Study: How forests changed during a hot period 56 million years ago

For their study, the researchers determined how much light a leaf received in fossil leaves and in present-day leaves, or conversely, how much shade there was at the tree's location. The study by a group led by paleobotanist Regan Dunn from the Natural History Museum of Los Angeles County in Los Angeles, USA, was published in the journal Science.

The investigated hot period in prehistory occurred at the border of the Paleocene and Eocene geological epochs. It probably happened because of a sharp increase in CO₂ levels in the atmosphere. Within 3,000 to 10,000 years, around 2,500 to 4,500 billion tons of carbon entered the Earth's air and oceans. This development has led to “massive disruptions to the carbon cycle, ocean acidification and an increase in the global average temperature of five to nine degrees Celsius,” write the study authors. The exact cause of the greenhouse gas release is not clear; The cause may have been massive volcanic eruptions.

“We are releasing CO₂ into the atmosphere faster than any known natural process.”

The CO₂ enrichment of the air occurred quickly back then, but not nearly as quickly as today. “We are releasing CO₂ into the atmosphere faster than any known natural process,” Dunn said, according to a statement from her institution. Nevertheless, the temperature maximum 56 million years ago offers the best opportunity to see how the Earth's climate and ecosystems react to massive carbon input.

The researchers gained insight into the plant world at this time through leaf fossils from the Hanna Basin in the US state of Wyoming. The leaves, contained in coal and other organic materials, are so well preserved that researchers were able to determine the size of the individual cells in the outer layer of tissue, the epidermis.

From this they were able to reconstruct the lighting conditions at that time. Because leaves in the shade have a different aspect ratio than leaves in the sun, they are longer. “This is how plants respond to shade: they elongate the leaf by stretching each individual cell,” says Dunn. The ratio of the long side to the narrow side provides information about how dense the forest canopy was during the plant's lifetime. The scientists determined the aspect ratio of the leaves of today's trees in relation to the forest canopy; From this they developed a model that can be used to determine how dense the forest was during the lifetime of fossil leaves based on the leaf cells. The researchers also analyzed fossils of pollen and spores.

The historical model shows where the earth could develop

The analysis of the data showed that the forest canopy thinned rapidly during warming 56 million years ago. In addition, the vegetation shifted from angiosperms, primarily flowering plants, to gymnosperms, especially ferns. The tree species increasingly dominated by palms and other plant species, whose modern relatives have their greatest biodiversity in subtropical or dry tropical climates.

“It turns out that excess carbon dioxide is harmful to forests because the associated warming and drying stresses the trees and causes many of them to die,” explains Ellen Currano from the University of Wyoming in Laramie, USA, co-author of the study. The historical model shows where the earth could potentially develop soon.

From around 1982, increased CO₂ levels in the air caused the leaf area index to increase; the index is a measure of how dense the forests are. Since plants need CO₂ to grow, the additional CO₂ made for greener forests. But around the year 2000, the development was reversed on around 90 percent of the areas with vegetation: the earth has become browner since then. The researchers interpret this as a response to increasing heat and drought stress in the plants. “We would be wise to protect and restore forests while they can still help mitigate the unprecedented pace of human-caused climate change,” Dunn emphasizes.