It had long been assumed that actively photosynthesising leaves -- using energy from sunlight to convert carbon dioxide and water into sugar -- are nearly as cold or hot as the air around them.

The new findings not only challenge long-held precepts in plant biology, but could upend climate models that use tree rings to infer or predict past and present temperature changes.

For decades, scientists studying the impact of global warming have measured the oxygen isotope ratio in tree-rings to determine the air temperature and relative humidity of historical climates.

Oxygen atoms within water molecules evaporate more or less quickly depending on the number of neutrons they carry, and the ratio between these differently weighted atoms in tree trunk rings has been used as a measure of year-to-year fluctuations in temperatures and rainfall.

"The assumption in all of these studies was that tree leaf temperatures were equal to ambient temperatures," lead researcher Brent Helliker told AFP. "It turns out that they are not."

Helliker and University of Pennsylvania colleague Suzanna Richter turned those assumptions upside down in examining 39 tree species, across 50 degrees of latitude ranging from sub-tropical Columbia to boreal Canada.

They compared current observed records of humidity and temperature against the isotope ratios in the trees, and found that tree leaves were internally cooler than surrounding air temperatures in warm climes, and warmer in cool climes.

Even more startling was that in all cases the average temperature -- over the course of a growing season -- was about 21 C.

"It is not surprising to think that a polar bear in northern Canada and a black bear in Florida have the same internal body temperature," because both animals have internal thermostats to prevent overheating or freezing to death, he said.

"But to think that a Canadian black spruce and a Caribbean Pine have the same average leaf temperature is quite astonishing," he added.

Tree leaves keep cool through constant evaporation and reducing sun exposure through leaf angles or reflective qualities. Warmth is gained by decreasing evaporation and increasing the number of leaves per branch.

All these tricks should be seen as evolutionary adaptations that help the trees attain a maximum of nutrients through optimal photosynthesis, Helliker said.

The fact that part of this adaptation occurs at the level of entire forest canopies, and not just within individual leaves, is one reason direct measurements of tree temperatures have been so hard.

The new findings, published in the British journal Nature, are bolstered by a recent study of a mixed species forest in Switzerland based on infrared thermal imaging.

Measured across an entire growing season, the forest canopy temperatures were found to be 4 C to 5 C (7 F to 9 F) higher than the cool, ambient air in the Swiss Alps.

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