Ferns Fuelled Vast Fires After Europe’s Mass Extinction 201 Million Years Ago

A new study has concluded that the ferns which spread across Europe after the mass extinction 201 million years ago provided fuel for vast fires. These fern savannahs repeatedly burned for tens of thousands of years.

The results assign fire a much greater part in one of the most severe biological catastrophes in Earth’s past. Fires opened up land for ferns, while the expanding fern cover increased the likelihood of further fires. This feedback cycle left north-west Europe burnt and without trees for millennia.

When the forests collapsed

The catastrophe began with volcanism on a scale rarely matched elsewhere. As the supercontinent Pangaea began breaking apart, magma surged through the crust. These eruptions emitted an estimated 100 trillion tonnes of carbon dioxide.

Global temperatures rose by approximately 5 to 10°C. Conifer forests, which had covered large parts of the region, declined and died. Ferns colonised the devastated landscape and remained there.

Fresh research by geologists at Utrecht University outlines both why ferns were so successful and what their expansion triggered. Professor Bas van de Schootbrugge, a senior author of the paper, investigates how life reacted to ancient greenhouse crises.

In an email to Earth.com, Van de Schootbrugge explained why ferns are such efficient colonisers. “The bottom line is that ferns can rapidly invade disturbed landscapes that have been destroyed by fire, and deal with altered soil or no soil conditions.”

Decades of pollen research had already demonstrated this takeover. However, the role played by fire had remained uncertain. Charred material within the rocks suggested burning, while previous work linked greater fire activity with forest loss.

His team collected samples from four drill cores across north-west Europe. The deepest core, recently drilled in England, reaches around 640 metres beneath the ground.

A dark zone in four cores

Two principal approaches are used to investigate ancient fires. Scientists either count fossil charcoal in sediment or measure the soot-derived molecules produced in wildfire smoke and deposited afterwards.

Neither technique is straightforward. Charcoal pieces can break apart during burial, exaggerating their apparent numbers. Smoke molecules may travel well beyond a fire or may not survive in the rock at all.

The researchers therefore examined a simpler clue. Under a microscope, fossil pollen and spores altered in colour through layers deposited during the extinction event. The older, deeply buried samples were light in colour, whereas grains from the extinction interval were nearly black.

This pattern is the reverse of what would normally be expected. Plant material generally darkens the deeper it is buried, as heat underground alters it, meaning the oldest samples ought to have been darkest. Above the extinction layer, however, the grains became pale yellow once again.

The darkening occurred at the same point in all four cores, despite the basins having very different geological histories. Burial was therefore not a viable explanation. Fern spores and conifer pollen darkened to the same degree, excluding a cause limited to a single plant group.

Van de Schootbrugge told Earth.com, “The biggest surprise was how consistent the data really are from site to site.”

The team has also identified less detailed signs of the same dark band at roughly a dozen additional European locations. This indicates a continent-wide signal rather than an isolated local feature.

Burning spores in the lab

The pattern needed to be tested. Collaborators heated spores from a living clubmoss in controlled conditions and monitored how their colour shifted at every temperature.

Discussing the experiment with Earth.com, Van de Schootbrugge said: “By placing spores in an oven and baking them at different temperatures, we reproduced the same darkening effects as what we saw 200 million years ago. It is always nice when something that happened so long ago can actually be reproduced in a lab under controlled conditions.”

The fossil grains corresponded with the range created by surface fires. Such fires move through low vegetation and pass rapidly, unlike more intense blazes that rise into the forest canopy. The researchers quantified the colour, and fire is the explanation they consider most consistent with the evidence.

Calculating the darkness required no unusual equipment. A light-microscope camera records each grain’s red, green and blue values, which are combined into a single grey value. The team generated 15,000 such measurements.

The dark zone coincided with the fern surge, the extinction event, and peaks in charcoal and smoke molecules. A 2010 study from East Greenland reported increasing fire as narrow, more flammable foliage displaced broad leaves. The new evidence expands that view far beyond a single region.

Ferns as fuel

Ferns form thick, low-growing mats that can dry into material much like kindling. Certain species also grow high enough to transfer flames upwards into any remaining standing vegetation. “When the ferns dry out, the thick mats act as the ideal fuel to trigger massive wildfires,” said Van de Schootbrugge.

Fires do little to halt their recovery. Although fronds burn away, the underground stems endure and send up fresh growth. Slower-growing plants do not have the same advantage.

Each fire consequently strengthened fern dominance, and greater fern cover supplied more fuel. The period dominated by ferns persisted for between 40,000 and 300,000 years.

Climate conditions sustained the cycle. Extended droughts dried the mats, while the intense rainfall that followed eroded burnt soil and carried it into coastal basins.

A separate study of one of the same German cores found volcanic mercury and distorted fern spores continuing for more than a million years afterwards.

A warning for modern landscapes

Wildfires at the end of the Triassic had been suspected for many years, but their scale had not been demonstrated. The darkened grains provide that evidence at four locations simultaneously and reveal that the plants replacing the forests were also fuelling the fires.

The same combination can be recognised today. When Earth.com asked about present-day parallels, Van de Schootbrugge pointed to living examples. “Ferns today are already invading certain disturbed areas and act as fire propellants.”

He cited invasive climbing ferns that smother native trees in the Everglades, where drying mats allow flames to reach the canopy. He said it remains uncertain how much global warming may intensify this effect.

The colour-based technique can be used widely. Requiring only a microscope and camera, it can be applied to any sediment that still contains pollen.

Van de Schootbrugge believes it could be useful for other extinction events marked by a fern increase, including the end-Permian crisis and the event that ended the age of dinosaurs. These dark zones can now be examined for evidence of fire.

Photo credit: Mark Garlick