ALBERT

Description: Connections between vegetation and soil thermal dynamics are critical for estimating the vulnerability of permafrost to thaw with continued climate warming and vegetation changes. The interplay of complex biophysical processes results in a highly heterogeneous soil temperature distribution on small spatial scales. Moreover, the link between topsoil temperature and active layer thickness remains poorly constrained. Sixty-eight temperature loggers were installed at 1–3 cm depth to record the distribution of topsoil temperatures at the Trail Valley Creek study site in the northwestern Canadian Arctic. The measurements were distributed across six different vegetation types characteristic for this landscape. Two years of topsoil temperature data were analysed statistically to identify temporal and spatial characteristics and their relationship to vegetation, snow cover, and active layer thickness. The mean annual topsoil temperature varied between −3.7 and 0.1 ∘C within 0.5 km2. The observed variation can, to a large degree, be explained by variation in snow cover. Differences in snow depth are strongly related with vegetation type and show complex associations with late-summer thaw depth. While cold winter soil temperature is associated with deep active layers in the following summer for lichen and dwarf shrub tundra, we observed the opposite beneath tall shrubs and tussocks. In contrast to winter observations, summer topsoil temperature is similar below all vegetation types with an average summer topsoil temperature difference of less than 1 ∘C. Moreover, there is no significant relationship between summer soil temperature or cumulative positive degree days and active layer thickness. Altogether, our results demonstrate the high spatial variability of topsoil temperature and active layer thickness even within specific vegetation types. Given that vegetation type defines the direction of the relationship between topsoil temperature and active layer thickness in winter and summer, estimates of permafrost vulnerability based on remote sensing or model results will need to incorporate complex local feedback mechanisms of vegetation change and permafrost thaw.

Description: Climate, vegetation, and permafrost are coupled through various positive and negative feedback loops in the Arctic and Subarctic. Many of these feedback mechanisms are still poorly quantified, in particular with respect to vegetation density or biomass. For instance, climate warming facilitates shrub densification and range expansion. The shrub canopies in-turn shade the ground surface during the summer, keeping permafrost cooler, while during the winter the canopies trap more snow, insulating the surface and keeping the ground (and permafrost) warmer. We investigated the feedback of vegetation change on permafrost conditions and local climate at the Trail Valley Creek study site, near tree-line, in Northwest Canada (133.50 ◦ W, 68.74 ◦ N). In particular, we quantified the effect of vegetation on the soil surface temperature and thaw depth through shading in summer and through snow collection in winter. We combine local field measurements of vegetation, climate, and permafrost with spatially resolved data from repeated aerial surveys of high resolution imagery and laser scanning. Our results show that winter ground surface temperatures below tall shrubs are on average 2 ◦ C warmer than below lichen tundra due to the snow layer being twice as deep. However, delayed spring onset and soil shading in summer result in shallower thaw depths below tall shrubs (47cm on average) as compared to lichen tundra (61cm on average). Our results highlight the complex interactions between vegetation and permafrost involving snow, the surface energy budget and soil properties.

Description: Rapid climate change in the northern high latitudes has a strong impact on permafrost stability, apparent as coastal erosion, subsidence, or lake dynamics with potentially severe consequences for local communities and ecology. In a rapidly warming Arctic, the monitoring of these processes is essential to understand and predict permafrost dynamics over the upcoming decades. These landscape dynamics are highly diverse, localized, but widely distributed and require datasets with very high spatial resolution, which are barely achieved by satellite data alone. Repeat observations over several years allow for unprecedented insights into highly critical landscape dynamics and the potential integration with and validation of more coarse resolution satellite data. AWI’s research aircraft (Polar-5 and Polar-6) were equipped with airborne LiDAR (full-waveform, multi-echo) as well with experimental modular sensors such as the DLR-developed multi-spectral optical Modular Airborne Camera System (MACS) with a spatial resolution of few cm, stereo capabilities and a very broad radiometric range. The incoming data stream of acquired laser return point cloud data as well as hundreds of thousands of high-resolution images for individual campaigns poses new challenges of handling and processing large data volumes. Here we present an overview about past and upcoming flight campaigns in Alaska and northwestern Canada. Furthermore, we will show applications of the acquired datasets, such as assessments of subsidence, coastal erosion or infrastructure development.

Description: Soils are warming as air temperatures rise across the Arctic and Boreal region concurrent with the expansion of tall-statured shrubs and trees in the tundra. Changes in vegetation structure and function are expected to alter soil thermal regimes, thereby modifying climate feedbacks related to permafrost thaw and carbon cycling. However, current understanding of vegetation impacts on soil temperature is limited to local or regional scales and lacks the generality necessary to predict soil warming and permafrost stability on a pan-Arctic scale. Here we synthesize shallow soil and air temperature observations with broad spatial and temporal coverage collected across 106 sites representing nine different vegetation types in the permafrost region. We showed ecosystems with tall-statured shrubs and trees (〉 40 cm) have warmer shallow soils than those with short-statured tundra vegetation when normalized to a constant air temperature. In tree and tall shrub vegetation types, cooler temperatures in the warm season do not lead to cooler mean annual soil temperature indicating that ground thermal regimes in the cold-season rather than the warm-season are most critical for predicting soil warming in ecosystems underlain by permafrost. Our results suggest that the expansion of tall shrubs and trees into tundra regions can amplify shallow soil warming, and could increase the potential for increased seasonal thaw depth and increase soil carbon cycling rates and lead to increased carbon dioxide loss and further permafrost thaw.

Description: The vegetation map distinguishes between five tundra vegetation types, trees, and open water at the forest–tundra transition north of Inuvik, Northwest Territories, Canada. The area is underlain by continuous permafrost. Vegetation types were distinguished based on vegetation height derived from airborne laser scanning, airborne orthophotos and observations from the field site. A detailed description of the data sources and processing steps is included.

Description: As a result of recent climate warming, permafrost thaw changes landscape hydrology and threatens infrastructure in the north. Topsoil temperature is an important indicator of how surface conditions translate to active layer thickness and permafrost temperatures. We measured topsoil temperature at 1-3cm depth at 68 locations within 0.5km²? at the Trail Valley Creek study site in the tundra-taiga transition zone, Northwest Territories, Canada. The sensors recorded temperature below six different vegetation types for two years (2016-2018). Topsoil temperature was highly spatially variable even within vegetation types and with mean annual temperatures between -3.7 and 0.1°C. Winter and spring topsoil temperatures clearly depended on the snow distribution, which was influenced by vegetation. On the other hand, summer and autumn temperatures were less variable in space and only weakly related with vegetation type or height, making vegetation a poor proxy for summer soil warming. Vegetation played a crucial part in the link between topsoil temperature and thaw depth. Cold winter temperature was associated with deep active layers in the following summer beneath lichen and dwarf shrub tundra, while we observed the opposite beneath tall shrubs and tussocks. Summer topsoil temperature was not important for thaw depth, in particular at tall shrub and tussock locations. Only beneath lichen and dwarf shrub tundra, we could observe a tendency towards deeper active layers at locations with higher cumulative positive degree days. Our study elucidates how vegetation mediates between above ground processes and permafrost thaw, likely in combination with soil properties and soil moisture. We highlight the importance of complex feedback mechanisms and spatial variability within a few meters for the overall permafrost response. Therefore, reliable estimates of permafrost vulnerability based on permafrost models or remote sensing observations will need to incorporate vegetation-permafrost interactions.