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000115 San Juan Ecology Study results agree with the Medicine Bow findings and state: "Bolewood production of spruce-fir stands was found to remain constant over a 20-year study period, and it was thought that productivity is mainly under genetic control. Tree biomass was shown to undergo severe perturbations attributable to catastrophic events such as windthrow, insect outbreak, or fire. It was considered debatable whether changes in snowpack could positively or negatively influence these disturbance events. "Measurements of water stress in forest vegetation resulted in the identification of two periods of elevated stress, one immediately preceding snowmelt, the other later in the season, after the exhaustion of soil moisture held over from winter. If precipitation management were to extend the date of moisture release from snow cover, this might extend the duration of the spring periOd of moisture stress. The opinion was also expressed in the findings that the major effect of this high-stress period would be to reduce photosynthesis rather than cell division, since the trees were found able to rehydrate at night. As long as the stress period occurs before the beginning of phenologi- cal activity, little change in growth would be expected. Significant alteration in growth would result only if precipitation management were to delay snowmelt into the period of active growth." [4, pp. 3-111) Two Sierra Ecology Project WorkshOps examined the effects of preclpl- tation increases on vegetation. Workshop I, "An Evaluation of Possible Effects of Weather Modification Upon Forest Insect and Diseases in the American River Basin, California," noted the impacts of a spectrum of events or activities upon the forest. These were similar to those considered in the Medicine Bow and San Juan studies, and included fire, grazing, logging, mining, recreation, and other water management activities. Assuming that cloud seeding would occur during years of below and near-normal snowfall and produce a 10 to 15 percent increase in precipitation, Workshop I reached the following conclusions regard- ing potential impacts on the forest. "In normal years, cloud seeding will probably result in (1) soil moisture equal to or greater than that in un augmented years, (2) snowpacks of longer duration, and (3) a total increase in precipitation that is very small compared to the total for the area over any decade. "Forest ecosystems at upper elevations are already adapted to handle more 'normal' wet winters than would presumably occur during weather modification. Therefore, an increase snowpack during 'dry' years probably will not induce 'abnormal' changes and might even result in an overall net benefit to forest stands. 18