WHAT’S NEW — This large-scale study represents the culmination of about 14 years of work in Colombia, Jaramillo says. Until this point, what happened to tropical forests in South America after the Cretaceous-Paleogene extinction event was relatively unknown.
The study suggests the end of the Cretaceous was a real turning point in the structure and composition of this environment. What we recognize as a lush, biodiverse tropical rainforest today did not exist before the Paleocene.
It also took millions of years for the diversity of the forest to recover after the mass extinction event, which “should be a clear warning against deforestation and the rate at which we are currently intervening on the planet,” says Mónica Carvalho, a postdoctoral fellow at the Smithsonian Tropical Research Institute and co-lead study author.
BACKGROUND — Neotropical rainforests are unique environments that have what is called a closed canopy.
“You go to the rainforest today, you realize that there is a green carpet at the top of the trees,” Jaramillo says. “Every single space at the top of the canopy is occupied by a leaf because this is where most of the photosynthesis is happening.”
To figure out what forests looked like in the past, paleobiologists need to consult the record of ancient plant life. Analyzing the structure and composition of forests involves a science called palynology, the study of pollen. Counting and identifying thousands of fossil pollen grains across columns and layers of rock gives an idea of how the composition of plants in an area changed broadly over time.
Pollen examination can also determine the abundance of flowering plants, also called angiosperms, versus other plants that release spores, such as ferns. Leaf fossils are useful for determining if the forest had a closed canopy or not.
HOW THEY DID IT — Unraveling records of terrestrial life in deep time isn’t easy: ancient climate records on land are typically lost to decomposition and erosion. Carvalho, Jaramillo, and 19 other study authors worked for over a decade, doing an enormous amount of fieldwork, searching out pollen and thousands of leaf fossils to figure out how Cretaceous forests were impacted by the extinction event.
They looked at 39 different sections of rock and analyzed thousands of microscopic pollen grains found in different layers under the microscope. This, plus the thousands of leaf fossils, allowed the researchers to plot changes in the diversity and composition of flora before, during, and after the extinction event.
WHAT THEY FOUND — Cretaceous rainforests were quite different than the rainforests today, the findings suggest. There was plenty of light, and many gaps in the forest. Neotropical rainforests, the forests we recognize, made their appearance during the Paleocene — but it took another six million years before plant diversity recovered. Before the extinction event, the forests were about half ferns and half angiosperms, but in the new rainforests of the Paleocene, flowering plants rose to dominance.
Why did the forests shift from open to closed canopies across the end of the Cretaceous to the beginning of the Paleocene?
Jaramillo says there are potentially three reasons why, and all of these factors could have been happening at the same time.
It could have been that dinosaurs, especially large plant-eating ones, ate so much biomass it kept lighting shining through the forest during the Cretaceous.
The change in the structure of the environment could have been from an increase in soil nutrients. It is even possible the ash fall from the asteroid impact provided a layer of fertilizer.
Angiosperms may have been able to rise to dominance because there was selective extinction of ferns and other non-flowering plants due to their lack of ecological diversity.
WHY IT MATTERS — Rainforests in Central and South America are some of the most biologically diverse locations on Earth, and this study shows how they got their start. The study also illustrates the fact that major changes to an environment can completely upend an ecosystem. It's an ominous lesson in the age of anthropogenic climate change.
More broadly, this research matters because it makes a critical discovery about deep time in an area of the world, South America, that is often underrepresented in paleontology. Jaramillo says many of the researchers who participated in this study are from Colombia and shows that “if you create a critical mass, we can produce good science.”