Fossils show Southern Hemisphere hothouse forests

Arid today, Australia was once covered by lush forests, according to new research. Credit: Unsplash/CC0 Public Domain

David Greenwood has spent decades collecting Australian fossil plants, some so beautifully preserved it’s hard to realise they’re millions of years old. Greenwood and a team of academics, including climate modeller David Hutchinson from the University of New South Wales and UConn paleobotanist Tammo Reichgelt, have begun piecing together the evidence to see what more they can learn from the collection. Paleoceanography & Paleoclimatology reported their findings.

Eocene fossils date back 55 to 40 million years. In those hothouse conditions, palms grew at the North and South Poles while desert landmasses like Australia were lush and green. Reichgelt and colleagues searched for changes in precipitation and plant productivity.

Different plants flourish in different settings, hence plant fossils can reveal their surroundings.

By studying the morphology and taxonomy of 12 different floras, the researchers gained a better understanding of the Eocene’s climate and productivity.

Reichgelt says the morphological method relies on the fact that angiosperm leaves adapt to climate.

“If a plant with large leaves is left in the sun without enough water, it shrivels and dies,” says Reichgelt. “Large-leafed plants also lose heat. Finding a huge fossil leaf implies this plant wasn’t growing in a dry or cold climate that caused excess evaporation or heat loss. These and other morphological traits can be quantified. We may relate fossils to present floras around the planet.”

Second, taxonomy. “Up a mountain, the flora’s taxonomy alters. Low on the mountain, maples and beeches may dominate the deciduous forest, while spruce and fir dominate higher up “Reichgelt Beech and maple fossils indicate a warmer climate than spruce and fir fossils. Such climatic preferences can be utilised to quantitatively reconstruct an ancient plant group’s climate.

The Eocene climate was substantially different from modern Australia’s. The continent needed consistent rainfall to maintain a verdant landscape. Warmth increases evaporation and rainfall in Australia’s interior. Carbon fertilisation, caused by higher quantities of carbon dioxide in the atmosphere, also contributed to the lushness. Reichgelt says that with so much CO2, plants gorged.

“Southern Australia was mainly forested, with primary productivity equivalent to seasonal forests,” Reichgelt explains. “In the Northern Hemisphere summer, much of carbon dioxide is taken down due to primary productivity in vast woods approximately 40 to 60 degrees north. No landmass exists at those latitudes in the Southern Hemisphere. Eocene Australia was 40-60 degrees south. During the Southern Hemisphere summer, a vast, highly productive continent would take down carbon, more so than arid Australia does today.”

Geological evidence implies the climate is sensitive to CO2 and that the effect may be higher than our models predict, adds Hutchinson. “Our climate models tend to under-represent polar warming amplification, according to the facts. If we enhance our simulations of the high-CO2 Eocene, we may improve our future predictions.”

Future initiatives will broaden the data set beyond Australia to examine global production in a greenhouse environment.

“We have vast collections of plant fossils collected around the world, so we can apply the same methods we use here,” explains Reichgelt.

With rising carbon emissions, scientists are researching what occurs in the biosphere when plants increase photosynthetic activity and water usage efficiency. Reichgelt says current plants haven’t evolved to shifting CO2 levels. However, we may learn from the past.

Fossil floras allow us to peep into the ecosystem of old hothouse worlds.

Further information: Tammo Reichgelt et al, Plant Proxy Evidence for High Rainfall and Productivity in the Eocene of Australia, Paleoceanography and Paleoclimatology (2022). DOI: 10.1029/2022PA004418

Source: University of Connecticut