Forests may take much longer than previously thought to recover from major climate warming. A study based on fossils from 56 million years ago reveals that forest ecosystems lost up to 60% of their coverage and needed more than 100,000 years to recover their structure.
The finding allows us to understand how forests responded to an episode of extreme increase in temperatures and offers a new perspective on the effects that drought, water stress and changes in precipitation can have on vegetation. Researchers have been able to reconstruct this response from fossil tree leaves preserved in Wyoming, United States, and analyze what happened to the forests before, during and after that period of warming.
The warming of 56 million years ago that allows us to study how the forests reacted
To understand what may happen to forests in the face of intense global warming, researchers have turned to one of the most important climatic episodes in Earth’s history: the Paleocene-Eocene Thermal Maximum (PETM). This event took place about 56 million years ago and was marked by a rapid release of large amounts of carbon into the atmosphere.
During that period, the average temperature of the planet increased between 5 and 8 °C, causing significant changes in the climate. Conditions were especially difficult for vegetation in some regions, where aridity and water stress increased.
Now, a team of researchers has been able to reconstruct part of the forests’ response to that warming by studying plant fossils found in the Bighorn Basin, in Wyoming, United States. The leaves preserved in the rocks allow us to analyze characteristics such as the cellular structure and the density of their stomata, small pores that intervene in the gas exchange of plants.
The results show that warming did not simply translate into greater abundance of vegetation. On the contrary, the forest cover suffered a significant deterioration and the tree canopy lost up to 60% of its coverage.
Wyoming fossils show how warming reduced forest cover
The researchers analyzed leaf fossils preserved from about 56 million years ago. These remains allow us to study how plants responded to the new climatic conditions and compare their state before, during and after the Paleocene-Eocene Thermal Maximum.
One of the elements that provides the most information are the stomata, small pores located in the leaves that allow plants to exchange gases with the atmosphere and control water loss. Their structure and density can be used as indicators of the environmental conditions in which those plants grew.
The analyzes point to a considerable deterioration of the forest cover. The increase in temperatures was accompanied by changes in precipitation and greater water stress, conditions that made the development of dense vegetation difficult. The forest canopy lost up to 60% of its usual coverage, transforming spaces that previously had more closed vegetation.
Recovery was also not quick when weather conditions began to improve. According to the fossil record studied, the forests needed more than 100,000 years to recover the density of their crowns and once again reach conditions similar to those they had before the warming episode.
The increase in CO₂ did not prevent the deterioration of forests
One of the questions raised by this study is whether a higher concentration of carbon dioxide (CO₂) can offset the negative effects of warming on forests. CO₂ is an essential raw material for photosynthesis and, under certain conditions, greater availability of this gas can promote plant growth.
However, the Wyoming fossil record shows that this effect has limits. During the Paleocene-Eocene Thermal Maximum, the increase in temperatures was accompanied by important changes in precipitation and greater water stress. These conditions ended up imposing themselves on the possible benefits that a higher concentration of CO₂ could provide to the vegetation.
The result was a landscape less covered by forests, with a greater presence of sparser vegetation and bushes. Aridity also affected the soil and reduced the ability of vegetation to maintain optimal photosynthetic activity and act as an effective carbon sink.
The finding shows that the effect of CO₂ on plants cannot be analyzed in isolation. Although the gas can stimulate photosynthesis under certain circumstances, extreme temperatures and lack of water can limit this benefit to the point that the final balance is negative for forest ecosystems.
More than 100,000 years to recover the forests
Data obtained from fossils indicate that it took more than 100,000 years for the forest cover to return to conditions similar to those before warming. The recovery was, therefore, much slower than the deterioration caused by climate change.
This gap is important because forest ecosystems do not respond solely to average temperature. The availability of water, the characteristics of the soil and the ability of plants to withstand prolonged periods of stress determine their survival and their ability to reoccupy a territory.
The study also shows that the effects of a warming episode can last long after temperatures have begun to drop. The loss of trees changes the structure of the ecosystem and can make it difficult for the forest to re-establish itself, especially when humidity conditions remain unfavorable.
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