Few living things seem as passive as trees. When the wind blows, they sway; when the rain falls, they get wet; and when temperatures get hot or cold, so do they. At least that's what scientists have generally assumed. But a controversial study suggests that trees may control the temperature of their leaves, keeping them at a balmy 21.4°C on average regardless of how cold the air gets.
The evidence comes from an isotopic analysis of 39 species of trees ranging from Inuvik, Northwest Territories, in Canada to Maricao, Puerto Rico. Environmental scientist Suzanna Richter and plant physiological ecologist Brent Helliker, both at the University of Pennsylvania, measured the ratio of two isotopes of oxygen--rarer oxygen-18 and more common oxygen-16--in samples of wood. Precipitation is rich in oxygen-18, so generally the ratio of oxygen-18 to oxygen-16 should be higher in warmer, wetter climates than in the colder north. The ratio should also be higher in drier climates, as there the lighter oxygen-16 diffuses more readily from a tree's leaves into the air, leaving more of the heavier oxygen-18 behind to be incorporated into wood.
The researchers collected their samples close to weather stations, so they had detailed temperature and humidity records for the regions. When they compared the isotope predictions with the weather-station data, the measured ratios in northern latitudes tended to be higher than expected, the team reports tomorrow in Nature. Helliker and Richter found that they could explain the ratios if they assumed that, even in the northernmost regions where the average annual temperature dips to -10°C, the leaves maintain an average temperature of 21.4°C. "I had the only jaw-dropping moment that I've ever had in science," Helliker says.
The leaves do not generate heat and are not made of some superinsulating material, Helliker stresses. Rather, trees in northern regions have probably evolved to clump their leaves together and trap a boundary layer of still air. The boundary layer acts like a blanket, holding in heat and moisture and aiding the tree in its metabolism, he says. In fact, says Helliker, infrared imaging of trees has already shown that their leaves can be warmer than the surrounding air by several degrees, as was reported last year by Christian Körner, a plant ecologist at the University of Basel in Switzerland.
Körner himself is not convinced by the new study, however. He says that Helliker and Richter's analysis relies on the average annual temperature; it doesn't take into account the fact that trees grow only during the warm season, and then only at warm times of day. Instead of controlling their own temperature, firs and other trees in the north may simply grow only during those few months when the temperatures reach at least 21.4°C, he says. Körner also argues that spindly conifer needles simply cannot trap heat.
But laboratory measurements on fir branches show that position is "absolutely wrong," says William Smith, a biophysical ecologist at Wake Forest University in Winston-Salem, North Carolina. The debate is more than academic, Smith says. To reconstruct accurate climate records for the past and forecast climate changes for the future, modelers need to know just how passive trees really are when it comes to air temperature.