HomeMy WebLinkAboutAPA4148Increased or Decreased
Energy for Moose?
The Susitna Hydroelectric Project
By
William B. Collins*
A moose walks out onto a Susitna River gravel bar a few
miles downstream from Talkeetna and begins to browse some
low -growing willow and balsam poplar shrubs. The animal has
spent approximately one-half of its day feeding on similar
vegetation. Approximately one-fourth of its day will be spent
rechewing its harvest, further converting the woody plants into
a form it can use as energy. Energy cannot be created or de-
stroyed, but only changed in form.
Forty-eight miles upstream, an engineer contemplates
another form of energy conversion. His plan will alter the flow
of the river to harness its energy. The man has selected the lo-
cation for a dam because it represents a very efficient site for
conversion of water power to electricity. The moose has se-
lected its feeding site because, there too, energy conversion is
most efficient.
During this study the Agricultural Experiment Station
(AES) at Palmer was under contract to Terrestrial Environmen-
tal Specialists (TES) of Phoenix, New York, to investigate the
possible Impacts on vegetation and related 'habitat values for
moose of hydroelectric development on the Susitna River. This
effort was part of an overall feasibility study conducted by
Acres American Incorporated for the Alaska Power Authority;
TES was subcontracted to Acres. A primary concern has been to
determine what effect regulated river flow may have on moose
habitat within the floodplain downstream of the proposed Devil
Canyon damsite. This land is frequently flooded in summer, a
phenomenon which some believe may be responsible for main-
tenance of the vegetation in early successional stages which are
highly productive of moose forage. With hydroelectric develop-
ment, the annual fluctuation in downstream water levels and
*Assistant Professor, Range Management, Agricultural Experiment Sta-
tion, Fairbanks.
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SPRUCE CHANNEL YOUNG
BALSAM POPLAR
flooding would be reduced, thereby possibly allowing prime
moose habitat to advance to later, less productive successional
stages.
In May 1981, AES personnel began a study of vegetation
succession on the Susitna floodplain from the Devil Canyon
site downstream approximately 92 miles to the Deshka River.
Through reconnaissance of the area and comparison of histori-
cal (1951) photographs with 1980 photographs, seven vegeta-
tion types were identified which were thought to represent
stages of succession from bare ground to climax forest.
Vegetation soils data from each type were collected and
analyzed to determine vegetation history as it may relate to
flooding. Per cent cover of plant species and density, dimensions,
and ages of trees and shrubs were estimated or measured to
form the basis for characterizing each vegetation type. Particular
effort was made to determine the point in time when each stand
began developing. Soils from several pits within each type were
analyzed to determine what relationships may exist between
vegetation type and soil substrate. Elevations of each stand
above the level of the river were measured at various times dur-
ing the summer to be related to rates of flow (simultaneously
being measured by R and M Consultants, another member of
the feasibility study team). Eventually this information can be
used to predict the extent of flooding under different hydro•
electric development options.
Vegetation succession on the floodplain generally occurs in
the following sequence (Figure 1). Commonly, horsetail (Equis-
etum spp.) is the first vegetation to become established on bare
ground. However, its occurrence is dependent on the presence
of fine sands and silts in the surface horizons of the soil. This
vegetation is readily invaded by willow (Salix spp.) and balsam
poplar (Populus balsamifera) seedlings. Simi,lar topographic sites
which have coarser substrate may never develop horsetail as the
dominant species but may be occupied by willow -balsam poplar
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Figure 1. Possible sequence of vegetation succession (left to right) on Lower Susitna River. Horsetail, dryas, or willow -young balsam poplar may occur
as the pioneering vegetation type, depending on the substrate, plant propagules present, and other environmental conditions.
42 January/1983 Agroborealis
Figure Z. Early successional vegetation. Horsetail stand (on left) is the
initial pioneering vegetation. At a slightly higher elevation (on right) is a
heavily browsed stand of feltleaf willow which is 8 to 10 years old.
directly, or in the case of gravelly or cobbley sites, be pioneered
by dryas (Dryas drummondii), a nitrogen -fixing plant. In any
case, the initial vegetation is important in holding the soil and in
reducing the velocity of water and wind for further deposition
of substrate.
In time, depending on a number of factors such as siltation
and elevation of the surface above the level of frequent flood-
ing, density of alder (Alnus tenuifolia and A. sinuata), another
nitrogen fixer, apparently increases and gains dominance over
the balsam poplar and willows. This may be in response to an
inherently faster growth rate of alder, or preferential browsing
of the ..then by moos- -- a ounbinati-._ f both. How-
ever, _
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ever, some time after the alder achieves maximum height, the
balsam poplar emerges through the alder canopy, once again re-
ceiving full sunlight and begins fast growth into large trees.
Addition of nitrogen to the soil by alder may contribute to the
balsam poplar's increased rate of growth. Balsam poplar is
shade -intolerant and does not reproduce in its own shade; con-
sequently, most stands are even -aged.
Figure 4. Substrate determines the type of pioneering vegetation which
becomes established. Horsetail and willow readily establish on silty sites
(top), while only sparse patches of dryas and balsam poplar are found on
gravelly sites (bottom).
