The diversity of ecosystems across boreal landscapes, successional changes after disturbance and complicated permafrost histories, present enormous challenges for assessing how vegetation, water and soil carbon may respond to climate change in boreal regions. To address this complexity, we used a chronosequence approach to assess changes in vegetation composition, water storage and soil organic carbon (SOC) stocks along successional gradients within four landscapes: (1) rocky uplands on ice-poor hillside colluvium, (2) silty uplands on extremely ice-rich loess, (3) gravelly–sandy lowlands on ice-poor eolian sand and (4) peaty–silty lowlands on thick ice-rich peat deposits over reworked lowland loess. In rocky uplands, after fire permafrost thawed rapidly due to low ice contents, soils became well drained and SOC stocks decreased slightly. In silty uplands, after fire permafrost persisted, soils remained saturated and SOC decreased slightly. In gravelly–sandy lowlands where permafrost persisted in drier forest soils, loss of deeper permafrost around lakes has allowed recent widespread drainage of lakes that has exposed limnic material with high SOC to aerobic decomposition. In peaty–silty lowlands, 2–4 m of thaw settlement led to fragmented drainage patterns in isolated thermokarst bogs and flooding of soils, and surface soils accumulated new bog peat. We were not able to detect SOC changes in deeper soils, however, due to high variability. Complicated soil stratigraphy revealed that permafrost has repeatedly aggraded and degraded in all landscapes during the Holocene, although in silty uplands only the upper permafrost was affected. Overall, permafrost thaw has led to the reorganization of vegetation, water storage and flow paths, and patterns of SOC accumulation. However, changes have occurred over different timescales among landscapes: over decades in rocky uplands and gravelly–sandy lowlands in response to fire and lake drainage, over decades to centuries in peaty–silty lowlands with a legacy of complicated Holocene changes, and over centuries in silty uplands where ice-rich soil and ecological recovery protect permafrost.

Ground ice is abundant in the upper permafrost throughout the Arctic and fundamentally affects terrain responses to climate warming. Ice wedges, which form near the surface and are the dominant type of massive ice in the Arctic, are particularly vulnerable to warming. Yet processes controlling ice wedge degradation and stabilization are poorly understood. Here we quantified ice wedge volume and degradation rates, compared ground ice characteristics and thermal regimes across a sequence of five degradation and stabilization stages and evaluated biophysical feedbacks controlling permafrost stability near Prudhoe Bay, Alaska. Mean ice wedge volume in the top 3 m of permafrost was 21%. Imagery from 1949 to 2012 showed thermokarst extent (area of water‐filled troughs) was relatively small from 1949 (0.9%) to 1988 (1.5%), abruptly increased by 2004 (6.3%) and increased slightly by 2012 (7.5%). Mean annual surface temperatures varied by 4.9°C among degradation and stabilization stages and by 9.9°C from polygon center to deep lake bottom. Mean thicknesses of the active layer, ice‐poor transient layer, ice‐rich intermediate layer, thermokarst cave ice, and wedge ice varied substantially among stages. In early stages, thaw settlement caused water to impound in thermokarst troughs, creating positive feedbacks that increased net radiation, soil heat flux, and soil temperatures. Plant growth and organic matter accumulation in the degraded troughs provided negative feedbacks that allowed ground ice to aggrade and heave the surface, thus reducing surface water depth and soil temperatures in later stages. The ground ice dynamics and ecological feedbacks greatly complicate efforts to assess permafrost responses to climate change.

The Tanana Flats in central Alaska are a hot spot for thermokarst that is rapidly transforming the landscape. Time series analysis of high‐resolution imagery showed that permafrost degradation increased the area of three large fens by 26% from 1949 to 2018, but surprisingly permafrost also aggraded in small areas. Permafrost soils in adjacent birch forests frequently had fen peat near the surface indicating recent permafrost degradation and aggradation. We attribute groundwater as the primary driver of recent permafrost degradation, while climate extremes (especially cold, less snowy winters) caused recent permafrost aggradation, but only after ecological succession provided conditions favorable for permafrost formation. Near‐surface groundwater temperatures during early winters (2–6°C during 2011–2014) contributed to a near steady rate of lateral permafrost degradation (0.36 m/yr) over three periods. Fairbanks climate records (1904–2019) showed large ranges in mean winter temperatures (12°C) and snow depths (69 cm) between cold, less snowy winters and warm, snowy winters. During ecological succession from the collapsing margins to the fen centers over a ~250‐yr period, vegetation transitioned from aquatic forbs to shrubs and mosses, water depths decreased, and the soil carbon stock of new peat increased. These interactions among permafrost, groundwater, climate warming, and succession complicate our ability to project future ecological transitions and soil carbon changes across this dynamic boreal lowland landscape. The more frequent warm, snowy winters since 2014 and model projections for warmer winters, however, indicate that the region has crossed a tipping point where permafrost will no longer form and permafrost loss is irreversible.

Accurate identification of the relationships between permafrost extent and landscape patterns can help to develop airborne geophysical or remote sensing tools to map permafrost in remote locations or across large areas. These tools will be particularly applicable in discontinuous permafrost where climate warming or disturbances such as human development or fire can lead to rapid permafrost degradation. We have linked field-based geophysical, point-scale, and imagery surveying measurements to map permafrost at five fire scars (1930, 1975, 1988, 2001, and 2010) on the Tanana Flats in central Alaska. Ground-based elevation surveys, seasonal thaw-depth profiles, and electrical resistivity tomography (ERT) measurements were combined with airborne imagery and light detection and ranging (LiDAR) to identify relationships between permafrost geomorphology and elapsed time since fire disturbance. ERT proved to be a robust technique for mapping the presence or absence of permafrost because of the marked difference in resistivity values for frozen versus unfrozen material. There was no clear relationship between elapsed time since fire and permafrost extent at our sites. However, we have found that the transition zone boundaries between permafrost soils and unfrozen soils in the collapse-scar bogs at our sites had complex and unpredictable morphologies. This result suggested that attempts to quantify the presence or absence of permafrost using aerial measurements alone could lead to incomplete results. Taken in total, the results from our study indicated that although ground-based ERT measurements were the most rapid means of mapping permafrost, we were still limited in being able to apply airborne surveying measurements at the landscape scale toward accurately estimating permafrost extent.