Mixed Forest (2.4%), Lacustrine Fen Meadow
whereas biological controls such as organic mat-
(0.3%), and Lowland Wet Tall Scrub (0.6%). Over-
ter accumulation and competition become more
all, ~31% of the study area has undergone some
important in middle and late stages.
degree of permafrost degradation, but degraded
While these simplified models explain most of
areas were much more prevalent on the Tanana
the variation that we observed, ecosystem devel-
Flats (41%) than within the YMA (5.9%).
opment on the floodplains is more complex than
Drury (1956) first described thermokarst pro-
the simplified models indicate. Collins (1990)
cesses in the upper Kuskokwim River region and
quantified changes in erosional and depositional
the changes in vegetation associated with them,
environments between 1938 and 1982 and found
but little attention has been paid to this dis-
the braided portion of the Tanana River near
turbance regime. Racine and Walters (1994)
Fairbanks to be highly dynamic. Mason and
described fens on the Tanana Flats and related
Beget (1991) analyzed long-term changes in depo-
them to permafrost degradation and ground-
sitional environmental through stratigraphic
water discharge from the Alaska Range. The per-
analysis and found that much of the floodplain
mafrost underneath the degrading birch forests,
sediments were deposited between 30002000
found adjacent to the thermokarst collapse scar
years BP. Deposition was much less after 2000
fens, has been found to be extremely ice-rich, in
year BP, although sand units deposited during the
contrast to the permafrost under black spruce for-
last few hundred years indicate a period of larger
ests, which tends to be ice-poor (Walters et al.
flooding events. Mann et al. (1995) contributed to
1998). At the Blair Lakes Training Facility on the
our understanding of the successional develop-
Tanana Flats, the permafrost table has retreated
ment of this complex fluvial landscape by provid-
to a depth of 715 m (Chacho et al. 1995). In mod-
ing a more detailed analysis of geomorphic pro-
eling the sensitivity of permafrost distribution in
cesses, chronological development of fluvial
interior Alaska, Jorgenson and Kreig (1983) con-
sediments, and changes in plant macrofossils as
cluded, however, that permafrost was stable in
indicators of paleoecosystems. Their analyses
some areas, particularly north-facing slopes, even
reveal that later stages of development are less
with substantial climatic warming. Overall, per-
straightforward than the Drury (1956) model sug-
mafrost degradation on the Tanana Flats has been
gests and fire becomes an important factor. In our
found to be widespread (50% of frozen or previ-
analysis, most of the ecotypes we mapped fit the
ously frozen areas are in some stage of permafrost
traditional successional models, but we also
degradation) and rapid (Racine et al. in prep.).
found small areas of Riverine Pond, Riverine Wet
Stratigraphic and photographic analyses suggest
Meadow, and Riverine Wet Low Scrub occurring
that the degradation has occurred primarily dur-
in abandoned meander channels. These ecotypes
ing the last 200 years since the Little Ice Age.
are not included in the simplified sequence of
Racine et al. (1998) described a sequence of veg-
Viereck et al. (1993), but are consistent with the
etation and soil changes as permafrost degrades
complexity described by Drury (1956).
in a central portion of the Tanana Flats. The most
rapid thawing occurs in the permafrost underly-
Thermokarst
ing the Lowland Wet Broadleaf Forest (dominated
Much more poorly understood are successional
by Betula papyrifera) located next to Lowland Fen
relationships related to permafrost degradation,
Meadow (dominated by Menyanthes trifoliata and
although a relatively large portion of the land-
Equisetum fluviatile). As the forest drowns along
scape, particularly on the Tanana Flats is affected
this margin, an open-water moat is colonized by
by thermokarst (Fig. 30 and 31). In the entire study
minerotrophic species (mostly Calla palustris and
area, the ecotypes that generally have developed
Carex rostrata). At the same time, thawing in the
in response to thermokarst include Lowland Fen
interior of the birch forests produces water-filled
Meadow (2.8% of area), Lowland Bog Meadow
pits (small patch size, dominated by Bidens cernua,
(0.1%), Lowland Forest Thermokarst Complex
Lemna minor, and Potentilla palustris) and Lowland
(8.4%), Lowland Scrub Fen (2.0%), Lowland
Bog Meadow (large patch size, dominated by
Scrub-Thermokarst Complex (10.6%), Lowland
Sphagnum spp., Eriophorum scheuchzeri, and
Wet Broadleaf Forest (0.6%)* , Lowland Wet
Oxycoccus microcarpus) in which ombrotrophic
vegetation develops through several stages. As
the thawing front moves into the birch forest from
the fen, these thermokarst features become incor-
* Most of this type was included in Forest Thermokarst
porated into the fen.
Complex.
59
Back to contents page
Previous Page
