Bog and Fen Meadows and shallowest in the grav-
ous disturbances and describe the general concep-
elly lowland and alpine ecotypes.
tual models that have been developed to relate
Soil pH (either in soil paste or soil water) was
ecological changes over time in response to these
highly variable among ecotypes. Soil pH was
disturbances. To facilitate the discussion, we have
highest (typically >6) in Lowland Fen Meadow,
developed simplified ecological profiles of the
Lowland Scrub Fen, Lowland Wet Mixed Forest,
occurrence of ecotypes across the landscape based
and Riverine Moist Tall Scrub. These ecotypes
on the results of our analyses (Fig. 28 and 29). Dis-
generally are affected by groundwater or sedi-
turbance processes are identified on the profiles
mentation and tend to be early successional types.
as they relate to ecosystem patterns.
Lowest values (typically <5) were found in Low-
land Tussock Bog, Upland Moist Broadleaf For-
Fluvial processes
est, Upland Moist Needleleaf Forest, and Upland
Channel migration associated with the Tanana
Wet Needleleaf Forest.
and Chena Rivers is a prominent feature of the
Soil electrical conductivity (EC) generally was
Tanana Valley landscape, but the relative proportion
low for all ecotypes because of the lack of soluble
of affected areas in the overall landscape was rela-
salts. Highest values (typically >200 S/cm)
tively small. Within the study area, the area covered
occurred in Lowland Fen Meadow, Lowland
by water in the Lower Perennial River was 0.6% and
Scrub Fen, Lowland Wet Mixed Forest, Riverine
Riverine Barrens covered 0.2% of the area. Early
Moist Tall Scrub, and Lower Perennial River.
(Riverine Moist Tall Scrub, 1.9% of area), mid-
These ecotypes generally are affected by ground-
(Riverine Moist Broadleaf Forest, 0.9%, Riverine
water movement or frequent sedimentation.
Moist Mixed Forest, 1.4%, Riverine Wet Low Scrub
Lowest values (typically <50 S/cm) occurred in
0.1%, Riverine Complex 0.9%), and late- (Riverine
alpine and upland ecotypes. These ecotypes gen-
Moist Needleleaf Forest, 1.8%) successional eco-
erally are subject to downward leaching of soluble
types, which have developed after the initial
salts and lack surface sediment or groundwater
disturbance, occupy 8% of the total landscape.
input.
Previous studies have found a characteristic
Water depths generally were above the surface
pattern of vegetation succession along riverbanks
for Lakes and Ponds, Lacustrine Fen Meadow,
in interior Alaska (Drury 1956, Viereck 1970, and
Lowland Fen Meadow, and Lowland Scrub Fen.
Viereck et al. 1993) that correspond well to the
Wet ecotypes had water depths within 50 cm
ecotypes that we mapped on river floodplains.
below the surface, except Lowland Gravelly Wet
Generally, these conceptual models of floodplain
Low Scrub, Lowland Wet Mixed Forest, and Up-
succession indicate that (1) plant colonization is
land Wet Needleleaf Forest. These latter types
initiated by willows (05 yr for establishment)
tended to have highly variable water depth and
after the accumulation of sufficient sediments
soil moisture conditions, and thus they were prob-
along the active channels occurs, (2) initial colo-
lematic in terms of assigning a general moisture
nizers proceed through a willow-alder stage (5
condition. Alpine and upland ecotypes generally
10 yr), (3) forest stands develop through overstory
had water depths >75 cm below the surface, but
dominance by balsam poplar (20100 yr), (4)
positive determinations were infrequent because
mixed stands with poplar and white spruce (100
water generally occurred at great depths or rocky
200 yr) then develop, (5) mature white spruce
material prevented deep sampling.
(200300 yr) replaces those stands, and (6) black
spruce (>500 yr) eventually becomes dominant
(Viereck et al. 1993). The principal factors affect-
Ecosystem dynamics
Ecosystems not only have a spatial component
ing this successional development are decreasing
as described in the previous results, but also
sedimentation and water table levels. These are
change over time in response to disturbance and
caused by increasing bank height, accumulation
successional change. We identified the principal
of organics from litter and later feather mosses,
factors affecting the dynamics of ecosystems
burial of organic layers by flooding that provides
within the study area to be fluvial processes
the characteristic soil sequence of interbedded
associated with channel migration and flooding,
organics, and the development of permafrost as
thermokarst in ice-rich permafrost, fires that
soils become insulated by the thick organic layer.
mostly are associated with lightning strikes, and
Viereck et al. (1993) conclude that life-history
human disturbances. In the following discussion,
characteristics and flooding events are more
we identify the ecotypes associated with the vari-
important during the early stages of succession,
54
Go to contents page
Previous Page
