The populations, species, and communities in high elevation mountainous regions at or above tree line are being impacted by the changing climate. Mountain systems have been recognized as both resilient and extremely threatened by climate change, requiring a more nuanced understanding of potential trajectories of the biotic communities. For high elevation systems in particular, we need to consider how the interactions among climate drivers and topography currently structure the diversity, species composition, and life-history strategies of these communities. Further, predicting biotic responses to changing climate requires knowledge of intra- and inter-specific climate associations within the context of topographically heterogenous landscapes. Changes in temperature, snow, and rain characteristics at regional scales are amplified or attenuated by slope, aspect, and wind patterns occurring at local scales that are often under a hectare or even a meter in extent. Community assemblages are structured by the soil moisture and growing season duration at these local sites, and directional climate change has the potential to alter these two drivers together, independently, or in opposition to one another due to local, intervening variables. Changes threaten species whose water and growing season duration requirements are locally extirpated or species who may be outcompeted by nearby faster-growing, warmer/drier adapted species. However, barring non-analogue climate conditions, species may also be able to more easily track required resource regimes in topographically heterogenous landscapes. New species arrivals composed of competitors, predators and pathogens can further mediate the direct impacts of the changing climate. Plants are moving uphill, demonstrating primary succession with the emergence of new habitats from snow and rock, but these shifts are constrained over the short term by soil limitations and microbes and ultimately by the lack of colonizable terrestrial surfaces. Meanwhile, both subalpine herbaceous and woody species pose threats to more cold-adapted species. Overall, the multiple interacting direct and indirect effects of the changing climate on high elevation systems may lead to multiple potential trajectories for these systems.

Annual mean ground temperatures (T-g) decline northward from approximately -3.0 degrees C in the boreal forest to -7.0 degrees C in dwarf-shrub tundra in the Tuktoyuktuk Coastlands and Anderson Plain, NWT, Canada. The latitudinal decrease in T-g from forest to tundra is accompanied by an increase in the range of values measured in the central, tall-shrub tundra zone. Field measurements from 124 sites across this ecotone indicate that in undisturbed terrain Tg may approach 0 degrees C in the forest and -4 degrees C in dwarf-shrub tundra. The greatest range of local variation in T-g (similar to 7 degrees C) was observed in the tall-shrub transition zone. Undisturbed terrain units with relatively high T-g include riparian areas and slopes with drifting snow, saturated soils in polygonal peatlands and areas near lakes. Across the region, the warmest permafrost is associated with disturbances such as thaw slumps, drained lakes, areas burned by wildfires, drilling-mud sumps and roadsides. Soil saturation following terrain subsidence may increase the latent heat content of the active layer, while increases in snow depth decrease the rate of ground heat loss in autumn and winter. Such disturbances increase freezeback duration and reduce the period of conductive ground cooling, resulting in higher Tg and, in some cases, permafrost thaw. The field measurements reported here confirm that minimum T-g values in the uppermost 10 m of permafrost have increased by similar to 2 degrees C since the 1970s. The widespread occurrence of T-g above -3 degrees C indicates warm permafrost exists in disturbed and undisturbed settings across the transition from forest to tundra. Copyright (c) 2017 Government of the Northwest Territories. Permafrost and Periglacial Processes (c) 2017 John Wiley & Sons, Ltd.

Mountain ecosystems are commonly regarded as being highly sensitive to global change. Due to the system complexity and multifaceted interacting drivers, however, understanding current responses and predicting future changes in these ecosystems is extremely difficult. We aim to discuss potential effects of global change on mountain ecosystems and give examples of the underlying response mechanisms as they are understood at present. Based on the development of scientific global change research in mountains and its recent structures, we identify future research needs, highlighting the major lack and the importance of integrated studies that implement multi-factor, multi-method, multi-scale, and interdisciplinary research.