Climate change is affecting U.S. forests, especially in western regions, reducing tree growth and challenging their ability to act as carbon sinks.
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 affecting trees, researchers warn that forests across the American West are bearing the brunt of the consequences.
Study reveals regional disparities in forest health
The study, conducted by biology researchers at the University of Florida, was published c. Aaron Hogan and Jeremy W. Lichstein, in the journal Proceedings of the National Academy of Sciences. The study reveals a clear regional imbalance in forest productivity, a key measure of forest health that measures tree growth and biomass accumulation. Over the past two decades, the western United States, grappling with more severe impacts of climate change, has shown a marked slowdown in productivity, while the eastern United States, experiencing milder climate impacts, has seen slightly accelerated growth.
Forests as carbon sinks and climate regulators
Forests play a crucial role in regulating the Earth's climate, acting as carbon sinks that sequester approximately 25% of human carbon emissions annually. However, their ability to store carbon depends on the delicate balance between the positive and negative impacts of climate change. The study, using national-level forest inventory data, models trends from 1999 to 2020, analyzing 113,806 measurements in non-planted forests.
“We are seeing changes in forest performance as forest ecosystems respond to global drivers of change, such as carbon dioxide fertilization and climate change,” Hogan said. “It is the future balance of these drivers that will determine forest performance in the coming years or decades.”
Carbon dioxide fertilization and tree growth
Some drivers, such as drought and forest pathogens, have negative impacts on productivity, but other drivers, such as carbon dioxide fertilization, are expected to have positive impacts. This phenomenon indicates that increased carbon dioxide levels enhance plant growth PhotosynthesisWhich inspired researchers to take a deeper look at its effect.
“The U.S. Forest Service has been monitoring the growth and survival of more than a million trees across the United States for decades,” Lichtstein said. “We were interested to know whether their data provided evidence of increased tree growth rates, as predicted by the CO2 fertilization hypothesis.”
Challenging assumptions about carbon storage
While tree growth in the eastern United States is in line with expectations, the western region shows extreme climate impacts that overwhelm any positive growth trends, challenging the common assumption that the capacity of forests to store carbon will continue to increase.
“Our study suggests that future projections for climate and sea level rise may be too optimistic because, in fact, ecosystems are likely to store less carbon in the future,” Lichstein said. “Reduced carbon storage in the ecosystem means more carbon in the atmosphere and thus more warming and accelerated climate change.”
Regional fluctuations and climatic thresholds
The results also highlight the fact that climate change is not a unifying force, but rather a dynamic factor with region-specific impacts. The study shows how the degree of climate change could push forests beyond the tipping point. Some forests are already approaching or exceeding climate thresholds, turning them into carbon sources, rather than sinks that remove carbon from the atmosphere.
“Ecosystem carbon sequestration is not guaranteed to be permanent and may be reversible due to climate change,” Lichtstein said. “This reversal is already occurring in the western United States, and there are indications that it may also occur in other drought-affected regions of the world, such as the Amazon.”
It may be tempting to attribute losses to extreme events. But according to the researchers, lower productivity in the western United States cannot be attributed to increased tree mortality rates.
Decrease in tree growth and productivity
“We hear a lot about wildfires in the western United States, which are killing a lot of trees and releasing carbon into the atmosphere,” Lichtstein said. “But our study shows that more ecosystem carbon loss in western forests occurs because of lower tree growth rates.”
As tree growth slows due to adverse climate change impacts, including reduced rainfall, the study suggests that even without increased wildfires, the carbon store in western forests will continue to weaken without urgent action to reduce human greenhouse gas emissions.
“We must have healthy forests in terms of emissions reductions to restore the global carbon balance and limit climate change,” Hogan said.
The urgent need to preserve forests and reduce emissions
The shifts observed in US 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,” Lichtstein said. “Without the emissions reductions that scientists have been urging for decades, forest carbon sinks are likely to weaken, accelerating the pace of climate change.”
Reference: “Climate Change Sets Mark for Productivity Trends in U.S. Forests” by J. Aaron Hogan, and Grant M. Domke, Kai Zhou, and Daniel J. Johnson, Jeremy W. Lichtstein, January 16, 2024, Proceedings of the National Academy of Sciences.
This study was developed with Grant Domke of the U.S. Forest Service's Northern Research Station, Kai Zhou of the University of Michigan, and Dan Johnson of the University of Florida's College of Forestry, Fisheries, and Geomatics.
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