The rainforest of Colombia looked like very different 66 million years ago. Currently, the moist and biodiverse ecosystem is packed with plants and covered in a thick, light-blocking canopy of leaves and branches. Notably, there are no dinosaurs. But before the dinosaurs left with the Chicxulub impact, which marked the end of the Cretaceous, things looked very different. Plant cover in the area was relatively sparse, and a bevy 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, was led by scientists from the Smithsonian Tropical Research Institute (STRI) in Panama and supported by scientists from the Negaunee Institute for Plant Conservation Science and Action at the Chicago Botanic Garden.
“Forests disappeared as a result of the ecological catastrophe … and then the returning vegetation was mainly dominated by flowering plants,” said Mónica Carvalho, lead author and joint postdoctoral fellow at STRI and the Universidad del Rosario in Colombia, in an interview with Ars.
The study began 20 years ago when parts of the team collected and analyzed 6,000 leaf and 50,000 pollen fossils from Colombia. By looking at these fossils, the team got an idea of the types of plants that were present both before and after the asteroid hit the planet. This series represents the region’s biodiversity between 72 million and 58 million years ago, both before and after the impact. “It took us a long time to gather enough data to get a clear picture of what happened during the extinction,” Carvalho told Ars.
While the study is on Colombian fossils, Carvalho said the researchers can get a good idea of what happened in rainforests elsewhere in Central and South America, although the effects of the asteroid’s impact are somewhat variable from region to region. . ‘It’s a bit variable. We still don’t know why some places are more affected than others, ”she said.
After the asteroid hit Earth, nearly half of Colombia’s plant species died – the pollen fossils for those species did not reappear after that point. The rainforest began to be taken over by ferns and flowering plants which, while present before impact, were less common than they are now. In comparison, the conifers actually died out.
Apart from 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 in them are close together, meaning more plants are letting water float into the atmosphere. This leads to higher 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 stayed in place until the asteroid struck. “Only after the impact do we see the forests change their structure,” she said.
The researchers have some hypotheses about how this change happened. The first is that the demise of the dinosaurs caused the forests to become denser – there could have been fewer animals consuming the plants or stomping through the bushes, allowing the foliage to grow relatively out of control. The second idea is that shortly after the asteroid collided with the planet, there was a selective extinction of conifers in the tropics – they just couldn’t have fared so well after impact than their flowering counterparts.
The third is that the aftermath of the disaster could have fertilized the soil. Tsunami events that occurred after the impact may have transported debris and sediment from nearby carbon-rich, shallow sea areas. Burning wildfires could have sent ash into the atmosphere, and when it finally settled to the ground, it could have served as a kind of fertilizer. Flowering plants tend to grow better than conifers in nutrient-dense soils, Carvalho said. She also noted that all of these hypotheses, or two of them, could be true at the same time.