Leaves have their own internal thermostat.Getty

Whether growing in the heat of Puerto Rico or in the icy chill of northern Canada, tree leaves are able to buffer against the outside temperature, new research has found.

A survey of 39 North American tree species over an area spanning 50° of latitude has shown that plants protect one of their most important functions – photosynthesis – by maintaining average leaf temperatures at around 21 °C, regardless of the weather.

The findings, published this week in Nature ¹, could have implications for how scientists use tree rings to model past climates, and how they predict future responses to climate change.

The research is based on the fact that atmospheric oxygen is made up of two isotopes, ¹⁶O and ¹⁸O. Temperature can affect the relative content of each isotope in rainfall, suggesting that the ratio of oxygen isotopes found in tree rings correlates with the annual temperatures experienced by the trees. Humidity also plays a part: the lighter isotope, ¹⁶O, evaporates more readily, meaning that low humidity can drive up the relative concentration of ¹⁸O because the rate of evaporation is increased.

Some researchers have used the ratio of different oxygen isotopes in ancient tree rings to deduce details of a region's climate dating back millions of years. But the technique worried researcher Brent Helliker from the University of Pennsylvania in Philadelphia. “I’ve always been bothered by that,” he says. “In order for that to work, the temperature of the leaves in the tree canopy needs to be identical to ambient temperature. As a plant ecophysiologist, I know that’s not very likely.”

Helliker understood that photosynthesis, the method that plants use to generate sugars from light and carbon dioxide, is remarkably sensitive to temperature. Cool a plant too much and the enzymes important for the process may perform too slowly; too much heat, by contrast, can disturb the structure of the cellular membranes in which photosynthesis is carried out.

Sunscreen and mittens

Helliker and his collaborator Suzanna Richter, also at the University of Pennsylvania, decided to use the oxygen isotope method to calculate the temperature of modern tree canopies, and then to compare that to weather data collected in that region. Although leaf temperatures may fluctuate during the day, the isotope content of the tree rings should represent the average temperature of the leaves when they incorporated carbon from CO₂ into sugars. The data revealed a pattern: average leaf temperatures hovered around 21 °C, even when trees were located in very warm or cold climates.

Plants use several mechanisms to adjust their temperature. Some cool off by changing the angle of their leaves relative to the sun, or using fine hairs as a kind of sunscreen. They can also ‘sweat’, sacrificing water for the cooling effects of evaporation.

Meanwhile, trees in cold climates may clump their leaves closer together on their branches. Helliker likens this approach to using mittens in cold weather. “Gloves are not nearly as warm as mittens,” he says, “because your fingers are spread apart and the wind can whip away all of that heat.” Clumping the leaves together allows the branch to act more like a mitten, keeping leaves close so that each is less affected by the weather conditions. Trees that have adjusted to cold temperatures in this way may have a particularly hard time coping with the warming brought on by climate change, Helliker speculates.

The work provides an important contribution to understanding plant physiology, says Karl Niklas of Cornell University in Ithaca, New York. “This draws attention to the need to consider the whole plant,” he says, “rather than focusing on one or two traits before drawing ecological conclusions.”

• References

  1. Helliker, B. R. and Richter, S. L. Nature doi:10.1038/nature07031 (2008).