Increasing air temperatures are expected to have pronounced effects on permafrost terrain ecological processes over the coming decades. A projected 5 degree increase in mean annual air temperatures for interior Alaska by 2100 is expected to initiate widespread permafrost degradation. The thermal state of permafrost is predominantly driven by local soil conditions and the vegetation composition. In many places the permafrost is protected against summer warmth by thick organic rich soils and mosses. When permafrost degrades from pulse disturbances like wildfire or infrastructure development or press disturbances like climate warming the soil moisture content and vegetation structure can change dramatically. The responses to disturbance include ground subsidence, changing surface and shallow subsurface hydrology, and shifts in vegetation composition. We undertook this six year study of seasonal thaw measurements, snowpack characteristics, thermal monitoring, and remote sensing to identify relationships between ecotype and permafrost stability/instability. Airborne hyperspectral measurements allow us to identify relationships between vegetation type and seasonal thaw. Repeat LiDAR acquisitions can identify areas exhibiting thaw driven subsidence. High resolution ground surveys pinpoint how and where rapid thaw features (thermokarst) affect vegetation. A manipulated patch of tussock tundra was cleared of surface vegetation to track the thermal and thaw responses to disturbance.
Results/Conclusions
We found strong relationships between snowpack characteristics, vegetation, and the ground thermal state that support other studies showing the role snow plays in the wintertime thermal regime. Seasonal thaw depths above permafrost were controlled predominantly by ecotype and wet precipitation. For example, disturbed sites where surface vegetation and organic matter had been removed from wildfire, trail building, and at our manipulation site yielded the deepest seasonal thaw. Extreme wet precipitation events during two summers were associated with significantly deeper seasonal thaw compared to drier summers. The response to enhanced wet precipitation was strongest in the disturbed sites. This suggests a potential future feedback to enhanced permafrost thaw as the length of the growing season increases. Low ice content soils with deciduous stands had deeper seasonal thaw than higher ice content permafrost underneath tussock and conifer stands. Whether the higher ice content is protected by the tussocks and conifers or whether the vegetation is better suited for shallower summer season thaw is unclear. Hyperspectral imagery and ground truth measurements allowed us to model the relationships between vegetation type and thaw depths. From this, we identified what locations are vulnerable to increased thaw with future warming.