| Summary: | Anthropogenic climate change is one of the biggest threats to biodiversity and ecosystem functioning in high-latitude ecosystems. These regions are warming faster than the global mean, prompting changes in soil temperature, precipitation, and the onset and length of seasons. Species-level responses to these changes, rooted in temperature-dependent metabolic processes, will have consequences which permeate all levels of biological organisation and ecosystem functioning. Using a naturally occurring soil temperature gradient spanning 5–30 °C in the Hengill valley in Iceland, I set out to provide insight into long-term responses of species, communities, and soil processes to soil temperature. Epigeal plant and invertebrate communities were sampled in the summers of 2013, 2015, and 2017. In 2018, in addition to sampling aboveground communities, belowground communities and soil physiochemical properties were also examined. Chapter I sets out to examine the effect of soil temperature on populationand communitylevel epigeal invertebrate and plant communities. We recorded a significant decrease in the α-diversity of plants and invertebrates with increasing temperature, apparently driven by warming-induced decrease in plant species richness, and changes in the dominance hierarchy of the invertebrate community. Warm-adapted species replaced species with lower thermal optima in warm patches, leading to significant turnover in community structure with warming. Mean body size decreased with warming, and together with an overall increase in the abundance invertebrates at warmer patches, led to no effect of temperature on community biomass. Despite clear community-level trends in diversity and biomass indices, population-level effects were inconsistent, driven by differential thermal tolerances. However, these baseline trends were not consistent throughout the active season, as revealed in Chapter II. Seasonal fluctuations in invertebrate diversity indices were dampened by warming, while variation in biomass increased. ...
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