CLEMSON, South Carolina – Research from more than 100 scientists across the world, including that of Clemson professor of biological sciences Saara DeWalt, has recently combined to show that the world’s tropical forests are more similar than scientists previously thought.

In 1994, DeWalt had just graduated with a bachelor’s degree in biology from Brown University. Fully funded by a Fulbright Scholarship, DeWalt was able to conduct an ethnobotanical study in the lowland tropical forest of Bolivia. There, she assessed how an indigenous people called the Tacana made use of different tree and vine species in their everyday lives. To conduct the study, DeWalt led a forest inventory of trees near two Tacana communities.

Madidi National Park

DeWalt’s contribution to the study stemmed from her time in the Amazon of Bolivia.
Image Credit: Wikimedia Commons

More than two decades later, DeWalt’s documentations are part of a new study, published in the Proceedings of the National Academy of Sciences, that indicates that tropical forests can be grouped into two major regions based on the similarity of their flora: American and African tropical forests versus Indo-Pacific forests.

In addition to helping scientists reclassify the world’s tropical forests, the discovery supports what geologists know about the breakup of west Gondwana, an ancient supercontinent that contained what has since become Africa and South America.

In prior studies, researchers have attempted to understand how closely related forests in different parts of the world are by comparing how many tree species they share.

“For example, if two sites were compared, each with 100 individuals, and they shared 20 species, then we’d say the similarity of the two sites is 20 percent,” said Ferry Slik, the lead author of the study and an associate professor at the Universiti Brunei Darussalam Herbarium in Brunei, Borneo.

But because American and Asian forests have very few species in common, this method of assessing forest similarity failed to provide accurate measurements of the world’s tropical forests.

Other methods compared forests using higher-level classifications, such as genera and families, but even these were problematic because they didn’t take into account how closely related different flora might be.

For instance, species in the laurel family (such as cinnamon, avocados and bay leaves) are more closely related to species in the nutmeg family than they are to species in the coffee family – a detail that previously used analyses ignored.

In recent years, techniques for studying the similarity of plant communities have shifted toward the use of phylogenetics, which is important in disciplines such as genetics and evolutionary biology. Phylogenetics uses molecular and morphological data, such as DNA sequences and the structural features of organisms, to determine the evolutionary history or degree of relatedness. A phylogeny, or phylogenetic tree, then depicts these relationships among the organisms.

“Ferry Slik was able to amass hundreds of tree surveys from around the world and, based on that, he created a phylogeny of each forest,” said DeWalt, a professor of biological sciences at Clemson. “Because of this, we’re not just comparing areas by asking what percentage of similarity two forests have between their species or genera. Now we can ask, for all of the species in a forest, how closely related are they to all the other species in a different forest?”

Prior to the team’s study, the established hypothesis was that the Neotropics – including Central and South America – were different from the Paleotropics of Africa, Asia and Australia.

“But when we make these phylogenetic trees for each forest, we actually find that American forests and west African forests are more closely related to each other than they are to the Indo-Pacific,” DeWalt said.

This result matches what researchers have found when they’ve studied animals in these regions. But perhaps more importantly, it also conforms with what happened when an ancient land split apart and gave rise to four of the seven present-day continents.

cluster graph of tropical forests

Classification maps of the world’s tropical forests, showing two (A), three (B), four (C), and five (D) clusters.
Image Credit: PNAS, doi: 10.1073/pnas.1714977115

Gondwana came together 600 million years ago, around the same time that multicellular animals were beginning to form in the oceans. The supercontinent encompassed present-day South America, Africa, Arabia, Madagascar, India, Australia and Antarctica. It later merged with northern Eurasia 300 million years ago to form the world’s most recent supercontinent, named Pangaea.

But when magma beneath Earth’s crust began pushing upward, Pangaea began to split, releasing Gondwana and the future southern hemisphere with it. About 140 million years ago –  at a time when dinosaurs still roamed the planet – Africa and South America drifted apart and the southern Atlantic Ocean filled in between them.

South America and Africa are now 1,600 miles apart, but the researchers have speculated that plants were able to disperse their seeds over the broadening Atlantic Ocean for a long period after Gondwana first began to split, giving further credence to their compositional similarity today.

The study also found that dry tropical forests appear very similar to each other across the world, possibly indicating a shared origin of these flora.

“Or there could have been selection for drought-resistance and fire-resistance in these dry forests so that maybe only certain taxa, certain plant families, certain genera can hack it in drier areas,” DeWalt said. “That’s pretty interesting, and you might not have been able to pick that up in any other analysis.”

Combined together, the findings will help researchers better understand how forests across the world operate.

“If we know how similar forests are, we may be able to conserve, manage or plan for environmental change in these forests more easily. Perhaps what we learn from one dry forest in South America might help us manage a dry forest in Southeast Asia, more so than a wet forest in the same region could,” DeWalt said. “This kind of analysis helps us to evaluate which regions really are distinct and which are more similar and plan accordingly.”