The Moon’s gravitational pull on our planet’s tides is just one of Earth’s many regular lives.
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 canopy cover.
Given the growing interest in mangroves as natural carbon sinks, the study findings could improve our understanding of how much carbon mangrove ecosystems are likely to draw down and sequester in the coming decades.
Understanding the seemingly strange connection between lunar cycles, tides and mangrove growth could also provide advanced warning about their vulnerability to severe drought.
Led by wetland ecologist Neil Saintilan from Macquarie University, the team of researchers measured mangrove canopies across the Australian continent using a new set of high-resolution satellite images taken between 1987 and 2020.
Digging into the data, the researchers found a surprising fluctuation in mangrove canopy cover.
Just as the Moon pulls tides back and forth—tides that strike the roots of coastal mangroves—long-term fluctuations in the moon’s orbit also affect the growth patterns of these salt-tolerant trees that occupy gently sloping mudflats, the study found.
This cycle, called the “lunar oscillation”, swings around every 18.61 years, pulling low tide lower and pushing high tide higher in two distinct phases, about 9.3 years long.
This wobble, the analysis reveals, appears to be a dominant factor controlling the expansion and contraction of mangrove canopy cover across much of the Australian continent.
“Until now, we haven’t had the length of annual records at an appropriate scale to see these patterns,” Saintilan told ScienceAlert, referring to the data set used.
“When we plotted the trend in annual canopy cover over time, that’s when we noticed a very interesting fluctuation on an approximate 18-year cycle.”
Fortunately, Saintilan had heard about the lunar swing after NASA scientists showed in 2021 how the next phase was set to cause an increase in coastal flooding. Putting two and two together, the team uncovered what appeared to be a strong connection between lunar cycles and mangrove canopies.
“When we looked in detail at the timing of the peaks and troughs of the lunar cycle, it matched perfectly with changes in the mangrove canopy cover – one of those ‘Eureka!’ moments you get a few times in your career,” he says.
When the lunar cycle is in its minimum phase, the researchers realized that mangrove ecosystems would be sucked dry, experiencing fewer days where their roots were saturated and greater water stress, leading to thinner canopies.
At its maximum, the lunar oscillation can make the tide higher, increasing mangrove growth.
The synchronicity between the lunar oscillation and the mangrove canopy stood out against a backdrop of steady climate change, whereby higher air temperatures, sea levels and CO22 levels 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 oceanic 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 about 40 million mangroves shriveled and died in the Gulf of Carpentaria in northern Australia in 2015—the biggest mangrove decline in recorded history—an intense El Niño coincided with a minimum of the moon’s wobble, the researchers found.
This goes some way to explaining why the Gulf suffered greater mangrove loss than adjacent regions, the researchers claim.
Not only was extremely dry weather from the intense El Niño event such that the average sea level in the Gulf plunged 40 centimeters (16 in), the trough in the lunar cycle drew the tides even lower, meaning large areas of mangroves were thirsty for water.
The analysis showed that mangrove decline was also most pronounced in mud flats bordering coastlines and river channels, areas exposed to the full range of tidal extremes, adding weight to the researchers’ conclusions.
“Observations from the 2015 die-off suggest that the gray mangrove Avicennia marina are particularly susceptible to canopy decline during reduced inundation,” Saintilan and colleagues write.
Provided that A. marina is the most widespread mangrove species in Australia, “this may explain the consistency of the nodal cycle’s influence on mangrove canopy cover across the continent,” they add.
Studies like this are important for testing out the Earth’s natural rhythms, and this one in particular can lead to future research that looks at whether the mangroves’ ability to absorb and store carbon in their soaked soils also changes with the lunar cycles that the mangroves cover. do.
It will be important to understand 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 at your fingertips. But there is a limit to what these resilient, adaptable plants can withstand, as they are pushed landward by rising sea levels. So we better move fast.
The research was published in The progress of science.