Like Arboreal Mittens

Brent Helliker explains how trees control leaf temperature.

From the Canadian hemlock to the Caribbean pine, a recent study published in Nature has found that different tree species maintain the same leaf temperature during photosynthesis. This finding, which contradicts the longstanding assumption that temperature in a photosynthesizing tree leaf is equal to ambient air temperature, came out of a fortuitous interdepartmental collaboration between Penn scientists.

“I had this idea that we could maybe solve for leaf temperature with isotopes,” says Assistant Professor of Biology Brent Helliker, “but it would have taken me years to gather the data to test it.” Luckily for Helliker, Suzanna Richter—a postdoctoral researcher in the Department of Earth and Environmental Science—had already collected the data he needed. “Suzanna was initially interested in doing climate reconstructions going back 40 to 50 million years. To that end, she started sampling all this wood from modern trees, and she had this odd finding. We found that tree leaf temperature actually explained her odd finding. It was incredibly synergistic.”

With a data set of tree-ring wood samples from 39 different species gathered from 25 sites—ranging from Puerto Rico to the boreal forest—Helliker and Richter analyzed oxygen isotopes in conjunction with information from weather stations to calculate average tree leaf temperatures during photosynthesis. What they discovered was that this temperature was always the same—about 21° C or 70° F—irrespective of latitude and average growing-season temperature.

“Ambient air temperature here in Philadelphia during the growing season is reasonably close to 21° C,” says Helliker, “but this leaf temperature homeostasis finding means that as you get farther north, leaf temperature must be considerably warmer than ambient air temperature. And as you go south, it must be cooler. This suggests that there is some push, some natural selection for trees to actually control the temperature of their leaves.”

Helliker is quick to emphasize that trees are not endothermic, but they are capable of what he calls “clever little physical tricks.” Many tree species in northern climates, for example, have leaves that are packed closely together to better retain heat received from the sun. “The analogy I like to use is the difference between a glove and a mitten,” Helliker explains. “A mitten is much warmer than a glove. In a glove your fingers are separated and the wind can easily whip the heat away, whereas in a mitten you have what we call a ‘higher boundary layer.’”

While presenting a challenge to scientists who have used oxygen isotopes in tree rings to reconstruct past atmospheric temperatures, Helliker and Richter’s study will be useful to many in the climate change community. “What we’ve offered is a mechanism to explain one of many reasons why trees won’t do well in a climate change scenario where the northern part of the planet warms quickly,” says Helliker. “A tree that has been selected through thousands upon thousands of generations to keep its leaves close together on its branches—if you warm up the climate, it’s going to overheat.  The question is, ‘Will any change occur slowly enough to select for those trees that don’t grow their leaves like this?’”