Higher autumn temperatures lead to contrasting CO 2 flux responses in boreal forests versus tundra and shrubland

Nina A. RandazzoAnna M. MichalakCharles E. MillerScot M. MillerYoichi P. Shiga & Yuanyuan Fang


Increasing atmospheric concentrations of greenhouse gases are driving climate change, which is happening particularly rapidly near the poles of the Earth. This rapid warming may help plants in these high-latitude regions take up more carbon through photosynthesis, which could decrease the amount of carbon dioxide (CO2) in the atmosphere, slowing climate change. However, rapid warming is also speeding up soil microbial activity and other processes that release more CO2 into the atmosphere at high latitudes, possibly accelerating climate change. Therefore, the balance of these different processes will determine the net impact of high-latitude ecosystems on climate change. We assess how much warming and associated conditions influence the total CO2 exchange across Alaska. 

Figure: The warmer conditions of October 2013 were associated with dramatic carbon losses in the tundra and shrubland, but not in boreal forests. Each data point represents the net ecosystem exchange for a single October day averaged over each biome. In tundra and shrubland areas (panel a), higher temperatures were associated with greater net release of carbon. In contrast, in boreal forests, temperatures above freezing were associated with a growing season extension and therefore decreasing carbon loss with increasing temperatures (panel b). Because most days of October 2013 were above freezing in boreal forests, the growing season extension in this biome appears to have offset much of the carbon loss that may have otherwise been associated with warmer or wetter conditions. 


The future carbon balance of boreal ecosystems under increasing temperatures is highly uncertain. In particular, the net effects of a longer growing season versus enhanced respiration are poorly understood. Here, we use a geostatistical inverse model from 2012 to 2014 to determine temperature sensitivity in Alaskan biomes throughout the growing season, in order to identify the relative effects of these competing phenomena. We find that temperature explains a large portion of the disparities in autumn carbon flux between 2013 and 2014. Boreal forests experienced a growing season extension during the warm October of 2013 that offset increased respiration into autumn in years with high temperatures. In contrast, increased temperatures in the tundra and shrublands led to a large respiration signal during October 2013, producing a greater net carbon release. These results suggest a greater vulnerability of Alaskan tundra and shrubland carbon stocks compared to boreal forest carbon stocks under warming.