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CHAPTER II A DISCUSSION AND SYNTHESIS !I INTRODUCTION 2/ Nel Caine - , C. P. Patrick Reid, P. J. Webber, and Donald A. Klein vation. With the exception of Wolf Creek Pass, no all-weather roads ~ross the mountain area and much of it lies within the Weminuche Wilderness. This chapter is intended to:draw together the main conclusions of Chapter IV of this report with regard to the influence of qloud seeding on the terrestrial ecosystems of the San Juan Mountains. Following a brief description of the field area, the ecosystem components are cortsidered separately and then the possibility of linked interactions among them, which may lead to eitlier damped or amplified responses to snowpack augmeqtation, is discussed. This treatment requires so~ speculation about poten- tial impacts, especially long term ones, since not all of the predictions made her~ are based upon statis- tically proven relationships. The speculation is, however, that of scientists 'who have worked in the San Juan Mountains for 5 yeqrs and so is usefully included in a general discussion of ecological effects, bearing in mind th~ increased risk of error which such speculation entails. I I During the 5 year life of tqe research reported here (1970-1975), the San JUan Moulltains have been the site of a pilot pr~ject designed to test the operational effectiveness of snowpack augmenta- tion through a randomized s~eding experiment. Originally, it was anticipatied that: the proj ect would give a 15 percent increase in winter snowpacks; that is about one half of the expected full poten- tial of winter cloud seeding in th~! area. Subsequent evaluation of the cloud deediing experiment has led to the conclusion that actual s~lOwpack augmentation was below the anticipated 15 percent level (Howell, this vol. p. 5). Any artificial~y caused increase in the snowpack of the San Juan Mountains has been imposed on a region in which precipi~ation and snow accumulation is naturally highly variable: annual snowfall there ranges betwee~ 35 percent less and 80 percent greater than the ~nnual mean of 40 cm water equivalent (w.e.) measured over a 25 year period. Many of the estimates of ecologic impact made in this study are basedl on the potential 30 per- cent increase in mean snowpack accumulation that was originally assumed to be possible in an operational phase ,of winter cloud seedin~. THE FIELD AREA In southwestern Colorado, the Continental Divide makes a wide bend to the west around the headwaters of the Rio Grande and through the San Juan Mountains. The target area for cloud se~ding has been a 3400 km2 area in the southern and eastern part of the range, extending from the Colorado-New Mexico state line to the Needle Mountains. Most of the field investiga- tions of the San Juan Ecology Project have been conducted in this target area (Figure 1, Chapter I). In the target area, the Continental Divide runs almost east-west and subsidiary ridges extend from it to the south or southwest between tributaries of the San Juan River. TIle topography ~f the area is generally rugged with some of the peak~ exceeding 4250 m ele- Geologically, this is a young part of the southern Rockies and is composed largely of Tertiary volcanic tuffs and lavas, although metamorphic materials of Precambrian age are exposed extensively in the Needle Mountains and Grenadier Range at the western end of the target area. Mesozoic sedimentary materials are found only at lower elevations. This diversity of parent materials, combined with the changes of climate and vegetation due to elevation and a his- tory of multiple late-Cenozoic glaciations, has produced a complex pattern of soils and surficial deposits; however, this complexity is typical of the southern Rocky Mountains. The vegetation of the San Juan Mountains is characteristic of that in the southern Rockies. At lower elevations (2200 m), pinon pine, juniper, and scrub oak vegetation is common and is replaced successively by ponderosa pine, Douglas-fir, aspen, Engelmann spruce, and subalpine fir with increasing elevation. About 20 percent of the area lies above tree line (3500 m) where a mosaic of alpine tundra plant communities and bare rock dominates the land- scape. The vegetative cover of large parts of the range has been extensively disturbed by man's activity in the past 100 years and future changes in the vegetation may reflect this disturbance rather than the effects of cloud seeding. Within this diverse region, detailed field studies have been concentrated in four areas: Wolf Creek Pass; Missionary Ridge; and the Eldorado Lake and Williams Fork Lakes basins. These areas are described in Chapter IV. Less detailed studies have been conducted in other parts of the San Juan Mountains and elsewhere in the Colorado Rockies. These supplementary study areas include situations which are typical of the forest and alpine tundra ecosystems of the San Juan Mountains. PHYSICAL ASPECTS The studies of the San Juan Ecology Project have centered on two sets of potential impacts from snowpack augmentation. The first set concerns the effect of the seeding agent (AgI) on the terrestrial ecosystems. In the pilot project, silver was intro- duced to the target area at a rate of 0.04 g/ha/yr and this rate might be approximately doubled if snow augmentation were conducted on an operational basis. The eventual disposition of this silver is examined below but it is worth noting that it is imposed on a system in which silver occurs in surface soils at concentrations of up to 0.1 ppm, i.e. four orders of magnitude greater than that in a seeded snow- pack. This "natural" silver is not known to occur in the form of silver iodide. 1/ In Steinhoff, Harold W. a~d Jack D. Ives (Eds.) 1976. Ecological impacts of snowpack augmentation in the - San Juan Mountains, Colorado. Final Report, San Juan Ecology Project, Colorado State University Publ., Fort Collins. ' 'l) Present Address: INSTAAR,I University of Colorado, Boulder, Colorado 80309. 7