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Abstract Alaska has diverse boreal ecosystems across heterogeneous landscapes driven by a wide range of biological and geomorphic processes associated with disturbance and successional patterns under a changing climate. To assess historical patterns and rates of change, we quantified the areal extent of ecotypes and the biophysical factors driving change through photo-interpretation of 2200 points on a time-series (∼1949, ∼1978, ∼2007, ∼2017) of geo-rectified imagery for 22 grids across central Alaska. Overall, 68.6% of the area had changes in ecotypes over ∼68 years. Most of the change resulted from increases in upland and lowland forest types, with an accompanying decrease in upland and lowland scrub types, as post-fire succession led to mid- and late-successional stages. Of 17 drivers of landscape change, fire was by far the largest, affecting 46.5% of the region overall from 1949 to 2017. Fire was notably more extensive in the early 1900s. Thermokarst nearly doubled from 3.9% in 1949 to 6.3% in 2017. Riverine ecotypes covered 7.8% area and showed dynamic changes related to channel migration and succession. Using past rates of ecotype transitions, we developed four state-transition models to project future ecotype extent based on historical rates, increasing temperatures, and driver associations. Ecotype changes from 2017 to 2100, nearly tripled for the driver-adjusted RCP6.0 temperature model (30.6%) compared to the historical rate model (11.5%), and the RCP4.5 (12.4%) and RCP8.0 (14.7%) temperature models. The historical-rate model projected 38 ecotypes will gain area and 24 will lose area by 2100. Overall, disturbance and recovery associated with a wide range of drivers across the patchy mosaic of differing aged ecotypes led to a fairly stable overall composition of most ecotypes over long intervals, although fire caused large temporal fluctuations for many ecotypes. Thermokarst, however, is accelerating and projected to have increasingly transformative effects on future ecotype distributions. 

Abstract. The formation, growth, and decay of freshwater ice on lakes andrivers are fundamental processes of northern regions with wide-rangingimplications for socio-ecological systems. Ice thickness at the end ofwinter is perhaps the best integration of cold-season weather and climate,while the duration of thick and growing ice cover is a useful indicator forthe winter travel and recreation season. Both maximum ice thickness (MIT)and ice travel duration (ITD) can be estimated from temperature-driven icegrowth curves fit to ice thickness observations. We simulated and analyzedice growth curves based on ice thickness data collected from a range ofobservation programs throughout Alaska spanning the past 20–60 years tounderstand patterns and trends in lake and river ice. Results suggestreductions in MIT (thinning) in several northern, interior, and coastalregions of Alaska and overall greater interannual variability in riverscompared to lakes. Interior regions generally showed less variability in MITand even slightly increasing trends in at least one river site. Average ITDranged from 214 d in the northernmost lakes to 114 d acrosssouthernmost lakes, with significant decreases in duration for half ofsites. River ITD showed low regional variability but high interannualvariability, underscoring the challenges with predictingseasonally consistent river travel. Standardization and analysis of theseice observation data provide a comprehensive summary for understandingchanges in winter climate and its impact on freshwater ice services. 

Abstract. Characterization of permafrost, particularly warm and near-surface permafrost which can contain significant liquid water, is critical to understanding complex interrelationships with climate change, ecosystems, and disturbances such as wildfires. Understanding the vulnerability and resilience of permafrost requires an interdisciplinary approach, relying on (for example) geophysical investigations, ecological characterization, direct observations, remote sensing, and more. As part of a multiyear investigation into the impacts of wildfires on permafrost, we have collected in situ measurements of the nuclear magnetic resonance (NMR) response of the active layer and permafrost in a variety of soil conditions, types, and saturations. In this paper, we summarize the NMR data and present quantitative relationships between active layer and permafrost liquid water content and pore sizes and show the efficacy of borehole NMR (bNMR) to permafrost studies. Through statistical analyses and synthetic freezing simulations, we also demonstrate that borehole NMR is sensitive to the nucleation of ice within soil pore spaces.