The rainforest of Colombia seemed very different 66 million years ago. Today, the humid and biodiverse ecosystem is teeming with plants and covered by a thick canopy of leaves and branches that blocks light. It should be noted that there are no dinosaurs. But before the dinosaurs left with the Chicxulub impact, which marked the end of the Cretaceous period, things looked very different. Plant cover in the area was relatively sparse, and a group of conifers called it home.
Using the fossilized remains of plants, a team of researchers studied the rainforest’s past and how the asteroid gave rise to today’s rainforests. The study, published in Science on April 1, it was led by scientists from the Smithsonian Tropical Research Institute (STRI) in Panama and supported by scientists from the Negaunee Institute of Science and Action for Plant Conservation at the Chicago Botanical Garden.
“The forests disappeared due to the ecological catastrophe … and then the vegetation that returned was mainly 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.
The research began 20 years ago, with parts of the team collecting and analyzing 6,000 leaves and 50,000 pollen fossils from Colombia. Looking at these fossils allowed the team to get an idea of the types of plants present before and after the asteroid hit the planet. This sequence represents the biodiversity of the region between 72 and 58 million years ago, covering both before and after the impact. “It took us a long time to gather enough data to have a clear idea of what was happening during the extinction,” Carvalho told Ars.
While the study deals with Colombian fossils, Carvalho said researchers may have a clear idea of what happened in rainforests in other parts of Central and South America, although the effects of the asteroid impact are somewhat variable from one region to another. “It is a bit variable. We still don’t know why some places were more affected than others, ”he said.
After the asteroid hit Earth, almost half of the plant species in Colombia perished; pollen fossils from those species stopped appearing after that point. The rainforest began to be invaded by ferns and flowering plants that, although present before impact, were less common than they are today. Coniferous trees, by comparison, did indeed go extinct.
Beyond the presence of conifers, the rainforests of the past were probably much rarer than their modern counterparts. Today’s rainforests have thick canopies and the plants within them are close together, which means that more plants are transpiring water into the atmosphere. This leads to higher levels of humidity and cloud cover. According to Carvalho, the relative lack of moisture in previous forests means that the regions were probably much less productive than they are today.
But the lowest-productivity forest remained in place until the asteroid hit. “It was only after the impact that we see the forests change their structure,” he said.
The researchers have some hypotheses about how this change occurred. The first is that the disappearance of the dinosaurs caused the forests to become denser; there could be fewer animals consuming the plants or trampling undergrowth, allowing the foliage to grow relatively uncontrollably. The second idea is that, shortly after the asteroid hit the planet, there was a selective extinction of conifers in the tropics; they simply could have fared worse than their blossoming peers after impact.
The third is that the aftermath of the disaster could have fertilized the soil. Post-impact tsunamis may have carried debris and sediment from nearby shallow, carbon-rich marine areas. The blazing wildfires could have sent ash into the atmosphere, and when it finally settled into the ground, it could have acted as a kind of fertilizer. Flowering plants tend to grow better than conifers in nutrient-rich soils, Carvalho said. He also pointed out that all of these hypotheses, or two of them, could be true simultaneously.