Siberia holds about 15% of the world's forest areas and the region has been exposed to rapid warming in recent decades. In response to warmer temperatures, a lengthening of the growing season and widespread ‘greening’ seen by optical remote sensing data have been reported. These changes are generally interpreted as an increase in aboveground biomass carbon (AGC) and CO₂ uptake, and the prevailing view is that Siberian forests have acted as a net sink for atmospheric CO₂ during recent decades. However, the shift to a drier climate, accompanied by an increased frequency of wildfires causing forest loss and degradation (Fig. 1), may have weakened the carbon sink of Siberian forests. Wildfires account for most of the  forest loss in Siberia, causing significant carbon emissions (from live biomass and dead wood) (Fig. 2) and causing parts of Siberia to already emit more carbon than they capture. Recent heat-waves and wildfire events in 2010, 2012 and 2015 may change the carbon balance of the Siberian forest.

However, as the available forest inventories are limited, the carbon balance of the Siberian forest remains uncertain. Thanks to a team of 20 coauthors and data contributors, we tried to fill this knowledge gap in our latest paper, just published in Nature Geoscience. We estimated wall-to-wall annual changes in the aboveground biomass carbon stocks including living biomass and dead wood, over Siberian forests from 2010 to 2019.

Spatially explicit live aboveground biomass carbon (AGC_live) at 25 km spatial resolution (Fig. 3A) was computed using L-band Vegetation Optical Depth (L-VOD) from passive L-band microwave observations, which are sensitive to the biomass of stems, branches and leaves, and do not saturate even in dense forests (Fan et al., 2019). Moreover, L-VOD is not sensitive to the effects of sun illumination and atmosphere (aerosols, clouds) which limit the capabilities of optical observations at high latitudes. Carbon stocks of dead wood were estimated using the ratio between live biomass carbon stocks and dead wood, collected from hundreds of in situ measurements over the Siberian forest.

 We find that during 2010-2019 the carbon balance of Siberian forests was close to neutral, with the forests acting as a small carbon sink of 0.02 Pg C yr ^-1 (Fig. 3B). Carbon storage in deadwood increased, but this increase was largely offset by a decrease in live biomass (Fig. 3C). Substantial losses of live aboveground carbon stocks could be attributed to fire and drought, such as the widespread fires in northern Siberia in 2012, and extreme drought in eastern Siberia in 2015. These losses of aboveground living carbon contrast with the "greening" trends observed in the leaf area index (LAI) time series over the same period, a decoupling explained by faster post-disturbance recovery of leaf area than live aboveground carbon (Fig. 3D and E). Our study highlights the vulnerability of large forest carbon stores in Siberia to climate-induced disturbances, challenging the persistence of the carbon sink in this region of the globe.