| Summary: | The Arctic is warming more than three times as fast as the rest of the globe, with serious consequences for high-latitude tundra ecosystems. In connection to warming, there is evidence for widespread changes in composition, productivity and structure of Arctic tundra vegetation, often connected to the expansion of shrub vegetation. Shrubs play a key role in tundra ecosystems, as they dominate plant biomass and can affect abiotic conditions through shading of the ground surface. Shrub expansion can thus influence vegetation dynamics and ecosystem processes such as litter decomposition and carbon cycling, with potential consequences for the global climate system. Considering the scale at which ecological processes and their influencing factors act is key to accurate assessments of ecological dynamics. We therefore need to improve our understanding of the controls and consequences of shrub dynamics across scales to enable more accurate projections of future climatic changes. In this thesis, I explore what factors predict the abundance of individual shrub species at regional scales, and how shrubs and other components of tundra vegetation affect micro-scale climate and, in turn, litter decomposition dynamics. The work is based on systematic monitoring of Arctic tundra plant communities and environmental conditions in two areas along the west coast of Greenland. I used downscaled regional climate (macroclimate) variables to explain shrub species abundance, and monitored local temperatures (microclimate) in different heights above and below the soil surface to assess micro-scale thermal effects of tundra vegetation. Shrub species varied strongly in their relationships to environmental predictors, with climatic and biotic variables being most influential to explain abundances. Shrub-dominated vegetation and snow cover were tightly linked to variation in local temperatures, and particularly to thermal differences between vegetation layers, during both summer and winter. Moreover, links between vegetation and summer soil ...
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