Scientists find surprising genetic differences between Brazil’s mangroves

  • Mangrove forests occupy tropical costal areas and provide important habitat for wildlife, as well as ecosystem services for human communities. They’re also carbon storage powerhouses, pound-for-pound capable of sequestering four times more carbon than a rainforest.
  • Researchers analyzed the genes of mangrove forests along the coast of Brazil. They found that trees in different forests show “dramatic” differences from one another, even when they belong to the same species.
  • They think these differences arose because an ocean current separates mangroves in northern and southern Brazil, making it so they can’t exchange genes.
  • The researchers suspect the genetic distinctiveness of mangrove populations extends beyond Brazil. They say their results highlight the importance of enacting conservation plans that give a higher priority to the preservation of genetic diversity – an endeavor they say is becoming more and more critical for mangroves as they continue to disappear and climate change ramps up.

Hugging tropical coastlines with masses of long, tangled branches and roots that stick up out of the mud at low tide, mangrove forests all have a similar look at first glance. But dig a little deeper into their genes and surprising differences pop up.

When researchers from institutions in Brazil did just that, they discovered that mangrove trees along the same coastline and of the same species were genetically distinct from each other. They think these differences likely arose because oceanic currents act as a barrier to mangrove tree seeds, effectively separating the two populations. In addition to genetic differences, some mangroves have physiological adaptations that make them better suited to their specific environments.

The researchers say their results, published in the journal Ecology and Evolution earlier this year, highlight the importance of enacting conservation plans that give a higher priority to the preservation of genetic diversity – an endeavor they say is becoming more and more critical for mangroves as they continue to disappear.

Compared to other kinds of forests, mangroves don’t cover a very large area, restricted to slim margins between land and sea in tropical areas of the world. But they provide crucial habitat for coastal wildlife, as well as important ecosystem services for human communities like flood control and maintenance of fishery stocks. Mangroves are also carbon storage powerhouses, pound-for-pound sequestering up to four times the carbon that rainforests can.

Black mangrove (Avicennia) seedlings growing among aerial mangrove roots in northern Brazil. Photo by Mariana Vargas Cruz

But mangroves aren’t faring well in the face of human expansion. Scientists estimate the world lost between 30 and 50 percent of its mangroves in the past half-century, replaced by shrimp farms, rice fields and oil palm plantations, and starved of freshwater by upstream dams. In the future, scientists worry many of those that survive the onslaught from land will drown in the sea as oceans rise due to global warming.

And when mangroves die, the carbon they’ve been storing in their vegetation and soil is released. A lot of it. Scientists estimate that the mangrove deforestation that happened between 2000 and 2012 alone released around 317 million metric tons of CO2 per year – more than the annual emissions of Poland.

In response to the decline of the world’s mangroves, conservationists have undertaken reforestation efforts to plant new mangroves in the hopes of re-establishing them. For instance, a massive reforestation project in Senegal reportedly resulted in the planting of nearly 80 million mangrove trees; more recently, Pakistan set a world record for the most trees planted in one day by planting more than 1 million mangroves (the word is used interchangeably to describe both individual trees and forests) in 24 hours.

But mangrove reforestation will only work if the trees are able to grow and flourish where they’re planted. And new research out of Brazil cautions that this may not be so simple, even though there are relatively few mangrove tree species compared to other types of forest.

In one of the first thorough genetic analysis of mangroves ever conducted, researchers at universities in Brazil investigated the relatedness of different populations of two species of mangroves along the country’s coastline. They discovered some big differences between some of these populations, even when they belonged to the same species.

Mariana Vargas Cruz and Stephany Karenina Bajay sampling black mangrove (Avicennia schaueriana) leaves in Northern Brazil. Photo by Mariana Vargas Cruz & Gustavo Maruyama Mori

Perhaps most notably, they found mangroves sampled in northern Brazil were genetically different than those along the southern coast. This, the researchers say, is likely because of a current that comes from Africa and hits Brazil at its midsection, where it separates into two currents: one that travels north up the coast, and another that flows south.

Mangroves spread by dropping their seeds into the ocean where they’re ferried to other shores. The researchers think that the current effectively made it so that seeds from northern populations stayed in the north and seeds from southern populations stayed in the south. With little to no genetic exchange between the two, they gradually evolved to be increasingly distinct.

“We found that the difference between the same mangrove species that occur in the North and the South is genetically dramatic,” said Anete Pereira de Souza, with the State University of Campinas and coauthor of the study.

Stilted mangrove (Rhizophora stylosa) seed floating in the South China Sea, off Iriomote Island, Japan. Photo by Gustavo Maruyama Mori

While some of these tree populations don’t appear distinct from one another, others seem to be adapted quite differently to their respective environments. For instance, the researchers found that mangroves in the Amazon River delta in northern Brazil do well in the region’s warm weather and abundant sunshine. Meanwhile, mangroves in southern Brazil are more suited to lower temperatures and more cloud cover.

“This means that mangrove reforestation projects in the North should not be done with seedlings brought in from the South, and vice versa. The seedlings would die,” Souza said.

Study coauthor Gustavo Maruyama Mori of the Institute of Biosciences at the State University of São Paul says these differences aren’t quite enough to redefine species at this point, but say that their findings “provide an important piece to the puzzle” of mangrove taxonomy.

Mori believes genetic differences in mangrove populations aren’t unique to Brazil, and that mangrove forests in other parts of the world may be more unique than previously assumed.

“There is evidence of the role of these currents shaping the population organization of mangrove trees in different parts of the world (for instance Brazil, US, Mexico, Cameroon and Southeast Asia) although sometimes different species respond differently,” Mori said.

Black magnrove (Avicennia germinans) growing in a human-impacted, high-salinity field in northern Brazil. Photo by Mariana Vargas Cruz & Gustavo Maruyama Mori

Gustavo told Mongabay that more attention be paid to ensuring mangroves maintain their genetic diversity. Without it, he says they will be less likely to successfully adapt to a changing environment.

But as mangrove populations continue to disappear, so do their unique assemblages of genes. Gustavo urges governments and conservation groups to give greater priority to preserving genetic diversity when deciding where to put protected areas, and that climate change is increasing the importance of doing so.

“As [genetic distinction within species] exists in many parts of the globe and different populations may respond differently to environmental pressures as [the] global climate changes, conservation units, Ramsar and UNESCO World Heritage site selection and restoration practices could have better results if they considered genetic variation a priori,” he said.

The researchers say that the methods and findings of their study will help scientists gain a better understanding the genetics of mangroves around the world. For Mori, he hopes to learn more about how mangroves will respond to climate change by figuring out which genes allow populations of the same species to inhabit different environments and how much of a role environment-influenced gene expression has in facilitating this.

And he remains hopeful that the world’s mangroves can be saved.

“I am aware of the many scientific and practical challenges involved, mainly in developing countries as Brazil,” Mori said, “but as technology progresses and the gap between conservation/restoration practitioners and researchers reduces, it becomes more feasible for mangrove forests to be effectively conserved or restored in the long term.”


Banner image: Great egret (Ardea alba) in a white mangrove tree (Laguncularia racemosa) in southern Brazil. Photo by Mariana Vargas Cruz & Gustavo Maruyama Mori

Citation: Francisco, P. M., Mori, G. M., Alves, F. M., Tambarussi, E. V., & de Souza, A. P. (2018). Population genetic structure, introgression, and hybridization in the genus Rhizophora along the Brazilian coast. Ecology and evolution8(6), 3491-3504.

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