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

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1. Report No. 2. 3. Recipient's Accession No.
CSU-FNR-7052-l
4. Title 5. Report Da te
March 1976
Ecological Impacts of Snowpack Augmentation in the San Juan 6.
Mountains of Colorado
7. Author(s) 8. Performing Organization Report No.
Harold W. Steinhoff and Jack D. Ives (Eds)
9. Performing Organization Name and Address O. Work Unit No.
College of Forestry and Natural Resources
Colorado State University 1. Contract or Grant No.
Fort Collins, Colorado 80523 l4-06-D-7052
2. Sponsoring Agency Name and Address 3. Type of Report
Bureau of Reclamation Final Report
Building 67, Denver Federal Center 4.
Denver, Colorado 80225
5. Supplementary Notes
This document represents a cooperative research effort between Colorado State University, the University of
Colorado, and Fort Lewis College. This report supersedes the previous Interim Progress Reports of 1971,
1973, and 1975. Also, this report has been designated as a contribution to the United Nations Man and the
Biosphere (MAB-6) Program.
6. Abstract
The following conclusions summarize the work of 33 scientists from Colorado State University, the University
of Colorado, and Fort Lewis College who studied the effects of varying snowpack on the ecosystems of the San
Juan Mountains in Colorado from 1970 to 1975. Initiation of shoot elongation was delayed for plants in the
tundra and forests as-a result of lower temperatures associated with deeper snowpack. The delay was most ap-
parent for herbaceous species, such as Thurber fescue (Festuca thurberi), whose growth points lay in the
- -~~ediate zone of lower temperature at ground level. The effect on trees such as Engelmann spruce (Picea
engelmanii) and quaking aspen (Populus tremuloides) was less. The result was a decrease in biomass production
of herbaceous species, but not of trees or of Gambel oak (Quercus gambellii), which was found in a zone that
became snow clear very early. The delay in beginning of growth persisted through the other stages of the
annual cycles of most plants but with less magnitude. There seemed universally a compensation factor which
permitted each species to compress its annual cycle in years with a shorter growing season caused by greater
depth of snowpack. Phytosociological studies in both forests and tundra showed that plant communities were
arranged along a snow clear date gradient. In the forest, Gambel oak gives way to quaking aspen, then to sub-
alpine fir (Abies lasiocarpa), ,and finally to Engelmann spruce, as the snow clear date extends later into
the year. A noticeable decline in forest populations of deer mice (Peromyscus maniculatus) and, to a lesser
extent, chipmunks (Eutamias spp.) and Microtus spp., followed winters of heavy snowfall. The basic reason
was a delay in breeding and in availability of essential foods. No similar effect was found for the pocket
gopher (Thomomys talpoides) or mountain toad (Bufo boreas). Elk (Cervus canadensis) were almost excluded
from regions with more than 70cm of penetrable snow depth, and their travel was noticeably restricted at
depths over 40 cm. Movement of elk to higher elevations in spring coincided with the initiation of growth of
herbaceous plants rather. than directly with the receding snow line, and adequate calving areas were always
present. Plant litter in the tundra decreased in areas df deeper snowpack. Local changes in the rate of
soil erosion may increase by a factor of 10 as a result of the greater area of bare soil left by thf' decre,ul<'
in vegetative cover around snowbanks. The area involved is small and thus the total impact is very small
regionally. The long term climatic trend since 1930 has been toward higher temperatures and less precipitatioll,
with short term fluctuations, of a few years, from relatively dry to wet modes. No significant increases in
silver concentration were detected, because of the small amounts added in cloud seeding and the high back-
ground levels of silver in the target area. No deleterious effects of silver iodide have been found at con-
centrations which could be expected due to cloud seeding.
17. Key Words 18. Distribution Statement
alpine tundra. climate, cloud seeding, Colorado, Available From:
aspen, National Technical Information Service
ecology, ecosystems, elk, environmental impact, fescue, Springfield, Virgi.nia 22151
forests, geomorphology, oak, phenology, phytosociology,
productivity, San Juan Mountains, silver iodide, small
mammals, snow, spruce, weather modification.
19. Secuirty Classif. of this report 20. Security Classif. of this page 21. No. of pages 22. Price
Unclassified Unclassified 489

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