The journey of rainforest trees

Shorea leprosula, one of the most widespread dipterocarp species in Southeast Asia. Jun Ying Lim

The rainforests of Southeast Asia are home to iconic animals like the orangutans, rhinoceros, tigers and elephants. The canopy of gigantic trees provides a safe refuge for several plants and animals, some of which are found nowhere else in the world. These rainforests have four of the world’s 25 biodiversity hotspots, and constitute 15% of the world’s tropical rainforests. In over 400 species of plants in these forests, the dipterocarp family of trees dominate the landscape.

Dipterocarps (meaning ‘two-winged fruit’ in Greek) are tropical trees widely harvested for timber, aromatic oils and resin. The culturally-significant Sal tree (Shorea robusta), native to India, belongs to this family. These trees are very tall, some as tall as 90 metres (for comparison, Delhi’s Qutub Minar is about 73 metres). The world’s tallest tropical tree Shorea faguetiana (or the yellow meranti) is also a dipterocarp.

In a new study, researchers from institutes in India, the UK, the US and Canada have probed the origins of dipterocarps using fossils that are at least 50 million years old. The study, published in the journal Science, found that these trees originated in Africa about 102 million years ago, hitched a ride to Asia when India separated from Africa, and then they penetrated the rainforests of Southeast Asia.

“The fossils discovered in the present study contribute to a better understanding of the impact of movement of the Indian Plate and changes in climate on the evolution of dipterocarps and hence Southeast Asian rainforests,” says Vandana Prasad, a paleobotanist and Director at the Birbal Sahni Institute of Palaeosciences, India, who led the study.

Vandana’s quest began in 2009, when she and her group began studying the rich diversity of pollens recovered from fossils found in western India. “At that time, we observed that a major fraction of the fossilised pollen resembles the pollen of living Dipterocarpaceae,” she says. Intrigued, she began searching for older fossils that could help bridge the gap in our understanding of where these plants originated and how they dispersed.

Existing fossil evidence had indicated that dipterocarps dispersed from India into other parts of Asia about 40-50 million years ago. But, their origins before they arrived in India remained a mystery until now due to lack of fossils. This study puts together the missing pieces using fossil evidence and molecular data to tell the journey of dipterocarps and discern their evolutionary history.

The researchers found that dipterocarps originated in Africa around 102 million years ago when India was still connected to Africa through the ancient Gondwana continent. However, its climate was too water-deficient for many flowering plants to survive. Around 30 million years later, the Indian landmass split from Gondwana and started its journey northwards, colliding with a series of island arcs—long chains of volcanic islands in the Tethys Ocean near the Equator. By then, the climate in India had turned wet and tropical, conducive to the growth of dipterocarps.

The series of islands acted as ‘stepping stones’ for the seeds of many flowering plants, including dipterocarps, to enter India, with animals, birds, wind or water dispersing the seeds. When India collided with the rest of Asia 40-50 million years ago, these thriving plants further spread into Southeast Asia, rapidly diversifying into different species, many of which are endemic. Subsequently, they conquered the wet lowlands in the region, forming the rainforests.

Window into history

When compared to dinosaur or mammoth fossils, plant fossils don’t hog the limelight. But, they provide a crucial window into the past. “Plants can tell us about past climatic conditions and are helpful when looking into past scenarios of global warming,” says Carina Hoorn, a paleoecologist at the Institute for Biodiversity and Ecosystem Dynamics, The Netherlands, who was not involved in the study. They can also paint a picture of the past environment, vegetation and origins of ancient continents.

Pollen is a powdery substance containing pollen grains, the ‘sperm cells’ of male plants. Although only as big as the thickness of human hair, pollen is a powerful tool in studying the geographical distribution of plants and animals of the past. “Pollen is made of sporopollenin, an extremely durable component that is resistant to temperature and pressure,” says Carina. “Because of the durability of sporopollenin, the pollen can remain in the sediment for millions of years.”

In this study, the researchers used pollen fossils of Dipterocarpaceae recovered from sediments in Sudan, Africa, that were about 68.5 million years old. They compared them with pollen fossils from India recovered from lignite coal mines in Rajasthan and Gujarat that were about 54 to 63 million years old.

“We were really fascinated to find similar pollen from northern Africa and Rajasthan,” says Vandana. “It gave us a sense that the flora of Africa and India have had a long evolutionary history together.” When they combined the pollen details with molecular data, they could derive an evolutionary tree of the different species that evolved in Africa, India and Southeast Asia in their dispersal journey.

Bleak future

Although the dipterocarps made a long inter-continental journey, it was marked by volcanic explosions, movement of tectonic plates, the fateful meteorite collision that killed the dinosaurs, and the formation of Deccan Traps from molten lava.

Yet, these hardy plants survived it all. Their future, however, is threatened by humans. “They are being extensively exploited for timber at a high rate,” Vandana says. Between 1990 and 2010, humans destroyed 32 million hectares of rainforests in Southeast Asia. At this rate, scientists fear that this region would lose about half of its animals and plants by the end of this century. Climate change and global warming further threaten the fragile rainforest ecosystems.

Studying millions-year-old plant fossils can help us in conserving them. Based on the evolutionary history of dipterocarp species and the life forms they support, they can be prioritised for conservation. Since the species-rich lowland dipterocarp rainforests also mitigate greenhouse gas emissions, “their conservation is of paramount importance and urgent,” Vandana concludes.