Scientists have traced the evolutionary history of the world’s tropical forests, with some surprising results – and new insights that may help improve responses to climate change.
To the uninitiated, tropical forests in Africa, Asia and Latin America might look the same: exuberant green vegetation from the forest floor to the canopy. Look closer, and they are actually quite different. Just 4% of tropical tree species are found on all three continents. There are very few palms in the African forests, for example, while in Asia and Latin America palm fronds are found throughout the understory.
Based on the few shared species, scientists initially made a distinction between the ‘old world’ forests of Africa and Asia and the more distantly related ‘new world’ American forests. But a new study has used a technique called ‘community phylogenetic similarity’, which involves digging back into plants’ evolutionary past to uncover new connections.
More than 100 scientists contributed data from forest plots across Asia, Africa and Latin America – almost a million different samples from 15,000 tree species. Lead author Ferry Slik, from the Universiti Brunei Darussalam, then analyzed the ‘relatedness’ of each tree species – how long since they shared a common ancestor – between all the forest plots.
“You take two locations, and in each location you have a list of 20 species,” Slik says.
“Even if none of the species are the same, using the phylogeny – the family tree – you can still compare the two forests, because at some level in their evolutionary past these species are all connected.
“For each species in location one, we look for the most closely related species in location two. You can then use the phylogeny to work out how long ago these species split up.
“Then you do that for all 20 species and calculate an average – and the ones that share a lot of closely related species become more similar in this analysis.”
When he crunched the numbers, they overturned some established ideas about the world’s biogeography. It was clear that actually the African and Latin American forests are close cousins, while the Asian and Pacific forests form another branch of the family.
Similarly, the huge islands of Madagascar and New Guinea – previously believed to be two separate major tropical regions – are really both part of a closely-related, broader Indo-Pacific family.
“If you look purely at the species composition of Madagascar and New Guinea, there is almost no species overlap, because they are both full of endemics that are found only there. But when you look at the patterns deeper in history, they are actually closely connected.”
The researchers also found a link between subtropical forests in East Asia and those in the Americas. To Slik, that’s suggestive that they are both remnants of the vast lost tropical forest that stretched all the way from North America to China, back when the world was warmer around 15 million years ago.
“I like the idea that those forests aren’t completely gone, and these are their descendants,” Slik says.
THE FUTURE’S IN THE PAST
The results support the theory that tropical forests share a common ancestor from 100 million to 66 million years ago during the Cretaceous period, when dinosaurs ruled the earth and the southern continents were joined together in the supercontinent, Gondwana.
As the land masses pulled apart, the forests were isolated. Over time, they adapted to different conditions and evolved into new species.
“When global weather patterns are drier, you get pockets of rainforest shrinking, and species becoming more specialized and then dispersing again,” says one of the paper’s co-authors, Center for International Forestry Research (CIFOR) Senior Associate and Professor at the University of British Columbia Terry Sunderland.
“For example, there is fossil pollen evidence for a rich palm flora in Africa, but it went through more and much harsher dry periods than the other regions, and so many of them went extinct.”
The new study is significant because it could help develop more region-specific responses to climate change, Sunderland says.
“The big message is the ‘so what?’ factor. Phylogenies give us really interesting evidence of change over time, and that could help us predict from a taxonomic perspective what might happen in the future.
“Obviously the climatic changes that are happening now are a lot more rapid than they’ve tended to be in the past, but this gives an idea of how forests adapt to an ever-changing climate and how they bounce back – and we need to know, because we need to project some of these things to help us deal with climate change.”
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