Breathing easier on climate change

Warming-induced release of CO2 from trees may be less than expected

A new study led by CFANS researcher Peter Reich offers a rare bright spot in the news about the effects of rising global temperatures. 

Reich and his colleagues studied carbon dioxide emissions by trees, which, like humans, produce CO2 through the process of respiration. They discovered that when suddenly exposed to temperature increases of 3.4 degrees C—6 degrees F, in the midrange of predicted increases by 2100—trees from northern temperate and boreal zones increased their CO2 emission rates by about 23 percent. However, rates for trees given a few weeks to acclimate to the higher temperature rose by only 5 percent. 

“Acclimation eliminated 80 percent of the expected increase in leaf respiration,” says Reich, a Regents Professor of forest resources.

“High latitude boreal and temperate forests account for approximately one third of Earth’s total forest area and play a significant role in terrestrial carbon cycling. If such dampening of effects … occur[s] generally, the increase in respiration rates of terrestrial plants in response to climate warming may be less than predicted, and thus may not raise atmospheric CO2 concentrations as much as anticipated.”

The study is published in the journal Nature.

Plants have to breathe, too

Much of the debate about the effects of rising temperatures concerns the balance between how much CO2 the world’s plant ecosystems can absorb and process via photosynthesis and how much is returned to the atmosphere by respiration of plants and soil organisms. Photosynthesis stores CO2 from the atmosphere as carbohydrates, but plants also have to burn carbohydrates for energy. This is what respiration does; it essentially reverses photosynthesis and releases some of the stored CO2. 

“The world’s plants and microbes each emit six times as much carbon into the air every year via their respiration as we do via fossil fuel emissions,” notes Reich. “Thus, even a small increase in plant respiration in response to warming could translate into a major increase in CO2 concentration and thus in the pace of climate change.

“Assuming photosynthesis does not change, all it takes is for global respiration to increase by a few percent to shift the land surfaces from a ‘sink’—where photosynthesis exceeds respiration—to a ‘source,’ which is the reverse.”

As the temperature rises, plants respire at faster rates. However, after continued exposure to higher temperatures, plants adjust their respiration downward, a process called thermal acclimation.

For example, suppose two similar plants are grown at 20 degrees C (a standard temperature for measuring respiration), with the “control” plant staying at 20 C but the other moved to 23 degrees C for several weeks. If the two plants’ respiration is then measured at 23 degrees, both will respire faster at that temperature than the control plant does at 20 degrees; however, the difference will be significantly smaller for the one previously exposed for weeks to the higher temperature.

“The degree of acclimation is extremely uncertain, especially for plants in natural settings,” the researchers write in the introduction to the Nature paper. “The greater the thermal acclimation of respiration globally, the smaller the positive feedback between climate warming and ecosystem CO2 release.”

Thus the big question: How much can acclimation dampen respiration increases due to warming?

Trees tell the tale

To find out, Reich and his colleagues studied acclimation in 1,200 trees representing 10 dominant North American species, including oak, poplar, pine, balsam fir, birch, white spruce, and maple. The seedlings grew outdoors at two northern Minnesota sites: the Cloquet Forestry Center and the Hubachek Wilderness Research Center near Ely. About half were grown under ambient (control) conditions. The others were artificially warmed by a system, including overhead heaters and underground cables, that kept their temperature 3.4 degrees C above ambient, which gave them the chance to acclimate to warmer conditions. The experiment ran from 2009 to 2013.

The key findings emerged when the researchers measured respiration rates for leaves from warmed and control trees at 20 and 23.4 degrees C. The baseline was the rate for leaves from the control trees at 20 degrees C.

  • At 23.4 degrees, the respiration rate for leaves from control trees averaged 23 percent above baseline, while the rates for leaves from warmed trees averaged only 5 percent above baseline.
  • As seasonal temperatures rose, plants needed only about five nights of exposure to a higher temperature to achieve a comparable level of thermal acclimation.

“If our realistic, long-term field results are broadly indicative, they suggest that leaf respiratory acclimation globally may have a larger ameliorating impact than expected on CO2 [increases] with rising temperature as climate changes,” Reich says.

“However, although this is good news in the sense that the fundamental physiology of plants is not going to make the warming of the planet dramatically worse, the problem we have created in the first place with our greenhouse gas emissions still exists. So we very much still need to cut our carbon emissions enough to stop climate change.”