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

alpine situations, such an acceleration was eound
to offset the eeeects of a shortened growth period.

Soil moisture ~ ~ may not be particularly impor-
tant to the vegetation during snowmelt or the period
immediately following snow clearing. However, the
influence of water in soils near field capacity was
found to retard soil warming and thus to affect the
physical and biological functioning of root systems.
This should be reflected in restricted water uptake
by roots, through decreased permeability of root
membranes, or through" inhibition of root growth.
Changes in actual physiological processes of root
systems such as carbohydrate utilization and storage,
growth hormone levels, and mineral uptake may also
occur. These effects may result in changes in plant
growth too subtle to have been observed in the period
of field work.

Snow cover/bare soil relationships may also affect
growth processes through an influence on air temper-
atures. This is of particular relevance, since energy
from solar radiation is near its annual maximum at
the approximate time of snow clearing. In the
alpine and subalpine, a delay in the start of the
growing season almost always results in the growing
season taking place at a time of decreasing solar
radiation. One might argue that increased snow will
tend to favor plants associated with moister habitats
through the additional water supply. However, be-
cause of lower soil temperatures this water may not
be available for plant growth.

ANI~S

Consumers are generally more difficult to study than
other ecosystem components because of their diverse
behavior and, in the case of those studied here,
their low densities. Nevertheless, three groups of
animals were studied in the San Juan Ecology Project:
big game, small mammals, and amphibians. The prin-
cipal species "involved were elk, pocket gopher,
Colorado chipmunk, vole (Microtus sp.), deer mouse,
and boreal toad. No systematic studies of
invertebrates, birds, or carnivorous vertebrates
were made, although some censusing was done and
species lists were compiled.

Very few effects of snow augmentation on animals
have been established with certainty. The winter
distribution of elk was found to be dependent on
snow depth and distribution. Elk activities are
impeded when the depth of penetrable snow exceeds
40 cm and almost eliminated at 70 cm. As winter
progresses, elk travel to lower elevations where
the snowpack allows easier movement, but where their
contact with man is increased. The consequence of
such contact may be detrimental to both elk and man.
"With winter snow covering grassy vegetation which
elk normally graze, their food habitats change be-
cause they are forced to browse. In years of high
snowfall, elk herds could be significantly affected.
Increased browsing by elk will affect the montane
oak brush vegetation but this impact is unlikely to
be great. In the tundra, gophers are located in
winter on sites with a moderate ( 50 cm depth at
peak accumulation) to deep snow cover. It is also
in these sites that their main influence in the eco-
system is made through burrowing and earth moving.

Studies of other small mammals were carried out in
both alpine tundra and subalpine forest situations.
The only species which showed clearly demonstrable
response to winter snow accumulation was the deer
mouse in the forest. These animals showed low

numbers in the spring after a winter with a heavy
snowpack and late melt. This seems the result of a
shortage of food of the proper quality, and a
shortened breeding period.

The boreal toad was studied as a potential indicator
species. Its usefulness in this regard was found to
be limited since its geographic distribution is
apparently not related to variable snow patterns.
Certainly deep winter snow is necessary for success-
ful hibernation, but the exact requirement or
specificity of this requirement has not been estab-
lished. Toad reproduction does seem to be limited
above 3350 m elevation and weather modification
could delay egg laying, embryogenesis, and subsequent
metamorphosis in these populations.

ECOSYSTEM INTERACTIONS

The main conclusions drawn from the San Juan Ecology
Project are based on consideration of components of
the terrestrial ecosystem. In this ecological impact
analysis, it is also necessary to evaluate the links
between components (or parts of them), particularly
if these links might provide mechanisms whereby the
impacts of silver and additional snow are attenuated
or amplified within the system. While it is easy
to suggest that a change in one component affects the
entire ecosystem, it is probably useful to consider
only the links corroborated by the substantive
evidence presented in Chapter IV.

Figure 1 shows the process-response links which our
work suggests might be influenced by snowpack aug-
mentation. Six environmental changes generated by
cloud seeding are identified as being capable of
producing responses in the terrestrial ecosystem.
They are represented by the centers of six "wheels"
in Figure 1. Some of these changes would be derived
more directly from cloud seeding than others and this
sequence is suggested by the first order arrows,
joining the wheels. For example, the imposition of
additional silver and snow accumulation should be
immediate impacts of cloud seeding. On the other
hand, changes in soil moisture and temperature regimes
seem to be contingent on a delayed snowmelt, which
is itself a response to the additional snow accumu-
lation and so is only indirectly derived from cloud
seeding. These indirect changes are also partial
responses to environmental factors other than those
associated with the winter snowpack, e.g., snow
cover duration is a function of snowmelt rates as
well as accumulated snow depth. This suggests that
impacts should be successively more damped as they
work through the system.

Within each wheel, second order arrows represent the
influence of particular environmental characteristics
on the defined ecosystem attributes which form the
rim of the wheel. These are the expected responses
to cloud seeding and silver iodide imposition which
have been substantiated in our work in the San Juan
Ecology Project. Indirect responses that are pro-
duced through intermediate ecosystem components are
shown as broken arrows around the outside of each
wheel where they link ecosystem attributes. The
direction of the change expected through both direct
and indirect responses is indicated by arithmetic
signs on Figure 1: (+) suggests an increase (e.g.
in the rates of leaching and litter decomposition
followin~an increase in snowfall; (-) suggests a
reduction from "natural" rates (e.g. in the activity
of soil microorganisms with the addition of silver
to the soil). Not all of these changes are consis-
tent; different species or plant communities respond

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