The glossy leaves and branching roots of mangroves are downright eye-catching, and now a study reveals that the moon plays a special role in the vigor of these trees.
Long term tidal cycles set in motion by the moon drive, in large part, the expansion and contraction of Australia’s mangrove forests, researchers report in the September 16 Scientists progress. This finding is key to predicting when stands of mangroves, which are good for carbon sequestration and could help fight climate changeare most likely to proliferate (SN: 11/18/21). Such knowledge could inform forest protection and restoration efforts.
Mangroves are coastal trees that provide fish habitat and a buffer against erosion (SN: 09/14/22). But in some places, forests face a range of threats, including coastal development, pollution and land clearing for agriculture. To get a big picture of these forests, Neil Saintilan, an environmental scientist at Macquarie University in Sydney, and his colleagues turned to satellite imagery. Using Landsat data from NASA and the US Geological Survey from 1987 to 2020, the researchers calculated how the size and density of mangrove forests across Australia have changed over time.
After taking into account the persistent increases in the growth of these trees – likely due to rising carbon dioxide levels, rising sea levels and increasing air temperatures – Saintilan and his colleagues noticed a curious trend. Mangrove forests have tended to expand and contract in both extent and canopy cover in predictable ways. “I saw this 18-year swing,” Saintilan says.
This regularity has led researchers to think of the moon. Earth’s closest celestial neighbor has long known to help drive the tides, which provide water and nutrients needed by mangroves. A rhythm called the lunar nodal cycle could explain the growth pattern of mangroves, the team theorized.
Over the course of 18.6 years, the plane of the moon’s orbit around the Earth slowly tilts. When the moon’s orbit is least inclined to our planet’s equator, semidiurnal tides – which consist of two high tides and two low tides each day – tend to have greater amplitude. This means that in areas that experience semidiurnal tides, higher high tides and lower low tides are generally more likely. The effect is caused by the angle at which the moon gravitationally pulls on Earth.
Saintilan and his colleagues found that mangrove forests experiencing semidiurnal tides tended to be taller and denser precisely when higher high tides were expected based on the moon’s orbit. The effect even seemed to outweigh other climatic drivers of mangrove growth, such as El Niño conditions. Other regions with mangroves, such as Vietnam and Indonesia, are likely experiencing the same long-term trends, the team suggests.
Access to data going back decades was key to this discovery, Saintilan says. “We never really grasped some of these longer-term drivers of vegetation dynamics before.”
It’s important to recognize this effect on mangrove populations, says Octavio Aburto-Oropeza, a marine ecologist at the Scripps Institution of Oceanography in La Jolla, Calif., who was not involved in the research.
Scientists now know when certain mangroves are particularly likely to thrive and should make an extra effort at those times to promote the growth of these carbon-sequestering trees, says Aburto-Oropeza. It could look like additional limitations on nearby human activity that could harm forests, he says. “We should be more proactive.”