Behind the fringe, mangroves hide surprising diversity
To any human eye, the mangrove fringes growing along shores and coastlines throughout tropical regions of the world all look alike. But the dense thickets of important coastal species like the red mangrove (Rhizophora mangle) harbor a surprising reservoir of distinct genetic diversity, according to new research led by a Smithsonian Marine Station scientist.
The findings could drive important questions about how mangroves are studied, and shift approaches for mangrove forest restoration.
One long-held scientific assumption about mangrove genetic diversity is that the trees of one forest are mostly related to one another, like a large collection of cousins at a family reunion. Mangrove forests typically feature a handful of older, larger individuals surrounded by many younger offspring. Self-fertile and self-recruiting, mangroves reproduce via propagules, pod-like embryonic trees that grow from parent plants. Once they drop to the ground or water below, propagules must rely on natural forces—wind and water—to transport them to a suitable place to take root.
Life is tough for a young mangrove. They compete for light and nutrients within a dense mangrove forest, have limited opportunities for dispersal, and are a favorite food of crabs. For these reasons and more, it had long been assumed that any propagules that do take root in a given area are likely to be from local parent trees and are potentially pre-adapted to local conditions.
In a new paper examining the genetic diversity of several hundred red mangrove forests in Honduras, the common wisdom did hold true—right along the seashore. There, trees were indeed mostly related to each other.
But outsiders lurked deeper in the forest, where clusters of a distinct “other” genetic group grew.
After conducting in-depth comparative genetic analysis of 269 trees across four sites, the lead author on the paper, Smithsonian Marine Station research marine biologist Steve Canty, was surprised to see a patchwork of distinctly different genetic groups occupying the interior of the forest. This pointed to the possibility of events in the past when propagules from other populations had been able to make their way past the barrier of the seaward fringe and establish their own small family groups.
“Rather than these giant family bubbles, we were seeing that the family circles are a lot smaller,” said Canty, who coordinates the Smithsonian Marine Conservation Program. “We weren’t expecting to see so much difference at just a 10-meter scale.”
“It’s a really clear demonstration that what you see along the coast is not the whole of the forest,” said Jennifer Rowntree, an associate professor of ecological genetics at the University of Plymouth and a co-author on the study. “We’re probably greatly underestimating genetic diversity.”
Initial drivers of genetic variation at and behind the mangrove fringe edge may be linked to the physical forces acting on mangroves’ coastal habitat: tides, ocean currents and tropical storms.
Tides and currents behave mostly predictably. They carry propagules along coastlines or across channels, where they would most easily take root in seaward locations. This would account for the relative homogeneity of the seaward fringe samples Canty observed in the analysis.
High-energy storms, on the other hand, can carry material from much further away, and push seedlings much further inland than they would otherwise float on their own. Between 1852 and 2016, a total of 52 tropical storms and hurricanes passed within 50 nautical miles of the study’s primary sampling site, on Fort Cay in Honduras’ Bay Islands. The hardest recent hit came from Hurricane Mitch in 1998, which severely damaged the fringe. Canty speculated that storm damage of the sea-facing mangrove fringe and interior canopies could have created opportunities for propagules from other areas to take root.
“Currents and winds are moving away what was there and allows a whole new load of mangrove material to get dumped in. It changes the whole seascape of the understory,” Canty said.
A Different Sampling Approach
The study’s finding has important impacts for basic scientific research methods on mangrove forests, as well as future work in mangrove habitat conservation and restoration.
To start, sampling methods may need to change.
Biological fieldwork is rarely easy, but working in mangrove forests involves extraordinary challenges. To sample mangroves, scientists like Canty compete with dangling branches, twisting trunks and haphazard prop roots, usually in waterlogged to outright flooded conditions – not to mention the mosquitos.
Hence, typical scientific surveys of mangroves forests are fairly straightforward: researchers collect study material from easier-to-reach trees, usually along a line at the waters’ edge. Sampling only from the seaward fringe could exclude a distinct reservoir of genetic diversity for scientific study.
The same can probably be said for the collection of propagules for restoration and conservation plantings: propagules are likely gathered from the seaward fringe simply because they are accessible. In the context of replanting schemes, this could inadvertently create genetic bottlenecks.
“The fringe area is a fish nursery ground, and it’s important to understand how that works,” Canty said. “But now we see there’s a bigger piece behind the fringe that becomes more important, that 50- to 100-meter buffer area for hurricanes and storm damage. Understanding what goes on behind the fringe is critical.”
“Tough” is often used to describe mangroves; after all, they thrive in some of Earth’s harshest conditions. They withstand inundation by salt water, low-oxygen soils, exposure to relentless wave and tidal forces, and the heat of the tropics. Canty suggested a slightly different view: these adaptations make mangroves fine-tuned.
“These are the Ferraris or Teslas of plants,” Canty said. “They live in unbelievably dynamic situations, and are hyper-tuned for that environment. You can’t just grab any old one, randomly plant it and hope it grows well there. You wouldn’t do that to a rose. Why would it work for a mangrove? Mangroves are highly specialized and consideration needs to be given to how and where you plant.”
From a conservation standpoint, this means that each tree could be adapted to its particular location in the forest. Propagules should be collected from a greater variety of sites within a mangrove fringe, Canty suggested, and replanted in a similar scheme. A genetic component may be at play that determines the ability of seaward trees to thrive so close to the water, and a different suite of genetic adaptations for trees growing more inland.
“If you’re collecting from 10 trees each from interior and exterior, you have propagules that are likely pre-adapted to that environment,” Canty said. “Trees from shorelines should be used on shorelines; trees from 10 meters in are adapted to conditions further back. Then you have mangroves that you know are likely to survive in these areas.”
A better understanding of the genetic drivers of why a particular tree thrives in one place could also inform replanting efforts, or landscape restoration to permit mangroves to re-colonize an area naturally.
“Natural restoration is probably the best way to get a stable forest, but we don’t always have time to wait for that,” Rowntree added. “Genetic diversity is important to consider when thinking about conservation and restoration, because you cannot predict what’s going to happen within a particular forest, or how communities are going to respond to various stresses. To give them the best chance of survival, including redundancy and diversity seems to be a good approach.”
--Michelle Z. Donahue, March 2022