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Mangroves exhibit remarkable tolerance to extreme heat compared to other species worldwide, making them less vulnerable to global warming, a new study has revealed.
Researchers from India studied 13 mangrove species from Kerala and found that the estimated maximum leaf temperatures were higher than the surrounding air temperatures. However, these leaf temperatures remained well below the species’ heat tolerance limits.
“This indicates that these mangroves maintain large thermal safety margins, which may allow them to deal with future warming. This capability is crucial in ensuring that their photosynthetic processes remain functional despite the increased heat, safeguarding the ecosystems they support,” said Abdulla Naseef, lead author of the research.
This is the first attempt to analyse the thermotolerance of mangroves in India.
Mangroves play a crucial role in providing ecosystem services such as coastal protection, carbon storage, livelihoods for local communities and enhancing biodiversity.
However, with climate change leading to more frequent and extreme temperature events, the survival of mangrove species may be at risk, the study noted.
The study, in press in the journal Science of The Total Environment earlier this month, was conducted by the Kerala Forest Research Institute (KFRI) in collaboration with the Indian Institute of Science Education and Research (IISER), Pune. It focused on mangrove species along the southwest coast of India and their ability to withstand rising temperatures.
The study examined 13 species, including the river mangrove (Aegiceras corniculatum), white mangrove (Avicennia marina), Indian mangrove (Avicennia officinalis), oriental mangrove (Bruguiera cylindrica), Burma mangrove (Bruguiera gymnorhiza), upriver orange mangrove (Bruguiera sexangula) and spurred mangrove (Ceriops tagal).
Other species studied included narrow-leaved Kandelia (Kandelia candel), black mangrove (Luminitzera racemosa), red mangrove (Rhizophora apiculata), Asiatic mangrove (Rhizophora mucronata), crabapple mangrove (Sonneratia alba) and mangrove apple (Sonneratia caseolaris).
The researchers quantified heat tolerance by measuring T50 values — temperatures at which photosynthetic function declines by 50 per cent — and thermal safety margins (TSM).
The T50 is the threshold where photosynthesis function decreases by 50 per cent, leading to irreversible damage and eventual leaf death.
The difference between T50 and maximum leaf temperatures defines the TSM. Deepak Barua, associate professor at IISER, Pune and co-author of the study, explained that leaf temperatures can be 10°C or more above air temperatures for some species.
The TSM helps assess vulnerability to future warming.
T50 values were lowest in White mangrove and highest in Upriver orange mangrove, ranging from 48.9 degrees Celsius to 55.32°C, the study noted.
The mean T50 for the 13 species examined was 53.3°C, with a range from 48.9°C to 55.3°C. The researchers found that species with broader leaves, such as Burma mangrove and crabapple mangrove, exhibited higher heat tolerance as they experienced greater leaf temperatures.
They also observed that the median heat tolerance was around 5°C higher than that of woody species from tropical, Mediterranean and temperate regions.
Our finding also suggests that leaf size plays a critical role in determining a species’ ability to cope with heat stress, although other leaf traits did not show a clear link to heat tolerance. These findings indicate that species with larger leaves might have an adaptive advantage in a warming climate
Sreejith Ashtamoorthy, principal scientist at KFRI
The study noted that maximum leaf temperatures for the species were significantly higher than the maximum air temperature of 35.4°C recorded at the site during the study period.
“Maximum leaf temperatures differed across species and ranged from 38.5°C in black mangrove to 41.4 °C in Asiatic mangrove,” it observed.
This high heat tolerance and the large TSMs suggest that mangroves in this region are likely well-equipped to withstand high temperatures, improving their chances of survival under future climate conditions, Barua said.
Ashtamoorthy noted the species studied are found globally and the findings of exceptional heat tolerance and TSMs have worldwide significance when considering mangrove species for ecosystem restoration.
“However, the outcomes mentioned are based on a single but significant factor, temperature alone; a combined effect of other abiotic stressors, such as high salinity, light, etc, may also need to be analysed,” he added.
