Management and Disturbance Effects on Water Yeild

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period, it was assumed to have been fully forested for the prior 20-year increment. This process allowed forest cover to be redefined for each 20-year increment from the year 1860 to 1997. In a similar, but reversed, procedure the current conditions were extrapolated in 20-year increments forward to 2060. A summary of the estimated size class distribution by species for 1860 to 2060 is presented in Table 2. Because the stand description data used in this analysis were collected over several years beginning in the 1990s; what is considered to be the current or existing condition (ca 2000) is also considered to synonymous with the reference year 1997. As was the case in the two previous efforts on the North Platte River Basin, basal area projections were done in two ways for the spruce-fir type. As presented in Table 2 changes in spruce-fir may be projected using the same procedure as other species. A preferred approach, however, is to hold the current spruce-fir component constant for the entire time. The logic is that since the spruce-fir type is a climax complex, little actual change in those stands may have occurred during the past 140 or so years. The hydrologic consequences of both alternatives are presented. 160 140 120 V N$ 100 ? 80 a ? 60 ? m 40 20 y=1.1141x+14.966 R = 0.8771 . • . F / # ?. . . Fj AR « • L? MBR • x PSI 20 40 60 80 100 120 Age Figure 3. Basal area/age relationship for aspen. The keyed symbols represent the average age and basai area of seedling-sapiings (youngest), poles (mid-point), and sawtimber (oldest), by Forest. Small symbols represent inventory data used to derive the line. The age and basal area of poles and sawtimber on the PSI are younger and less dense than on the other forests. 8 2/2/2007