Trees are definitely our allies when it comes to taking in greenhouse gases and thus aiding in the fight against climate change. But new research suggests that forests might not be quite as helpful as we'd hoped.

Computer models that predict how climate change will play out assume that as greenhouse gas concentrations go up, forests will take advantage of the additional carbon dioxide and grow a bit more, increasing their capacity to mitigate global warming.

But after analyzing tens of thousands of tree rings taken from tropical forests in Bolivia, Cameroon and Thailand, an international team of scientists is calling this assumption into question. Their research, published yesterday in the journal Nature Geoscience, found no correlation between increased atmospheric carbon dioxide concentrations over the past 150 years and forest growth as evidenced in trees' rings.

Tropical forests "are very important carbon stocks," said lead author Peter van der Sleen of Wageningen University's Forest Ecology and Management Group in Wageningen, Netherlands. But, van der Sleen said, his research does call into question tropical forests' capacity to mitigate climate change.

This finding has the potential to change our climate predictions, explained Lucas Cernusak of the College of Marine and Environmental Sciences at James Cook University in Cairns, Australia.

"Current model formulations predict increasing tropical forest biomass through this century," Cernusak, who was not involved in the study, wrote in an email. "If this does not occur, the atmospheric CO2 growth rate will increase, and global warming will accelerate."

Tree rings tell 'surprising' story
It's likely that the new study came to a different conclusion because of differences in research methods. Earlier studies were based on analysis of biomass material contained in small forest plots rather than a random sampling from trees throughout a forest as with van der Sleen's study. Also, without the long-term data provided in tree rings, earlier experiments looked at tree growth on a shorter time scale.

Van der Sleen and his co-authors thought that if trees were indeed growing more with increasing CO2 in the atmosphere, their rings would thicken over time.

They did find evidence that trees reacted to more CO2 in the atmosphere. By analyzing the carbon isotopes in the wood, they found trees were using water more efficiently and likely also became more efficient at photosynthesis, the process where light is converted into energy.

For some reason, though, neither of these things translated into thicker tree rings or, the researchers suspect, bigger trees.

"An increase in water-use efficiency is one of the most reliably observed responses of trees to increased atmospheric CO2 concentrations," Cernusak wrote in an accompanying "News and Views" piece to van der Sleen's study, also published yesterday in Nature Geoscience. "But the increase in water-use efficiency observed in these forests perhaps makes it all the more surprising that growth rates stagnated."

Could there be more trees?
Why might this be the case? The study offers three ideas. First, it's possible that another climate change-related stressor, like rising temperatures, is keeping trees from growing more. The second theory is that other parts of trees like the fruit or the roots did grow more, just not the tree rings.

The third theory, which both van der Sleen and Cernusak favor as likely, is that tree growth is limited by other resources not having to do with carbon dioxide or water, like the amount of nutrients in soil.

But it's not time to give up on the idea that forests may be compensating for increasing emissions just yet—van der Sleen cautioned his results are "not conclusive."

He said it's possible that even if individual trees aren't growing more, the number of trees could be increasing in reaction to more CO2 in the atmosphere.

"You can think that maybe CO2 is not increasing tree growth, but tree growth is not the only thing that determines biomass," van der Sleen said.
