Small-scale climate variability and its ecosystem impacts in the sub-Arctic

Sammanfattning: Abstract in Undetermined To improve our knowledge of climate change and its impacts on ecosystem services, and to inform local people and governments that need assistance with the development of mitigation and adaptation strategies, climate impacts need to be understood urgently at the smaller scale. On one hand, the microclimate/topoclimate is known to be strongly decoupled from the general macroclimate of the freely circulating atmosphere and cannot reliably be inferred from climate station data or conventional climate models. On the other hand, it is the microclimate that directly influences local ecological processes and reflects subtle environmental variability. Thus, capturing and assessing climate and environmental variability at the small scale is an important subject that has been at the forefront of climate research for some years. The main objective of this thesis is to model small scale climate variability and explore its implications for vegetation distribution and nutrient cycling at Abisko, Northern Sweden. Comparison of the regional climate reanalysis product (i.e., ERA 40) and measurements from weather stations demonstrated that the coupling between the regional climate pattern and field-work measurements was affected by local terrain features. The microclimate pattern in terms of temperature distribution within the mountainous environment was driven mainly by the surface-atmosphere energy exchange and the influence of Lake Torneträsk through the lake-land breeze system, while cold air ponding tended to dominate during the nighttimes. Assuming a ‘static’ nature of these short-term microclimate characteristics, a fine-scale (50-m), long-term surface-air-temperature dataset was developed based on an empirically-based topoclimate model. The results showed that the extent of the area in which permafrost, if present, could be potentially at risk from thawing after the year 2000 exceeded that of the earlier warming period during the 1930 – 1940s by more than 10%. The coverage of favourable areas for potential tree establishment at the treeline decreased after the mid-1930s before increasing again after the 1960s. Modelling results demonstrated that the distribution of lowland birch forest and more restrictedly distributed snowbed community were strongly associated with temperature. However, the temperature and soil variables were poor predictors of the distribution of plant communities such as wetland communities partly due to the absence of key ecological drivers (e.g., hydrological processes, snow, wind). I further explored the association between forest structure/plant community composition and nutrient cycling in two co-occuring birch forest communities. Lack of explanatory power of temperature at the 50m scale for nutrient cycling variability in the birch forest is compensated by the association between site factors at a smaller scale (<10 m) and the ecological processes of nutrient cycling. Within this context, this study has improved our understanding of small-scale climate variability and its relationships with local ecological processes. The fine-scale climate dataset of this study has numerous ecological and other applications beyond the modelling exercise that identified substantial potential impacts of the climate variability on lowland permafrost dynamics and treeline movement in the Abisko region.