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

In 9ur analysis snow duration was shown to be highly
correlated with the major gradien~ in the vegetation.
But how is snow duration related to other snow para-
meters? Generally speaking, snow duration at a site
is a function of the water equivalent of the snowpack
and the energy budget of the site. That 1s to say,
the amount of heat (calories) needed to melt the snow
is equal to the mass of the snow times the latent heat
of fusion for water (80 calories/gram). Furthermore,
the energy budget at the site determines the rate at
which those calories are made available to melt the
snow. Two parameters determined in this study that
are related to the energy budget are Radiation Index
and elevation. Radiation Index is an estimate of
solar beam radiation, and elevation 1$ related to air
temperature through the adiabatic lapse rate.

An understanding of how snow melts is important in
interpreting. the effects of snow on vegetation.
During the winter, the volume under the snow surface
is protected: from extremes of temperature by the
insulating properties of the snow. The snow-ground
interface at this time is warmer than anywhere in the
snow itself (Geiger, 1971; Wardle, 1968). Snow melt
water formed does not generally reach the soil, but is
held within the snow, tending to move upward by capil-
lary movement and vapor diffusion. This movement is
caused by the temperature gradient within the snow-
pack (Geiger, 1971).

As the snow begins to melt in the spring, snow melt
water begins' to reach the ground where it infiltrates.
This maintains a nearly constant temperature of about
DoC at the soil surface throughout the melt period
(Wardle, 1968; Geiger, 1971).

Later in the snow melt period, when the snowpack is
only a few centimeters in depth, snow tends to melt
from the bottom up. Snow is relatively transparent to
higher energy, short wavelength radiation which passes
through the snow and is absorbed by the substrate.
This results in a warming of the substrate and so a
reradiation of energy of a longer wavelength. Snow
absorbs these longer wavelengths and melts from the
bottom upwards. Water from the melting snow infil-
trates the soil, saturates it, and further melt water
runs off at the surface. When the site is snow-free,
soil temperatures begin to rise and moisture depletion
may occur, subject to subsequent precipitation or
runoff and seepage (Geiger, 1971).

These observations of snowmelt help in understanding
the effects that snow might have on vegetation. The
effects are different for each species, and vary in
magnitude across the snow duration gradient. In
general, we would expect (from the PCA) aspen trees
and those species near the top of the understory
species list. in Table 1 to be more :lmportant in stands
where snow is early to disappear. In fact, when aspen
densities and understory species frequencies were
plotted as a function of snow duration, this behavior
was evident (Figures 1, 2, and 3). On the other hand,
spruce trees and understory species near the botton of
Table 1. respond positively to increased snow duration.

Where the snow disappears early the ground is early
exposed to the non-snowpack environment. Soil mois-
ture is depleted and soil temperatures rise. Those
forest sites' where snow is early to disappear now
support only aspen trees. Conifer survival may be
limited due to drought periods during the growing
season, warm night temperatures, or high light inten-
sities. The conifer seedling, with its short root
system, cannot sustain the plant during long periods
of drought (Noble, 1973). Spruce survival has been

shown to be very poor in areas where night tempera-
tures are warm (Helmers et al., 1970) and solariza-
tion at high light intenS1t:Les may cause death of
conifer seedlings (Ronco, 1970). In any case, aspen
suckers, with their connections to deep root systems
and their inherent tolerance of high light intenSi-
ties, are more capable of surviving in such stands
than are conifers.

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As the anow lies later, conifer seedling survival may
be enhanced. Patten (1963) found that spruce, fol-
lowed by subalpine fir, became established in cool,
shaded and moist areas which collected deep snow in
the winter and maintained soil moisture through much
of the growing season. He noted that spruce seed
germination was rare under snowbanks, but that seed-
lings already established may grow under shallow snow
because of the high shade tolerance of the species.
He also noted that a year with heavy snows followed
DY a wet summer favors conifer establishment. Sur-
vival during the next winter is poorer in areas where
the seedlings are blown free of snow, due perhaps to
frost damage. Patten found that spruce seedling
survival under cool, moist conditions is especially
high when these conditions are maintained under high
light intensities.

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Tree seedling establishment of all kinds may be sup-
pressed in areas of still later snow clearance.
Brink (1959) investigated subalpine tree establish-
ment in British Columbia and attributed their estab-
lishment to a recent climatic shift, which caused a
decrease in snowpack. Subalpine trees, including
subalpine fir, are becoming established in areas that
are free of snow earlier than the surrounding heath.
Franklin et al. (1971) investigated the invasion of
subalpinelneadows by trees in the Cascades. They
state:
"The snow-free period in certain subalpine
meadow communities is probably the most
critical factor affecting tree establish-
ment. ...Snowpack, through its influence
on length of growing season, is known to
be one of t~e most important factors in
determining the position of forest~eadow
ecotones in subalpine zones. A climatic
flux which reduced duration of snow cover
.. .would favor conifer invasion."

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Other studies have shown s:lmilar suppression of
seedling success at the upper extreme of the snow
duration gradient. Wardle (1968) found that late
lying snow delayed the growth of seedlings in the
Front Range in Colorado. A month delay in snow free
date resulted in a 15 day difference in the date of
max:lmum shoot growth. He found for Niwot Ridge that
although there seemed no less seedling establishment
in areas of late lying snow (except where there was
prolonged flooding by melt water from above), there
was a higher incidence of snowmolds (Herpotrichia sp.
and possibly ~ sp.) here than in areas clear of
snow earlier. Thus, seedling survival is affected by
late lying snow. However, Wardle did not find a
relationship between the date of snow disappearance
(from around the stem base) and the phenology of
either saplings or mature trees.

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Snow persisted longest in stands near alpine timber-
line, represented in our study by those stands on
time Mesa (those stands not covered by aerial photo
survey). Billings (1969) studied vegetational
pattern in such stands in the Medicine Bow Mountains
of Wyoming,where he found the forest to be in ribbons
and patches, a physiognomy similar to that present to
a large extent on Lime Mesa. Billings explains that

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