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US Forests Are "Less Productive" Because of Climate Change

A forest from above.
Credit: Geranimo / Unsplash.
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Climate change is reshaping forests differently across the United States, according to a new analysis of U.S. Forest Service data. With rising temperatures, escalating droughts, wildfires and disease outbreaks taking a toll on trees, researchers warn that forests across the American West are bearing the brunt of the consequences.

The study was led by University of Florida researchers and includes a University of Michigan co-author. It is scheduled to publish online the week of Jan. 15 in the journal Proceedings of the National Academy of Sciences.

The study reveals a pronounced regional imbalance in forest productivity, a key barometer of forest health that gauges tree growth and biomass accumulation. Over the past two decades, forests in the western United States, grappling with more severe climate change impacts, have exhibited a notable slowdown in productivity, while forests in the Eastern U.S., experiencing milder climate effects, have seen slightly accelerated growth.

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Forests play a critical role in regulating Earth’s climate, acting as carbon sinks that sequester approximately 25% of human carbon emissions annually. However, their ability to store carbon hinges on the delicate balance between the positive and negative effects of climate change.

The study, using national-scale forest inventory data, models trends from 1999 to 2020, analyzing 113,806 measurements in nonplantation forests.

“We are witnessing changes in forest functioning as forest ecosystems respond to global change drivers, such as carbon dioxide fertilization and climate change,” said study lead author J. Aaron Hogan, former postdoctoral researcher at the University of Florida who is now a postdoc fellow at the U.S. Forest Service. “It is the future balance of these drivers which will determine the functioning of forests in the coming years to decades.”

Understanding changes in forest productivity in a rapidly changing global environment is a challenging task, says study co-author Kai Zhu, an ecologist at the Institute for Global Change Biology, based at U-M’s School for Environment and Sustainability.

“This difficulty arises due to the abundance and interaction of various global change factors,” said Zhu, who assisted Hogan in framing the study’s research question and provided computer code from a previous study of forest growth.

“Predicting forest productivity becomes even more daunting, yet critical, because it determines their ability to absorb carbon dioxide from the atmosphere. Our research aims to address this challenge with rigorous design and comprehensive analysis to shed new light on this crucial subject.”

Some drivers of change in forests, such as droughts and forest pathogens, have negative effects on productivity. But other drivers, such as carbon dioxide fertilization, are predicted to have positive effects. This phenomenon suggests that increased carbon dioxide levels enhance plant growth by increasing photosynthesis, which inspired the researchers to take a deeper look at its impact.

“The U.S. Forest Service has been monitoring the growth and survival of over a million trees across the U.S. for multiple decades,” said study senior author Jeremy Lichstein, professor of biology at the University of Florida. “We were interested to see if their data provided evidence for increased rates of tree growth, as predicted by the carbon dioxide fertilization hypothesis.”

While tree growth in the eastern U.S. aligns with expectations, the western region shows extreme climate effects overshadowing any positive growth trends, challenging the prevailing assumption that forests’ carbon-storing ability will continue to increase.

“The current scientific understanding is that forests have been absorbing a significant amount of carbon from the atmosphere, which has helped to slow down the pace of climate change,” said Zhu, who is also an associate professor in the U-M Department of Ecology and Evolutionary Biology. “However, our latest study suggests that their capacity to absorb carbon is likely to decrease in the future.

“As a consequence, more carbon will remain in the atmosphere, leading to a faster rate of global warming. This accelerated warming will exacerbate climate change, and this is a cause for concern.”

The findings also illuminate the fact that climate change is not a uniform force but rather a dynamic agent with region-specific influences. The study illustrates how the degree of climate change can push forests past a tipping point. Some forests are already approaching or surpassing climate thresholds which shift them to become sources of carbon, rather than sinks that remove carbon from the atmosphere.

“For a long time, people have been taking for granted the services that forests provide, such as their ability to absorb carbon dioxide and slow down global warming,” Zhu said. “However, as climate change accelerates, we are seeing that forests may no longer be able to offer these services to the same extent as before. In recent years, we have observed these reversals in the western U.S. and the Amazon.”

It might be tempting to chalk the losses up to extreme events. But, according to the researchers, the decline in productivity in the western U.S. cannot be attributed to increased rates of tree mortality.

“Recent news coverage of the western U.S. forests predominantly centers on wildfires. Without a doubt, wildfires act as canaries in coal mines, signifying the effects of climate change on forests,” Zhu said.

“However, our study reveals that there’s more to the story than just wildfires; forests are also losing carbon due to a decrease in growth rates,” he said. “This lagging growth, in conjunction with a rise in tree mortality, is causing a significant reduction in the capacity of forests to absorb carbon dioxide.”

With trees growing slower due to adverse climate change effects—including decreased precipitation—the study implies that, even without the intensifying wildfires, the carbon sink in western forests will continue to weaken without urgent action to reduce human greenhouse gas emissions.

The transformations observed in U.S. forests raise concerns about their future resilience and sustainability. The researchers hope their findings highlight the urgent need for governments and industry to work together to reduce greenhouse gas emissions and achieve net-zero emissions as soon as possible.

“Our results highlight the need for reduced global greenhouse gas emissions,” said Lichstein. “Without the emissions reductions that scientists have been urging for decades, forest carbon sinks will likely weaken, which will accelerate the pace of climate change.”

Reference: Hogan JA, Zhu K, Lichstein J. Climate change determines the sign of productivity trends in US forests. Proc Nat Acad Sci. 2024. doi: 10.1073/pnas.2311132121

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