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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OIH 57'

- Direct gradient analYsis

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The result (Table 1) that the major gradient or first
principal component in the vegetation is highly re-
lated to snow duration was further investigated by
plotting the response of vegetational parameters
directly on a snow duration gradient. Figure 1
illustrates how some species. frequency values re-
spond to snow duration. For example, Achillea
lanulosa in Figure l(a), FraRsria vir~iniana in
Figure l(b) and Svmphorlcarpcs oreophilus in Figure
1 (d) all show decreases in frequency as snow lies
later. Vaccinium spp. and Polemonium pulcherrimum in
Figure l(b) increase in frequency with increased snow
duration. Some species, such as Geranium richardsonii
in Figure lea), show maximum frequencies somewhere
between the extremes of snow duration. Other species
with this latter behavior were included in Table 1.

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Species which show maximum frequencies at intermediate
positions along the snow duration gradient may have
very little of their variance accounted for by the peA
analysis in Table 1. The analysis assumes a statisti-
cal model that may not fit the response of every
species to the major gradients. This is not to say,
necessarily, that the species is not responding to
factors determining the major gradients in the vegeta-
tionj the species may be responding in a manner not
accounted for by the analytical model. The ecological
significance of the analysis should not be judged
wholly on the basis of correlation to the assumed
model. In this instance, however, an important envi-
ronmental parameter has been found" over which many
species do seem to respond. With many more stands
with snow duration data, deterministic models might be
derived that could predict species behavior with
statistical certainty.

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If the understory species are related to the snow
duration gradient, the question of how the tree
species behave with respect to snow duration naturally
arises. Figure 2 shows the response of the densities
of important tree species to snow duration. At sites
where snow is clear very early, aspen is the only
species present. As the snow lies later spruce and
fir begin to make up a larger part of the stems
present, and the number of aspen stems decreases. As
snow lies even later fir becomes the most dense
species. Finally, as snow continues to persist,
spruce becomes the most dense species and aspen is no
longer present. At the latest dates of snow duration
provided for in the data, fir becomes a minor compo-
nent of the dominant vegetation and spruce reaches its
maximum densities.

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Figure 3 shows the relationship of important tree
seedling densities to snow duration. Aspen seedling
densities reach their highest values in those stands
where snow clears early. Aspen seedling densities
decrease as snow lies late while densities of spruce
and fir increase, fir with the higher densities. As
snow lies very late, seedling densities of all species
decrease considerably, the density of fir falling
below that of spruce.

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In addition to the 57 subalpine stands, 4 stands were
selected that occur near timberline on Lime Mesa.
These stands are beyond the extent of aerial photo
coverage and no quantitative estimate can be made for
their position along a snow duration gradient. In
general subalpine fir is not present in these timber-
line stands. Here Eng1emann spruce forms stands in
ribbons and patches, associated with rock outcrops,
topography, and apparent prevailing weather influ-
ences. The parent material is limestone of the Ouray

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Formation. This limestone is essentially different
from the parent material of the remainder of Mission-
ary Ridge, which is prtmarily a limestone of the
Hermosa Formation (Larsen and Cross, 1956).

The vegetational composition of these timberline
stands is vastly different from the subalpine stands.
Within each stand, species from subalpine and alpine
meadows are found with subalpine forest species.
Species diversity is consequently very high, and
these stands represent a wide range of species
composition. The inability of obtaining snow data
for these stands, the difficulty in understory
species identification due to a much belated phenol-
ogy, and the apparent wide range of composition with-
in this structural type forced the investigators, in
the interest of time and economy, to concentrate
studies on the lower, subalpine stands. Although no
snow duration data were available for these stands,
personal on-site observation in the 1973 season
suggest snowmelt and phenological events several
weeks later than sites 300 to 400 meters lower. A
discussion is given below of the apparent influence
of snow on this vegetation.

4.3. D Discussion

The slow response of communities dominated by 10ng-
lived perennials to manipulative experiments pre-
cluded its use in studies as short-lived as the
present one. Instead, this project was organized as
a process study. An understanding of how the envi-
ronment is related to community composition and
structure was sought and from this the effects of an
increased snowfall might then be evaluated.

It was assumed that as the snowfall increased, there
would be an increased snowpack resulting in a delayed
snowmelt. Concomitant with this longer snow duration
at a site would be the following factors:

(1) delayed soil moisture depletion.
(2) delayed rise in soil temperatures.
(3) lower light levels until site is snow free.
(4) increase in and longer duration of physical
pressure on vegetation.
(5) longer duration of protection of vegetation
from extremes of non-snowpack environment.
(6) increased susceptability to pathogens
(snowmo1ds) .

Each species would, of course, react differently to
the above factors; as a result there would be an
alteration in community structure.

It was evident from the 1972-73 snowfall that large
increases in snowpack can delay snowmelt several
weeks. As stated above, the same general pattern of
snowmelt was noted in 1973 as in 1971. Such observa-
tions were also made in studies cited by Geiger
(1971). On page 451, Geiger states:

"The way snow melts in mountain areas is
closely related to topography...; that is
to say, even if the snow melts at different
times in different years, the melting pattern
is always the same."

In addition, Geiger cites relevant studies which
relate the melting pattern of snow to phenological
and vegetational differences. (These references
include Friedel, 1952; Kreeb, 1954; Aulitzky, 1958;
and Waldmann, 1959).

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