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Trees from the Caribbean to Canada maintain a constant leaf temperature regardless of the ambient air temperature, according to a new study by researchers at the University of Pennsylvania. The findings could dramatically alter interpretations of data used to approximate past climate from the composition of tree rings, the researchers say.

Scientists who measure cellulose composition in tree rings to reconstruct past climate conditions have long assumed that the temperature of photosynthesizing leaves was essentially the same temperature as the surrounding air, said Brent Helliker, a U. Penn ecologist and coauthor of the study. "Trees aren't good thermometers," said Christopher Field, a global ecologist at the Carnegie Institution for Science who wasn't involved with the study. Field said that instead of looking at tree rings as faithful records of ambient temperature, modelers now had to introduce a correction factor into the equation going from growth and photosynthesis to climate.

"When you look at tree rings," Helliker said, "you need to take the actual physiology and ecology of the tree into account."

Helliker and his coauthor Suzanna Richter, a postdoc at U. Penn, sampled cellulose from 39 tree species across 50 degrees of latitude, from broadleaf trees in the Caribbean to pines and firs in the boreal forests of Northern Canada. The two then measured the ratio of isotopic oxygen, a proxy for growth, in the trees' cellulose and found that photosynthesis was occurring when the leaves were about 21˚ C, irrespective of latitude or ambient temperature.

"When I did [the calculations] with the full dataset and actually solved for what the temperature was, my jaw dropped," Helliker said. The result suggested that the leaf were actively maintaining leaf temperature homeostasis.

Rather than regulating their temperature through physiological mechanisms, as mammals do, trees seem to utilize structural and morphological attributes, Helliker explained. For example, conifer trees, which have needle-like leaves that lose heat readily, pack branches and needles together in the canopies of boreal forests to retain heat through cold winters.

"It's really important for the climate community to realize that plants are strategists," said Field. "The things we use as proxies for past climate conditions are complicated systems. They're not just passively soaking up what is dished out."

William Smith, a Wake Forest University biophysical ecologist, said that the finding bridges the worlds of climate modelers and plant physiologists by injecting thermal biophysics at the leaf level into broader climate modeling and prediction. "This contribution is important because a lot of ecosystem modeling types don't really have a grasp of this because they're working on a higher spatial scale," Smith said. "This will bring more ecosystem people down to the leaf level."

Helliker said that his correction to past models that assumed a tight coupling of air temperature to leaf temperature can not only improve reconstructions of past climate, but may be able to help scientists better predict future climate change and how Earth's vegetation may respond. "Now we have the potential opportunity to pull out how the trees themselves respond to both year to year weather variation but also the warming that we've seen over the last hundred years," he said.