Forests are becoming less productive due to climate change

Climate change is increasingly recognized as a critical threat to the world’s forests, and a recent study highlights the severity of this issue for forest carbon sequestration, particularly in the United States. 

The research, conducted by University of Florida researchers J. Aaron Hogan and Jeremy W. Lichstein, involved a comprehensive analysis of U.S. Forest Service data. 

Forest productivity 

The experts analyzed data from 1999 to 2020, including 113,806 measurements in non-plantation forests. The goal of the study was to assess how forests across the country are responding to the changing climate.

The analysis revealed a pronounced difference in forest health and productivity between the eastern and western United States. 

In the West, severe climate impacts have led to a marked decline in tree growth and biomass accumulation. By contrast, the East has experienced a slight increase in growth, attributed to milder climate effects.

Carbon sequestration

Forests are crucial in regulating Earth’s climate by absorbing a significant portion of CO2 emissions. However, the ability of forests to store carbon hinges on the delicate balance between the positive and negative effects of climate change, noted the researchers.

Changes in forest functioning 

“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.”

While some drivers, such as droughts and pathogens, have negative effects on forest productivity, carbon-dioxide fertilization could potentially boost productivity.

“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.”

Accelerating climate change 

The findings challenge the assumption that forests will continue to increase their carbon storage capabilities. In reality, ecosystem carbon storage might decrease, contributing to more atmospheric carbon and accelerated climate change.

“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,” explained Lichstein. “Less ecosystem carbon storage means more carbon in the atmosphere and therefore more warming and accelerating climate change.”

Regional variation

The results of the study highlight the fact that the impacts of climate change vary by region. This variation can push forests beyond climate thresholds, potentially flipping them from carbon sinks to carbon sources.

“Ecosystem carbon sequestration is not guaranteed to be permanent, and it can be reversed by climate change,” said Lichstein. “This reversal is already happening in the Western U.S., and there are signs that it may also be happening in other drought-impacted regions of the world, such as the Amazon.”

Declining productivity 

While wildfires are a significant concern, the experts found that the decline in western forests’ productivity is not solely due to tree mortality from fires but also from declining growth rates caused by adverse climate conditions.

“We hear a lot about wildfires in the Western U.S., which kill a lot of trees and release carbon to the atmosphere,” said Lichstein. “But our study shows that additional ecosystem carbon loss in western forests is occurring due to declining tree growth rates.”

Greenhouse gas emissions 

The researchers emphasize the need to reduce global greenhouse gas emissions to preserve forest carbon sinks and slow the pace of climate change.

“We must have healthy forests in connection with emissions reduction to restore the global carbon balance and limit climate change,” said Hogan.

“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.”

The study is published in the journal Proceedings of the National Academy of Sciences.

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