Earth’s ancient forests carry a warning for forests today

Fossils from Wyoming show how extreme warming stripped away a third of the forest canopy 56 million years ago, echoing changes now underway in forests today.

Carbon dioxide is plant food, so more of it in the air should mean fuller forests. From the early 1980s on, satellite records showed exactly that. Then, around the year 2000, the greening reversed across most of the world’s vegetated land.

A new study finds the same turn 56 million years fago, the last time the planet took a huge dose of carbon. Forests in what is now Wyoming lost about a third of their leaf cover. Past a point, the authors conclude, heat and drought kill trees faster than extra carbon dioxide can boost their growth.

Regan Dunn, who studies fossil plants at the La Brea Tar Pits in Los Angeles, led the work. Her team has been collecting fossils in Wyoming’s Hanna Basin for more than a decade.

Geologists call that heat spike the Paleocene-Eocene Thermal Maximum, or PETM, and the authors treat it as the closest natural comparison to what people are doing now. It’s an imperfect one.

Palms replaced Wyoming’s dense forests

Before the heat arrived, dense forest covered the basin, several layers of leaves deep. Then the pollen record changes in a hurry: ferns for a short interval, then palms as the dominant plants, while relatives of today’s walnuts, elms, and birches declined.

In an interview with Earth.com, Dunn described open woodland: low trees standing far apart, many of them palms. They may have resembled the very open dry tropical forests of Central and South America, she said.

“A walk through the PETM forests would have been hot and dry, with little shade cover and parched soils.”

Newcomers arrived from warmer places: pollen from plants whose closest living relatives now grow along the Gulf Coast and through Central and South America. Pooled across the event, the PETM layers hold at least 40% more kinds of pollen and spores than the layers above or below.

It wasn’t only Wyoming. Pollen records from the same ancient warming show plant communities reorganizing in eastern North America, the far north, and Eurasia.

Leaf cells reveal ancient forest cover

A leaf’s outer skin leaves behind a tough, clear film that outlasts the leaf. Botanists call it cuticle, and it holds the outlines of the cells that made it for tens of millions of years.

Those outlines depend on light. Shade leaves build long, narrow cells. Sun leaves build shorter, rounder ones.

Turning that into a number took fieldwork in living forests. At 41 sites across Costa Rica, Ecuador, and Argentina, the team photographed the canopy overhead through a fisheye lens. Then they measured cells from the leaf litter underneath, at least 33 fragments per site. Cell shape tracked the canopy closely.

What a fisheye photograph measures is leaf area index, the amount of leaf stacked over a patch of ground. Five means roughly five layers’ worth.

Dunn said the surprise was that broken leaf fragments work at all as a way to measure ancient forest cover. That has been a hard question in the study of ancient ecosystems.

The team then measured fossil cuticle from 82 Wyoming rock samples the same way: 32 from before the event, 16 from during it, and 34 from after.

Fragments of fossilized leaves like these helped the authors recreate the forest canopies of 56 million years ago. Credit: Dr. Regan Dunn

Wyoming’s canopy thinned by a third

The team estimates 4.3 for the Wyoming forest before the warming, inside the range of a modern broadleaf forest, 3.9 to 5.7. The last sample before the carbon spike is the densest reading in the whole record, 5.9.

Then the canopy came apart.

Inside the event, the first sample comes in at 2.3, a drop of 61%, and the dominant species turn over completely between the two layers. Across the event the average is 2.8, about 35% below the older forest, and for long stretches it stays near 1.5, what modern shrubland and dry scrub measure.

That order matters. The densest canopy in the record comes right before the crash, which may be extra carbon dioxide doing its fertilizing work up until the heat arrived. Big trees that had just built extra canopy had the most to lose.

“It was also surprising how closely the forest canopy’s response matched what we know about the changes in the carbon cycle during the PETM,” Dunn told Earth.com. When the carbon release ran heaviest, she said, the forests suffered most.

Cell shapes shown here indicate greater exposure to sunlight in open vegetation. Scale bar is 50 microns (0.05 millimeters) in length. Credit: Dr. Regan Dunn

Heat and drought killed large trees

The authors attribute the collapse to trees dying in heat and drought. No temperature record survives at this site, so they used measurements from a nearby basin: about 9 °F (5 °C) of average warming, with summers above 104 °F (40 °C).

Hotter air heats leaves and soil. A hot leaf loses water faster, and the pores that should hold it in work less well. Big trees built for plenty of water are the ones that suffer most.

“It turns out that too much carbon dioxide is a bad thing for forests because the accompanying warming and drying stresses the trees and kills many of them,” said Ellen Currano, a co-author at the University of Wyoming.

Currano said forests are beginning to change the same way today, particularly in the Amazon.

The ground changed with the trees. Before and after the event this was swamp country, waterlogged and peaty, the source of the basin’s coal beds. During it, the soils drained and reddened.

Rivers wandered more, leaving thick sheets of sand and coarse pebbles from the highlands around the basin. The samples with the least leaf cover are the ones packed with far older marine plankton fossils, washed out of eroding hillsides and dumped into the floodplain.

Forest recovery took 100,000 years

The low readings persist through most of the event. The canopy closed again as the carbon cycle settled, and the densest forests in the record came afterward, in the early Eocene, averaging about 5.2. The highest of those readings run past anything the team measured in a living forest.

Asked by Earth.com how long the canopy needed to come back, Dunn was specific.

“In the case of the PETM, it took well over 100,000 years to begin recovering, followed by several tens of thousands of years subsequently to make a full recovery,” she said.

Forests can return to what they were. The speed depends on how much carbon dioxide is in the air and how quickly that carbon is locked away again. The forests growing back were probably a major driver of that, cooling the planet and restoring water cycles.

Losing forests does the reverse. Other researchers put the cost of losing half the carbon in plants and soils at about 600 extra parts per million in the air. The erosion and soil evidence from this interval fits forests that had turned from carbon sinks into sources.

Are forests losing canopy today?

Satellites have measured the same quantity since 1982, and for about 18 years the planet got greener. Around 2000 that reversed across roughly 90% of vegetated land, and the sharpest reversals were in the tropics and mid-latitudes.

Dunn said tree deaths are happening everywhere on Earth right now, from heat, drought, insects, disease and fire.

The authors are explicit about the comparison’s limits. The carbon released then is comparable in total to what people are projected to emit this century, but it came out over thousands of years, not a few hundred. Nothing back then was clearing forests or changing fire regimes on top of the heat.

What nobody can say yet is how fast the canopy came apart. The first few thousand years are sampled too coarsely to tell. That’s also where the canopy and the ferns don’t line up: the canopy fell first, and it had partly recovered by the time ferns took over.

“We should recognize that forests are key to moderating the impacts of our emissions, and we should act to protect and restore them for our own sake and the sake of the planet,” Dunn said. “It’s not too late.”

Settling the timing would take denser sampling through those first millennia, from rock that holds leaf fragments as well as the Hanna Basin does. The measurements behind this one are already public.

The full study was published in the journal Science.

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