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OJ2~3~ CHAPTER II A DISCUSSION AND SYNTHESIS !I Nel Caine II C. P. Patrick Reid, P. J. Webber. and Donald A. Klein vation. With the exception of Wolf Creek Pass, no all-we~ther roads cross the mountain area and much of it lies within the Weminuche Wilderness. INTRODUCTION This chapter is intended to draw together the main conclusions of Chapter TV of this report with regard to the 1nfluence of cloud seeding on the terrestrial ecosystems of the San Juan Mountains. Following a brief degcri~tion of the field area, the ecosystem components are considered separately and then the possibility of linked interactions among them, which may lead to either damped or amplified responses to snowpack augmentation, 19 discussed. This treatment requires some speculation about poten- tial impacts. especially long term ones, since not all of the predlct!ons made here are based upon statis- tically proven relationships. The speculation is, however, that of scientists ~ho have worked in the San Juan Mount~ins for 5 years ~nd so is usefully included in a general discussion of ecological effects, bearing in mind the increased risk of error which such speculation entails. During the 5 year life of the research reported here (1970-197,), the San Juan Mountains have been the site of a pilot project designed to test the operational effectiveness of snowpack augmenta- tion through a randomized seeding experiment. Originally, it was anticipated that the project would giVe a 15 percent increase in winter sno~acks; that is about one half of the expected full poten- tial of winter cloud seeding in the area. Subsequent evaluation of the cloud deeding experiment has led to the conclusion that actual sn~ack augmentation was below the anticipated 15 percent level (Howell, this vol. p. 5). Any artificially caused increase in the snowpack of the San Juan Mountains has been imposed on a region in which precipitation and snow accumulation is naturally highly variable: annual snowfall there ranges between 35 percent less and 80 percent gre8t~r than the annual 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 based 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 seeding. rHE FJELD 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 seeding 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 I, Chapter I). In the target area, the Continental Divide runs almost esst-west and subsidiary ridges extend from it to the south or southwest between tributaries of the San Juan River. The topography of the area is generally rugged with some of the peaks exceeding 4250 m ele- In Steinhoff, Harold W. and Jack D. Ives (Eds.) 1976. San Juan MOUl1tains, Colorado. Final Report, San Juan For t Collins. ~/ Present Address: y 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 salls 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 8c~ub oak vegetation is common and is replaced successively by ponderosa pine, Douglas-fir, aspen, Engelmann spruce. and subalpine fiT ~itb increasing elevation. About 20 percent of the area lies above tree line (3500 m) where a mosaic of al~ine tundra plant communities and bare rock dominates the land- sca~e. The vegetative cover of large parts of the range has been extensively disturbed by manls 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 ate typical of the forest and alpine tundra ecosystems of the San Juan Mountains. P~SlCAL ASPECTS Xhe studies of the San Juan Ecology Project have centered on two sets of potential impacts from snovpac~ augmentation. Tne 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 5no~ augmentation were conducted on an operational basis. The eventual disposition of this silver ia examined below but it is ~orth 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. Ecological impacts of snowpack Ecology Project, Colorado State augmentation in the UniverSity Publ., INSTAAR, University of Colorado, Boulder, ColoYado 80309. )