From Palms to the Poles: Fossil Plants Reveal the Lush Forests of the Southern Hemisphere in the Climate of Ancient Greenhouses

For decades, paleobotanist David Greenwood has collected plant fossils from Australia – some so well preserved it’s hard to believe they’re millions of years old. These fossils contain details of the ancient world in which they thrived, and Greenwood and a team of researchers, including modeller and climate researcher David Hutchinson, from the University of New South Wales, and paleobotanist from the Department of Geosciences of UConn, Tammo Reichgelt, began the process of gathering the evidence to see what more they could learn from the collection. Their findings are published in Paleoceanography & Paleoclimatology.

Fossils date back 55 to 40 million years, during the Eocene era. At that time, the world was much warmer and wetter, and these greenhouse conditions meant that there were palm trees at the North and South Poles and that mostly arid landmasses like Australia were lush and green. Reichgelt and his co-authors searched for evidence of differences in rainfall and plant productivity between yesterday and today.

Since different plants thrive in specific conditions, plant fossils can tell what kinds of environments these plants lived in.

By focusing on the morphology and taxonomic characteristics of 12 different floras, the researchers developed a more detailed view of what the climate and productivity was like in the old greenhouse world of the Eocene era.

Reichgelt explains that the morphological method is based on the fact that the leaves of angiosperms – flowering plants – generally have a strategy to react to the climate.

“For example, if a plant has large leaves and it’s left in the sun and doesn’t get enough water, it starts to shrivel up and die from excessive evaporation,” Reichgelt says. “Big-leaved plants also lose heat to their environment. So finding a large-leaf fossil means that most likely this plant was not growing in an environment that was too dry or too cold for excessive evaporation or sensible heat loss. These and other morphological characteristics can be related to the environment that we can quantify.We can compare the fossils to modern floras around the world and find the closest analogy.

The second approach was taxonomic. “If you travel in the mountains, the taxonomic composition of the flora changes. Further down the mountain there may be hardwood forest dominated by maple and beech and the higher you go up the mountain the more spruce and fir forest you see,” says Reichgelt. “Finding beech and maple fossils therefore probably means a warmer climate than if we find spruce and fir fossils.” Such climatic preferences of plant groups can be used to quantitatively reconstruct the ancient climate in which a plant group in a fossil assemblage grew.

The results show that the Eocene climate would have been very different from modern Australia’s climate. To maintain a verdant landscape, the continent needed a steady supply of rainfall. The heat meant more evaporation and more precipitation was available to move into Australia’s continental interior. Higher levels of carbon dioxide in the atmosphere at the time, 1,500 to 2,000 parts per million, also contributed to the lushness via a process called carbon fertilization. Reichgelt explains that with the abundance of CO2, the plants stuffed their mouths.

“Southern Australia appears to have been largely forested, with primary productivity similar to seasonal forests, much like those here in New England today,” Reichgelt says. “In the northern hemisphere summer today, there is a big change in the carbon cycle, as a lot of carbon dioxide is being extracted due to primary productivity in the huge expanse of forests that exist in a great belt around 40 to 60 degrees north. In the southern hemisphere, no such landmass exists today at these same latitudes. But Australia during the Eocene occupied 40 degrees to 60 degrees And as a result, there would be a large, very productive landmass during the Southern Hemisphere summer, attracting carbon, more than Australia does today because it is largely arid.

Hutchinson says that the geological evidence suggests that the climate is very sensitive to CO2 and that this effect may be larger than our climate models predict, “The data also suggests that the polar amplification of warming was very strong, and our models climatic conditions also tend to under-represent this effect. So if we can improve our high CO models2 Eocene world, we could improve our predictions of the future.

Future projects will extend the dataset beyond Australia to ask what global productivity does during a greenhouse climate on a global scale.

“We have large datasets of plant fossils that have been collected around the world, so we can apply the same methods we use here to ask what’s happening to the global productivity of the biosphere,” says Reichgelt. .

With increasing carbon emissions, more and more research is being done to study what is happening in the biosphere with increased photosynthetic activity and water use efficiency in plants. Reichgelt explains that modern factories have not had time to adapt to changing CO2 conditions. However, by looking to the past, we can glean some of this information.

“It will obviously take a long time for plants to adapt to changing CO2 levels, but fossil floras allow us to peek into the biosphere of ancient greenhouse worlds.”