James C. Walters

PERMAFROST DEGRADATION AND ECOLOGICAL CHANGES ASSOCIATED WITH A WARMING CLIMATE IN CENTRAL ALASKA

Studies from 1994–1998 on the TananaFlats in central Alaska reveal that permafrostdegradation is widespread and rapid, causing largeshifts in ecosystems from birch forests to fens andbogs. Fine-grained soils under the birch forest areice-rich and thaw settlement typically is 1–2.5 mafter the permafrost thaws. The collapsed areas arerapidly colonized by aquatic herbaceous plants,leading to the development of a thick, floatingorganic mat. Based on field sampling of soils,permafrost and vegetation, and the construction of aGIS database, we estimate that 17% of the study area(263,964 ha) is unfrozen with no previous permafrost,48% has stable permafrost, 31% is partiallydegraded, and 4% has totally degraded. For thatportion that currently has, or recently had,permafrost (83% of area), ∼42% has been affected bythermokarst development. Based on airphoto analysis,birch forests have decreased 35% and fens haveincreased 29% from 1949 to 1995. Overall, the areawith totally degraded permafrost (collapse-scar fensand bogs) has increased from 39 to 47% in 46 y. Based on rates of change from airphoto analysis andradiocarbon dating, we estimate 83% of thedegradation occurred before 1949. Evidence indicatesthis permafrost degradation began in the mid-1700s andis associated with periods of relatively warm climateduring the mid-late 1700s and 1900s. If currentconditions persist, the remaining lowland birchforests will be eliminated by the end of the nextcentury.

Groundwater-Discharge Fens in the Tanana Lowlands, Interior Alaska, U.S.A.

Large expanses of herbaceous floating mat wetlands (FMW) bordered by slightly higher uplands with forest or scrub occur in the northwest corner of the Tanana Flats between the Alaska Range and the Tanana River. Five major FMW systems, together with other outliers and extensions, are linear in shape and cover over 20 km2. Nutrient-rich and circumneutral water flows slowly through these areas toward the northwest and through outlets to the Tanana River. The floating mat vegetation consists of tall emergent macrophytes; mosses, in particular Sphagnum spp., are conspicuously absent and shrubs are infrequent. Although species dominance shifts over short distances on the mat, four community types can be recognized: (1) Menyanthes trifoliata, (2) Carex aquatilis, (3) Typha latifolia, and (4) Calla palustris. Below the water surface, the mat extends to a depth of 0.5 to 1.0 m and consists of rhizomes and roots in a matrix of well-decomposed peat and water. The mat then either directly overlies unfrozen gray silts at a depth of 1 m, or more commonly, floats on a clear-water or loose peat zone above more consolidated peat lying on unfrozen silt at a depth of 1.5 to 2.5 m. No permafrost or frozen ground was detected in late August or late winter below these floating mats but it is ubiquitous on the bordering uplands, 0.5 to 2 m above the FMW. The topographic location, apparent absence of permafrost, water chemistry, and vegetation composition suggest that these areas are fens fed by groundwater sources flowing out of the Alaska Range. Permafrost degradation and lateral expansion of these FMW is indicated by slumped blocks of forest peat, dead trees, and open water moats along the upland margin. Although floating mats are frequently described in the literature as occupying the edge of northern ponds and lakes, the FMW described here do not and they appear to be unique because of their large extent, absence of mosses, physiographic position, and presumed origin.

Polygonal patterned ground in central New Jersey

Abstract Polygonal patterned ground and associated sediment-filled wedges occur in thin-bedded shale in central New Jersey. During a dry growing season, numerous areas of polygonal ground were visible owing to differential growth of vegetation over the sediment-filled fractures. Construction sites where material was removed from the surface prior to excavation also revealed areas of polygonal ground. Measurements of the patterns show networks of polygons with diameters ranging from about 3 m to over 30 m, with an average of approximately 20 m. The wedges examined in vertical exposures range in depth from 25 to 260 cm (average, 125 cm) and in width from 10 to 240 cm (average, 50 cm). The infilling material of the wedges is mostly a sandy loam, although this material is not homogeneous throughout the wedges. The presence of ventifacts and frosted sand grains within the wedges indicates eolian activity during the formation of these features. The polygonal ground and wedge structures exhibit several characteristics similar to those of ice-wedge casts, and available evidence suggests they may have originated as ice-wedge polygons during the last glacial maximum.