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The impact of the asteroid that killed dinosaurs radically altered the Earth's tropical forests

The day before a giant asteroid hit Earth 66 million years ago, a very different type of rainforest thrived in what is now Colombia. Unfolded ferns and flowering shrubs bathed in sunlight flowing through large openings in the canopy between tall conifers.

Then the fireball hit and everything changed (SN: 01/06/20). That impact not only triggered a mass extinction event that wiped out more than 75 percent of life on Earth, but also redefined the Earth’s tropical rainforests, transforming them from open forests with sunny canopies into dark, dense, lush and drizzle. Amazon's current forests, researchers report on April 2 in Science.

Researchers analyzed tens of thousands of fossils of pollen, spores and leaves, collected at 39 sites across Colombia, dating back 70 to 56 million years. The team then assessed the overall diversity of forest plants, dominant species, and insect-plant interactions and monitored the change in these factors. Researchers found that plant diversity decreased by 45 percent immediately after the asteroid attack and it took 6 million years before the rich rainforest diversity was restored. Still, the forests have never been the same.

"A single historical accident has changed the ecological and evolutionary trajectory of tropical rainforests," says Carlos Jaramillo, a paleopalinologist – someone who studies ancient pollen – at the Smithsonian Tropical Research Institute in Panama City. "The forests we have today are really the byproduct of what happened 66 million years ago."

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Just before the extinction event, tropical forests were a mixture of about 50-50 angiosperms, or flowering trees and shrubs, and other plant species such as conifers and ferns. “The competition for the light wasn’t that intense,” Jaramillo says. Afterwards, ferns and conifers largely disappeared and angiosperms took over to account for about 90 percent of the forest’s plant species.

The reasons for this are not entirely clear. The climate of the region at the end of the Cretaceous period 66 million years ago was similar to how it is today: hot and humid. But other factors are likely to work. The huge plant-eating sauropods, the long-necked dinosaurs, would help keep the ponds open by letting in light, says Jaramillo (SN: 17/11/20). Once the asteroid arrived, those dinosaurs were left out of the picture. The extinction of certain plant families due to the impact may also have played a role, he said.

A third likely factor was a change in the chemical composition of forest soil. Frequent rains during the warm, humid Cretaceous leached the soils from many nutrients, which would favor gymnosperms like conifers, Jaramillo says. "Gymnosperms had this incredible ability to grow with little food and could outgrow angiosperms."

Falling ash fluttering to the ground as a result of the asteroid’s impact may have added phosphorus to the soils, effectively fertilizing them, the team suggests. With more food available, angiosperms easily outnumbered gymnosperms, growing rapidly into the sky and blocking sunlight.

This thick closed canopy appeared shortly after the impact, but overall forest diversity took much longer to recover, as new species began to evolve to occupy new ecological niches. The researchers found that this also happened with the many species of insects that had previously celebrated the plants. Leaf fossils bear traces of the different types of interactions between herbivorous insects and plants, from creatures stripping leaves to their skeletons to those that pierce careful holes.

fossils of leaves and pollenFossils of leaves (shown) and pollen from all over Colombia reveal how rainforests changed due to the extinction event in the late Cretaceous. Prior to the event, the forests were a sunny mix of conifers, ferns, and flowering trees (examples in the lower half of this image). After the event, the forests were dominated by flowering trees and developed a thick, heavy, light canopy (examples in the upper half of this image).Carvalho et al / Science 2021

"Generalist" insects and other groups that eat plants that can conform to many different types of food plants "seemed to scream largely unaffected," says co-author Conrad Labandeira, a paleoecologist at the Smithsonian National Museum of Natural History in Washington. DC (SN: 25/01/17). “The specialized interactions that were actually created were things like leaf miners, drills, and suction cups,” which depend more on particular plant types, Labandeira says.

Jaramillo di (SN: 9/4/20). The road to recovery has a valuable lesson for the lasting impact of modern human activities such as deforestation. “Generating new diversity takes geological time,” he says. "It's not just about planting trees."

This is the first complete picture of what happened in tropical ecosystems just after the extinction event, says paleoecologist Elena Stiles of the University of Washington in Seattle, who was unrelated to the study. Most previous work on the chunks of time immediately before and after the extinction event – the end of the Cretaceous and the beginning of the Paleogene period – come from North America or much further south, as in Patagonia, says Stiles (SN: 2/04/19). "In the tropics, there is no place where we have the boundary (between periods) preserved, (and) we have the limitation of a very fragmentary fossil record."

Also striking, says Stiles, is the possibility that this finding may help answer a long question about South America’s amazing biodiversity. “For a long time, people have been wondering where all this diversity comes from,” he says. Researchers have speculated, for example, that the continent’s climate or its long isolation from other continents may be responsible. “So it’s really interesting that this mass extinction event could have been one of the mechanisms that set it up to be this unique region,” he says.

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