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and other vegetation have become more stressed and more susceptible. The apparent <br />result is that the infestation is widespread and severe, and increasingly smaller trees are <br />aggressively attacked. As a result, trees in pole stands, as well as sawtimber stands, are <br />being impacted. Although, current estimates of the severity and extent of the infestation <br />are somewhat speculative, the consensus is that the epidemic is widespread and a <br />significant percentage of smaller, pole size, trees will be killed along with the larger trees. <br />Tree mortality, whether the result of fire, timber harvest, or insects and disease attack, <br />changes the water balance on-site and can result in a change in water yield. <br />Table 5. Acreage of lodgepole pine stands on NFS land in the South Platte River Basin, <br />by Forest and size class. Pole stands are those with an average DBH from 4.9 to 9.0 <br />inches, and sawtimber stands are those with an average DBH greater than 9.0 inches. <br />Pole and sawtimber stands represent nearly 95 percent of all lodgepole pine stands on <br />the forests. <br /> Area (acres) <br />Forest Sawtimber Pole Total <br />Arapaho-Roosevelt 113,486 231,745 345,231 <br />Medicine Bow-Routt 3,045 885 3,930 <br />Pike-San Isabel 29,900 50,570 80,471 <br />Total 146,431 283,201 429,632 <br />Although speculative, we simulated the potential hydrologic impact of several beetle kill <br />scenarios in the lodgepole pine forest type on NFS land in the South Platte River Basin <br />that are quite feasible given the severity of the epidemic. Troendle and Nankervis (2000) <br />simulated the potential hydrologic impact of a beetle outbreak in spruce-fir stands in the <br />North Platte River Basin and determined the impact on water yield could be significant. <br />Based on input from R2 staff, we simulated three scenarios in lodgepole pine. First, we <br />simulated the hydrologic impact of a 50-percent kill in all lodgepole pine sawtimber and <br />pole size stands. Second, we simulated the impact of a 90-percent kill throughout both the <br />sawtimber and the pole stands. Third, based on the consensus of the R2 staff that a 50- <br />percent kill in the pole size stands (4.9 to 9.0 inch DBH) and a 90-percent kill in the <br />sawtimber stands was the most likely expectation of what may actually occur during the <br />next few years, we simulated that impact. <br />Given the significant percentage of the forest land occupied by the lodgepole pine type <br />and the severity of expected mortality; the hydrologic impact of the beetle infestation is <br />expected to be significant for any of the three scenarios (Table 6). If 90 percent of the <br />trees in the sawtimber stands are killed along with 50 percent of the trees in the pole size <br />stands; the increased annual water yield from the impacted area in the lodgepole pine <br />type will average nearly 3.6 area inches or 129,000 acre-feet. The potential annual <br />increase of 129,000 acre-feet of water resulting from the beetle kill in lodgepole pine <br />represents one-half of the total declines in water yield in the South Platte River Basin that <br />has been simulated to have occurred since 1920 because of increases in forest density. <br />23 2/2/2007