Many reading this article when you hear the word tropical rain forest, you may assume that it is a place where giant lush green trees dominate and large animals like tigers and elephants roam free. A few of you may have seen the rainforest when you visited Amazon, Borneo, trekked through the Western Ghats mountains in India, or had a holiday vacation in Andaman or the Nicobar Islands. But, for most tropical rainforest seems like a faraway remote place removed from everyday life. Nonetheless, our lives are intertwined with tropical forests, as archaeologist Patrick Roberts says in his book Jungle: How tropical forests shaped the world – and us “the history of tropical forests is human history too”. With the evolution of the first land plants and the origin of trees, tropical forest-affected dinosaurs influenced the evolution of early mammals, the first primates, and modern humans. Despite rainforests being the powerhouse for the evolution of plants and animals including humans, we know little about how rainforests initially developed on planet earth and spread across the continents.
A new collaborative study published in the prestigious journal Science, explores the origin of the modern-day tropical rainforests of Asia using dipterocarp tree evolution as a model. Dipterocarps are giant, often emergent trees frequently exceeding 45 meters in height and these dominate the canopy of tropical rainforests in Southeast Asia. They also hold the record of being some of the world’s tallest tree species reaching heights of over 100 meters. They belong to the plant family called Dipterocarpaceae and produce winged fruits which are dispersed by wind. Dipterocarps are best known for their commercial timber and mass flowering and fruiting which traditionally occur on a roughly 4 or 5year basis coinciding with dry weather conditions linked with the ocean-atmosphere phenomenon called the El Niño-Southern Oscillation (also known as ENSO or “El Niño”). In Bornean rainforests nearly 200 species of Dipterocarpaceae flower and fruit synchronously over an area of 150 million hectares producing colorful and breathtaking views of tropical rainforest. This is a boon for seed predating animals, especially wild boars. The local people call this mass fruiting year a time of plenty as they get to export dipterocarp fruits, and harvest plenty of pork for consumption. The tropical rainforest dominated by dipterocarps are rich in biodiversity, providing the habitat for many critically endangered animal species including orangutans, rhinoceros, and elephant, and mitigating greenhouse gas emissions by sequestering twice the CO2 from the atmosphere compared to any trees in Amazonian Forest. Thus, their conservation is of paramount importance and urgency.
Since the Asian tropical forests are mostly dominated by dipterocarp trees, it is believed that dipterocarp’s origin and diversification also fueled the rise of highly biodiverse tropical rainforests,s especially in the Southeast Asian region. In this collaborative study recently published in Science, researchers from The Nature Conservancy-India, Birbal Sahni Institute of Palaeosciences-India, Institut Français de Pondichéry-India, Palynova-UK, Indian Institute of Technology Bombay-India, Petroleum Laboratories, Research & Studies (PLRS)-Sudan, Concordia University-Canada, and Harvard University-USA analyzed the fossil pollen and amber of Dipterocarpaceae collected from western India and Sudan and conducted molecular dating analysis based on DNA sequence of half of the known species within the plant family and evaluated morphological characters of fossil and living pollen of species to determine where dipterocarps first originated, how they traveled to other continents, whether past climate change and geological events shaped their evolution, and when dipterocarps started to proliferate in Southeast Asia and fuel the rise of their tropical rainforests.