Prior to the Chicxulub impact, rainforests looked very different

Image of a tropical forest.

Colombia’s rainforest looked very different 66 million years ago. At present, the humid and biodiverse ecosystem is jam-packed with plants and is covered in a thick, light-blocking canopy of leaves and branches. Notably, there are no dinosaurs. But prior to the dinosaurs going away with the Chicxulub impact, signaling the end of the Cretaceous Period, things looked very different. The area’s plant coverage was relatively sparse, and a bevvy of conifers called it home.

Using the fossilized remains of plants, a team of researchers studied the past of the rainforest and how the asteroid gave rise to the rainforests of today. The study, published in Science on April 1, was led by scientists at the Smithsonian Tropical Research Institute (STRI) in Panama and supported by scientists at the Negaunee Institute for Plant Conservation Science and Action at the Chicago Botanic Garden.

“Forests disappeared because of the ecological catastrophe… and then, the returning vegetation was mostly dominated by flowering plants,” said Mónica Carvalho, first author and joint postdoctoral fellow at STRI and at the Universidad del Rosario in Colombia, in an interview with Ars.

A change of scenery

The research began 20 years ago, with parts of the team collecting and analyzing 6,000 leaf and 50,000 pollen fossils from Colombia. Looking at these fossils allowed the team to get a sense of the types of plants present both before and after the asteroid struck the planet. This sequence represents the region’s biodiversity between 72 million and 58 million years ago, covering both before and after the impact. “It took us a long time to gather enough data that we could have a clear picture of what was going on during the extinction,” Carvalho told Ars.

While the study deals with Colombian fossils, Carvalho said the researchers can get a fair idea of what happened in rainforests elsewhere in Central and South America, though the effects of the asteroid’s impact are somewhat variable from region to region. “It’s a little bit variable. We still don’t know why some places were affected more than others,” she said.

After the asteroid hit the Earth, nearly half of the plant species in Colombia perished—the pollen fossils for those species stopped appearing past that point. The rainforest began to be taken over by ferns and flowering plants that, while present pre-impact, were less common than they are today. The coniferous trees, by comparison, effectively died out.

Beyond the presence of conifers, the rainforests of the past were likely much sparser than their modern counterparts. Current rainforests have thick canopies, and the plants within them are spaced closely together, meaning more plants are transpiring water into the atmosphere. This leads to higher levels of humidity and cloud coverage. According to Carvalho, the relative lack of humidity in earlier forests means that the regions were likely much less productive than they are today.

But the lower-productivity forest remained in place until the asteroid hit. “It was only after the impact that we see the forests change their structure,” she said.

Explaining the changeover

The researchers have some hypotheses about how this change occurred. The first is that the demise of the dinosaurs caused the forests to grow more dense—there could have been fewer animals consuming the plants or stomping through the brush, allowing foliage to grow relatively unchecked. The second idea is that, shortly after the asteroid collided with the planet, there was a selective extinction of conifers in the tropics—they could have simply fared less well than their flowering peers post-impact.

The third is that the aftermath of the catastrophe could have fertilized the soil. Tsunami events that occurred after the impact could have carried debris and sediment from carbon-rich, shallow marine areas nearby. Burning wildfires could have sent ash into the atmosphere, and when it finally settled on the ground, it could have acted as a kind of fertilizer. Flowering plants tend to grow better than conifers in high-nutrient soils, Carvalho said. She also noted that all of these hypotheses, or any two of them, could simultaneously be true.

“This is something we continue to explore as we search for more fossil sites and when we keep on studying the tropics,” she said.

After the massive dinosaur-killing catastrophe, the rainforests also began seeing an increase in legume plants (which includes everything from trees to peas), both in terms of raw amount and biodiversity. Legumes are particularly good at fixing nitrogen into soil, thanks to symbiotic bacteria that associate with their roots. This added nitrogen could have given the plants an edge as the rainforests began to grow again and, in turn, could have made the soil more fertile, benefitting other types of plants and making the region richer for animals.

Carvalho said that this research can also provide a glimpse into the future of anthropogenic climate change. Around 10 million years after the Cretaceous ended, the world entered the Paleocene-Eocene Thermal Maximum (PETM), a period in which the atmosphere was inundated with carbon dioxide and the world saw a warming of 5°C.

Comparatively, the extent of greenhouse gas production and warming the world is seeing now leaves conditions like those of the PETM still a ways off. However, the PETM lasted around 100,000 years, and humans have made headway on this path in the past 200 years, which is incredibly short in geological terms, Carvalho said.

“The changes we are seeing today in relation to climate and deforestation are so rapid that we haven’t really seen them in any other scenario in the history of the planet,” she said. “Extinction is something that occurs really fast.”

Science, 2021. DOI: 10.1126/science.abf1969  (About DOIs).

Doug Johnson (@DougcJohnson) is a Canadian freelance reporter. His works have appeared in National Geographic, Undark, and Hakai Magazine, among others.