How did the catastrophe that sealed the age of the dinosaurs affect the plant world? An examination of fossil leaves and pollen now shows what happened in the rainforests of what is now South America. Accordingly, around 45 percent of the plant species there died out as a result of the asteroid impact and biodiversity only recovered over the course of millions of years. In contrast to the Cretaceous Period, the result was a rather dense vegetation dominated by flowering plants, which still characterizes the tropical rainforests today, the researchers report.
As is well known, the Cretaceous period ended with a bang: Even today, the extensive traces of the Chicxulub asteroid, which crashed into the earth about 66 million years ago, can still be seen in the area of the Mexican Yucatan peninsula. The massive impact is estimated to have released the energy of more than ten million Hiroshima atomic bombs. Fire storms and gigantic tsunamis raced across the planet and the sky darkened. Presumably this was followed by a period of severe climate cooling that lasted for years. Many of the creatures of the Cretaceous were unable to cope with the abrupt changes in their environment and the collapse of the food chains: paleontological finds show that as a result of the impact, around 75 percent of all animal species disappeared from the stage of evolutionary history.
The fate of the rainforests in sight
The most famous victims were the dinosaurs – but not only animals but also plants were affected by the catastrophe, as is already known from fossil records. So far, however, there has been hardly any information about the fate of the plant community in the region, which today houses one of the most important and biodiverse ecosystems on earth: tropical South America. “We wondered how the tropical rainforests changed after the drastic ecological disturbance from the Chicxulub felling,” says Mónica Carvalho of the Smithsonian Tropical Research Institute in Panama. As part of the study, she and her colleagues examined over 50,000 fossil pollen finds and more than 6,000 leaf fossils from before and after the impact. The plant fossils came from different sites in Colombia.
As the researchers report, the pollen and spores from rocks older than the impact show that the rainforests of the late Cretaceous period were characterized by a mixture of some flowering plants (angiosperms), but mainly ferns and naked-seed plants (gymnosperms). Accordingly, the dinosaurs lived there in the shade of many conifers. The asteroid impact on the Cretaceous-Paleogene border then resulted in a collapse of the entire plant diversity by 45 percent, according to the investigations. Only in the course of the next ten million years did biodiversity return to a level similar to that before.
New types and structures
The new rainforests differed significantly from those of the Cretaceous, however, the researchers report: The representatives of the gymnosperms had almost completely disappeared from the tropical vegetation of the New World – the flowering plants had taken over. The investigations of the leaf fossils also provided the researchers with indications that the structural structure of the forests had changed significantly in the period after the disaster. Certain features of the leaf structures show that the trees were comparatively far apart before the felling. There was therefore no shot canopy so that the light could reach the forest floor. But in the course of the ten million years after the impact, the rainforests shaped by the flowering plants became significantly denser. This is how the vegetation structures emerged as we know them today.
As the researchers point out, the representatives of a certain plant group in particular had a successful career in the new rainforests: According to fossil traces, there was a great abundance of species and population density in the legumes, which to this day represent an important family of plants in the tropical rainforests. Paleobotanists say they have probably also greatly altered the nutrient cycle in the ecosystems. Because the legumes have a special adaptation: They live in symbiosis with bacteria that give them access to the nitrogen in the air. In this way, the legumes were able to enrich the material cycle of the new forests with this important fertilizer.
Why so different?
But why didn’t the forests just grow in a similar way after the felling? There are three possible explanations for this, the scientists say, and their mixture may have led to the change. One aspect may have been that before the impact, the dinosaurs kept the forests open by devouring large masses of plants and trampling through the landscape. A second factor could have been the ash deposits after the felling. Possibly they enriched the soils in the tropics with nutrients that gave the fast-growing flowering plants an advantage. Thirdly, the conifers may also have reacted particularly sensitively to the changes after the disaster, which gave the flowering plants a decisive starting point in the recovery of the system.
“In any case, it is becoming apparent that the tropical ecosystem did not simply react to the abrupt disturbance by simply regressing. Instead, it was replaced by something new – albeit in the course of a rather lengthy process, ”says Carvalho, summing up the results of the study.
Source: Smithsonian Tropical Research Institute, article: Science, doi: 10.1126 / science.abf1969