How the meteor that killed dinosaurs created modern forests

A cataclysmic asteroid strike 66 million years ago off the Yucatán Peninsula changed tropical forests in distinct ways revealed in extensive new fossil evidence.

Why it matters: The new study may help to determine how the tropical forests of South America — contributors to the global climate, rich with species and a lifeline for tens of millions of people — emerged and how they may be shaped by the pressures of climate change today.

"This single historical accident changed the ecological and evolutionary trajectory of the rainforest forever," says Carlos Jaramillo, a paleontologist at the Smithsonian Tropical Research Institute in Panama and co-author of the study published today in Science.

• "Without the meteorite, living in the tropical forest today would be very different."

What they found: Over 12 years, Jaramillo and his colleagues collected and analyzed more than 6,000 specimens of fossilized leaves and 50,000 fossilized pollen samples in Colombia spanning the Late Cretaceous and early Paleogene period.

• The researchers measured changes in the density of leaf veins, the diversity of insect damage and other attributes to quantify how the forest plants' diversity and structure changed from the impact.
• Before the Chicxulub impact event, the forests were a mix of flowering plants, ferns and conifers with an open canopy that allowed light to pour into the forest.
• After the impact, 45% of the plant species went extinct (pollen fossils for the species vanished in the record) and flowering plants came to dominate a more closed canopy, forming the forests seen today.

The big question: Why did the forests go in that new direction instead of returning to their pre-impact state, especially since the climate was similar in both periods?

Jaramillo offers three possibilities:

• The disappearance of dinosaurs, which flattened the forest floor and fed on the canopy, reduced competition for light between plants and allowed flowering flora to take off.
• Ash from the impact brought phosphorus to the ground, creating a rich niche for fast-growing, nitrogen-fixing flowering plants to take off. Jaramillo says the team plans to collect samples of soil preserved in rocks to measure their nutrient content.
• Another is that conifers that evolved to be canopy trees in the tropics went extinct, whereas flowering plants were diversifying and "doing all sorts of things," he says.

The big picture: It took 6 million to 7 million years for the diversity of plants in the forest to return to levels before the collision.

• "The lesson that comes out of these studies looking at rebuilding is that diversity doesn't come back right away," says Sean Gulick, who studies the Chicxulub impact and its effect on marine ecology at the University of Texas Institute for Geophysics and wasn't involved in the new research.
• The findings are in line with other studies that found it took millions of years to reestablish the diversity of marine plankton after the impact, he says.

The bottom line: "The modern-day climatic crises we’re creating are geologically going to appear very fast in the record millions of years from now," Gulick says, pointing to the current rates of species extinction.

• "We’re going to take a very long time to come back to the same level of diversity, and it is very likely it won’t look the same."