geosphere-Biosphere Programme to understand the impact of climate change on 12 different forest types in the country. But these observations started only a few years ago. 

Similarly, the Indian Institute of Science, Bangalore rolled out its Long Term Ecological Observatories programme in 2015. In its first phase, the programme is monitoring a range of themes, one of which is forests, across six different regions of India — the Western Himalayas, Eastern Himalayas, Northwestern Arid Zone, Western Ghats, Central India and Andaman and Nicobar Islands.

This was first published in the State of India’s Environment 2024

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