Moon’s ‘wobbles’ mysteriously linked to mass mangrove tree deaths in Australia, study finds

Mangrove deaths co-influenced by other factors such as El Niño events, scientists say

A lunar cycle that happens once in two decades plays a “significant role” in the expansion and contraction of mangrove forests across Australia, a new study finds.

The research, published on Wednesday in the journal Science Advances, that a Moon orbital cycle that happens once every 18.61-years, called the “lunar wobble,” regulate the maximum tide heights along coastlines, and affect the mangrove canopy cover in the continent.

Scientists, including Neil Saintilan from Macquarie University in Australia, say the findings can help improve the understanding of how mangrove forests impact the rate of atmospheric carbon storage over decades.

Studies have shown that mangroves are an important source of coastal flood control and are also an important sink that captures planet-warming carbon dioxide emissions.

Previous research has also shown that long-term lunar cycles regulate the maximum tide heights along coastlines and some have found that mangroves flourish under higher tide heights.

In the new study, researchers analysed mangrove canopy cover drawn from satellite images across Australia taken between 1987 and 2020.

They found that the mangrove extent in Australia is controlled in part by whether the lunar wobble is at its maximum or minimum.

The study found that the canopy cover is determined by whether a coast experiences one of two tide types: diurnal – one high and low tide every lunar day, or semidiurnal – two high and low tides in that period.

These features, scientists say, also interact with other factors such as El Niño events, which are climate patterns linked to the unusual warming of surface waters in the eastern Pacific Ocean.

Citing an example, they said the death of mangroves at one Australian gulf was around 50 per cent in 2015 when an intense El Niño coincided with a lunar wobble minimum.

“Observations from the 2015 dieback event suggest that the gray mangrove Avicennia marina is particularly susceptible to canopy decline under reduced inundation,” scientists wrote in the study.

“A. marina is the most widely distributed species in Australia, dominating subtropical and temperate mangroves, and this may explain the consistency of the nodal cycle influence on mangrove canopy cover across the continent,” they added.

Researchers say the lunar wobble’s influence on mangroves could be comparable in Central America, Vietnam, Thailand, the Philippines, and Indonesia.

These regions, together with Australia, host more than half of the world’s remaining mangroves, they said.

“Long-phase tidal cycles regulate maximum tide heights, are an important control over mangrove canopy cover, and may influence mangrove ecosystem services including forest productivity and carbon sequestration at regional scales,” scientists concluded in the study.