Climate change threatens global forest carbon sequestration, study finds

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 the forests of the American West are bearing the brunt of the consequences.

The study, led by UF Biology researchers J. Aaron Hogan and Jeremy W. Lichstein, was published in the Proceedings of the National Academy of Sciences. The study reveals a pronounced regional imbalance in forest productivity, a key barometer of forest health that measures tree growth and biomass accumulation.

Over the past two decades, the western United States, facing more severe climate impacts, has experienced a notable slowdown in productivity, while the eastern United States, facing milder climate impacts , experienced slightly accelerated growth.

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

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

“We are seeing changes in forest functioning as forest ecosystems respond to drivers of global change, such as carbon dioxide fertilization and climate change,” Hogan said. “It is the future balance of these factors that will determine how forests function in the years or decades to come.”

Some factors, such as droughts and forest pathogens, have negative effects on productivity, but others, such as carbon dioxide fertilization, are expected to have positive effects. This phenomenon suggests that increased carbon dioxide levels improve plant growth by increasing photosynthesis, prompting researchers to delve deeper into its impact.

“The U.S. Forest Service has monitored the growth and survival of more than a million trees across the United States for several decades,” Lichstein said. “We wanted to see if their data provided evidence of increased tree growth rates, as predicted by the carbon dioxide fertilization hypothesis.”

While tree growth in the eastern United States matches expectations, the western region shows extreme climate effects overshadowing any positive growth trends, calling into question the prevailing assumption that forests’ ability to store carbon will continue to increase.

“Our study suggests that future projections for climate and sea level rise may be overly optimistic because, in reality, ecosystems are likely to store less carbon in the future,” Lichstein said. “Less carbon storage in ecosystems means more carbon in the atmosphere and therefore more warming and accelerated climate change.”

The results also highlight that climate change is not a uniform force but rather a dynamic agent with region-specific influences. The study illustrates how the scale of climate change can push forests beyond a tipping point. Some forests are already approaching or exceeding climate thresholds that turn them into carbon sources rather than sinks that remove carbon from the atmosphere.

“Carbon sequestration in ecosystems is not guaranteed to be permanent and can be reversed by climate change,” Lichstein said. “This reversal is already happening in the western United States, and there are signs that it could also happen in other drought-affected regions of the world, such as the Amazon.”

It might be tempting to attribute these losses to extreme events. But the decline in productivity in the western United States cannot be attributed to increasing tree mortality rates, researchers say.

“We hear a lot about wildfires in the western United States, which are killing many trees and releasing carbon into the atmosphere,” Lichstein said. “But our study shows that additional carbon loss in western forest ecosystems is occurring due to declining tree growth rates.”

As trees grow more slowly due to the harmful effects of climate change, including reduced precipitation, the study implies that even without intensifying wildfires, the carbon sink of Western forests will continue to weaken without urgent action to reduce human greenhouse gas emissions.

“We must have healthy forests linked to reducing emissions to restore global carbon balance and limit climate change,” Hogan said.

The transformations observed in American 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 to reduce global greenhouse gas emissions,” Lichstein said. “Without the emissions reductions that scientists have been calling for for decades, forest carbon sinks will likely weaken, accelerating the pace of climate change.”

This study was developed with Grant Domke of the U.S. Forest Service’s Northern Research Station, Kai Zhu of the University of Michigan and Dan Johnson of the UF School of Forestry, Fisheries and Geomatics Sciences.