The rainforest of Colombia seemed very different 66 million years ago. Currently, the humid and biodiverse ecosystem is full of plants and is covered by a thickness of leaves and branches that blocks light. It should be noted that there are no dinosaurs. But before the dinosaurs left with the impact of Chicxulub, which marked the end of the Cretaceous period, things looked very different. The vegetation cover of the area was relatively scarce and a bunch of conifers called it home.
Using fossilized plant remains, 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 with the support of scientists from the Negaunee Institute for Plant Conservation and Science and Action of the Chicago Botanical Garden.
“The forests disappeared due to the ecological catastrophe … and then the vegetation that returned was dominated mainly by flowering plants,” said Mónica Carvalho, first author and joint postdoctoral fellow at STRI and the University of 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. Observing these fossils allowed the team to know the types of plants present both before and after the asteroid hit the planet. This sequence represents the biodiversity of the region between 72 and 58 million years ago, spanning both before and after the impact. “It took us a long time to gather enough data to be able to have a clear picture of what was going on during the extinction,” Carvalho told Ars.
Although the study deals with Colombian fossils, Carvalho said researchers may have a good idea of what happened in the rainforests of other Central and South American countries, although the effects of the ‘impact of the asteroid are slightly variable from one region to another. “It simply came to our notice then. We still don’t know why some places were affected more than others, “he said.
After the asteroid hit Earth, nearly half of the plant species in Colombia died; pollen fossils of these species ceased to appear beyond this point. The rainforest began to be taken over by ferns and flowering plants that, while having a previous impact, were less frequent than they are today. In comparison, coniferous trees disappeared.
Beyond the presence of conifers, the rainforests of the past were probably much scarcer than their modern counterparts. Today’s rainforests have thick canopies and the plants inside are spaced apart, which means more plants transpire water into the atmosphere. This leads to higher levels of humidity and cloud cover. According to Carvalho, the relative lack of moisture in earlier forests means that the regions were probably much less productive than they are today.
But the lower productivity forest remained in place until the asteroid struck. “It was after the impact that we see forests change their structure,” he said.
Researchers have some hypotheses about how this change occurred. The first is that the extinction of the dinosaurs caused the forests to grow denser: there could be fewer animals consuming the plants or stepping on the brush, which allowed the foliage to grow relatively uncontrollably. The second idea is that shortly after the asteroid collided with the planet, there was a selective extinction of conifers in the tropics, which could have been less effective than their flowering counterparts after the impact.
The third is that the consequences of the catastrophe could have fertilized the soil. The tsunami events that occurred after the impact could have brought debris and sediment from carbon-rich, shallow marine areas. The burning of forest fires could have sent ash into the atmosphere, and when it finally settled on the ground, it could have acted as a kind of fertilizer. Carvalho said flowering plants tend to grow better than conifers in nutrient-rich soils. He also noted that all of these hypotheses, or two of them, could be true simultaneously.