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Goldilocks tree leaves

Sweating in the heat or huddling in the cold keeps temperatures favorable

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12:01pm, June 11, 2008

Tree leaves do a pretty good job of achieving temperatures that are just right for photosynthesis, even if it’s too hot or too cold where they live, a new study shows.

From roughly the top to the bottom of North America, across some 50 degrees of latitude, trees all do their photosynthesizing at leaf temperatures around 21.4° Celsius plus or minus 2.2 degrees, says physiological ecologist Brent Helliker of the University of Pennsylvania in Philadelphia. That conclusion was based on a broad survey of the ratios of two forms of oxygen that vary depending on the temperature and humidity of leaves. Those properties control evaporation and make a signature in the cellulose of the tree rings, Helliker and colleague Suzanna Richter re

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