Research suggests trees absorb less carbon dioxide as the world’s temperature rises

The world’s forests play an important role in mitigating climate change. They are carbon sinks, which means they absorb more carbon dioxide than they emit. But according to new research, the most prolific tree in North America, the Douglas fir, will absorb less atmospheric carbon dioxide in the future, and therefore will do less to curb climate change than ever thought.

“More warming for trees could mean more stress, more tree death, and less ability to curb global warming,” said Margaret Evans, assistant professor of dendrochronology at the Tree Ring Research Laboratory at the University of Arizona. “Until now, forests have stabilized the climate, but as they become more stressed by drought, they could become a destabilizing source of carbon.”

Evans is the lead author of the study, which is published in Global Biology Change and it was an international effort led by UArizona researchers.

“We chose to study Douglas firs because they have a great environmental niche,” said Evans.

Douglas firs grow in the western half of North America, from the mountains of southern Mexico to the mountain peaks that mark the Sonoran Desert, the tropical forests of the Pacific Northwest, and the cold peaks of the Rocky Mountains.

To study the impact that Douglas firs might have on future climate, the researchers gathered a wealth of data to understand the relationship between the width of tree rings and the climate to predict future growth.

Tree rings are annual growth layers made of carbon. When the rings are thinner, it suggests that the trees removed less carbon dioxide from the atmosphere that year. The researchers’ results suggest:

• The worst decreases in tree ring width are forecast for the desert sky islands of Sonora, the southwestern United States, and northwestern Mexico. The team predicts a 15-30% decrease in the width of the tree ring.
• The growth of the Douglas fir in the Pacific Northwest Cascade Range and along the west coast, considered the center of the species’ ecological niche and where its growth is currently highest, is forecast to decline by approximately 10%.
• High-altitude Douglas firs in the Rocky Mountains of the northern US are expected to be least affected. The team expects a 2-11% decrease.
• At high elevations in Montana, there is a surprising increase in the projected growth of tree rings. This is because it is so cold that warmer temperatures will allow more growth.

“The treasure trove of tree-ring data, previously unavailable publicly available data from researchers’ hard drives, and the Forest Inventory and Analysis program of the United States Forest Service, really had an impact on the analysis. “said lead author Stefan Klesse, a former Arizona postdoctoral fellow who is now at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL in Birmensdorf, Switzerland.

The data set combined more than 2.7 million tree rings spanning the past century from nearly 2,700 sites within the ecological range of Douglas firs.

“Tree rings have proven to be a powerful tool for investigating past climate. Projecting future tree growth is an obvious extension of the same logic,” Evans said.

The problem, he said, is that the future doesn’t look like that past, and predictions about the ability of forests to absorb carbon dioxide in the future have varied widely.

Some models predict that a warmer world will lead to stressed trees that grow more slowly or die under new conditions.

Other models predict that trees will absorb carbon dioxide more efficiently and grow more, a controversial idea called the carbon fertilization effect. Here’s how it works: Trees open tiny pores in their leaves to let in carbon dioxide, but as they do, water escapes by evaporation. Trees are constantly balancing the cost and benefit of opening those pores. The idea is that by increasing the concentration of atmospheric carbon dioxide, trees can absorb carbon dioxide more efficiently through these pores and lose less water.

But most scientists have doubts about the size of the carbon fertilization effect in forests.

“The question is whether those one-sheet scale experiments are realistically applied on a global scale,” Evans said. “This is where tree rings could be a complementary source of data. A tree ring records the environmental conditions surrounding an entire tree. Knowing how trees respond in nature can better inform the models currently based on measurements made on a very small scale “This is something we like to do in science; we like to find different data sources and models, and if everyone agrees, then we feel more confident in our future predictions. “

The researchers have relied on trees in warmer climates to predict future tree responses in colder climates for now. For example, Douglas firs in Arizona are used to predict how Douglas bees will grow in Montana in decades, when temperatures have risen, Evans said. This is known as space-for-time substitution, and the researchers were able to test this idea with their new dataset and model.

Their research revealed that the assumptions underlying the substitution of space for time are too simple and produce poor predictions.

“We can’t take the average tree growth rates in Arizona now to predict the average tree growth rate in Montana in the future,” said Klesse, “and the reason for that lies in the genetics of the trees.” .

A Douglas fir in Montana does not have the same genetic makeup as trees in Arizona. You cannot treat them as equals because they have adapted to their local climate.

“A more accurate prediction for future tree ring growth in Montana is actually the sensitivity of an Arizona tree to the current climate,” said Klesse.

Factors that affect sensitivity include average temperatures and average amount of precipitation. The warmer and drier the local climate is on average, the more trees suffer from increased temperatures and dryness.

Almost all Douglas firs will grow with thinner tree rings, meaning they will absorb less carbon dioxide, due to rising global temperatures and increased drought.

“Forest ecosystems would become positive feedback loops, which is not a good thing,” Evans said.

Harold “Hal” Fritts, a professor emeritus of dendrochronology at UArizona who died last year at age 90, predicted the link between a tree’s climate sensitivity and ring growth more than 50 years ago.

“In the Northwest states, each ring on a tree looks the same. They look like a railroad track because they just aren’t very sensitive to change,” Evans said. “But in more desert terrain, like Arizona, the average width of the ring is much smaller, and the variability or sensitivity from one year to the next is much greater. In a wet year in the desert, trees can grow large rings and Fat. He (Fritts) predicted this relationship between ring width and sensitivity long ago. Our model proved he was right. “

The team said that better predicting how trees will respond to climate change will ultimately require genetic studies. They concluded that many of the differences in average ring width between Douglas firs are likely due to evolutionary adaptation, but exactly how much is still unknown.

“Possible future work is to go back to genetics,” said Klesse.