The Roles of Climate Extremes, Ecological Succession, and Hydrology in Repeated Permafrost Aggradation and Degradation in Fens on the Tanana Flats, Alaska

Abstract

AbstractThe 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.