Tree biomass does not correlate with soil carbon stocks in forest-tundra ecotones along a 1100 km latitudinal gradient in Norway

Due to climate warming, forests are expanding to higher elevations and latitudes at the expense of tundra vegetation. While the subsequent increase in aboveground biomass is well-documented, there is much speculation regarding the effects on soil organic carbon (SOC) stocks. To provide insight into the consequences of tree encroachment into treeless tundra, we sampled SOC stocks across 36 forest-tundra ecotones along a 1100 km latitudinal gradient in Norway. Our results show that SOC stocks vary greatly within, as well as among treeline ecotones, and that tree biomass and tree species are not correlated with this variability. Instead, SOC stocks increase with temperature, and vary with slope steepness, slope aspect, and soil parent material. Applying a ‘space-for-time substitution’ perspective, our findings suggest that tree encroachment into tundra is unlikely to have immediate consequences for SOC stocks.