WSP00036

• File Number: 8200.750
• Description: San Juan River General
• State: CO
• Basin: San Juan/Dolores
• Water Division: 7
• Date: 3/1/1975
• Author: BOR
• Title: San Juan Ecology Project - Interim Progress Report - March 1975
• Water Supply Pro - Doc Type: Report/Study

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the pattern is a result of the removal of snow by
wind from open areas and its deposition into drifts.
The melt of the drifts provides a good moisture
regime for seedling establishment up to where its edge
occurs in June or early July. That part of the area
where the drift melts later produces a wet or moist
meadow vegetation typical of the "snow glade. n This
produces the ribbon-like pattern of the forest.
Billings suggests that the ribbon forest and snow
glade areas can be expected "just below climatic
timberline wherever plateaus or gentle slopes are
exposed to heavy snowfall, strong winter winds, and
cool sununers."

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4.3. 6 Conclusions

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The results of this study attest to, the significance
of snow associated parameters in determining subalpine
vegetation. This is supported by the work of other
investigators: Aulitzky, 1958; Billings, 1968; Brink,
1959; Franklin et al., 1971; Friedel, 1952; Geiger,
1971; Patten, 1963;- Waldmann, 1959; and Wardle, 1968.
Of special significance in forests is the effect snow
has on seedling establishment. Spruce and fir re-
generation may be limited at sites now supporting only
aspen trees because of the long dry periods during the
growing season, warm night temperatures, or high light
intensities. An increase in snowpack at these sites,
which would result in increased snow duration and
improved soil moisture and temperature conditions,
should increase the survival of spruce and fir seed-
lings. Solarization effects on spruce seedlings are
not as severe when moisture conditlons are favorable
(Ronco, 1970). Increased snowpack need not be
increased continuously to sssure establishment of
conifer seedlings in this habitat. Seedlings estab-
lished during one or two favorable growing sea,ons
may survive if the long-term'climatic conditions favor
their growth. In other words, with increased snowpack
there should be a tendency for stable aspen stands
with little or no conifer reproduction to succeed to
spruce-fir forest. Also, succession should proceed
more rapidly in successional aspen stands as condi-
tions become more favorable for the survival of the
spruce and fir seedlings.

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Our results indicate that within the spruce-fir forest
spruce seedlings may be more tolerant of increased
snow duration than are fir seedlings (Figure 3). In
this case, survival of spruce seedling relative to fir
seedlings would be increased, resulting in stands of
higher relative densities of spruce. The resultant
absolute densities that would occur as a result of
increased snowpack would depend where along the snow
duration gradient a given site occurs. As is typical
of biological responses, the densities of spruce and
fir increase up to some optimal densities and then
decrease as the snow lies later. As the snow per-
sists, conditions become less favorable for all tree
seedling survival. The growing season may be short-
ened, the site may remain too wet from snowmelt for
seedlings to survive, pathogenic attack (snowmolds)
may cause increased mortality of seedlings that are
established. As the snow lies very late all tree
seedling establishment may be precluded. At this
extreme, tree seedling establishment depends on a
coincidence of good seed production and short term
weather conditions.

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Consistent with this response of tree species to
increased snow duration, the vegetation as a whole
tends to respond to increased snow duration. All
species have their unique responses to heat, light
end moisture regimes, and their own tolerances of
extremes of environmental conditions. As some species

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are more adapted to what would be a slightly altered
environment, they would have a tendency to respond
more favorably to that environment. This would
result in a change in community structure as inter-
specific interactions would become altered, stabiliz-
ing with respect to that environmental change only
after some considerable time. As was pointed out
above, many of these species are long-lived peren-
nials. The eventual structure of a stand should be
one of plants that are more highly positively corre-
lated with snow.duration. Stands would lose some
species and gain others, depending on the species
tolerances to snow duration and associated factors,
and the outcome of the new species interactions.
These may be predicted from Table 1.

4.3.7 Summary

The objectives of this study are to provide a quanti-
tative descr~ption of the subalpine forest of the San
Juan MOuntains within which the results of the other
investigations may be expressed and to predict the
effects that weather modification may have on the
composition and dynamics of this vegetation. The
establishment of experiments was not feasible, due
to the very slow response of communities dominated by
long-lived perennials. Principal environmental
gradients along which the subalpine forest responds
were sought. This information was then used to
predict possible changes caused by weather modifica-
tion. Principal components analysis was used to
determine the major gradients in the vegetation of
the subalpine forests. The analysis provided cor-
relations of measured environmental parameters with
the major vegetation gradients. The gradient which
accounts for the most variation in the vegetation was
found to be the snow duration gradient.

The responses of. several understory species over a
snow duration gradient are described. Some species
attain their highest values of frequency of occur-
rence in stands where snow disappears early. Other
species had maximum frequency values where snow is
very persistant. Still other species have maxima
between these two extremes.

It was found that quaking aspen, subalpine fir, and
Englemann spruce trees vary in their response to the
snow duration gradient. Aspen is present at higher
densities where the snow leaves early. As snow lies
later more spruce and fir are found with the aspen.
First fir and then spruce trees reach dominance in
numbers as the snow persists. A similar response to
snow duration was found for the seedlings of these
subalpine tree species.

A species will succeed in an environment where it has
the ability to become established and to maintain
itself. An increased snowfall resulting in a larger
snowpack and a delayed snowmelt will result in
delayed soil moisture depletion and temperature rise,
lower light levels until the snow is gone, an in-
crease in and longer duration of physical pressure on
the vegetation, a longer duration of protection of the
vegetation by the snowpack environment, and an in-
creased susceptabi1ity to snowmolds. Each species
reacts differently to these factors~ The result is
an alteration in community structure.

It was concluded that an increased snowfall which
resulted in a longer snow duration would favor spruce
and fir seedling establishment in those aspen stands
presently stable (without coniferous reproduction).
Conifer seedling success would be favored more in
successional aspen stands, resulting in more rapid

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