Introduction


As the planet warms, biological feedbacks can either cool or further heat the planet.
Major biological processes
Major biological processes
When Charles Keeling began recording CO2 concentration in the atmosphere, the growth rate was only about 0.75 ppm per year. Recently the annual increase in CO2 concentration has been 2 ppm, but for the past four years the annual increase has exceeded 2 ppm. In 2015 El Niño caused a huge leap in CO2 levels. Over 2015 CO2 concentrations grew by 3.05 ppm, the largest jump on record.

An important determinant of whether increasing CO2 levels are amplified or slowed down is the global carbon cycle. Plants both remove CO2 from the atmosphere through photosynthesis and contribute CO2 to the atmosphere through respiration. Plant respiration results in an annual flow of carbon dioxide (CO2) to the atmosphere that is six times as large as that from emissions from burning fossil fuels. Although warming stimulates photosynthesis and net primary production, it causes a sharp increase in CO2 emissions from ecosystem respiration. This results in a heightened net CO2 release from plants which may further atmospheric warming.

It has been known for decades that over the short-term (minutes to hours), respiration increases rapidly with temperature. But it is also known that plant respiration can gradually adapt (acclimate) to higher temperatures, which tends to weaken CO2 emissions stimulated by warming. However, there has been little reliable quantitative information about the long term effect of warming on plant respiration rates. This article reports quantitative results on the effect of long term (months and years) warming on plant respiration rates.

Description of the experiment


Northern boreal and temperate forests account for about one-third of Earth’s total forest area. These forests play an important role in the Earth's carbon cycling. In this article the researchers report on an experiment that assessed the adaptation capacity of more than 1,200 trees from 10 dominant North American tree species. The trees were divided into two groups. Some were grown in normal outdoors conditions. The remainder were warmed by +3.4 °C compared to normal outdoor conditions. The experiment ran from 2009 to 2013 in forest habitats at two sites 150 km apart at the boundary between boreal and temperate forest zones in Minnesota, USA.

Effect of adaptation on plant respiration
Effect of adaptation on plant respiration
The experiment involved taking trees grown at normal outdoor temperatures, warming them by +3.4 °C for 30 minutes and then measuring the respiration rate. This was compared with the respiration rate of similar trees raised in a warmed environment (+3.4 °C above normal outdoors). As a simplified example, a plant grown under normal outdoors conditions might show leaf respiration which is increased by 23% after being warmed (to normal + 3.4 °C) for 30 minutes. In contrast, a plant grown under warmer conditions (normal + 3.4 °C) and measured at warmer (normal + 3.4 °C) temperature might only exhibit an increase of 5% in respiration rate. In this case plant adaptation (called acclimation) has eliminated some of the increase in respiration and CO2 emissions with rising temperature that would be expected of plants that did not have time to adapt.

Results


The authors report the results of 1,620 leaf respiration–temperature response measurements. They performed a number of statistical analyses which confirmed their conclusion that adaptation reduces CO2 emissions stimulated by higher temperature by 80% in northern and temperate forests.

Respiration in land plants (including CO2 from roots, stems and leaves) releases about 64 gigatonnes ( Gt) of carbon per year to the atmosphere. This offsets about half of the CO2 removed from the atmosphere by photosynthesis by land plants. The adaptation of respiration to rising temperature is crucial to maintaining the global carbon balance as the climate warms. If plant adaptation occurs generally in northern forests, the increase in respiration rates of terrestrial plants in response to climate warming may be less than predicted, and thus may not represent as strong a feedback mechanism which accelerates atmospheric CO2 concentrations as much as anticipated.

Sources


Boreal and temperate trees show strong acclimation of respiration to warming, Peter B. Reich et al., Nature 531, 633–636(31 March 2016)

Terrestrial ecosystem carbon dynamics and climate feedbacks, Martin Heimann & Markus Reichstein, Nature 451, 289-292 (17 January 2008)

Terrestrial biogeochemical feedbacks in the climate system, A. Arneth et al., Nature Geoscience 3, 525 - 532 (2010).