Graduate Thesis Or Dissertation
 

Habitat factors affecting pronghorn use of playas in south central Oregon

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  • Pronghorn (Antilocapra americana) use of four playas (intermittent lakebeds) on Hart Mountain National Antelope Refuge (NAR) in south central Oregon was studied during the summers of 1974 and 1975. Percent canopy cover of playa vegetation, plant phenology, percent desiccated vegetation, soil moisture, available surface water and pronghorn use of range sites adjacent to playas were measured for each study area; the relationship between these parameters and pronghorn use of each lakebed was determined. A total of 14,316 pronghorn observations was recorded on the four playas during the two summers. Pronghorn observations totalled 4510 on range sites adjacent to the playas. Desert Lake received significantly (P < 0.01) more pronghorn use than the other playas during 1974 and 1975. No significant differences in use existed among Dobyns, Long and Spanish Lakes during either study year. Pronghorns demonstrated a greater dependence on lakebeds during a dry year than during a wet year. The importance of succulent vegetation to pronghorns was reflected in the relationship between soil moisture and pronghorn use. Pronghorns indicated a strong preference for leafy arnica (Arnica foliosa), particularly at Desert Lake. On Spanish Lake during the wet year, 1975, prostrate knotweed (Polygonum aviculare) was an important pronghorn food. Other forbs eaten by pronghorns were poverty sumpweed (Iva axillaris) and Oenothera flava. As soil moisture decreased, desiccation of vegetation increased, and pronghorn use on playas increased, primarily on Desert Lake where the lowest percent desiccated vegetation occurred. Early leaf growth and early flowering stages of forbs were the most important developmental stages causing a selective response by pronghorns. Pronghorns terminated use of a lakebed if drinking water was unavailable, but the mere presence of water on a playa did not insure use. As upland range sites, dominated by low sagebrush (Artemisia arbuscula), dried, pronghorn use of playas intensified throughout summer and early fall. Patterns of use by pronghorns of range sites adjacent to playas were similar between years, but increased pronghorn use during the wet year on all sites except Desert Lake supported the idea that during the wet year pronghorns were less dependent on playa vegetation. At Desert Lake, pronghorns indicated a preference for the Intermittent Lake (fair condition) range site, dominated by Bolander's silver sagebrush (A. cana bolanderi). At the other three lakebeds, pronghorns showed a preference for Rocky Terrace (poor condition) range site. Pronghorns appeared to prefer the Desert Lake study site because of the combination of habitat factors which the other three playa sites lacked.
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