Figure 3. Feltleaf willow, balsam poplar, and alder seedlings gain domi-
nance over initial horsetail stand. Alder fthe large shrubs) are 3 years old
and already overtopping the willow and balsam poplar which are 7 to 10
years old.
As the balsam poplar forest matures, spruce (Picea glauca)
may appear in the canopy (a few may become evident as early
as the alder stage). Eventually, the balsam poplar becomes deca-
dent and falls, leaving space for development of more balsam
poplar or spruce and birch (Betula papyrifera). The factors re-
sponsible for development of the birch -spruce stands versus
continuation of balsam poplar are still unclear but may be
better understood after further analyses of soils. Balsam poplar
does have a better chance of continuing if the disturbance
exposes mineral soil. Elevation surveys of the different types did
not indicate that the birch -spruce stands were any less likely to
be inundated by high water than were mature balsam poplar
stands. Much work still needs to be done to understand more
fully the mechanisms underlying each of the apparent succes-
sional sequences.
Initial observations by both AES and Alaska Department of
Fish and Game (ADF and G) personnel indicate that the wil-
iow-baisam poplar sapling type may be the most valuable
feeding habitat for moose living on the floodplain. This is being
Figure 5. Alder thicket with balsam poplar emerging through the top.
Agroborealis January/1983 43
Figure 6. After 25 years, a dryas stand (see Fig-
ure 4) may accumulate enough alluvial and
windblown silt to support a more productive
and diverse community of plants. However,
the vegetation on siltier soils (background) has
advanced to an immature balsam poplar forest
in the same amount of time.
Figure 7. Balsam poplar browse is elevated far
above the heads of moose as saplings grown
into trees as seen in this immature forest.
studied by the ADr and G which is responsible for the evalu-
ation of the big game populations and distribution as part of the
feasibility study. Since the willow -balsam poplar sapling type
occurs early in vegetation succession, it is probably dependent
on some form of disturbance to create conditions favorable for
its establishment. Such disturbance could result from flooding
and subsequent siltation, erosion of banks by ice and redeposi-
tion, wind throw, or fire. The relative influence of each of these
factors thus is also an important consideration in defining the
impact of flooding.
Mature and decadent balsam poplar and birch -spruce stands
also produce abundant moose browse and provide security
cover. Highbush cranberry (Viburnum edule) and prickly rose
(Rosa acicularis) are important browse species in these forested
types, whereas willow species have become much less abundant
than in the open types. Paper birch saplings provide additional
browse for moose in birch -spruce stands, and alder species may
be browsed occasionally in all stands. Devilsclub (Oplopanax
horridus), which often leaves numerous painful spines imbedded
in the legs and hands of hikers, and ostrich -fern (Matteuccia
struthiopteris) are highly preferred as forage by lactating cows
and calves using mature and decadent balsam poplar stands in
spring and early summer.
Birch -spruce stands appear to be in dynamic equilibrium or
climax for vegetation occurring on the lower Susitna River
floodplain. These stands characteristically have four phases
which repeat themselves in the following cyclic sequence. The
tallest, oldest part of the forest, having a well -developed under -
story of grasses and forbs but relatively few shrubs, begins to
44 January/1983 Agroborealis
Figure 8. Birch -spruce may be the climax
vegetation Susitna River floodplain.
lose paper birch trom the canopy as heart rot takes its toll. As
the canopy opens up, the spruce apparently becomes more
susceptible to windthrow, and large patches of overstory are
completely lost. This then leads to the decrease of some shade -
tolerant species and the increase of paper birch saplings, high -
bush cranberry, prickly rose, willow, and thinleaf alder (Alnus
tenuifolia). As brush fields age, spruce begins to appear and
eventually this phase advances to birch -spruce forest. The early
Figure 9. After roughly 150 years, balsam poplar can become decadent,
creating space for emerging spruce and/or birch. Newly deposited bar in
the foreground shows the successional sequence starting over.
birch -spruce stands characteristically retain more browse in the
understory than do more mature stands. As birch -spruce stands
age, the cycle apparently is repeated. The close association of
brush fields with mature forest in the birch -spruce type appears
to provide good overall moose habitat.
AES personnel will begin estimating the forage productivity
of each vegetation type. This information, coupled with habitat -
preference data collected by ADF and G and vegetation succes-
sion information, will be used to assess the effects of changes in
vegetation which may occur if hydroelectric development takes
place. Then it may be known how development of Susitna
hydroelectric energy will affect the availability of an efficient
source of energy for moose.
ACKNOWLEDGMENTS
Funding for this project was provided by Terrestrial Envi-
ronmental Systems. TES was subcontracted to Acres American
Incorporated who was the prime contractor to the Alaska Power
Authority.
THE DYNAMIC FORCES OF THE SUSITNA RIVER AT WORK
Figure 10. Siltation during summer flood occurring on dryas, horsetail,
and balsam poplar sapling communities. AES personnel discuss the im-
pacts with a wetlands specialist and forester from Sweden.
Figure 12. Young alder and balsam poplar which were "bulldozed" by ice
during spring breakup. Rocks were deposited as the ice block melted
from beneath them.
Figure 11. A black spruce peat bog (not considered part of the flood-
pla,*-) is being undercut and pulled into the river.
Figure 13. Young balsam poplar trees (center) exhibit "flood.trained"
shape as a result of past ice jamming.
Agroborealis January/1983 45
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