CHANGES IN THE LANDSCAPE STRUCTURE OF A SOUTHEASTERN WYOMING RIPARIAN ZONE FOLLOWING SHIFTS IN STREAM DYNAMICS

Throughout western North America, flood regimes have been altered as a result of large-scale water impoundments and diversions, yet the effects on riparian landscape structure have not been quantified. Using aerial photographs and a GIS, we examined changes in the Rawhide Wildlife Management Area in southeastern Wyoming between 1937 and 1990 after shifts in the frequency and intensity of flooding of the North Platte River. The river declined in wetted area by 75% between 1937 and 1990. Also, the areal proportion occupied by cottonwood (Populus spp.) stands with <30% canopy closure increased while stands with >70% canopy closure decreased during this period, indicating a shift from young, dense stands to older, more open stands. Some traditional measures of landscape structure (i.e. richness, diversity, dominance, average patch perimeter length, average patch shape), however, appeared insensitive to these changes. Finally, the proportion of the landscape that changed land types declined with increasing distance from the river. We expect further modification of the landscape structure, associated with continued declines in cottonwood recruitment and increases in cottonwood mortality.