Studies from 1994-1998 on the Tanana Flats in central Alaska reveal that permafrost degradation is widespread and rapid, causing large shifts in ecosystems from birch forests to fens and bogs. Fine-grained soils under the birch forest are ice-rich and thaw settlement typically is 1-2.5 m after the permafrost thaws. The collapsed areas are rapidly colonized by aquatic herbaceous plants, leading to the development of a thick, floating organic mat. Based on field sampling of soils, permafrost and vegetation, and the construction of a GIS database, we estimate that 17% of the study area (263,964 ha) is unfrozen with no previous permafrost, 48% has stable permafrost, 31% is partially degraded, and 4% has totally degraded. For that portion that currently has, or recently had, permafrost (83% of area), similar to 42% has been affected by thermokarst development. Based on airphoto analysis, birch forests have decreased 35% and fens have increased 29% from 1949 to 1995. Overall, the area with totally degraded permafrost (collapse-scar fens and bogs) has increased from 39 to 47% in 46 y. Based on rates of change from airphoto analysis and radiocarbon dating, we estimate 83% of the degradation occurred before 1949. Evidence indicates this permafrost degradation began in the mid-1700s and is associated with periods of relatively warm climate during the mid-late 1700s and 1900s. If current conditions persist, the remaining lowland birch forests will be eliminated by the end of the next century.
