Just as athletes stressed by exercise breathe more efficiently over time, trees stressed by a slowly warming climate can alter how much carbon dioxide (CO2) they release into the atmosphere, a new study suggests. And they may be doing it in a way that could slow climate change: Compared with trees suddenly exposed to hot temperatures, these acclimated trees release far less CO2 at night. This hints that future CO2 emissions from Northern Hemisphere forests won’t be as large as scientists thought, even though they would still be on the rise.
“We’ve known that trees respond to changing temperatures over the course of a day and over the course of the seasons, but we didn’t know how well they acclimated over longer periods of time,” says Kevin Griffin, a forest ecologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York. “This really needs to be included in climate models.” Yet some researchers say the new findings won’t significantly alter these models.
When they breathe, trees and other plants take in CO2 during the daytime, combining the planet-warming gas with water and sunlight to make sugars and build tissues. Then at night, the reverse occurs. Plants burn some of the sugars for energy and release CO2 into the atmosphere via respiration. Previous studies have shown that the rate of respiration is strongly linked to temperature: The higher the ambient temperature, the more quickly a leaf’s sugars are burned and the more CO2 returns to the atmosphere during nighttime hours.
But plants can also adjust their respiration if exposed to high temperatures for long periods of time, the studies suggested. However, that rate of adjustment appeared to vary significantly and wasn’t easily pinned down, says Peter Reich, a forest ecologist at the University of Minnesota, St. Paul. He adds that many of the previous studies analyzed respiration in trees younger than 1 year grown in a lab setting. So to get a better idea of how trees respond to higher temperatures, he and his colleagues decided to conduct a long-term field study.
The study took place at two sandy-soiled sites in northeastern Minnesota, with 24 plots at each location. Each circular, 3-meter-diameter plot contained 110 trees of 10 different species—five that dominate high-latitude forests and five that are common in temperate forests. Most of the trees were 2 years old when transplanted to the plots in 2008. During the 5-year study, researchers heated half the plots with underground cables and infrared heat lamps. At all hours of the day and night, the heated trees were kept about 3.4°C (6.1°F) warmer than those growing in unheated plots, Reich says. Because the plots weren’t enclosed, all trees received similar amounts of rain and wind.
“The scale and scope of the measurements in this study is unprecedented,” says Mary Heskel, a plant ecologist at the Marine Biological Laboratory in Woods Hole, Massachusetts, who was not involved in the work. She says she also appreciated that the team studies tree species that naturally occur in the area where the study was conducted. Furthermore, the trees were grown at much closer to natural conditions than those in a lot of previous studies, says Mark Vanderwel, a forest ecologist at the University of Regina in Canada. “That’s a good reason to expect that the team’s results will be more realistic.”
Reich and his colleagues clipped small samples of foliage from their trees on a regular basis and measured the leaves’ rates of respiration at temperatures ranging from 12°C (53.6°F) to 37°C (98.6°F). Over 5 years, the team analyzed more than 1600 samples, vastly more than in previous studies. “If there was a Guinness Book of World Records entry for analyzing leaf samples, we’d have it,” Reich says.
The researchers then estimated the rates of respiration for each species at 20°C and 23.4°C, a common temperature in respiration studies and the amount of extra warming in the heated plots, respectively. From those data, the team could calculate how well the heated trees had acclimated to their warmer-than-normal conditions. Whereas the unheated trees, on average, respired about 23% more quickly at 23.4°C than at 20°C, the trees exposed to warmer temperatures over long periods increased their rates of respiration an average of only 5%, the researchers report online today in Nature. Surprisingly, the findings held for both deciduous trees, which shed their leaves, and evergreens.
The difference in respiration rates is much larger than presumed in current climate models, and Heskel says it’s important to account for this kind of acclimatization. Otherwise, she adds, the amount of CO2 being returned to the atmosphere by heat-stressed trees will be overestimated, resulting in higher-than-expected levels of climate warming.
But others warn that the new findings might not make much of a dent in those warming estimates, because leaf respiration accounts for only a very small portion of carbon being released into the atmosphere. “This is an interesting study, but its results only relate to leaves,” says Pierre Friedlingstein, a climate modeler at the University of Exeter in the United Kingdom. He adds that the paper ignores many other sources of CO2 that could be exacerbated by a warming climate, such as the decomposition of organic matter residing in forest soils or long trapped in high-latitude permafrosts.
“I’m afraid this paper is not a game-changer,” he says. “My cautious recommendation would be not to take this study as a demonstration that global warming will not induce carbon loss from ecosystems.”