Goldilocks tree leaves

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

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.

JUST RIGHT Tree leaves can do plenty to keep their temperatures just right for photosynthesis. Corbis

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 report in an upcoming Nature.

Such temperature control undermines the assumption that the insides of leaves have the same temperature as the air, Helliker says. That’s an assumption underlying studies that check oxygen ratios in old tree tissue to reconstruct past climates, he says.

The tree-ring community is just starting to sort out what the finding means. “I, and I am sure my colleagues in isotope dendroclimatology, will welcome this paper because it improves our understanding of the complex relationship between climate and the stable isotope ratios in wood,” says Danny McCarroll of the University of Wales Swansea. However, he objects to Helliker’s claim that paleoclimatologists’ approaches have relied so heavily on whether leaf temperatures match those of the surrounding air.

Those paleoclimatology methods for using isotopes in tree rings to reconstruct climate have been validated by observations, says Jan Esper of the Swiss Federal Research Institute in Birmensdorf. “From this perspective, the findings by Helliker and Richter are indeed surprising, as I would have expected a closer association between leaf and surrounding air temperature,” he says.

Helliker says he has been bugged for years by the assumption that a tree leaf photosynthesized at whatever the local air temperature might be. Trees release water, and during hot times, that botanical sweat cools them down. And trees that grow in cold places tend to cluster their leaves. These tight formations can affect the rate at which leaves lose heat on cold days, just as fingers pressed together in mittens stay warmer than fingers separated by space in gloves.

Physiologists, of course, could measure the temperature on individual leaves, but measuring enough leaves to give a picture of the canopy has been difficult. Helliker estimates that scientists would need at least 140 leaves to get a valid reading for the temperature of photosynthesis of a single tree.

His colleague Richter, however, had collected tree ring data for another project, and Helliker realized it would be perfect to test his idea. Richter had not only recorded oxygen ratios in the tree rings, but had also collected data from nearby weather stations on relative humidity. Since she knew the humidity, the researchers could calculate what the leaf temperature must have been to produce particular ratios of oxygen isotopes. When the leaf is photosynthesizing, the sugars it produces include oxygen in the temperature-sensitive ratio. The cellulose in tree rings made from these sugars thus indicates the leaf temperature during photosynthesis.

“What I like about this paper is the fact that it highlights the need to account for actual life conditions,” says Christian Körner of the University of Basel in Switzerland.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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