Widespread shrubification across the Arctic has been generally attributed to increasing air temperatures, but responses vary across species and sites. Wood structures related to the plant hydraulic architecture may respond to local environmental conditions and potentially impact shrub growth, but these relationships remain understudied. Using methods of dendroanatomy, we analysed shrub ring width (RW) and xylem anatomical traits of 80 individuals of Salix glauca L. and Betula nana L. at a snow manipulation experiment in Western Greenland. We assessed how their responses differed between treatments (increased versus ambient snow depth) and soil moisture regimes (wet and dry). Despite an increase in snow depth due to snow fences (28-39 %), neither RW nor anatomical traits in either species showed significant responses to this increase. In contrast, irrespective of the snow treatment, the xylem specific hydraulic conductivity (Ks) and earlywood vessel size (LA95) for the study period were larger in S. glauca (p < 0.1, p < 0.01) and B. nana (p < 0.01, p < 0.001) at the wet than the dry site, while both species had larger vessel groups at the dry than the wet site (p < 0.01). RW of B. nana was higher at the wet site (p < 0.01), but no differences were observed for S. glauca. Additionally, B. nana Ks and LA95 showed different trends over the study period, with decreases observed at the dry site (p < 0.001), while for other responses no difference was observed. Our results indicate that, taking into account ontogenetic and allometric trends, hydraulic related xylem traits of both species, along with B. nana growth, were influenced by soil moisture. These findings suggest that soil moisture regime, but not snow cover, may determine xylem responses to future climate change and thus add to the heterogeneity of Arctic shrub dynamics, though more longterm species- and site- specific studies are needed.

The quantification of vegetation height for the circumpolar Arctic tundra biome is of interest for a wide range of applications, including biomass and habitat studies as well as permafrost modelling in the context of climate change. To date, only indices from multispectral data have been used in these environments to address biomass and vegetation changes over time. The retrieval of vegetation height itself has not been attempted so far over larger areas. Synthetic Aperture Radar (SAR) holds promise for canopy modeling over large extents, but the high variability of near-surface soil moisture during the snow-free season is a major challenge for application of SAR in tundra for such a purpose. We hypothesized that tundra vegetation height can be derived from multispectral indices as well as from C-band SAR data acquired in winter (close to zero liquid water content). To test our hypothesis, we used C-band SAR data from Sentinel-1 and multi-spectral data from Sentinel-2. Results show that vegetation height can be derived with an RMSE of 44 cm from Normalized Difference Vegetation Index (NDVI) and 54 cm from Tasseled Cap Wetness index (TC). Retrieval from C-band SAR shows similar performance, but C-VV is more suitable than C-HH to derive vegetation height (RMSEs of 48 and 56 cm respectively). An exponential relationship with in situ height was evident for all tested parameters (NDVI, TC, C-VV and C-HH) suggesting that the C-band SAR and multi-spectral approaches possess similar capabilities including tundra biomass retrieval. Errors might occur in specific settings as a result of high surface roughness, high photosynthetic activity in wetlands or high snow density. We therefore introduce a method for combined use of Sentinel-1 and Sentinel-2 to address the ambiguities related to Arctic wetlands and barren rockfields. Snow-related deviations occur within tundra fire scars in permafrost areas in the case of C-VV use. The impact decreases with age of the fire scar, following permafrost and vegetation recovery. The evaluation of masked C-VV retrievals across different regions, tundra types and sources (in situ and circumpolar vegetation community classification from satellite data) suggests pan-Arctic applicability to map current conditions for heights up to 160 cm. The presented methodology will allow for new applications and provide advanced insight into changing environmental conditions in the Arctic.

Rapid climate warming has resulted in shrub expansion, mainly of erect deciduous shrubs in the Low Arctic, but the more extreme, sparsely vegetated, cold and dry High Arctic is generally considered to remain resistant to such shrub expansion in the next decades. Dwarf shrub dendrochronology may reveal climatological causes of past changes in growth, but is hindered at many High Arctic sites by short and fragmented instrumental climate records. Moreover, only few High Arctic shrub chronologies cover the recent decade of substantial warming. This study investigated the climatic causes of growth variability of the evergreen dwarf shrub Cassiope tetragona between 1927 and 2012 in the northernmost polar desert at 83 degrees N in North Greenland. We analysed climate-growth relationships over the period with available instrumental data (1950-2012) between a 102-year-long C.tetragona shoot length chronology and instrumental climate records from the three nearest meteorological stations, gridded climate data, and North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) indices. July extreme maximum temperatures (JulT(emx)), as measured at Alert, Canada, June NAO, and previous October AO, together explained 41% of the observed variance in annual C.tetragona growth and likely represent insitu summer temperatures. JulT(emx) explained 27% and was reconstructed back to 1927. The reconstruction showed relatively high growing season temperatures in the early to mid-twentieth century, as well as warming in recent decades. The rapid growth increase in C.tetragona shrubs in response to recent High Arctic summer warming shows that recent and future warming might promote an expansion of this evergreen dwarf shrub, mainly through densification of existing shrub patches, at High Arctic sites with sufficient winter snow cover and ample water supply during summer from melting snow and ice as well as thawing permafrost, contrasting earlier notions of limited shrub growth sensitivity to summer warming in the High Arctic.

