In Minnesota, trees were expected to release 23% more CO2 as climate change warms the planet, but a new study found a surprising result

Plants release more carbon dioxide as temperatures rise. Scientists have long worried that this could cause forests to absorb less carbon and eventually become a source of carbon, making climate change worse.

However, a new University of Minnesota study of more than 1,000 young trees suggests the impact may be much smaller than previously thought. The researchers found that trees can adjust to warmer temperatures and release far less additional carbon dioxide than expected.

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Scientists warmed Minnesota trees by 3.4°C

The five-year study, published in Nature, involved 10 boreal and temperate tree species grown in 48 outdoor plots in two northern Minnesota forests. Researchers raised temperatures in the plots by about 3.4°C, a level that could occur by the end of the century.

Trees exposed to the warmer conditions increased their leaf respiration, the amount of carbon dioxide they released, by an average of just 5%. Without time to adjust to the warmer temperatures, their respiration would have increased by about 23%.

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The findings suggest that earlier studies using small plants in laboratories may have overestimated how much carbon forests could release as the climate warms. In the Minnesota experiment, trees had several years to acclimate to the higher temperatures, making the results more representative of what could happen in real forests.

"This work is important because most global C cycle models ignore this respiratory adjustment and project accelerated climate warming because of elevated respiratory CO2 release," says Peter Reich, professor of forest resources at the University of Minnesota, who led the project and is the paper's lead author. "Now, with better data we can make those models more realistic. "

"Although these results are 'good news' in the sense that the underlying physiology of plants is not going to make the warming of the planet radically worse, the problem we have created in the first place with our greenhouse gas emissions from fossil fuel burning still exists," he says. "So, we very much still need to cut our carbon emissions in the coming decades by enough to stop climate change."

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Who conducted the research?

"B4Warmed" was funded primarily by the U.S. Department of Energy; the research team included scientists from the University of Minnesota, as well as institutions in China and Maryland.

The research was part of the B4WarmED experiment in northern Minnesota, which was designed to recreate future warming conditions in real forests. Researchers studied tree seedlings in both open and closed-canopy areas and planted 11 tree species.

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The plots were exposed to different temperatures: normal conditions, about 1.7°C warmer, and about 3.4°C warmer. Infrared heaters warmed the tree canopies, while cables buried in the soil heated the ground.

During the 2009–2011 growing seasons, the researchers were able to maintain temperatures close to their targets. The warming also reached at least one metre below the soil surface, allowing the experiment to examine how both trees and soil responded to higher temperatures.

What it means for climate change

The study could change how scientists understand the role forests play in the climate system. Trees constantly absorb and release carbon dioxide, making forests an important part of the global carbon cycle.

Scientists have worried that rising temperatures could make trees release much more carbon dioxide. If that happened, forests could lose some of their ability to absorb carbon and could add to the warming of the planet.

But the Minnesota experiment shows that trees can adjust to warmer conditions over time. The response seen after years of warming was much smaller than what shorter laboratory experiments had suggested. The trees also adjusted to changes in temperature across different seasons.

This does not mean climate change will have little impact on forests. Instead, the findings suggest that trees may have a stronger ability to adapt to long-term warming than previously believed.

If trees in other forests respond in a similar way, the amount of extra carbon dioxide released by plants as temperatures rise could be lower than some climate models currently predict. This could help scientists build more accurate models of how forests and climate change will interact in the future.