Digging Deep: How mangroves are affected by climate change

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Marine ecosystems harbour a wide range of species – both that live on the surface of the water body and those that thrive on the bottom of the ocean floor. Temperature and density play a key role in determining the kind of community that comes to inhabit the waterscape; and yet this relationship is not well-understood. A recent study in Nature, therefore, seeks to examine how mangroves may respond to global changes in seawater density due to climate change.

Mangroves are tropical forest ecosystems that occupy shallow, warm water in the intertidal region between land and sea. Mangrove ecology is quite unique: the dominant trees in a mangrove ecosystem are capable of excreting salt from their leaves, and can tolerate the high salt content of seawater. Another characteristic that mangroves are known-by is that of aerial roots. This is a system of extensively developed adventitious roots – i.e. formed from non-root tissue – that extend above the water. Aerial lend physical support to the tree.

But the trait that is uniquely characteristic of mangroves is the way their seeds/propagules germinate. The seeds actually germinate on the tree itself, and then fall of from the tree into the water, taking root only when encumbered by mud and sediment. This is very unique adaptation in the mangroves that helps them perpetuate in the harsh saline conditions. Scientifically, this condition is known as vivipary. What van der Stoken and his collaborators found was that the behaviour of these propagules – how far they disperse and where they get established – is affected by changes in sea surface temperature and salinity.

The premise behind this is fairly straightforward. The propagules float or sink depending on whether they are denser than the water beneath them or not. In fact, this has been known for more than a hundred years, thanks to the British botanist Henry Brougham Guppy’s surveys in the Pacific in the late nineteenth century. In his seminal Observations of a Naturalist in the Pacific Between 1896 and 1899 (Macmillan 1907), which carry a detailed account of his experiments on seed buoyancy, he notes that the ‘non-buoyancy in the great majority of plants has had a far-reaching influence not only on plant-distribution, but on plant-development.’

In the present study, van der Stoken et al. (2022) employ the Bio-ORACLE – a database on marine ecosystems – to compare ‘global- and species-level data on the distribution of mangrove forests with sea surface properties. Both present (2000-14) and projected future (2090-2100) parameters (temperature, salinity, density) were considered. The study predicts that towards the turn of the century, coastal mangrove waters may witness a decrease in sea surface density, and that this will be more pronounced in the West Pacific than in East Pacific. This will, as the aforementioned logic goes, influence propagule buoyancy and their floating orientation. An expected change is the increase in sinking rate, which would mean that seeds/propagules will not be able to travel long distances to take root and establish themselves, sinking much before reaching a ‘suitable establishment zone.’ This, the study argues, will have cascade effects as the forest structure of a place – influenced by seed dispersal – will inevitably impact far-reaching impacts on biogeography and ecology.

‘Mangrove propagule density is somewhere between that seawater and freshwater. Increasing surface temperature and reduced density can lower the floatation time, inhibiting the long-distance distribution of the mangrove propagules. Sensitivity to sea surface density varies between different mangrove species. For example, the propagules of Rhizophoraceae and Avicennia may be more affected by such changes as their densities are closer to that of seawater,’ Dr Ajay Kumar, a mangrove-ecologist based at the Central University of Kerala, clarified in an email communication with The Indian Express. Not only that, temperature and salinity (apart from density) will also exert their own pressures on seed dispersal. ‘Decreasing seawater salinity can increase the risk of fungal attacks on the buoyant propagules, further posing a threat to mangrove cover,’ Dr Kumar highlighted.

Van der Stoken and co-workers assert that these findings are important not just from the perspective of mangroves, but most plant communities that rely on water-based seed dispersal mechanisms, such as seagrasses and coastal strand communities. Rising temperatures, for instance, could push mangrove distribution towards higher latitudes, and could even provide more impetus to seed germination. It remains to be seen whether plants will be able to devise strategies for seed dispersal that overcome changing climatic conditions – a veritable question for future studies.

The author is a research fellow at the Indian Institute of Science (IISc), Bengaluru, and a freelance science communicator. He tweets at @critvik. 

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