been studied on silty floodplains on the lower Tanana
that later stages of development are less straightforward
and Yukon Rivers. Generally, these conceptual models
than the Drury model suggests and that fire can be an
of floodplain succession are as follows:
important factor. In our analysis, the ecotypes often
differed from the predicted successional sequence; for
Plant colonization is started by willows (05 years
example, we observed initial colonization by Dryas
for establishment) after sufficient sediments accu-
drummondii and Elaeagnus commutata followed di-
mulate along the active channels.
rectly by Populus balsamifera saplings. We lacked a
Initial colonizers proceed through a willowalder
tall scrub stage, presumably because of the lack of sedi-
stage (510 years).
mentation of fine-grained cover deposits during
Forest stands develop through overstory dominance
overbank flooding.
by balsam poplar (20100 years).
Mixed stands with poplar and white spruce (100
Fire. Fire is a frequent and widespread disturbance
200 years) then develop.
in interior Alaska that causes well-documented stages
Mature white spruce (200300 years) replaces those
of vegetation succession (Lutz 1956, Viereck 1973, Van
stands.
Cleve et al. 1983). In our study area, a compilation of
Black spruce (>500 years) eventually becomes domi-
forest fires through remote sensing by the Alaska Fire
nant (Viereck et al. 1993).
Service revealed that 59% (153,812 ha) of the study
area has burned since 1950, although a substantial por-
The principal factors affecting this successional de-
tion of this area has burned more than once (Fig. 12).
velopment are decreasing sedimentation and water-table
The abundance of early successional ecotypes related
levels, owing to increasing bank height; accumulating
to fire (Upland Moist Low and Tall Scrub, Upland
organics from litter and later feathermosses; burial of
Moist Low and Tall Scrubdisturbed, Upland Rocky
organic layers by flooding, which provides the charac-
Dry Low Scrub, Lowland Low Scrubdisturbed, Low-
teristic soil sequence of interbedded organics; and the
land Moist Tall Scrub, Lowland Gravelly Moist Low
development of permafrost as soils become insulated
Scrub) tells us that approximately 16% of the entire
by the thick organic layer (Van Cleve et al. 1993). Viereck
study area has been burned recently (within approxi-
et al. (1993) concluded that life-history characteristics and
mately 30 years). Mid-successional ecotypes (broad-
flooding events are more important during the early stages
leaf and mixed forest types) occupy approximately 5%
of sucession, whereas biological controls such as organic
of the area. Late successional types (upland and low-
matter accumulation and competition become more im-
land needleleaf forests) occupy approximately 19% of
portant in middle and late stages.
the area. Two types, Lowland Tussock Scrub Bog and
While these simplified models explain most of the
Lowland Wet Low Scrub, which occupied 35% of the
variation that we observed, ecosystem development on
area, have little tree cover and their composition does
the floodplains is more complex than the simplified
not appear to change substantially after fire. Overall,
models indicate. Collins (1990) quantified changes in
more of the area is covered by early successional stages
erosional and depositional environments between 1938
or lowland tussock scrub bog than by late successional
and 1982 and found the braided portion of the Tanana
stages.
River near Fairbanks to be highly dynamic. Mason and
The effects of fire on ecosystem development de-
Beget (1991) used stratigraphic analysis to evaluate
pend on the nature of the ecosystem (i.e., species, life-
long-term changes in depositional environment and
history characteristics, soils), and the severity and fre-
found the following:
quency of the fire (Viereck 1973, Van Cleve et al. 1983).
The severity of the fire will affect how much of the
Much of the floodplain sediments were deposited
organic matter on the forest floor is burned and subse-
between 30002000 years BP.
quent regeneration pathways. In general, forest stands
Deposition was much less after 2000 years BP.
are replaced by the same tree species (Viereck 1973,
Sand units deposited during the last few hundred
Van Cleve et al. 1983). On moist upland sites (white
years point to a recent period of larger flooding
spruce sites), Foote (1983) identified six distinct suc-
events.
cessional stages:
Newly burned stage during 03 years.
Mann et al. (1995) contributed to our understanding of
Herbtree stage, when fast growing mosses, herbs,
the successional development of this complex fluvial
landscape by providing a more detailed analysis of geo-
and tree seedlings become established after 310
morphic processes, chronological development of flu-
years.
Tall shrubsapling stage occurring 330 years after
vial sediments, and changes in plant macrofossils as
indicators of paleoecosystems. Their analyses reveal
fire.
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