Soil carbon stocks in different vegetation classes across the treeline ecotone in central- and southern Norway

The expansion of the treeline into the currently treeless alpine tundra may have considerable impacts on carbon storage. Recent studies have shown that the expansion of trees into the alpine tundra could lead to an increase in CO2 emissions, having positive feedback on global warming. However, few studies have assessed differences in carbon storage between vegetation classes in the forest-tundra ecotone. Here, I have done that by classifying the vegetation according to the Nature Types in Norway (NiN) classification system. Top organic soil was sampled in five different vegetation classes: forest, lee side, ridge, subxeric heath, and xeric heath along a 600-kilometer latitudinal gradient from the south to the middle part of Norway from late July to late August 2021. From the soil samples both carbon stock and carbon concentration were estimated in the lab, and statistical analyses were carried out to see if there were any differences in carbon stock and carbon concentration between the vegetation classes in the alpine tundra and treeline ecotone. To account for spatial dependencies caused by the sampling design (study sites and sample lines), a linear mixed effects model was used. Elevation was also included in the model to account for the possible effect of elevation on soil carbon storage. All vegetation classes found in the alpine tundra had larger soil carbon stock sizes and higher carbon concentration value than the vegetation class in the treeline ecotone, but not all were significantly different. Elevation had only a very minor influence on carbon concentration, and non-significant effect on carbon stock. Altogether, the results indicate that carbon stock and carbon concentration in the organic soil layer is larger for the vegetation classes in the alpine tundra than the vegetation class in the treeline. An expansion of the treeline could further lead to higher CO2 emissions and have positive feedback on global warming. The study illustrates how the expansion of trees into alpine tundra could affect the carbon storage in the top organic soil layer