Rapid transformation of tundra ecosystems from ice-wedge degradation

Abstract

Ice wedges are a common form of massive ground ice that typically occupy 10–30% of the volume of upper permafrost in the Arctic and are particularly vulnerable to thawing from climate warming. In assessing the patterns and rates of ice-wedge degradation in northeastern Alaska, we found degradation was widespread and rapidly transforming the microtopography, hydrology, soils, ground ice, and vegetation of tundra ecosystems through a sequence of degradation and stabilization stages. Across an extensive mapping area (30 km2), thermokarst troughs and pits with open water (degradation-advanced) covered 0.7% overall and 1.6% in the oldest terrain in 2018. Within an area (0.5 km2) of concentrated thermokarst, undegraded and degraded ice wedges together covered 29% of the area, and all degradation stages combined increased from 2% in 1950 to 19% area in 2018. Initial degradation peaked at 9% in 2000 and initial stabilization was trending upward at 12% area in 2018, indicating slowing degradation and a transition to stabilization. Integration and reorganization of the drainage network as troughs expanded and connected reduced impounded surface water and helped slow degradation. Degradation created large changes in microtopography, trough widths, water depths, depth to wedge ice, pH, soil and ground ice characteristics, and thermal regimes among stages. Community composition completely shifted from dominance of deciduous and evergreen shrubs in tussock tundra in the undegraded stage to dominance of aquatic mosses and forbs in flooded depressions in degradation-advanced stage. Soil slumping along trough margins and rapid colonization by aquatic mosses halted the degradation, and aquatic sedges became dominant in the stabilization-initial stage. In stabilization-advanced, aquatic mosses persisted and the diversity of wet-adapted forbs, sedges, and mosses increased. Recovery back to tussock tundra, however, is unlikely. This decadal-scale transformation has major implications for arctic land cover, tundra productivity, lake expansion and drainage, soil‑carbon balance, trace-gas emissions, and caribou and bird populations.