The Arctic experiences rapid climate change, but our ability to predict how this will influence plant communities is hampered by a lack of data on the extent to which different species are associated with particular environmental conditions, how these conditions are interlinked, and how they will change in coming years. Increasing temperatures may negatively affect plants associated with cold areas due to increased competition with warm-adapted species, but less so if local temperature variability is larger than the expected increase. Here we studied the potential drivers of vegetation composition and species richness along coast to inland and altitudinal gradients by the Nuuk fjord in western Greenland using hierarchical modelling of species communities (HMSC) and linear mixed models. Community composition was more strongly associated with random variability at intermediate spatial scales (among plot groups 500 m apart) than with large-scale variability in summer temperature, altitude or soil moisture, and the variation in community composition along the fjord was small. Species richness was related to plant cover, altitude and slope steepness, which explained 42% of the variation, but not to summer temperature. Jointly, this suggests that the direct effect of climate change will be weak, and that many species are associated with microhabitat variability. However, species richness peaked at intermediate cover, suggesting that an increase in plant cover under warming climatic conditions may lead to decreasing plant diversity.

In the Low Arctic, a warming climate is increasing rates of permafrost degradation and altering vegetation. Disturbance associated with warming permafrost can change microclimate and expose areas of ion-rich mineral substrate for colonization by plants. Consequently, the response of vegetation to warming air temperatures may differ significantly from disturbed to undisturbed tundra. Across a latitudinal air temperature gradient, we tested the hypothesis that the microenvironment in thaw slumps would be warmer and more nutrient rich than undisturbed tundra, resulting in altered plant community composition and increased green alder (Alnus viridis subsp. fruticosa) growth and reproduction. Our results show increased nutrient availability, soil pH, snow pack, ground temperatures, and active layer thickness in disturbed terrain and suggest that these variables are important drivers of plant community structure. We also found increased productivity, catkin production, and seed viability of green alder at disturbed sites. Altered community composition and enhancement of alder growth and reproduction show that disturbances exert a strong influence on deciduous shrubs that make slumps potential seed sources for undisturbed tundra. Overall, these results indicate that accelerated disturbance regimes have the potential to magnify the effects of warming temperature on vegetation. Consequently, understanding the relative effects of temperature and disturbance on Arctic plant communities is critical to predicting feedbacks between northern ecosystems and global climate change.