Higher Autumn Temperatures Lead to Contrasting CO₂ Flux Responses in Boreal Forests Versus Tundra and Shrubland

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.