How the Chicxulub impactor gave birth to modern rainforests

Tropical rainforests are biodiversity hotspots today and play an important role in global climate systems. A new study published today in Science sheds light on the origins of modern rainforests and can help scientists understand how tropical forests will respond to a rapidly changing climate in the future.

Study by researchers at the Smithsonian Tropical Research Institute (STRI) shows that the impact of asteroids that ended the dinosaur reign 66 million years ago also caused the extinction of 45% of plants. of what is now Colombia, and gave way to the reign of flowering plants in modern rainforests.

“We wondered how the rainforests changed after a drastic ecological disruption such as the impact of Chicxulub, so we looked for fossils of tropical plants,” said Mónica Carvalho, first author and joint postdoctoral fellow at STRI and the ‘Universidad del Rosario in Colombia. “Our team examined over 50,000 fossil pollen records and over 6,000 leaf fossils before and after impact.”

In Central and South America, geologists scramble to find fossils exposed by road cuts and mines before heavy rains wash them away and the jungle hides them again. Prior to this study, little was known about the effect of this extinction on the evolution of flowering plants that now dominate the American tropics.

Carlos Jaramillo, STRI paleontologist and his team, mostly STRI fellows – many of them from Colombia – studied pollen grains from 39 sites including rock outcrops and cores drilled for oil exploration in Colombia, in order to paint a large regional picture of forests before and after impact. Pollen and spores obtained from rocks older than impact show that rainforests were also dominated by ferns and flowering plants. Conifers, such as the relatives of the Kauri pine and Norfolk Island pine, sold in supermarkets at Christmas (Araucariaceae), were common and cast their shadows on the dinosaur trails. After the impact, conifers almost completely disappeared from the New World tropics, and flowering plants took over. Plant diversity has not recovered for about 10 million years after impact.

The leaf fossils told the team a lot about the past climate and the local environment. Carvalho and Fabiany Herrera, postdoctoral research associate at the Negaunee Institute for Conservation Science and Action at the Chicago Botanic Garden, led the study of more than 6,000 specimens. Working with Scott Wing at the Smithsonian National Museum of Natural History and others, the team found evidence that rainforest trees before impact were widely spaced, allowing light to reach the ground from the forest. Less than 10 million years after the impact, some rainforests were dense, like those of today, where the leaves of trees and vines cast a deep shadow on smaller trees, bushes and herbaceous plants. below. The more sparse covers of pre-impact forests, with fewer flowering plants, would have displaced less soil water into the atmosphere than those that grew in the millions of years that followed.

“It was just as rainy in the Cretaceous, but the forests functioned differently.” Carvalho said.

The team found no traces of legume trees before extinction, but thereafter there was great diversity and abundance of legume leaves and pods. Today, legumes are a dominant family in tropical rainforests and, through associations with bacteria, take nitrogen from the air and turn it into fertilizer for the soil. The rise of legumes would have considerably affected the nitrogen cycle.

Carvalho also worked with Conrad Labandeira at the Smithsonian National Museum of Natural History to study insect damage on leaf fossils.

“Insect damage to plants can reveal in the microcosm of a single leaf or in the expanse of a plant community the basis of trophic structure in a rainforest,” Labandeira said. “The energy residing in the mass of plant tissue that is transmitted along the food chain – ultimately to boas, eagles and jaguars – begins with the insects that skeletonize, chew, pierce and suck, undermine, scuff and dig into plant tissue. proof of this food chain of consumption begins with all the diverse, intensive and fascinating ways in which insects consume plants. “

“Before impact, we see that different types of plants have different damage: the diet was specific to the host,” Carvalho said. “After impact, we find the same types of damage on almost all plants, which means the diet was much more general.”

How the aftermath of the impact transformed the sparse, evergreen rainforests of the dinosaur era into the rainforests of today – towering trees dotted with yellow, purple and pink blossoms, dripping with orchids ? Based on evidence from both pollen and leaves, the team come up with three explanations for the change, all of which may be correct. One idea is that dinosaurs kept forests open before impact by foraging and moving around the landscape. A second explanation is that the ash fall from the impact has enriched soils in all the tropics, giving faster growing flowering plants an advantage. The third explanation is that the preferential extinction of coniferous species has created an opportunity for flowering plants to take over the tropics.

“Our study follows a simple question: How are tropical rainforests changing?” Carvalho said. “The lesson learned here is that under rapid disturbances – geologically speaking – tropical ecosystems don’t just bounce back; they’re replaced and the process really takes a long time.”