| Summary: | The Arctic is experiencing strong increases in air temperature during the last decades. High-latitude tundra regions are very responsive to changes in temperature and may cause a shift in tundra vegetation composition towards greater dominance of deciduous shrubs. With increasing deciduous shrub cover, the surface albedo (proportion of sunlight that is reflected to the atmosphere) may be reduced and lead to air warming by trapping more solar radiation into the Arctic ecosystem. As a result of this warming, thawing of carbon-rich permafrost soils may increase and cause a large greenhouse gas flux to the atmosphere, thus contributing to global warming. In my thesis I studied how climate influences shrub growth in the Siberian tundra and how climate-induced changes in shrub cover affect summer permafrost thaw and surface albedo. I investigated these interactions between climate, permafrost and Arctic shrub growth using a combination of shrub ring width analysis, field experiments and remote sensing techniques. I measured and compared growth ring widths with meteorological station data and observed that shrub growth is stimulated by higher summer air temperatures. By performing a shrub removal experiment, I demonstrated that a temperature-induced increase in shrub cover may reduce summer permafrost thaw. Shading by the shrub canopy reduced the transfer of energy to the soil. A denser shrub cover thus effectively reduces summer permafrost thaw, despite leading at the same time to a lower surface albedo. These results indicate it is important to incorporate feedbacks between shrub growth, climate and permafrost thaw in model predictions on the Arctic climate and stability of permafrost in a future warmer world.
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