A forest buried in Wyoming for 56 million years is delivering a warning about the limits of nature’s ability to absorb carbon. Fossils from the Hanna Basin suggest that rapid, sustained greenhouse warming during the Paleocene–Eocene Thermal Maximum, or PETM, did not simply make ancient forests grow faster under elevated carbon dioxide. Instead, the warming was associated with increasingly open canopies, major changes in plant composition, greater landscape erosion and a decline in the amount of vegetation packed into the forest. The result is a rare long-term view of what can happen when a carbon-rich atmosphere is accompanied by persistent heat and environmental stress.
The PETM began about 56 million years ago, when a massive release of carbon into the atmosphere and oceans drove a sharp global temperature rise. Although the exact sources and sequence of carbon emissions remain the subject of continuing research, the event is widely regarded as one of the closest geological analogues to modern human-driven climate change. The ancient warming unfolded over thousands of years rather than decades, but it produced profound ecological consequences. Temperatures increased, rainfall patterns shifted, soils were disturbed and ecosystems reorganized across continents. Because the PETM lasted long enough for forests and other plant communities to adjust, its fossil record offers scientists an opportunity to examine responses that cannot be captured by short-term experiments.
Regan Dunn and colleagues investigated these responses in the Hanna Basin, a sedimentary region in southern Wyoming that preserves a detailed record of ancient landscapes. The researchers combined several independent types of evidence, including fossil pollen, plant remains, sediment characteristics and geochemical measurements. Together, these records allowed them to track not only which plants lived in the region, but also how the structure of the forest changed as the climate warmed. The study focuses on a crucial distinction in climate biology: a forest can contain more productive individual leaves while simultaneously becoming less dense, storing less carbon overall and providing less continuous habitat.
A central element of the work is a new method for estimating Leaf Area Index, or LAI, from fossil leaf cuticles. LAI describes the total one-sided leaf surface area relative to the area of ground beneath it. In modern ecology, it is a key measurement of canopy density, light interception, water use and plant productivity. High LAI generally indicates a thick, multilayered canopy, while lower values point to a more open forest. Leaf cuticles are the waxy, chemically resistant outer layers that protect leaves from water loss and environmental damage. Even when the soft tissues and much of the plant have decayed, cuticles can remain preserved in sediments. By analyzing their abundance and characteristics, the researchers reconstructed changes in ancient canopy cover that would otherwise be nearly impossible to observe directly.
The fossil evidence indicates that the PETM was accompanied by rapid forest canopy opening. The decline in canopy density was not an isolated botanical change: it coincided with increased erosion across the landscape, suggesting that vegetation became less effective at shielding soil from rainfall and runoff. Dense forests slow precipitation, bind soil with roots and organic matter and reduce the energy of water moving across the ground. When canopies thin and plant cover becomes more discontinuous, soil can be exposed, making erosion more likely. The sedimentary record therefore provides an environmental link between forest structure and broader landscape instability during prolonged warming.
The plant community itself also underwent a dramatic transformation. Temperate broad-leaved angiosperms, which had formed an important component of the pre-PETM vegetation, declined as heat-tolerant plants became more prominent. Palms and ferns expanded in the altered environment, reflecting a shift toward species capable of tolerating warmer conditions and, in some cases, different combinations of moisture and seasonality. Such changes are more than a simple replacement of one set of species by another. Plant identity determines how much carbon is stored in trunks, branches, roots and soils, how efficiently water is used, how quickly organic matter decomposes and how habitats are structured for animals and microorganisms.
The findings challenge a common assumption about the relationship between carbon dioxide and forests. Elevated CO2 can stimulate photosynthesis by increasing the raw material available for plants to manufacture sugars. It can also improve water-use efficiency in some species because plants may partially close the microscopic pores, known as stomata, through which they exchange gases. But these benefits are not unlimited. High temperatures can damage photosynthetic machinery, increase respiratory losses and raise atmospheric demand for water. Drought can restrict the supply of water needed to transport nutrients and maintain leaf function. Over time, heat and water stress can overwhelm the initial CO2 fertilization effect, causing forests to thin even while atmospheric carbon dioxide remains high.
That possibility is especially important today because modern vegetation has already shown signs that its capacity to absorb carbon is under pressure. For decades, rising CO2 and longer growing seasons helped increase plant growth in many regions, allowing forests and other ecosystems to remove a portion of human emissions from the atmosphere. In recent years, however, extreme heat, drought, wildfire, insect outbreaks and land-use change have weakened or reversed some of those gains. The Wyoming fossils cannot provide a direct forecast for any particular modern forest, and the PETM climate was not identical to today’s climate. Nevertheless, the record demonstrates that the apparent resilience of vegetation under elevated CO2 can erode when warming persists long enough to alter water availability, species composition and ecosystem structure.
The study’s broader message is that forests should not be treated as permanent carbon-storage machines. Their ability to remove carbon depends on the interaction of atmospheric chemistry, temperature, precipitation, soils and biodiversity. A forest may initially grow more rapidly under higher CO2, yet later lose canopy cover and carbon-storage capacity as heat and drought intensify. Once plant communities shift toward more open and heat-tolerant vegetation, the consequences can extend beyond the carbon cycle, affecting erosion, habitat quality, local climate regulation and the movement of water through the landscape. By combining fossil cuticles with pollen, sediments and geochemical signals, Dunn and colleagues have reconstructed a detailed example of this process from deep time. The ancient forest’s decline suggests that the most important question is not whether plants can benefit from extra carbon dioxide, but whether forests can remain structurally intact as the planet continues to warm.
Subject of Research: Ancient forest structure, plant community change and carbon storage during the Paleocene–Eocene Thermal Maximum
Article Title: Forest canopy decline under elevated CO2 during the Paleocene-Eocene Thermal Maximum
Web References: https://doi.org/10.1126/science.aec4776
References: Dunn et al., “Forest canopy decline under elevated CO2 during the Paleocene-Eocene Thermal Maximum,” Science. DOI: 10.1126/science.aec4776
Keywords: Paleocene–Eocene Thermal Maximum, PETM, forest canopy, Leaf Area Index, fossil leaves, fossil pollen, Wyoming, Hanna Basin, elevated carbon dioxide, climate change, forest carbon storage, global warming, palms, ferns, vegetation response, erosion
