A study in Science developed a quantitative method to reconstruct the density of ancient forests and found that heat and dryness outweighed the effect of carbon dioxide “fertilization.” The findings illustrate why the expansion of foliage in some areas does not offset the overall damage of the Climate crisis
Higher concentration of Carbon dioxide can encourage photosynthesis and leaf growth, but it does not guarantee that forests will continue to thrive in a warmer, drier world. A new study, published in the journal Science, provides evidence from the distant past that when warming and falling precipitation are strong enough, they overwhelm the “fertilizing” effect of carbon dioxide, leading to thinning of forest canopies and the loss of large trees.
Researchers have developed a new method that allows them to calculate the density of an ancient forest canopy using the microscopic shape of cells in fragments. Fossilized leavesThey ran it on sediments from the Hanna Basin in Wyoming, which recorded the heat record of thePlauken Eocene, a sharp warming event that occurred about 56 million years ago.
The findings showed that during the warming period,forests The area is more open and sparse. Less of the sky was covered with leaves, the number of large trees decreased, and the landscape changed even though the atmosphere contained high concentrations of carbon dioxide.
A small fossil that reveals the shape of the forest
Until now, fossil plant researchers have been able to identify which species lived in a particular area and offer a general description of their environment: dense forest, open woodland, or sparse vegetation. However, such descriptions depend to some extent on the interpretation of the researchers and do not necessarily provide a numerical measure that allows for comparisons between forests from different periods and regions.
The new study focused on the Leaf Area Index, or LAI. The index describes the total area of leaves relative to the ground surface, or in simpler terms, how much of the sky is obscured by foliage when looking down. Forest canopyThe higher the index, the denser the canopy generally is.
To link cell structure to forest density, the researchers studied modern forests in Central and South America. They photographed the forest canopy with a wide-angle lens and collected fragments of cuticle—the outer layer that protects leaves—from the ground.
It turned out that the shape of the epidermal cells in a leaf is related to the amount of light it receives during its growth. In leaves grown in shade, under a dense canopy, the cells were generally longer and narrower. In leaves exposed to more light, as occurs in an open canopy, the cells were wider and less elongated.
After calibrating this relationship in modern forests, the researchers were able to measure thousands of cells in fossilized leaf fragments and calculate the density of the ancient forest canopy. This turned microscopic particles preserved in rocks into a quantitative measure of the structure of an ecosystem tens of millions of years old.
A warm world that forests have failed to take advantage of
The Paleocene–Eocene thermal record, known by the acronym PETM, is considered one of the most important examples from the past of warming caused by the injection of large amounts of carbon into the climate system. During this period, global temperatures rose by several degrees, the oceans warmed and acidified, precipitation regimes changed, and many species migrated to new areas.
Before the event, the Hanna Basin was covered in forests of giant tussocks, sycamores, alders, palms, and other subtropical species. Instead of the sagebrush that characterizes parts of Wyoming today, a rich, moist forest system existed.
But the increase in carbon dioxide did not protect the forest from climate change. According to the new reconstruction, warming and decreased precipitation opened the canopy and reduced the number of large trees. Plant species migrated north, the soil became more exposed to rain and wind, and erosion increased.
The loss of forest canopy has effects that go beyond the trees themselves. A dense forest regulates temperatures near the ground, returns water to the atmosphere through transpiration, moderates stormwater runoff, and provides habitats at different elevations. When the canopy opens up, water cycling, soil conservation, nutrient cycling, and tree-dependent species are compromised.
The half-true claim about a “greener planet”
One of the common arguments in the discourse that denies or downplays the seriousness of Climate crisis The argument is that the increase in carbon dioxide concentration is “greening” the Earth, and therefore also has significant benefits. There is a factual basis for this claim: carbon dioxide is a raw material for photosynthesis, and under the right conditions, an addition of it can increase the rate of plant growth.
Satellite measurements have indeed detected an increase in leaf area over large parts of the world for several decades. A study reported by NASA in 2016 attributed much of the trend to the fertilizing effect of carbon dioxide. But jumping from this fact to the conclusion that the climate crisis is beneficial to vegetation is wrong.
First, a satellite that detects more green foliage does not necessarily measure a healthy forest. An agricultural field that produces several crops per year, a commercial plantation, or a dense planting of trees of the same species may appear very green from space, even though they do not provide the species diversity, carbon storage, or resilience of an old-growth natural forest. NASA found that a significant portion of the greening measured in China and India was due to forest plantings and intensive agriculture, not just to increased carbon dioxide concentrations.
Second, a plant doesn’t just need carbon dioxide. It also needs water, nitrogen, phosphorus, a suitable temperature range, and functioning soil. When any one of these resources is lacking, increasing the amount of carbon in the air doesn’t allow the plant to continue growing indefinitely. Extreme heat increases water loss, drought causes stomata in leaves to close, and fires and pests can quickly release carbon that a forest has stored for decades or centuries.
