Hidden ‘Highways’ Connect Brazilian Rainforests, Aiding Dispersal of Tree Species, Research Reveals

Forests growing along the edges of rivers in Brazil act as “highways” allowing tree species to traverse between the Atlantic and Amazon rainforests, a phenomenon that has been occurring for millions of years, according to new research led by University of Exeter and Royal Botanic Garden Edinburgh (RBGE).

Hundreds of miles of savanna and dry forest separate the two forests, where most rainforest tree species are unable to survive, a press release from University of Exeter said.

“Rather than tree species being exchanged during specific wetter periods in the past, we found that species have dispersed consistently over time,” said Dr. James Nicholls, a biodiversity genomics scientist with RBGE, in the press release. “This probably happens slowly, by generations of trees growing along the ‘highways’ provided by rivers that run through Brazil’s dry ecosystems.”

Before the study, scientists had believed that tree species only moved between the Atlantic and Amazon rainforests long ago, when Earth’s climate was wetter and a large swath of South America was rainforest.

However, the new study tells a different tale.

The research team — which included Brazilian scientists — studied 164 Inga tree species, commonly found in Latin American rainforests.

Through DNA analysis, the team was able to reconstruct the “family tree” of the trees, which enabled them to see when each of the species split from its ancestors. The researchers then mapped the location of each tree species, allowing them to discover patterns of movement between rainforests.

Images of Inga demonstrating biological characteristics and the humid forest adaptations of this genus. Clockwise from top left: flowers of I. sessilis from the Mata Atlântica; I. cinnamomea from Amazônia showing the fleshy sarcotesta, an adaptation facilitating primate dispersal; germinating naked I. edulis seeds, Amazônia, showing lack of drought adaptations; I. affinis growing alongside a river in Central Brazil, showing riverside habitat traversing drier cerrado vegetation; large edible legume of I. spectabilis, Amazônia; flowers of I. lineata, Amazônia. All photos by R.T. Pennington.

From 16 to 20 “dispersal events” were discovered, when tree species that came from the Amazon to the Atlantic rainforest successfully established themselves. These events happened throughout the Inga tree species’ evolutionary history, not only during periods of humid forest cover over much of Brazil.

On the other hand, the research team found just one or two occurrences of species moving from the Atlantic rainforest to the Amazon. They believe this may be a reflection of the forests’ relative size — the vast Amazon produces a bigger outflow of tree seeds.

The findings of the study highlight the importance of riverside forest conservation. Riverside forests are protected under Brazilian law.

“This legal protection – and efforts to preserve these riverside forests – are highly valuable for long-term habitat connectivity,” said professor of tropical plant diversity and biogeography Toby Pennington with University of Exeter’s Global Systems Institute and RGBE, in the press release. “The study also tells us something fundamental about the history of the incredible biodiversity of the Atlantic rainforest, which contains about 3,000 more plant species than the Brazilian Amazon. Only 20% of the Atlantic rainforest now remains intact. In the short term, we need to protect these precious rainforests. In the long term, our study shows that we must also conserve the connections between them.”

The paper, “Continuous colonization of the Atlantic coastal rain forests of South America from Amazônia,” was published in the journal Proceedings of the Royal Society B.