There’s a climate change conundrum centered on trees.

The debate in scientific circles goes something like this: Plants need carbon dioxide for photosynthesis, and more CO2 in the air can give them a boost. But rising carbon dioxide levels are also driving global warming—and rising temperatures can cause water stress, limit plant growth and increase the risk of die-offs.

As carbon dioxide levels keep climbing, and the Earth’s temperatures keep warming, which effect wins out?

It’s a top question for climate scientists. Trees suck carbon dioxide out of the air as they grow, helping to slow the impact of climate change. If forests become stressed or begin to die off, they’ll remove less carbon—or even send it pouring back into the atmosphere—potentially speeding up global warming.

Now, new research may help fill in some of the gaps. A study published this week in Proceedings of the National Academy of Sciences suggests that the total amount of warming or carbon dioxide may be less important than the ratio between them—that is, how much warming occurs compared to how much CO2 levels increase.

As the research demonstrates, increasing carbon dioxide concentrations can compensate for rising temperatures up to a point. But if temperatures rise faster than CO2, and the proportion between the two of them crosses a certain threshold, forests may begin to suffer.

There were some differences among various types of forests across the country, but the results were broadly true for most environments, the researchers say.

“It’s one extra piece in the puzzle that can help us solve and understand what forests are gonna do in the future,” said study co-author Martin Venturas, a postdoctoral researcher at the University of Utah.

Exactly where that magic ratio lies depends on how quickly forests can naturally adjust to climate change.

Some trees physically respond to drought or other stressful environmental conditions in ways that help them maximize their resources and retain water. They may change the shape of their leaves, for instance, or adjust the rate at which they perform photosynthesis. These processes are known as “acclimation,” and they can help trees better tolerate the effects of climate change.

The new study investigates the effects of both rising temperatures and rising CO2 on forests across the United States. It also looks at whether acclimation makes a difference.

The research relies on a new model that accounts for the trees’ basic physiology—how they pull water from the soil and move it up to their leaves, and how these processes change with rising carbon dioxide or the increased water stress caused by warming temperatures.

The study suggests that if forests can acclimate to climate change, carbon dioxide concentrations must rise by at least 67 parts per million per degree Celsius of warming in order for the trees to keep growing. In model runs, about 71% of the future warming forecasts showed this scenario occurring.

If they fail to acclimate, CO2 must rise by at least 89 ppm per degree of warming. It’s a scenario that occurs in just over half the climate forecasts used in this study—meaning the scales could tip either way, Venturas said.

“We could fall on either side of that ratio,” he told E&E News. “So we have to really be concerned. That could mean we still have a huge uncertainty of what’s gonna happen to our forests, and the consequences can be major depending on if it goes the wrong way.”

Even if the ratio falls out of proportion for only a brief time, just a season or so, forests can experience dramatic consequences, he added. They can stop growing or even begin to die off. The response of Western forests to recent droughts is a prime example, he noted.

The new study may put scientists a step closer to accurately predicting these kinds of responses in advance. But there are still more questions remaining.

For one thing, predicting if forests can acclimate in the first place, and how strongly they’ll adjust, requires a lot more research in different types of forest environments. The study also doesn’t account for other side effects of rising temperatures, like outbreaks of tree parasites or increasing wildfires.

“I think this is a substantial improvement to our current modeling schemes,” Venturas said. “There’s a lot of room for improvement and future research.”

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news a