Changes in shrub fecundity in fourwing saltbush browsed by cattle

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.