Everyone knows plants need CO2 to grow. So it seems logical that the extra carbon we're spewing into the atmosphere will make plants grow more, capturing more carbon and mitigating climate change, right? That's something argued by climate change skeptics and assumed by some scientific models used to predict future changes in the Earth's vegetation.

But growing evidence suggests that extra CO2 in the atmosphere isn't making trees grow more. And, in fact, climate change is generating warmer, drier conditions that could make them grow less in many places.

A recent study of extraordinary Quebec cedars that are between 600 and 1,000 years old adds to that evidence.

It found rising CO2 levels in the atmosphere since pre-industrial times made trees more efficient at using water, but didn't increase the growth of their trunks — and therefore the amount of carbon they stored, the researchers reported in the Proceedings of the National Academy of Sciences last week.

"What we bring as a hypothesis is if you don't have the water and nutrients to consume this supplementary CO2, well, you cannot grow faster," said Claudie Giguère-Croteau, who conducted the research while she was a master's student at the University of Quebec in Montreal.

The cedars, between 600 and 1,000 years old, are found on rocky islands on Lac Duparquet. (Submitted by Claudie Giguère-Croteau)

That's consistent with what's been found in other studies of tree growth with climate change, both in the tropics and in northern ecosystems.

Oldest boreal trees

What's special about the trees in the recent Canadian study is they're North America's oldest boreal trees, allowing researchers to track changes much farther back in time, said Étienne Boucher, a geography professor at the University of Quebec in Montreal who co-authored the study.

The researchers drilled into the trees and took core samples about the width of a pencil so they could examine the growth rings. Then they ground samples from each ring for chemical analyses.

The rare trees grow on small, very rocky islands in Lac Duparquet in western Quebec. Despite their impressive age, their trunks max out at about the width of a water cooler. That means the rings are extremely small — fractions of millimetres wide — and need to be measured with a microscope.

The trees' growth patterns showed no obvious changes before and after the industrial revolution, Giguère-Croteau said.

Étienne Boucher, left, and Claudie Giguère-Croteau carefully pack a core drilled from an ancient cedar. Back in the lab, they measured the trees' growth rings and did a chemical analysis of each ring. (Submitted by Claudie Giguère-Croteau)

But the chemical analysis showed something interesting.

Plants typically lose water during photosynthesis through the leaf pores that they open to let the CO2 in. The researchers noticed that after the start of the industrial revolution, as CO2 levels started climbing, the ancient cedars became dramatically more efficient at using water during photosynthesis.

That's something that's been previously shown to happen in other trees when they're exposed to increased CO2 levels, allowing them to photosynthesize more efficiently.

But the increased efficiency didn't lead to any increased growth in the ancient cedars.

And the efficiency gains started levelling off after 1965, when the region began seeing more extreme heat events in summer that cause trees to shut their leaf pores to prevent water loss, slowing down photosynthesis.

Boucher said that's what we can expect to happen in a drier climate. Trees will rely more on their own reserves of carbon instead of sucking it from the atmosphere.

Despite their impressive age, the cedar trees are not that big. Their trunks are about as wide as a water cooler. (Submitted by Claudie Giguère-Croteau)

The researchers said the findings about trees' water use mean we may need to revise our understanding of how the water cycle will be affected by rising CO2 and climate change, as a lot of the Earth's water moves through trees and plants.

The results also imply that scientific models used to predict future vegetation growth may need to be adjusted. Many assume trees will grow more quickly and capture more carbon with higher CO2 levels in the atmosphere.

"Actually, in nature, it's more complicated than that," Boucher said. "Maybe trees will not grow as much as expected."

That also suggests a need to plant new trees if we want to capture more carbon from the atmosphere, Giguère-Croteau said. "We cannot count so much on the already existing trees."

The study was funded by the Natural Sciences and Engineering Research Council and the Fonds de Recherche Québécois Nature et Technologies, the Quebec government's non-profit agency that provides grants for scientific research.