Trees from the linkurl:Caribbean; to linkurl: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 linkurl:photosynthesizing; leaves was essentially the same temperature as the surrounding air, said linkurl:Brent Helliker,; a U. Penn ecologist and coauthor of the study. "Trees aren't good thermometers," said linkurl: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...

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