The concentration of CO₂ in the air has reached 600 parts per million (ppm). At first, it is manna for plants: the greater availability of carbon dioxide (their oxygen) boosts trees that grow and grow without restraint. But that concentration of greenhouse gases keeps heating the planet. The atmosphere loses more and more water and has to steal moisture from leaves, stems, and trunks. Plant mortality multiplies and the forest thins out. This allows solar radiation and heat to penetrate the understory, feeding back the process. Temperate climate species are displaced by xerophilous (drought-resistant) and thermophilous (heat-loving) species. Walnuts, elms, and birches give way to palms and shrubs. This process, which occurred at high and mid latitudes around the planet, happened 56 million years ago. But, according to a new study published in Science, it looks too much like what is happening with current forests.
Geologists, paleobotanists, and other scholars of the planet’s past call that process the Paleocene-Eocene Thermal Maximum (PETM). Fueled by a growing amount of CO₂ in the atmosphere, probably volcanic in origin, it caused global warming with temperature increases above 5°. Its impact was such that it marks the end of one geological period (the Paleocene) and the beginning of another (the Eocene). It is one of the examples used by current climate change deniers who have read it to insist that the climate always changes.
Just like during the PETM, deniers would recall, the amount of CO₂ in the atmosphere keeps growing; it is now at 423 ppm. As 56 million years ago, droughts multiply, with the atmosphere drier than in centuries; countless animals and plants die, causing changes in the landscape; and as then, the oceans acidify… What they do not say is that the PETM was a process lasting several millennia, while the current one is happening in decades. But, except for the speed and origin, anthropogenic this one, volcanic that one, they are similar climate changes.
“We discovered that, during the PETM, forests in Wyoming [United States] experienced an average 35% decrease in leaf area index (LAI),” says Regan Dunn, researcher at the Natural History Museum of Los Angeles County (United States) and lead author of the study, in an email. “This does not mean the forest disappeared, but that it lost about a third of the vegetation cover that drives many of its ecological processes,” she clarifies. “More sunlight reached the forest floor, temperatures rose, soils dried, less water was returned to the atmosphere through transpiration, and the forest became less effective at regulating climate and storing carbon,” Dunn adds.
The decline of a forest that in the Paleocene was dominated by juglandaceae, the family of walnuts and pecans, betulaceae (birches), or cupressaceae like junipers was not an isolated phenomenon in Wyoming, nor even in what is now North America. It occurred worldwide at mid and high latitudes, those dominated then, as now, by temperate forests. Gradually, they gave way to environments more like Mediterranean or dry tropical regions, such as the Amazonian cerrado. “In a way, the PETM landscape in Wyoming, USA, would have resembled a mix of the structure of Spanish shrublands and the structure and species composition of the dry tropical forests of the Neotropics,” explains Dunn.
The traditional fossil record shows the change in flora between the end of the Paleocene and the beginning of the Eocene. But it does not capture well what happened in between. Dunn and her colleagues devised an original method to do so. “We reconstructed the tree canopy of the past by measuring the shape of microscopic cells of the leaf epidermis,” explains Dunn. These cells change shape depending on the amount of sunlight they receive during growth. Leaves that develop in the shade have longer and more elongated cells than those growing in full sun. And they could see that in fossilized leaves, in their phytoliths.
After validating their idea by analyzing leaf litter from about twenty current forests and jungles, they could estimate the LAI, the amount of leaves in those Wyoming forests and how they thinned during the PETM. “This contrasts with the multi-layered wet forests before and after the PETM,” says the researcher, adding: “Spain also has some notable sites that preserve information from the PETM period, such as Zumaia, which documents changes in marine depths at that time, and terrestrial records in the Tremp-Graus basin,” a set of sedimentary layers excavated in the Pyrenees.
According to Dunn, “it is feasible that our new method could be applied to investigate this in Spanish deposits.” In fact, she adds: “The next extension of our work is to obtain a global view of how vegetation structure changed worldwide during this important climate event.” The final result of that climate change was a landscape upheaval in Wyoming and many other places at the same latitude. Temperate forest species gave way to thermophilous woody plants such as burseraceae (like palo santo), various shrub species, sequoias, and all kinds of palms. Such change caused many others, in a cascading effect, such as the appearance of new animal species better adapted to the new environment.
“Understanding how forest structure changed during the PETM is fundamental, as it reveals how plant growth, biomass, and productivity are affected by the amount of carbon dioxide in the atmosphere,” says Ellen Currano, paleobotanist at the University of Wyoming and coauthor of the study. “Excess CO₂ is harmful to forests, as the accompanying warming and drought weaken trees, causing many of them to die,” she adds.
Currano sees clear similarities between past and present: “Our work shows that during the PETM, tree canopies became more open, with fewer large trees, and this change affected climate, nutrient cycling, weathering, and, of course, the animals living in the forests,” and concludes: “We are beginning to observe similar changes in current forests, especially in the Amazon, and the fossil plant record from Wyoming gives us an idea of where Earth might be headed.”