WMOD00593

• Contract/Permit #: 14-06-D-7052
• Title: Ecological Impacts of Snowpack Augmentation in the San Juan Mountains, Colorado
• Date: 3/1/1976
• State: CO
• Weather Modification - Doc Type: Report

The disposition of silver in soil and vegetation was
measured in spring and fallion 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 in fall, as pas been reported in
studies from other areas. In stream water, the data
from other studies (United ~tates Geological Survey
1970, 1972, 1973) suggest a'direct relationship be-
tween stream discharge and silver concentration, but
this undoubtedly derives from naturally occurring
silver in the field area. Puring the study period,
the highest discharge of both silver and water
occurred in 1973. This followed a winter with less
cloud seeding than the previous two, but with a
higher than normal snowfalL

Silver concentrations in th~ soil and vegetation
around a generator show thei highest levels adjacent
to the generator itself with a decline to background
levels in less than 200 m d~stance downwind. Even
with an extended period of bperational cloud seeding
(up to 100 years) silver ac~umulation in the soil
will not reach a 1 ppm levei. Field observations
and laboratory studies on spil columns both show that
most silver iodide is adsorbed sufficiently strongly
on surface soil and litter particles to minimize
further movement, at least ~n the short-term. No
impact by silver iodide on plant growth or animal
populations has been observ~d in this study.

Field and laboratory studieS do show some uptake of
silver by plants from appli~ation of the free ions
of silver nitrate, reagent-grade silver iodide, and
silver iodide which has pas~ed through a seeding
generator. However, in gen~ral, less uptake occurs
from silver iodide than fraln the more soluble silver
nitrate, and the resulting 'concentrations tend to be
lower in the above-ground pprtion of subalpine mea-
dow plants than in the appliied solution. Within the
plant community, silver is ~ost concentrated in the
litter, roots, and foliage ~ith highest concentration
in the surface soil zone of, the plant-soil system.
,

This surface soil zone, in ~hich most silver accumu-
lation occurs, has a high microbial biomass and
activity in terrestrial eco~systems, so there is the
potential for a decrease in' the rates of organic
decomposition. This could broduce an accelerated
rate of organic matter accumulation at the soil
surface, if plant,growth co~tinues at normal rates.
However experimental evidence suggests that silver
concent;ations of at least :2 ppm (one or two orders
of magnitude higher than those presently found in the
litter zone) would be neede:d to initiate this process.

PLANTS

In the San Juan Mountains, plant studies have been
made in the alpine tundra; ,the subalpine forest of
spruce, fir, and aspen; th~ subalpine meadows; and in
the oak brush of the montane zone. These investi-
gations were concerned pri~arily with the effects
of snow cover on the phenolpgy and productivity of
selected plant species, and on phytosociology of the
plant communities.

Phytosociological studies ~ere conducted in both the
subalpine forests and in tHe alpine tundra. They
have established a quantita!tive description ot the
vegetation, placing it in s,ome perspective with plant
communities elsewhere, and have indicated the types
of vegetation responses tha:t may be anticipated with
particular environmental ch~nges. 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
snowpack 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 reflecting 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 was delayed
under experimental conditions in almost all species
examined. In the subalpine, the relationship between
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 was associ~ted
with 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 herbaceous
plants, and trees. With shortened active growth
periods, 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
seeds of those species which had an abbreviated
growing season.

In the alpine, a delay in the initiation of gro,wth
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 delay 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 cgres 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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