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ECOLOGICAL IMPACTS OF SNOWPACK AUGMENTATION IN THE SAN JUAN MOUNTAINS OF COLORADO CHAPTER I INTRODUCTION J. D. Ives, J. M. Sweeney, H. W. Steinhoff, and W. W. Howell This document is the final' report of the San Juan Ecology Project (SJEP) whi~h has been supported by the United States Bureau of Reclamati.on under Contract No. l4-06-D-7052 from 1970 to 197'5. An earlier study, that was prepared as a problem analysis, has been re- ported previously (SJEP, Phase I Final Report: Teller et al. 1970). The present: report (Phase II) does not include any of the early, preparatory material. SUMMARY OF KEY CONCLUSIONS This Summary condenses the evaluation of ecologic impacts due to varying snowpack in the San Juan Moun--- tains. It is based on a st~dy of the influence of a snowfall which is quite variable in both space and time. Direct observation of the effects of snowpack augmentation were not possible during the 5-year life of the San Juan Ecology Project, but the relationships established between ecosys'tem components and snow conditions allow the esti~ation of possible effects of long-term successful cloud seeding. The statements which follow treat the main ecosystem components in order. The ,impact of snowpack augmen- tation on each ecosystem component is arranged accord- ing to the nature of the e:nvironmental change which produced the effect. The basic environmental changes are the addition of more snow and more silver. Such changes could initiate a c,hain of effects that would radiate throughout the ecosystem. The first links in the chain which have a siinificant impact on ecosystem components are: (1) lowe:r soil temperature in the spring, (2) more moisture, in the spring, (3) deeper snowpack, and (4) more sil~er. , This summary includes only statements verifiable from the data, with causal connections and conclusions that would be accepted by 'scientific peers in environ- mental disciplines. Spec~lation about causal links in the chain, or extension of, implications beyond effects which were studied in the chain, and where the extension involves application of ge~eral ecologic understanding, are avoided in this summary. Such appropriate scien- tific conjecture can be found in Chapters II and III and in segments of individual reports that are spec- ulative by implication through the use of terms such as "may be" or"probably", or which are labelled as "broad significance" or "iptplications." Many other findings, which: are not directly related to the effects of snowpack au'gmentation on the indicated ecosystem components are ~ natural part of such studies. For a summary of these findings reference is made to the abstracts of individua~ reports or to the details of the individual reports 'themselves in Chapter IV. Plants Only two environmental changes which might be induced by increased snowfall have been found to relate signif- icantly to plants in the San Juan Mountains; lower soil temperatures and more soil moisture. - Lower Soil Temperatures Initiation of shoot elongation was delayed for plants both in the tundra and forests as a result of lower soil temperatures associated with a deeper snowpack for the species studied. Thes'e included Englemann spruce (Picea engelmannii), quaking aspen (Populus tremuloides), Thurber fescue (Festuca thurberi), and numerous herbaceous species in both the tundra and forest mea- dows. The delay was most apparent in species wi th growth points in the immediate zone of lower temperatures at the ground level. This includes pri- marily the herbaceous species. The growth regions of trees, in the cambium and terminal buds, were affected more indirectly, possibly through soil temperature effects on root function. From this a less clear re- lationship between growth inception and snowpack was anticipated although there was a general correlation of bud burst dates for both spruce and aspen with the snow clear dates. Moisture stress in spruce was un- usually high during the period of snowmelt and decreased dramatically at about the snow clear date. This did not affect the annual total radial growth of spruce, which initiated 2 to 4 weeks earlier than bud burst, apparently independent of snowpack. Resultant annual biomass growth of spruce was remarkably constant from year to year, unrelated to snowfall. Aspen was more variable in annual biomass production. Aspen growth inception was delayed by a later snow clear date, but we did not determine whether or not this was directly linked to variations in biomass production. The delay in beginning of growth persisted through the other stages of the life cycle of most plants, but with less magnitude. There seemed universally a compensation factor which permitted every species to complete its annual cycle of development each summer, regardless of snowpack depth. Except for spruce, there was no similar compensatory mechanism for biomass production. In most herbaceous species of the tundra, biomass production was inversely related to depth of snowpack. No effect was found on .either initiation of development or biomass production of oak (Quercus gambellii), pre- sumably because even in a deep snow year the snow had always disappeared from oak plots before the earliest date of growth initiation. - More Moisture Phytosociological studies in both forests and tundra showed that plant communities were arranged along snow clear date gradients. In the forests oak gives way to aspen, then to subalpine fir (Abies lasiocarpa), and finally to Englemann spruce as the snow clear date extends later into the year. The change was less obvious in the drier communities, i.e. in oak. The tundra changed from xeric (drier) to more mesic or even hydric communities. This could be expected to result in an increase in frequency of the rarer species. A dendrochronologic survey of ponderosa pine (Pinus ponderosa) immediately outside the target area showed its growth on dry sites to be associated with December precipitation. The relationship is a direct one and pine would presumably show an increase in growth if cloud seeding were to add to mid-winter snowfall. No such relationship was found for Engelmann spruce, the only other species on which dendrochronologic work has been conducted in this study. . Animals Only the lower soil temperature and greater snow depth, which might be expected to follow an increase in snow- fall, have been found influential on animal activity. 1