Ancient trees’ inefficient photorespiration may have helped stabilize Earth’s atmosphere during last ice age

Ancient trees may have played a key role in regulating Earth’s climate during the last ice age, by “breathing” less efficiently.

A new study, led by a Penn State researcher and published in Natural geosciencesexamined the chemical fingerprints of preserved subfossil wood or trees from across North America to understand how plants responded to low carbon dioxide (CO₂) levels and cooler temperatures of the last ice age, about 20,000 years ago.

The researchers found that as temperature and CO₂ levels fell, trees in many places increased their photorespiration, a process similar to labored respiration for plants and a sign that they are potentially wasting energy and releasing carbon dioxide into the atmosphere.

The increase in CO emissions₂ It may have inadvertently kept the climate just warm enough with enough carbon in the atmosphere for plants to survive, acting as a sort of natural handbrake helping to keep the Earth’s environment habitable.

“When we think about what will happen with climate change, a big question arises: whether we continue to increase atmospheric CO2₂how will the plant world respond?’” said Max Lloyd, assistant professor of geosciences at Penn State and lead author of the paper.

“We discovered a clear link between climate change and biosphere responses. Like atmospheric CO₂ levels and temperatures fell, many plants became less efficient at fixing carbon, further slowing CO uptake₂ of the atmosphere. There is a natural feedback loop that we are just beginning to understand. »

To study plant behavior during the last ice age, which lasted about 115,000 to 12,000 years ago, Lloyd and his colleagues used a new technique to reconstruct the photorespiration rates of ancient trees.

Photorespiration is the process by which plants take in oxygen and release carbon dioxide, thereby undoing some of the work of photosynthesis, the process by which plants produce energy from sunlight and carbon dioxide.

The new technique used a chemical process that measures molecules of several rare varieties, called clustered isotopes, in wood. Isotopes have similar chemical properties but different physical properties, and clumped isotopes act like a fingerprint for photorespiration, Lloyd explained.

By comparing isotopic analyzes of Ice Age trees with modern trees, the team found that trees from warmer regions during the Ice Age had higher rates of photorespiration than their modern counterparts, suggesting that low CO₂ The levels seen during the last ice age significantly hampered plant productivity, reduced the amount of carbon they could store in wood and soils, and plunged plants into distress.

Some of the key samples in the study came from the La Brea Tar Pits in Southern California, where researchers analyzed ancient juniper wood preserved in tar. The team found clear signs of high photorespiration in the samples, meaning the trees were releasing CO.₂ back into the atmosphere almost as quickly as they took it out.

“The relatively understudied plant fossils from the La Brea Tar Pits provide an excellent resource for understanding plant responses to climate change, not only in the past, but also in the future,” said Regan Dunn, deputy assistant director of the La Brea Tar Pits and Museum and co-author of the paper. “We’re only scratching the surface of what these ancient plants can tell us.”

The results help explain why previous studies have shown that atmospheric carbon dioxide levels never fell below the threshold of about 185 to 210 parts per million during ice ages, Lloyd said.

“To our knowledge, this is the first time we can test the long-standing hypothesis that elevated photorespiration helped keep atmospheric carbon dioxide at these levels tens of thousands of years ago,” Lloyd said. “To test this, you had to measure trees that were actually growing at that time.”

He added that photorespiration is a key control of the amount of carbon in the atmosphere. At a time when there is an urgent need to model climate scenarios, Lloyd said it is essential to understand and consider the effect of plants on the atmosphere. One way to look to the future is to look to the past and study how the Earth’s biosphere was able to self-regulate during previous periods of climate stress.

“We’re trying to understand how plants respond to dramatic changes in their world by looking at a time when the climate was changing relatively quickly,” Lloyd said.