Scientists find strange connection between lunar wobble and mangrove canopies

The Moon’s pull on our planet’s tides is just one of Earth’s many steady pulses of life.

Now researchers have found that lunar cycles not only synchronize the timing of animal migrations and mass coral spawning, but can also influence fluctuations in mangrove cover.

Given the growing interest in mangroves as natural carbon sinks, the study results could improve our understanding of how much carbon mangrove ecosystems are likely to absorb and absorb in the coming decades.

Understanding the seemingly odd connection between lunar cycles, tides and mangrove growth could also provide an early warning of their vulnerability to severe droughts.

Led by Macquarie University wetland ecologist Neil Saintilan, the research team measured mangrove canopy across the Australian continent using a new series of high-resolution satellite images taken between 1987 and 2020.

Digging through the data, the researchers found a surprising variation in mangrove cover.

Just as the moon pulls back and forth tides — tides that wash the roots of coastal mangroves — long-term fluctuations in the lunar orbit also affect the growth patterns of these salt-tolerant trees that populate gently sloping tidal flats, the study found.

This cycle, referred to as the “lunar wobble,” oscillates around every 18.61 years, moving in two distinct phases that are about 9.3 years long, low tide and high tide.

This wobble, the analysis shows, appears to be a dominant factor driving the expansion and contraction of mangrove cover over much of the Australian continent.

“Until now, we haven’t had the length of annual records on a reasonable scale to see these patterns,” Saintilan told ScienceAlert, referring to the data set used.

“When we plotted the annual canopy trend over time, we noticed a really interesting oscillation on a roughly 18-year cycle.”

Luckily, Saintilan had heard about the moon’s wobble after NASA scientists showed in 2021 how its next phase would lead to an increase in coastal flooding. He put two and two together, and the team discovered what appeared to be a strong connection between lunar cycles and mangrove canopies.

“When we looked in detail at the timing of the highs and lows of the lunar cycle, it aligned perfectly with the changes in mangrove cover – one of those ‘Eureka!’ Moments that you experience a few times in your career,” he says.

When the lunar wobble is at its minimum phase, the researchers hypothesized that mangrove ecosystems would be sucked dry, experience fewer days when their roots are saturated, and greater water stress, resulting in a thinner canopy.

At its maximum, the lunar wobble could drive tides higher and boost mangrove growth.

The synchronicity between the lunar wobble and the mangrove cover stood out against a backdrop of ongoing climate change, resulting in higher air temperatures, sea levels and CO₂ Plains are associated with mangrove expansion and canopy thickening.

That’s not all. The team discovered that these factors of lunar cycles and daily tides also interact with other climatic events such as the El Niño-Southern Oscillation, which affects alternating periods of heavy rainfall and prolonged drought in eastern Australia and the western coasts of North and South America.

For example, when around 40 million mangroves shriveled and died in northern Australia’s Gulf of Carpentaria in 2015 — the largest mangrove die-off in recorded history — an intense El Niño coincided with a moon-waggle minimum, the researchers found.

To some extent, this explains why the Gulf has suffered greater mangrove loss than neighboring regions, the researchers posit.

Not only did the extremely dry weather associated with the intense El Niño event cause the average sea level in the Gulf to drop by 40 centimeters (16 inches), but the low point in the lunar cycle pulled the tides even lower, meaning large areas of mangrove swamped by water thirsted.

The analysis showed that mangrove dieback was also most pronounced in tidal flats that lined coastlines and river channels, areas exposed to the full range of tidal extremes, reinforcing the researchers’ conclusions.

“Observations from the 2015 die-off suggest that the gray mangrove Avicennia Marina is particularly vulnerable to canopy decline when flooding is reduced,” write Saintilan and colleagues.

Given that A. Marina is the most widespread mangrove species in Australia, “this may explain the persistence of node-cycle influence on mangrove canopy across the continent,” they add.

Studies like these are important in elucidating the Earth’s natural rhythms, and this one in particular could stimulate future research into whether mangroves’ ability to absorb and store carbon in their waterlogged soils also changes with lunar cycles changes as the mangrove canopy is covered.

It will be important to understand this in detail as the world grapples with how to pull carbon dioxide out of the atmosphere to reverse global warming.

Protecting mighty mangroves is just one strategy available to us. But there is a limit to what these hardy, adaptable plants can tolerate as they are pushed landward by rising sea levels. So let’s do it quickly.

The study was published in scientific advances.