Tracing an ancient voyage of tropical African trees that travelled to Southeast Asia via India

From the chirps of birds and croaks of frogs to the roars of elephants and growls of tigers, Southeast Asia’s vibrant rainforests are home to a wide range of animals, many of which are found nowhere else on earth.

The rainforests of Southeast Asia comprise 15 percent of the world’s tropical rainforests and are spread across Indonesia, the Malay peninsula, which includes Malaysia, Thailand and Myanmar, and Laos and Cambodia. Boasting four out of 25 of the world’s biodiversity hotspots, the region hosts a treasure trove of species, including critically endangered, charismatic animals such as orangutans and rhinoceros. In 2020, scientists discovered 224 new species in the Greater Mekong region, which included 155 species of plants.

Southeast Asian lowland and hill rainforests are dominated by a family of pantropical trees called Dipterocarpaceae represented by more than 400 species. Dipterocarps —meaning ‘two-winged fruits’ — are valuable for their timber. But the origin of these slow-growing, economically-important trees has remained a mystery due to the lack of fossil evidence of their early history.

An international team of researchers studied newly discovered fossilised pollen to reveal that dipterocarps in Southeast Asian rainforests originated thousands of miles away in tropical Africa around 102 million years ago — more than 30 million years before the extinction of the dinosaurs.

Millions of years later, these trees dispersed to the Indian subcontinent, where they managed to survive two catastrophic events that affected much of life on earth. The subsequent collision of the Indian Plate with the Asian landmass paved the way for the dispersal of dipterocarps into Southeast Asia, where they underwent explosive diversification.

“For more than a century or so, people believed that dipterocarps originated in Southeast Asia and then they went all the way from Southeast Asia to Africa,” says Shivaprakash Nagaraju, a senior scientist at The Nature Conservancy in India and co-author of the study. An alternative hypothesis was that dipterocarps originated in Africa and dispersed out of India.

Scientists believed that Dipterocarpaceae must have originated sometime during the Miocene, which coincides between 40 to 35 million years before the present, Nagaraju explains. “Most of the fossil records earlier discovered [from Southeast Asia] were always less than 30 million years.” But the pollen fossils discovered recently from India and Africa date back to 60 to 70 million years.

Many species in the Dipterocarpaceae family can grow pretty tall. Located in a protected tract of rainforest in Sabah, Malaysian Borneo, the world’s tallest tropical tree, the yellow meranti (Shorea faguetiana), an endangered species, was first discovered in an airborne survey in 2014.

In 2019, climbers measured the tree to be about 100 metres tall — about a third of the height of the Eiffel Tower in Paris. Scientists believe that future airborne surveys may discover even taller trees, likely of this species, in the region.

While exploring mines in the western states of Gujarat and Rajasthan, a different group of researchers unearthed pollen fossils. In Sudan, they also found pollen fossils. To unravel the origin and journey of these trees, the team analysed specific morphological characters of fossil pollen and combined this with molecular dating analysis using the DNA sequence of half of all known species within the family.

India: an ancient corridor

The microscopic pollen fossils showed that the origin of the dipterocarp tree family could be traced back to Africa during the mid-Cretaceous period, around 102 million years ago. At this time, India was farther down south, where the climate was dry and unfavourable for dipterocarps. But, 30 million years later, the Indian subcontinent had moved northward where the climate was warm and humid — favourable for dipterocarps. The trees then dispersed to the Indian subcontinent, likely via an ancient island arc connecting Africa and India, known as the Kohistan Ladakh Island Arc, that acted as stepping-stones. India continued its northward journey and eventually collided with the Asian landmass. This provided a gateway for the dispersal of these towering trees to Southeast Asia.

“What this study nicely illustrates is the intimate evolutionary link between tropical rain forests worldwide,” says Thomas Couvreur, a tropical botanist at the French National Research Institute for Sustainable Development (IRD), who was not involved in the study.

“In fact, Africa is the birthplace of numerous other major tropical plant families too, and as such plays a central role in explaining the origin and evolutionary history of these uniquely hyper-biodiverse ecosystems globally.” Currently, rainforests in the African tropics stretch from West Africa into the Congo Basin with small patches along the eastern coast and the ancient Eastern Arc Mountain chains that extend from Tanzania to Kenya.

Pamela Soltis, a distinguished professor at the University of Florida and curator at the Florida Museum of Natural History says, “This sort of complex evolutionary pattern may be more common for Southeast Asian plants than previously appreciated, and it took a combination of molecular phylogenetics and discovery of new microfossils to make this inference.”

