11a). Mean elevations of upland, lowland, and riverine
ganic lowland areas (Fig. 11c). Water depths in the
ecotypes, while distinctly lower than alpine ecotypes, were
Lowland Moist Mixed Forest, Lowland Wet Needleleaf
highly variable, ranging from 300 to 700 m.
Forest, Lowland Wet Low Scrub, and Upland Wet
Permafrost usually was present in the alpine and
Needleleaf Forest often ranged below 50 cm, indicat-
loamy lowland ecotypes, whereas upland and riverine
ing that wetland status for these types sometimes can
ecotypes rarely had permafrost (Fig. 11a). Two excep-
be uncertain. Depths greater than 50 cm, however,
tions included Upland Wet Needleleaf Forest, which
should be considered minimum values because it was
occurs on north-facing slopes, and Riverine Wet
not always possible to determine depth to water when
Meadow, which generally occurs adjacent to small head-
water depths extended below the sampling pit.
water streams in loamy lowland areas.
Site pH (usually free soil water but occasionally soil
Thaw depths were difficult to measure consistently
paste) was highest for gravelly floodplains and
in many areas because of the presence of rocky soils
lowest for Alpine Rocky Dry Dwarf Scrub, Lowland
(Fig. 11a). Thus, thaw depths greater than 100 cm gen-
Wet Low Scrub, Lowland Dwarf Scrub Bog, and Up-
erally indicate a lack of permafrost and often were esti-
land Moist Low and Tall Scrub (Fig. 11c). Overall, most
mated by rounding up to the nearest 50 cm when per-
sites were slightly (6.16.5) to moderately (5.66.0)
mafrost was not positively identified. Thaw depths
acidic.
are reliable where permafrost was present. Thaw
Electrical conductivity (EC), overall, was relatively
depths were least in Alpine Wet Tussock Meadow,
low with no saline areas evident (Fig. 11c). The high-
Alpine Wet Low Scrub, Lowland Wet Tussock Scrub,
est EC values were for rivers, Riverine Gravelly Bar-
Lowland Dwarf Scrub Bog, and Lowland Moist
rens, and Riverine Gravelly Dry Dwarf Scrub, Low-
Mixed Forest. Shallow thaw depths generally were
land Wet Broadleaf Forests, Lowland Moist Mixed
associated with wet loamy soils with thick organic
Forests, and Alpine Rocky Dry Barrens. For the over-
accumulations.
whelming majority of ecotypes, EC values were less
than 100 S/cm.
Surface organic matter depth (uninterrupted O hori-
zons at the surface) is a general indicator of how
Ecosystem dynamics
geomorphically stable an area is. Depths were greatest
in loamy alpine and organic lowland ecotypes (Fig.
Ecosystems not only have a spatial component, as
11b). Surface organic matter accumulation essentially
described above, but also change over time in response
was absent in steep Alpine Rocky Dry Barrens and
to disturbance and succession. We identified the prin-
Upland Rocky Dry Meadow, and in gravelly riverine
cipal factors affecting the dynamics of ecosystems
ecotypes affected by scouring and sedimentation.
within the study area to be fluvial processes associated
Cumulative organic matter depths (sum of O hori-
with channel migration and flooding, fires associated
zons within the top 40 cm) are a good indicator of over-
with lightning strikes and military training, thermokarst
all decomposition rates and avoid the problems associ-
in ice-rich permafrost, and human disturbances. In the
ated with irregular sediment deposition. The greatest
following discussion, we identify the ecotypes associ-
depths were in organic and loamy lowlands, and loamy
ated with the various disturbances and discuss the gen-
alpine ecotypes (Fig. 11b). In contrast, depths were rela-
eral conceptual models that have been developed to
tively shallow for upland, gravelly lowland, and grav-
describe ecosystem dynamics.
elly riverine ecotypes.
Fluvial processes. Channel migration associated
Depth to gravel is important for assessing the accu-
mulation of eolian and fluvial fine-grained material and
with glacial rivers is a prominent feature of the land-
for evaluating drainage and soil moisture. Depths to
scape on Fort Greely, but the relative proportion of af-
gravel were the least for alpine rocky, upland rocky,
fected areas in the overall landscape was relatively
lowland gravelly, and riverine gravelly ecotypes (Fig.
small. Within the study area, the area covered by water
11b). Depths to gravel were greatest for loamy and or-
in upper perennial rivers was 3.1%, and riverine bar-
ganic lowlands. Depth to rocks or gravel was a crite-
rens covered 1.9% of the area. Early (scrub types, 1.2%
rion used for differentiating ecotypes, so it is no sur-
of area), mid- (broadleaf and mixed forests, 2% of the
prise that the depths are consistent with the classifica-
area), and late- (needleleaf forests, 2.6%) successional
tion. Depths greater than 50 cm, however, should be
ecotypes that have developed after disturbances occu-
considered minimum values because depth to gravel
pied 5.8% of the total landscape.
was not always determined independently of the soil-
Previous studies have found a characteristic pattern
sampling pit.
of vegetation succession along riverbanks in interior
Water depths (positive when above ground, nega-
Alaska (Drury 1956, Viereck 1970, Viereck et al. 1993),
tive when below ground) were nearest the ground sur-
although the gravelly floodplains on the glacial outwash
face for the wet ecotypes in alpine, and loamy and or-
are somewhat different from the ecosystems that have
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