Management and Disturbance Effects on Water Yeild

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:? Cd F'Ou"a?????T;?'ntAct?v?????? ori Wai'?aru''leld In contrast to the area affected by fire and insect mortality, forest management activities on the three Forests appear to have been relatively minor since 1997. The activities implemeilted consist mostly of partial stand removal practices primarily intended to reduce fuels, foster tree release, or to improve stand condition. About 38,000 acres of the 1.7 millions acres of NFS land has been treated in recent years. Of that, almost 24,000 acres were treated using a natural fuels broadcast or understory burn with the objective of impacting 20 to 40 percent of the residual basal area of the stand. The available data on disturbance activities are not polygon or year specific so site-specific simulations of hydrologic response are not possible. Data indicate that 78 percent of the activity occurred on the PSI National Forest with the balance occurring on the AR National Forest. In general, the management activities were not stand replacing. Only 2,600 of the 38,000 acres treated resulted in 50 to 100 percent of the basal area removed. The balance of the acreage treated received a less invasive partial removal, often understory, where less than 40 percent of the basal area was removed. About 80 percent of the activity (30,000 acres) on both forests occurred in ponderosa pine sawtimber stands and consisted of salvage cuts, understory burns, and thinning operations where 20 to 40 percent of the basal area was removed. Such treatments would be marginal in producing a measurable increase in water yield (Bosch and Hewlett 1982; Stednick 1996). Because the activity data cannot be related to specific polygons, it is not possible to characterize the initial stand conditions or the basal area reduction that actually occurred in the impacted stand. Therefore, we cannot simulate and can only speculate on the impacts of forest management activities on water yield. Given that most of the activity occurred on the PSI National Forest and in the ponderosa pine type, the potential increase in water yield that might result from forest management is minimal relative to the simulated fire and insect impacts. The potential increase in water yield resulting from the net effect of recent and current management activities is about 5,000 acre-feet per year. If the current level of activity is projected into the future, the net increase would average 10,000 to 20,000 acre-feet per year over time. U mmary Two significant conclusions were drawn from the assessment of the historical trends in water yield from NFS land in the South Platte River Basin. First, as was the case in the North Platte study, simulations suggest that a significant decline in water yield occurred from NFS land as a result of increases in forest density. However, apparent differences in specie composition and age distributions between the North and South Platte River Basins indicate that the cycle from high to low water yield is shorter in the South Platte River Basin. In both basins, however, the data indicate that the process is cyclical. As stands mature naturally, water yield declines; as stand regenerate, water yield increases. Without human intervention, this natural process is largely propagated by fire and insects. The second conclusion is that although water yield is expected to increase after catastrophic fires, such as the Hayman Fire, the potential effect of the current insect infestation, particularly in the lodgepole pine type, is a far more extensive and may be a more significant factor in effecting water yield. Mortality from the current mountain pine 26 2/2/2007