The shiny leaves and branching roots of mangroves are downright eye-catching, and now a study finds that the moon plays a special role in the vigor of these trees.
Long-term tidal cycles set in motion by the moon driving much of the expansion and contraction of Australia’s mangrove forests, researchers report in the September 16 issue Progress of science. This discovery is key to predicting when mangroves, which are good at sequestering carbon and could help combat climate changeare more likely to proliferateSerial number: 11/18/21). Such knowledge could inform efforts to protect and restore forests.
Mangroves are coastal trees that provide habitat for fish and buffer erosion (Serial number: 09/14/22). But in some places, forests face a variety of threats, including coastal development, pollution, and land clearing for agriculture. To get a bird’s-eye view of these forests, Neil Saintilan, an environmental scientist at Macquarie University in Sydney, and his colleagues turned to satellite imagery. Using Landsat data from the US Geological Survey and NASA from 1987 to 2020, the researchers calculated how the size and density of mangrove forests in Australia changed over time.
After accounting for persistent increases in the growth of these trees, likely due to rising carbon dioxide levels, rising sea levels, and rising air temperatures, Saintilan and colleagues noticed a curious pattern. . Mangrove forests tended to expand and contract in both extent and canopy cover in predictable ways. “I saw this 18-year oscillation,” says Saintilan.
That regularity got researchers thinking about the moon. Earth’s closest celestial neighbor has it has long been known to help drive the tides, which deliver water and necessary nutrients to the mangroves. A rhythm called the lunar nodal cycle could explain the mangroves’ growth pattern, the team hypothesized.
Over the course of 18.6 years, the plane of the moon’s orbit around Earth slowly tilts. When the moon’s orbit is less tilted relative to our planet’s equator, semidiurnal tides, which consist of two high and two low tides each day, tend to have a larger range. That 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 pulls gravitationally on the Earth.
Saintilan and colleagues found that mangrove forests experiencing semi-diurnal tides tend to be largest and densest precisely when highest high tides are 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 mangrove-bearing regions, such as Vietnam and Indonesia, are likely to experience the same long-term trends, the team suggests.
Having access to data going back decades was key to this discovery, Saintilan says. “We’ve never really picked up on some of these long-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, California, who was not involved in the research.
Scientists now know when some mangroves are likely to bloom and should go the extra mile at those times to promote the growth of these carbon-sequestering trees, says Aburto-Oropeza. That might seem like additional limitations on nearby human activity that could harm forests, she says. “We should be more proactive.”