Ancient trees may have played a key role in regulating Earth’s climate during the last Ice Age — by breathing less efficiently.
A new one, recently published in Nature Geoscienceanalyzed chemical “fingerprints” in subfossil woodthat is, preserved trees from various regions of North America, to understand how plants responded to low levels of carbon dioxide (CO₂) and the coldest temperatures of the last glacial period, around 20,000 years ago.
Researchers found that as temperatures and CO₂ levels dropped, trees in many locations increased their photorespiration — a process similar to labored breathing for plants, and a sign that potential energy waste was occurring, releasing carbon dioxide back into the atmosphere.
Increased CO₂ release may have inadvertently kept the climate enjoyable enough and the atmosphere with enough carbon to allow plants to survive — functioning as a kind of natural brake which helped keep the Earth’s environment habitable.
“When we think about what might happen as the climate changes, one of the big questions is: if we continue to increase atmospheric CO₂, how will the plant world respond?”, explains Max Lloyd, assistant professor of geosciences at Penn State and lead author of the paper, in a statement published on .
“We found a clear link between climate change and biosphere responses. As CO₂ levels and temperatures fell, many plantsbecame less efficient at fixing carbonwhich in turn slowed the further reduction of CO₂ in the atmosphere. There is a natural feedback loop that we are just beginning to understand,” explains Lloyd.
To study how plants withstood the last Ice Age, approximately 115,000 to 12,000 years agoLloyd and his colleagues used a new technique to reconstruct the photorespiration rates in old trees.
Photorespiration is the process by which plants absorb oxygen and release carbon dioxide, partially undoing the work of photosynthesis — the process by which plants produce energy from sunlight and carbon dioxide.
The innovative technique used a chemical process that measures molecules with multiple rare varieties, called clustered isotopespresent in the wood.
Isotopes have similar chemical propertiess, but different physical properties, and the grouped isotopes work like a fingerprint of photorespiration, Lloyd explained.
By comparing isotope analyzes of ice age trees with modern trees, the team found that trees from warmer regions during the Ice Age had higher photorespiration rates to their modern counterparts.
This behavior suggests that low CO₂ levels during the last Ice Age significantly harmed plant productivityreduced the amount of carbon they could store in wood and soils — and forced plants into a state of stress.
Some of the study’s key samples come from Alcatres of the Breain Southern California, where researchers analyzed ancient juniper wood preserved in tar.
The team found clear signs of elevated photorespiration in the samples, meaning the trees released carbon dioxide back into the atmosphere at almost every same speed at which they removed it.
“The relatively little-studied plant fossils from the La Brea Tar Rocks are an excellent resource for understanding plant responses to climate change, not only in the past but also in the future,” he says. Regan Dunnassociate director of the La Brea Tar Pits & Museum and co-author of the article. “We are just beginning to understand what these ancient plants can teach us.”
The findings help explain why previous studies found that atmospheric carbon dioxide levels never dropped below the limit of about 185 to 210 parts per million during glacial periods, according to Lloyd.
“As far as we know, this is the first time we were able to test the long-held hypothesis that elevated photorespiration helped maintain atmospheric carbon dioxide at these levels tens of thousands of years ago,” said Lloyd. “To test this it was necessary to measure trees that actually grew at that time.”
At a time when there is an urgency to model climate scenarios, Lloyd argues that It is essential to understand and account for the effect of plants in the atmosphere
One of the ways to look to the future is to return to the past and study how the Earth’s biosphere may have self-regulated in previous times of climate stress.
“We are trying to understand how plants respond to dramatic changes in their world, looking back to a time when the climate changed relatively quickly,” concluded Lloyd.
