WSP09641

• File Number: 8200.750
• Description: San Juan River General
• State: CO
• Basin: San Juan/Dolores
• Water Division: 7
• Date: 3/9/1976
• Author: Steinhoff and Ives
• Title: Ecological Impacts of Snowpack Augmentation in the San Juan Mountains - Colorado - March 9 1976
• Water Supply Pro - Doc Type: Report/Study

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The disposition of silver in Boil and vegetation was
measured 1n spring and fall on five transects which
included subalpine meadows, and spruce and aspen
communities. Over the 4 years of field study, no
increases In silver concentration were found. How-
ever, seasonal fluctuations indicate higher levels
in spring than 1~ fall, as has been reported in
studies from other areas. In stream water, the data
from other studies (United States Geological Survey
1970, 1972, 1973) suggest a direct relationship be-
tween stream discharge and silver concentration, hut
thi9 undoubtedly derives from naturally occurring
silver In the field area. During the study period,
the highest discharge of both silver and water
occurred in t973. This follo~ed a ~inter with less
cloud seeding than the previous two, but ~ith a
higher than normal snowfall.

Silver concentrations in the soil and vegetation
around a generatDr show the highest levels adjacent
to the generator itself with a decline to background
levels in less than 200 m distance down~ind. Even
with an extended period of operational cloud seeding
(up to 100 years) silver accumulation in the soil
will not reach a 1 ppm level. Field observations
and laboratory studies on soil columns both show that
most silver iodide is adsorbed sufficiently strongly
on surface soil and litter particles to minimize
further movement, at least in the short-term. No
impact by silver iodide on pldnt growth or animal
populations has been observed in this study.

Field and laboratory studies do show some uptake of
silver by plants from application of the free ions
of silver nitrate, reagent-grade silver iodide, and
silver iodide which has passed through a seeding
generator. However, in general, less uptake occurs
from silver iodide than from the more soluble silver
nitrate, and the resulting concentrations tend to be
lower in the above-ground portion of subalpine mea-
dow plants than in the applied solution. Within the
plant community, silver Is most concentrated in the
litter, roots, and folfage ~ith highest concentration
in the surface soil zone of the plant-soil system.

This surface soil zone, in which most silver accumu-
lation occurs, has a high microbial biomass and
activity in terrestrial ecosystems, so there is the
potential for a decrease in the rates of organic
decomposition. This could produce an accelerated
rate of organic matter accumulation at the soil
surface, if plant-growth continues at normal rates.
However, experimental evidence suggests that silver
concentrations of at least 2 ppm (one or two orders
of magnitude higher than those presently found in the
litter zone) would be needed to initiate this process.

PLANTS

In the San Juan Mountains, plant studies have been
made 1n the alpine tundra; the subalpine forest of
spruce, fir, and aspen; the subalpine meadows; and in
the oak brush of the montane zone. These investi-
gatiOns were concerned primarily with the effects
of snow cover on the phenology and productivity of
selected plant species, and on phytosociology of the
plant communities.

Phytosodological studies were conducted in both the
subalpine forests and in the alpine tundra. They
have established a quantitative description of the
vegetation, placing it in some perspective with plant
communities else~here, and have indicated the types
of vegetation responses that may be anticipated with
particular environmental changes. Gradients in snow

duration and moisture were shown to relate quite
strongly to vegetation species composition. In the
subalpine forests. spruce is associated with a longer-
lasting snow cover than fir, while aspen lies even
further down the contInuum of snoW cover duration.
Similar relationships to snow cover were found for
the tundra plant communities. This suggests that a
long-term increase in the duration of the winter
snowpsck in both forest and tundra situations will
induce a change in the species composition in favor
of those plants which are more tolerant of a late-
lying snow cover.

Observations of snow cover duration, air and soil
temperature, soil moisture, solar radiation, and
subalpine tree moisture stress indicate that the
sequence of plant growth and development in spring
and early summer is also related to the duration of
snow cover. or to local conditions re[lecting it. In
the alpine, snow fences were used to augment snow
accumulation on experimental siteS. Comparative ob-
servations on experimental and control plots showed
that initiation of vegetative growth ~as delayed
under experi~ental conditions in almost all species
examined. In the subalpine, the relationship bet..,een
snow and initiation of stem elongation growth in
Englemann spruce was not clear, although rate of
elongation, initiation of radial growth, vegetative
bud burst. and floral bud burst are all negatively
correlated with the snow cover duration. Tree
moisture stress showed a strong positive correlation
with the snow depth during the snowmelt period.
Relatively high daytime moisture stress ~as associated
..,ith cold soils resulting from late-lying snow. The
initiation of shoot growth of oak also followed date
of snow clearning.

Late-lying snow resulted in shorter active growth
periods for tundra plants, subalpine herbac~ous
plants, and trees. With shortened active gro~th
periuds, both alpine and subalpine herbaceous plants
demonstrated a "catch up" faCility, I.e. the ability
to comp~ete a sequence of phenological events in a
shorter period of time. However, data indicate that
the capacity for seed germination was inhibited in
seeda Qf those species which had sn abbrevi~ted
gro....nng season.

In the alpine. a delay in the initiation of gro~th
resulted in lower productivity on some sites. Simu-
lation modelling predicts that this effect would be
slightly greater on plant communities occupying
xeric sites than on those of more mesic areas. It
amounted to a reduction in aboveground productivity
of 17 percent in the former case and 14 percent in
the latter, assuming a 15 day del~y in snowmelt in
both cases.

Examination of bolewood biomass production by spruce
over a 20 year period showed very little varia-
tion in annual growth increment. This indicates
little response to temporal changes in the snow
cover. However, the technique used in this analysis
seemed to lack sensitivity. The lack of response
was not supported by the microscopic examination of
wood cores taken during the growth period, or by
phenological observations of radial growth.

Results from both alpine and subalpine situations
indicate that the cessation of plant growth in late
summer and fall was quite regular from year to year,
suggesting a photoperiod control. Thus a late-lying
snow cover causes a shorter growing period. This
could result" in reduced productivity if not compen-
sated by an increase in the growth rate. In some

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