| Summary: | Current climate change is amplified over northern high latitudes, affecting the biogeochemical interactions and feedback between vegetation and climate. One step towards a more complete understanding of vegetation-climate interactions at high latitudes is to study the contribution from nonvascular photoautotrophs (NVPs), that is lichens, bryophytes, and biological soil crust communities. While NVPs have received less attention than their vascular counterparts, their contribution to boreo-arctic ecosystems is considerable. Mat-forming species often dominate vegetation and NVPs encompass great variation in physiology and adaptions that enables them to persist in a wide range of environmental conditions. This thesis focuses on the ecological and climatological interactions of the group of bright macrolichens commonly known as reindeer lichens. These relations are approached through three research objectives, where each objective is explored in a corresponding research paper. The first objective was to quantify the effect of reindeer lichens on surface albedo, i.e., solar reflectance, and investigate how surface albedo varies between vegetation types and along environmental gradients. This was investigated through collecting and analysing field observations on a vegetation type scale. The second objective was to estimate current and historic reindeer lichen volumes across the pan-Arctic and relate changes in lichen volumes to drivers of environmental change. This was investigated with a remote sensing approach on a landscape scale, looking at how lichen abundance varies spatially and temporally. The last objective was to investigate the effects of winter heatwaves on growing season ecophysiology in lichen rich vegetation, and how the effects may differ between species and type of winter stress. This was done through a field experimental approach, studying the responses on a species scale. The results show how albedo of bright reindeer lichens is likely the highest albedo of common, naturally occurring vegetation, ...
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