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The Colombian rainforest looked very different 66 million years ago. At present, the humid and biodiversity-rich ecosystem is filled with plants and is covered with a thick canopy of leaves and branches that blocks the light. In particular, there are no dinosaurs. But before the dinosaurs vanished with the impact of Chicxulub, signaling the end of the Cretaceous, things looked very different. The vegetation cover of the region was relatively sparse and a multitude of conifers inhabited it.
Using the fossilized remains of plants, a team of researchers studied the rainforest’s past and how the asteroid gave birth 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.
“The forests disappeared because of the ecological disaster … and then the vegetation that returned was mainly dominated by flowering plants,” said Mónica Carvalho, first author and joint postdoctoral fellow at STRI and 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. Examining these fossils gave the team an idea of the types of plants present before and after the asteroid struck the planet. This sequence represents the region’s biodiversity 72 million to 58 million years ago, spanning both before and after impact. “It took us a long time to put together enough data to be able to get a clear picture of what was going on during the extinction,” Carvalho told Ars.
Although the study deals with Colombian fossils, Carvalho said researchers can get a fair idea of what happened in rainforests elsewhere in Central and South America, although the effects of the impact of l asteroid are somewhat variable from region to region. “It’s a bit variable. We still don’t know why some places have been more affected than others, ”she said.
After the asteroid struck Earth, nearly half of the plant species in Colombia perished – pollen fossils from these species ceased to appear beyond this point. The tropical rainforest began to be overrun with ferns and flowering plants which, although present before impact, were less common than they are today. Conifers, by comparison, have effectively become extinct.
Beyond the presence of conifers, the rainforests of the past were probably much more sparse than their modern counterparts. Today’s rainforests have thick blankets and the plants they contain are closely spaced, which means 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 earlier forests means that the regions were probably much less productive than they are today.
But the low-productivity forest remained in place until the asteroid struck. “It is only after the impact that we see the forests change their structure,” she said.
The researchers are speculating on how this change happened. The first is that the disappearance of the dinosaurs made the forests denser – there could have been fewer animals consuming the plants or trampling on the brush, allowing the foliage to grow relatively unchecked. The second idea is that soon after the asteroid collided with the planet, there was a selective extinction of conifers in the tropics – they simply might have fared worse than their flowering peers after the ‘impact.
The third is that the consequences of the disaster could have fertilized the soil. Tsunamis that occurred after the impact could have carried debris and sediment from shallow, carbon-rich marine areas nearby. Scorching 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. Flowering plants tend to grow better than conifers in nutrient-rich soils, Carvalho said. She also noted that all of these assumptions, or two of them, could be true simultaneously.
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