Aboveground vegetation biomass in northern treeless landscapes - peatlands and Arctic tundra - has been modelled with spectral information derived from optical remote sensing in several studies. However, synthesized overviews of biomass patterns across circumpolar sites have been limited. Based on data from eight study sites in Europe, Siberia and Canada, we ask (1) how biomass is divided between plant functional types (PFTs) and (2) how well biomass patterns can be detected with widely available, moderate spatial resolution (3-10 m) satellite imagery and topographic data. We explain biomass patterns using random forest regressions with the predictors being spectral bands and indices calculated from multi-temporal Sentinel-2 and PlanetScope imagery and topographic information calculated from ArcticDEM data. Our results indicate that there are notable differences in vegetation composition between northern landscapes with mosses, graminoids and deciduous shrubs being the most dominant PFTs. Remote sensing data detects biomass patterns, but regression performance varies between sites (explained variance 36-70%, normalized root mean square error 9-19%). There is also variability between sites whether Sentinel-2 or PlanetScope data is more suitable to detect biomass patterns and which the most important predictors are. Topographic information has a minor or negligible importance in most of the sites. Our results suggest that there is no easily generalizable relationship between satellite-derived vegetation greenness and biomass.

Effects of permafrost aggradation on greenhouse gas (GHG) dynamics and climate forcing have not been previously quantified. Here, we reconstruct changes in GHG balances over the late Holocene for a sub-arctic peatland by applying palaeoecological data combined with measured GHG flux data, focusing on the impact of permafrost aggradation in particular. Our data suggest that permafrost initiation around 3000 years ago resulted in GHG emissions, thereby slightly weakening the general long-term peatland cooling impact. As a novel discovery, based on our chronological data of bare peat surfaces, we found that current sporadic bare peat surfaces in subarctic regions are probably remnants of more extensive bare peat areas formed by permafrost initiation. Paradoxically, our data suggest that permafrost initiation triggered by the late Holocene cooling climate generated a positive radiative forcing and a short-term climate warming feedback, mitigating the general insolation-driven late Holocene summer cooling trend. Our work with historical data demonstrates the importance of permafrost peatland dynamics for atmospheric GHG concentrations, both in the past and future. It suggests that, while thawing permafrost is likely to initially trigger a change towards wetter conditions and consequent increase in CH4 forcing, eventually the accelerated C uptake capacity under warmer climate may overcome the thaw effect when a new hydrological balance becomes established.

Wetlands, including peatlands, supply crucial ecosystem services such as water purification, carbon sequestration and regulation of hydrological and biogeochemical cycles. Peatlands are especially important as carbon sinks and stores because of the incomplete decomposition of vegetation within the peat. Good knowledge of individual wetlands exists locally, but information on how different wetland systems interact with their surroundings is lacking. In this study, the ability to use a depression-based digital elevation model (DEM) method to inventory wetlands in northern landscapes and assess their hydrological connectivity was investigated. The method consisted of three steps: (1) identification and mapping of wetlands, (2) identification of threshold values of minimum wetland size and depth, and (3) delineation of a defined coherent area of multiple wetlands with hydrological connectivity, called wetlandscape. The results showed that 64% of identified wetlands corresponded with an existing wetland map in the study area, but only 10% of the wetlands in the existing map were identified, with the F1 score being 17%. Therefore, the methodology cannot independently map wetlands and future research should be conducted in which additional data sources and mapping techniques are integrated. However, wetland connectivity could be mapped with the depression-based DEM methodology by utilising information on upstream and downstream wetland depressions, catchment boundaries and drainage flow paths. Knowledge about wetland connectivity is crucial for understanding how physical, biological and chemical materials are transported and distributed in the landscape, and thus also for resilience, management and protection of wetlandscapes.