Permafrost degradation and ecological changes associated with a warming climate in central Alaska

Studies from 1994-1998 on the Tanana Flats in central Alaska reveal that pe rmafrost degradation is widespread and rapid, causing large shifts in ecosy stems from birch forests to fens and bogs. Fine-grained soils under the bir ch forest are ice-rich and thaw settlement typically is 1-2.5 m after the p ermafrost thaws. The collapsed areas are rapidly colonized by aquatic herba ceous plants, leading to the development of a thick, floating organic mat. Based on field sampling of soils, permafrost and vegetation, and the constr uction of a GIS database, we estimate that 17% of the study area (263,964 h a) is unfrozen with no previous permafrost, 48% has stable permafrost, 31% is partially degraded, and 4% has totally degraded. For that portion that c urrently has, or recently had, permafrost (83% of area), similar to 42% has been affected by thermokarst development. Based on airphoto analysis, birc h 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 degradatio n occurred before 1949. Evidence indicates this permafrost degradation bega n in the mid-1700s and is associated with periods of relatively warm climat e during the mid-late 1700s and 1900s. If current conditions persist, the r emaining lowland birch forests will be eliminated by the end of the next ce ntury.