Soltis, who was not connected to the study, wonders how many other groups might have this sort of history.

Against the odds

Remarkably, dipterocarps in India survived two cataclysmic events around 66 million years ago: the Cretaceous-Paleogene asteroid impact at the Gulf of Mexico; and the Deccan Traps in west-central India — the second largest volcanic eruption on land, which covers an area about the size of France. These events have been thought to have triggered a mass extinction, wiping off 75 percent of life on earth, including the dinosaurs. After these events, dipterocarps diversified on the Indian Plate, possibly facilitated by the rich soils from the volcanic eruptions.

Tropical rainforests once flourished in western India, but much of that region is now desert. Once India collided with Asia, the climate started to change. “The rise of the Himalayas drastically changed the climate of India,” explains Nagaraju. The “humid, wet climate changed into arid and dry in many places, and that’s one of the reasons scientists claimed that probably a lot of dipterocarps went extinct in India.”

Now, dipterocarps in India are found in the south, from Karnataka to the tip of southern India and in the northeast and the Andaman and Nicobar Islands. “You find them mostly in the Western Ghats,” where “there are around 15 to 16 species. In the northeast, you may find around 5 to 6 species,” says Nagaraju. Sal trees (Shorea robusta) are dipterocarps that are more suited to a drier habitat and are found in the sub-Himalayan region.

The rise of the rainforests of Southeast Asia

Dipterocarpaceae diverged into two main lineages, Monotoideae and Dipterocarpoideae where the former is adapted to tropical dry seasonal habitats while the latter is suited to wet seasonal habitats. That is the reason that Monotoideae only stuck to Africa while Dipterocarpoideae family started diversifying in Southeast Asia where you have more of a wet climate, explains Nagaraju.

The wet tropical climate of Southeast Asia contributed largely to the successful establishment and rapid diversification of Dipterocarpaceae into different species in Southeast Asia. “High diversification of this Dipterocarpaceae family coincided with this Miocene wet climate maximum when there was the maximum peak of this wet climatic period,” Nagaraju says. “It’s all because of the favourable climatic conditions, they started diversifying crazily in Southeast Asia.”

Jun Ying Lim, assistant professor at the National University of Singapore, says that “from the perspective of dipterocarps, Southeast Asia must have presented a tremendous ecological opportunity: the heterogeneous, complex and ever-changing geophysical landscape has provided many ways in which speciation may occur.” One example, he explains, is the periodic flooding and draining of the Sunda shelf during the ice ages, which would have led to repeated cycles of isolation driving “formerly contiguous populations to become fragmented, with each fragment diverging from the others to become new species.”

Another reason for the dominance of dipterocarps in the region is their ability to forge ties with fungi. “Dipterocarps rely on mutualistic relationships with root fungi for nutrient acquisition,” explains Lim, who was not associated with the study. “While many plant families have similar associations with fungi in their roots, dipterocarps stand out in that many species seem to specialise on different soils across the region.”

Hanging in the balance

Tropical rainforests worldwide, which play a role in climate change mitigation, face immense pressure from human activities such as forest clearing and global climate change. Some regions are more severely threatened than others, says Couvreur, citing the Western Ghats as an example. Timber obtained from dipterocarps has contributed to the economy of several Southeast Asian nations. A 2014 study estimated that between 1990 to 2010, forest cover in Southeast Asia dropped by 32 million hectares — an area the size of Ukraine. In a worst-case scenario, forest cover in Southeast Asia is projected to shrink by 5.2 million hectares by 2050, according to a 2019 modelling study.

“In a nutshell, the two main drivers of rainforest destruction are poverty and overexploitation driven by profits,” says Couvreur, who is currently based in Ecuador as an invited researcher at the Pontificia Universidad Católica del Ecuador. “Thus, their future will depend on transdisciplinary projects based on sustainable exploitation and poverty alleviation conserving not just the incredible biodiversity and evolutionary processes they harbour but the intimate win-win link that can exist between humans and rainforests.” Couvreur is optimistic that this can be achieved, adding that there are “wonderful examples of such sustainable systems shown by indigenous communities’ management of rainforest resources across the world.”

African tropical forests tend to receive less attention compared to those in South America and Southeast Asia. But Couvreur stresses that “from an evolutionary perspective, efficiently protecting African rainforests will be fundamental for conserving the enormous legacy they represent at a global scale.”

Read more: New study traces the evolution of South Asia’s bulbuls to other parts of Asia and even Africa

CITATION:

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Banner image: Shorea leprosula, a common dipterocarp that can be found throughout Southeast Asia. Photo by Jun Ying Lim.