Climate oscillations are a constant threat to mangroves

A study shows for the first time how temperature changes in the Pacific Ocean impact mangroves as they follow a specific pattern. In degraded areas, their ecosystem services are at risk

by Matteo Cavallito

 

Large-scale climate phenomena such as El Niño and La Niña significantly impact mangroves globally. This allegedly emerge clearly from decades-long analysis of satellite data raising concern about the resilience of these plants and the maintenance of their crucial role in protecting coastal ecosystems. The findings are suggested by a study conducted by Tulane University in New Orleans and published in Nature Geoscience.

The Oscillation and its effects

The investigation focused on the effects of the El Niño and La Niña, which constitute the so-called Southern Oscillation or ENSO that occurs on average once every five years between December and January. In summary: During the phase known as El Niño, the waters of the eastern Pacific warm up; during La Niña, they cool down. This effect alters precipitation, storms and temperatures across the Planet. Thus causing floods, droughts and changes in hurricane activity.

But what are the effects on mangroves? In the past, a statement from the researchers says, impacts on these plants had been documented as individual events.

These included the dramatic mangrove mortality that occurred in 2015 in northern Australia, with more than 40 million affected in a coastal stretch of nearly 2,000 kilometers. “We wanted to know whether these events were isolated or part of a broader pattern,” explained Zhen Zhang, a researcher at the Tulane School of Science and Engineering and co-author of the research. “Our findings confirm that ENSO has large-scale, recurring effects on mangrove ecosystems around the world.”

Impacts follow a global pattern

Using satellite information on Leaf Area Index in combination with ocean and climate data to assess the health of mangroves from 2001 to 2020, the study – which also involved Xiamen University and the National University of Singapore – identified for the first time a pattern. Describing, in other words, how Oscillation affects the growth or degradation of these plants globally.

In the research, “we show that more than 50% of global mangrove areas experience significant variations during ENSO events,” the authors explain.

Prominent, the study notes, is “a seesaw-like pattern across the Pacific Basin where mangrove leaf area decreases in the western Pacific but increases in the eastern Pacific during El Niño. With the reverse occurring during La Niña.” The so-called Indian Ocean Dipole, or the Indian Ocean surface temperature oscillation, affects the Indian Ocean in a similar way but with less evident effects. “These patterns,” the authors point out, “are driven by corresponding sea-level fluctuations across the Pacific and Indian ocean basins, with local contributions from lunar nodal cycles.”

Mangrove ecosystem services at risk

The results, in short, show how oscillation affects on a large scale these ecosystems. Which are already threatened by other factors such as urbanization and climate change. In this framework, then, the survival of these plants is confirmed as essential, especially given the services they provide. Such as, notoriously, carbon sequestration, biodiversity conservation, and tackling erosion.

The U.S. organization One Earth estimates that between 1980 and 2000, the world lost about 35 percent of these trees.

Although the situation has subsequently improved, the problem seems not to have been solved. The latest report by the NGO Global Mangrove Alliance, in particular, notes that in the first two decades of the 21st century, the Planet experienced a net loss (the balance between loss and regrowth) of more than 2,800 km2 of mangrove forests. Major factors involved include conversion of areas for aquaculture, palm oil production and rice cultivation.