Article by Morgan Erickson-Davis.
Though they may lie thousands of miles apart and house different species, the world’s tropical forests may not be as different from each other as they seem to be. A new study shines light on the evolutionary relationships between tropical trees, revealing some surprising connections.
For their study, an international team of several dozen researchers genetically analyzed the evolutionary relatedness of tree species that live in tropical and sub-tropical forests around the world. Their results were published today in Proceedings of the National Academy of Sciences.
In line with previous research, their study found that tropical forests share a common ancestry that dates back to between 100 million to 66 million years ago. During this time, dinosaurs roamed the planet and the supercontinent of Gondwana began breaking apart.
But then, sometime after 66 million years ago, tropical trees started diverging into more distinct groups. As continents drifted away from one another, oceans opened up, making it increasingly difficult for plants to spread their pollen and seeds to different landmasses. As trees became more isolated to their respective continents, they began to change into new species.
The world’s tropical forests are home to a vasty array of species, like this broad-billed tody (Todus subulatus).
When the researchers analyzed the more recent evolutionary relationships between tropical tree species living today, they uncovered some surprises that overturn traditional assumptions.
For one, according to the analysis, the world’s tropical forests seem to be divided into two main “floristic regions,” one that comprises most of Africa and the Americas and another in the Indo-Pacific region. The researchers write that despite now being around 1,800 miles (2,900 kilometers) apart, plants may have been able to disperse between Africa and South America over a widening Atlantic Ocean for some time after the breakup of Gondwana, thus maintaining their genetic similarity.
Tropical trees in the Indo-Pacific, a region that encompasses eastern Africa and southern Asia to the islands dotting the Pacific Ocean, also appear to comprise their own group.
“Given the diverse geologic history of Asia and the Indo-Pacific, it is surprising to find a similar forest type covering most of the region,” the researchers write. They say this might be due to exchanges of plants and animals that happened starting around 45 million years ago between Southeast Asia and Australia and 15 million years ago between Australia, New Guinea and the Pacific Islands.
The analysis also indicates dry tropical forests around the world – from Madagascar and India to Africa and South America – are unexpectedly similar to one another. The researchers aren’t sure whether this is because they have a common evolutionary origin or because climatic conditions have simply favored certain related species.
The study also uncovered a distinct group of trees that live in cooler climates and higher elevations in Asia and the Americas, which the researchers called the “Subtropical floristic region.” While in Asia this group is mostly confined to the subtropical zone – a region between temperate and tropical zones – in the Americas, the scientists found several areas of forest that belong to this group extending deep into the tropics. They write that this is likely due to the cooler, mountainous climate of Central America and mountain ranges like the Andes that run from the north to the south and act as a conduit for wildlife.
The researchers write that their findings “may necessitate reconsideration of established biogeographic ideas” and could help in the development of more region-appropriate responses to climate change.
Ferry Slik, J. W., Franklin, J., Arroyo-Rodriguez, V., Field, R., Aguilar, S., Aguirre, N., … & Avella, A. (2018). A phylogenetic classification of the world’s tropical forests. Proceedings of the National Academy of Sciences of the United States of America.