Mangroves are underappreciated. Around 100 species of these trees and shrubs blanket tropical estuaries and tidal zones across the globe. Their partially exposed, stilt-like roots extend deep into waterlogged soil, sequestering carbon and protecting coastlines against storm surges. They would seem like the hardiest of plants, but new research shows their tough living conditions may make it hard for mangroves to adapt to climate change.
Researchers from Sun Yat-sen University in Guangzhou, China, and several other universities, recently looked at the genetics of six mangrove species taken from two dozen populations around Southeast Asia. They found that diversity was so low within each species that individual trees are basically indistinguishable from each other at the genetic level. This is a major disadvantage when it comes to adaptation, and could spell disaster for populations unable to adjust to climate change and sea level rise.
Published in the journal Global Change Biology, the study found that while tropical mangrove forests “appear to be vibrant at present...optimism about the resilience of these ecosystems is premature.”
Chung-I Wu, an evolutionary biologist at the University of Chicago and coauthor of the paper, told Earther that DNA sequences like the ones they looked at are an organism’s “living fossils.”
“It is prudent to look at the records of DNA sequences and suggest that mangroves could suffer under the joint onslaught of sea level rises and human encroachment of their retreats upland,” he said.
Mangroves have survived for hundreds of thousands of years in harsh, saltwater conditions that many other trees and shrubs couldn’t tolerate. While they have adapted to withstand regular changes in salinity and temperature, most mangroves died at the end of the Last Glacial Maximum 20,000 years ago, when sea levels rose particularly quickly. According to the new study, these six species haven’t yet had enough time to recover from this genetic bottleneck.
Mangrove forests are critical natural barriers against floods and storm surges, and could help prevent a lot of costly projected damage from coastal flooding as sea levels rise. They are also unique ecosystems, home to a wide range of species found nowhere else.
In an effort to glimpse how this lack of genetic diversity might already be affecting mangroves, the researchers observed how the trees responded after a series of major floods hit China’s Hainan Island in the South China Sea. They found that when seawater rose by a meter, up to 95 percent of some of the island’s mangrove species were killed.
“Significantly, less genetically diverse mangrove species suffered much greater destruction,” write the authors. “The dieback was accompanied by a drastic reduction in local invertebrate biodiversity.”
The study is, of course, limited in scope, having looked at just six species and sea level rise in one area, so we should be cautious extrapolating everywhere.
According to the study, mangroves can adapt to sea level rise either by building up local sediment or migrating inland along river deltas. However, if sea levels rise too quickly, these defenses might not hold up. These retreats also require the existence of inland habitats that have been spared extensive human development, which are becoming increasingly rare.
“Coastal wetlands are being rapidly reclaimed to construct industrial
zones, ports, and other infrastructure,” states the study. “Seawalls
cover up to 60% of the total coastline length in mainland China.”
Another study in 2015 found that sea level rise could submerge many mangrove forests within the 50 years, especially in the Indo-Pacific region, home to most of the world’s mangrove forests. Activities such as dam destruction are cutting off crucial sediment accumulation in mangrove habitats, making it harder for the trees to survive sea level rise.
“On highly developed coasts, one can see resorts built next to mangrove forests,” said Wu. “Ironically, these resorts are built for human’s enjoyment of being next to the mangroves.”
One obvious route to helping mangroves survive would be to stop developing them, for business or for pleasure.