University of Florida:

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, 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 gauges tree growth and biomass accumulation. Over the past two decades, the Western U.S., grappling with more severe climate change impacts, has exhibited a notable slowdown in productivity, while the Eastern U.S., experiencing milder climate effects, has seen slightly accelerated growth.

Forests play a critical 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 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 non-plantation 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 Hogan. “It is the future balance of these drivers which will determine the functioning of forests in the coming years to decades.”

Some drivers, 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 Lichstein. “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.

“Our study suggests that future projections of climate and sea-level rise may be too optimistic because, in reality, ecosystems are likely to store less carbon in the future,” said Lichstein. “Less ecosystem carbon storage means more carbon in the atmosphere and therefore more warming and accelerating climate change.” [Continue reading…]