Rainforest trees use rivers as highways for migration

Forests along Brazil’s rivers have played a crucial role in connecting the Amazon and Atlantic rainforests for millions of years. These green corridors have allowed tree species to migrate between the two forests, even though they are separated by vast expanses of dry forest and savanna.

Until now, researchers believed that tree migration only occurred during wetter climatic periods when rainforests expanded across South America. However, a new study from the Royal Botanic Garden Edinburgh (RBGE) and the University of Exeter reveals a different story.

The research challenges the long-held assumption that tree species only moved between the Amazon and Atlantic forests during rare climate shifts. Instead, the study suggests that species have been dispersing continuously over time, using riverside forests as natural highways.

This discovery reshapes the understanding of how rainforest biodiversity has evolved and highlights the ongoing importance of these ecological corridors.

Tree migration through generations

Rather than short-term bursts of movement, tree species have migrated slowly over many generations. The researchers found that trees growing along rivers helped species move gradually across the dry landscapes of Brazil.

“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 of RBGE.

The study was focused on 164 species of Inga trees, a genus found widely in Latin American rainforests. By analyzing the DNA of these trees, the researchers reconstructed their evolutionary history, determining when different species diverged from their ancestors.

Mapping the locations of various Inga species further revealed patterns of movement between the Amazon and Atlantic forests.

This detailed genetic analysis uncovered a consistent pattern of species migration that spanned millions of years. The findings suggest that rainforest connectivity has depended not just on large-scale climate changes but also on localized ecological networks along rivers.

Amazon trees migrate more

The study identified 16 to 20 instances where tree species from the Amazon successfully established themselves in the Atlantic forest. These events were spread across evolutionary history rather than being concentrated in specific periods when Brazil had a wetter climate.

The findings indicate that trees have been able to migrate across dry regions at a slow but steady pace, rather than relying on rare climate-driven rainforest expansions.

Interestingly, the researchers found only one or two cases where tree species moved in the opposite direction—from the Atlantic rainforest to the Amazon. This suggests an imbalance in species migration, likely due to the Amazon’s vast size and greater seed production.

Because the Amazon is much larger, it likely sends out a greater number of seeds, increasing the chances of successful migration to the Atlantic forest.

This discovery reshapes the understanding of rainforest dynamics, showing that species movement is not just a relic of past climate changes but an ongoing process. It also suggests that even in modern times, forest corridors remain vital for biodiversity.

Protecting Brazil’s riverside forests

The research highlights the importance of conserving forests along Brazil’s rivers. These green corridors allow tree species to continue moving between the two rainforests, maintaining biodiversity and ecological balance.

Brazilian law already provides legal protection for riverside forests, recognizing their critical role in preserving the country’s natural ecosystems.

“This legal protection – and efforts to preserve these riverside forests – are highly valuable for long-term habitat connectivity,” said Professor Toby Pennington from Exeter’s Department of Geography and the Global Systems Institute.

Beyond their role in species migration, these forests are essential for sustaining the extraordinary biodiversity of the Atlantic rainforest trees.

The Atlantic rainforest is home to approximately 3,000 more plant species than the Amazon, yet only 20% of it remains intact. Habitat destruction has put many species at risk, making conservation efforts more urgent than ever.

Maintaining rainforest connections

“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,” said Pennington.

Protecting the Amazon and Atlantic rainforests requires more than just safeguarding individual patches of forest. It also depends on maintaining the ecological links that have allowed tree species to migrate and adapt over millions of years.

Ensuring the survival of riverside forests will not only help preserve the unique biodiversity of both rainforests but also support the broader health of Brazil’s ecosystems.

The study was funded by the United States National Science Foundation, emphasizing the global importance of understanding and protecting these critical rainforest connections.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–