For millions of years, trees have journeyed through regions too dry for survival. How have they managed to thrive in such challenging conditions?

The Amazon is located in the heart of South America, while the Atlantic forests lie along the eastern coast of the continent. These regions are separated by hundreds of kilometers of dry forest and savanna, which are too harsh for the survival of most tropical rainforest trees. Despite this gap, many different species of trees inhabit both ecosystems, as reported by IFLScience.

For millions of years, various tree species have spread from the Amazon to the Atlantic forests through a region that is too arid for their survival. How exactly they undertake this complex journey has been unclear, but a group of scientists has proposed a new idea: trees constantly use rivers as hidden "highways."

Previously, researchers suggested that trees crossed this dry area during historical periods when the region was much more humid and accommodating to passing trees. However, the results of the study have given rise to a new theory: trees from the Atlantic tropical forests are continually populated by trees from the Amazon. Scientists have also discovered that trees spread by growing along rivers, which act as hidden "arteries" between the two separated ecosystems.

According to the lead author of the study from the University of Edinburgh and the Royal Botanic Garden Edinburgh, Dr. James Nichols, they found that trees did not change during specific wetter periods in the past, but rather have been continuously colonizing over time. The scientists believe that this migration process occurs slowly, across generations of trees growing along the "highways" provided by rivers flowing through the dry regions of Brazil.

In the course of the study, researchers examined the genetics of 164 species of Inga trees, which are prevalent in the tropical forests of Latin America. Their DNA was used to reconstruct a "family tree" of the plants, showing the team when each species diverged from their numerous ancestors and how they migrated across the continent.

The results indicate between 16 and 20 instances of species migrating from the Amazon to the Atlantic forest and successfully establishing themselves. These "dispersal events" unfolded over the evolutionary timeline of Inga, rather than being confined to epochs when Brazil was covered in moist forest. At the same time, the scientists found only 1 or 2 examples of tree species moving in the opposite direction—from the Atlantic forest to the Amazon. This may be due to the intense outflow of seeds from the Amazon, which overwhelms the Atlantic forest.

According to co-author of the study, Professor Toby Pennington from the Department of Geography at Exeter and the Global Systems Institute and RGBE, the findings are a vivid demonstration that nature does not adhere to the national borders established by humans. These ecosystems are intertwined and deeply connected, often spreading across countries without regard for border checks and passports. This perspective, the team notes, could have significant implications for the conservation of tropical forests in South America and beyond.