| Andrés F. Cibils, David M. Swift, and Richard H. Hart |
| Authors are Research Specialist, School of Renewable Natural Resources, The University of Arizona, Tucson, Ariz. 85721, email: acibils@Ag.arizona.edu (on leave from INTA-EEA Santa Cruz and Universidad Nacional de la Patagonia Austral, Río Gallegos, Argentina); associate professor, Rangeland Ecosystem Science Department, and senior Scientist, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colo. 80523; and rangeland scientist, USDA-ARS High Plains Grasslands Research Station, Cheyenne, Wyo. 82009. At the time of the research, the senior author was PhD candidate, Rangeland Ecosystem Science Department and Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colo. |
Abstract |
| Shrub fecundity is critical to long term persistence of fourwing saltbush (Atriplex canescens (Pursh) Nutt.) populations at our research site on the shortgrass steppe in Colorado. We conducted a 2-year experiment to test hypotheses concerning the impact of cattle-browsing on fecundity-related variables in fourwing saltbush. Protection from cattle browsing was significantly associated with floral phenotype shifts toward femaleness, occurring mostly in monecious shrubs (1% and 13% of grazed and protected shrubs, respectively). Sex shifts observed at the individual shrub level did not translate into detectable alterations of sex ratios at the pasture level. Shrubs exhibiting no flowers were considerably more abundant in browsed pastures (26.5%) than in exclosures (1.5%). Nonflowering occurred as frequently in female (3.6%) as it did in male (1.8%) phenotypes. Percent utricle fill was not related to previous year's cattle browsing regime (39% and 44% in protected and grazed shrubs, respectively) but rather to crown volume of the fruiting female and to the gender of and distance to the nearest neighboring shrub. The influence of cattle-browsing on reproductive output of fourwing saltbush occurred mainly through its inhibition of flowering. |
| Key Words: Atriplex canescens, sex change, nonflowering, utricle fill, cattle browsing |