Tree Leaves Keep Inner Temp Steady

June 11, 2008 -- The internal temperature of leaves, whether in the tropics or a cold-clime forest, tends toward a nearly constant 71 degree Fahrenheit, reports a study released Wednesday.

It had long been assumed that actively photosynthesizing 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 said. "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.