Increases in the frequency and magnitude of disturbances associated with the thawing of ice-rich permafrost highlight the need to understand long-term vegetation succession in permafrost environments. This study uses field sampling and remote sensing to explore vegetation development and soil conditions following catastrophic lake drainage in Old Crow Flats (OCF). The data presented show that vegetation on drained lake basins in OCF is characterized by two distinct assemblages: tall willow stands and sedge swards. Field sampling indicates that these alternative successional trajectories result from variation in soil moisture following drainage. Increased willow mortality on older drained basins suggests that intraspecific competition drives self-thinning in shrub thickets. This finding, combined with data from paleoecological studies and contemporary vegetation in OCF, suggests that willow stands on drained lake basins are seral communities. These results also indicate that the increase in number of catastrophic drainages that occurred between 1972 and 2010 will alter regional vegetation in ways that affect wildlife habitat, permafrost conditions, and local hydrology.

The influence of Arctic vegetation on albedo, latent and sensible heat fluxes, and active layer thickness is a crucial link between boundary layer climate and permafrost in the context of climate change. Shrubs have been observed to lower the albedo as compared to lichen or graminoid-tundra. Despite its importance, the quantification of the effect of shrubification on summer albedo has not been addressed in much detail. We manipulated shrub density and height in an Arctic dwarf birch (Betula nana) shrub canopy to test the effect on shortwave radiative fluxes and on the normalized difference vegetation index (NDVI), a proxy for vegetation productivity used in satellite-based studies. Additionally, we parametrised and validated the 3D radiative transfer model DART to simulate the amount of solar radiation reflected and transmitted by an Arctic shrub canopy. We compared results of model runs of different complexities to measured data from North-East Siberia. We achieved comparably good results with simple turbid medium approaches, including both leaf and branch optical property media, and detailed object based model parameterisations. It was important to explicitly parameterise branches as they accounted for up to 71% of the total canopy absorption and thus contributed significantly to soil shading. Increasing leaf biomass resulted in a significant increase of the NDVI, decrease of transmitted photosynthetically active radiation, and repartitioning of the absorption of shortwave radiation by the canopy components. However, experimental and modelling results show that canopy broadband nadir reflectance and albedo are not significantly decreasing with increasing shrub biomass. We conclude that the leaf to branch ratio, canopy background, and vegetation type replaced by shrubs need to be considered when predicting feedbacks of shrubification to summer albedo, permafrost thaw, and climate warming. (C) 2014 Elsevier Inc. All rights reserved.

Thaw of ice-rich permafrost soils on sloping terrain can trigger erosional disturbance events that displace large volumes of soil and sediment, kill and damage plants, and initiate secondary succession. We examined how retrogressive thaw slumps (RTS), a common form of thermo-erosional disturbance in arctic tundra, affected the local loss and re-accumulation of carbon (C) and nitrogen (N) pools in organic and surface mineral soil horizons of 18 slumps within six spatially independent sites in arctic Alaska. RTS displaced 3 kg C and 0.2 kg N per m(2) from the soil organic horizon but did not alter pools of C and N in the top 15 cm of the mineral horizon. Surface soil C pools re-accumulated rapidly (32 +/- 10 g C m(-2) yr(-1)) through the first 60 years of succession, reaching levels similar to undisturbed tundra 40-64 years after disturbance. Average N re-accumulation rates (2.2 +/- 1.1 g N m(-2) yr(-1)) were much higher than expected from atmospheric deposition and biological N fixation. Finally, plant community dominance shifted from graminoids to tall deciduous shrubs, which are likely to promote higher primary productivity, biomass accumulation, and rates of nutrient cycling.

In arctic Alaska, air temperatures have warmed 0.5 degrees Celsius (degreesC) per decade for the past 30 years, winter. Over the same period, shrub abundance has increased, perhaps a harbinger of a conversion of tundra to shrubland. Evidence suggests that winter biological processes are contributing to this conversion through a positive feedback that involves the snow-holding capacity of shrubs, the insulating properties of snow, a soil layer that has a high water content because it overlies nearly impermeable permafrost, and hardy microbes that can maintain metabolic activity at temperatures of -6degreesC or lower. Increasing shrub abundance leads to deeper snow, which promotes higher winter soil temperatures, greater microbial activity and more plant-available nitrogen. High levels of soil nitrogen favor shrub growth the follow- summer. With climate models predicting continued warming, large areas of tundra could become converted to shrubland, with winter processes like those described here possibly playing a critical role.