Another study reported by NASA found that the average efficiency of the fertilization effect has steadily declined since 1982. According to the analysis, 86% of terrestrial ecosystems have become less efficient at utilizing the added carbon dioxide. Also IPCC Sixth Assessment Report Notes that the impact of fertilization is already weakening, while heat, droughts and extreme events are increasing carbon loss from terrestrial systems.
Third, the average impact of the climate crisis is not determined by a single positive outcome in a particular region. Greening cold regions or irrigated agriculture does not eliminate the decline in carbon sequestration in tropical forests, tree mortality, biodiversity loss, crop damage, wildfires, sea level rise, and ocean acidification. NASA has previously found that increases in carbon sequestration by vegetation in the Arctic are offset by decreases in water-scarce tropical regions.
In other words, “greener” is one visual or physical metric. It is not a comprehensive balance of the state of the climate and ecosystems.
Carbon both fertilizes and warms
There is no contradiction between the fact that carbon dioxide promotes photosynthesis and the fact that it warms the Earth. Both effects occur simultaneously. The question is which one dominates in a particular system and for how long.
In the early stages, and where water and nutrients are available, fertilization may increase leaf area and carbon uptake. But as temperatures rise, the air becomes more thirsty for moisture, and the soil dries out, the climate cost increases. At some point, it may outweigh the direct benefit to the plant.
This is exactly the process that the new study identifies in the forests of the Hana Basin 56 million years ago. Carbon dioxide was abundant, but the forest did not remain dense. Heat and dryness thinned it out and changed the functioning of the ecosystem.
The researchers note that similar trends are now being seen in some forests, including parts of the Amazon. Trees are simultaneously facing warming, drought, fires, logging, habitat fragmentation, invasive species, and pests. This combination of pressures could weaken their ability to recover even if carbon dioxide concentrations continue to rise.
The analogy to the past is limited – but worrying
The Pliocene-Eocene heat peak is not a replica of the current climate crisis. The continents were arranged differently, cities and modern agriculture did not exist, and the species that lived then were different. The source of carbon and its rate of emission were also not the same as today.
Furthermore, the direct reconstruction in the study was done in a single geological basin in Wyoming. It should not be concluded from this alone that all forests in the world responded in exactly the same way. The new method will need to be tested at additional sites and with different types of fossil forests.
However, there is one difference that makes the comparison with the present particularly troubling: According to the researchers, humans are now pumping carbon dioxide into the atmosphere at a rate about an order of magnitude faster than the rate of release during the ancient event, leaving ecosystems with less time to migrate, adapt or reorganize.
The study doesn’t prove that every modern forest is about to collapse. It does show that even in a carbon-rich world, forests are not immune to warming and drying. Presenting canopy expansion as evidence that emissions are beneficial to the planet focuses on one part of the process and ignores the overall balance.
According to lead researcher, paleobotanist Dr. Regan Dunn of the Natural History Museum of Los Angeles County, the early warming event is the best window into understanding how ecosystems responded to a large influx of carbon. She says that when trees are damaged, one of the important carbon pools that currently moderates the rate of warming is also damaged.
Questions and Answers
Does carbon dioxide really make plants grow faster?
Yes, under the right conditions it can increase photosynthesis and leaf growth. However, the effect is limited by water and nutrient availability, temperature, pests, and other factors. Studies show that the effectiveness of fertilization decreases over time.
If the Earth is becoming greener, why doesn't that offset the warming?
An increase in leaf area does not measure the entire state of an ecosystem. It does not necessarily tell us how much carbon is stored in the long term, what species live there, what the soil conditions are, or how resilient the vegetation is to drought and fire. Greening in one area can be offset by a loss of vegetation and carbon in another.
How can we know how dense a forest was 56 million years ago?
The researchers found a relationship between the shape of cells in the outer layer of a leaf and the amount of shade it grew in. After calibrating the relationship in modern forests, they used fossilized leaf fragments to calculate leaf area index and ancient canopy density.
Is the Paleocene-Eocene heat record the same as the current climate crisis?
No. The geological conditions, ecosystems, and source of emissions were different. However, the event provides an important example of the Earth's response to a large increase in greenhouse gas concentrations. The rate of human emissions today is much faster.
More on the subject on the science website
- Super El Niño: Earth is gaining heat at a record rate, the energy balance has doubled in recent decades
- NASA: Climate crisis doubles the extremes of forest fires worldwide
- The scientist leading the Amazon rescue
- There were no “syrupy oceans”: The study that undermines the myth that “there used to be a lot more carbon”
- Humanity did not live with a carbon dioxide-saturated atmosphere until the mid-twentieth century.
For the original publication: Opening the original publication
One response
Important fact: Trees in the Arctic Circle are absolute proof of the failure of the system because they replace ice and snow that reflect sunlight and slow down global warming. Seeing them as a fix for warming is simply an unfunny joke.