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Accueil du site → Doctorat → États-Unis → 1999 → Cattle-shrub interactions in an Atriplex canescens dominated community on the shortgrass steppe

Colorado State University (1999)

Cattle-shrub interactions in an Atriplex canescens dominated community on the shortgrass steppe

Cibils, Andres Francisco

Titre : Cattle-shrub interactions in an Atriplex canescens dominated community on the shortgrass steppe

Auteur : Cibils, Andres Francisco

Université de soutenance : Colorado State University

Grade : Doctor of Philosophy (PhD) 1999

The proportion of female Atriplex canescens (Pursh.) Nutt. shrubs in long-term ungrazed exclosures at a site on the shortgrass steppe in Colorado, was significantly higher than in adjacent stands grazed by cattle. While browsed female shrubs appeared to be younger than their male counterparts, ages of males and females in exclosures were apparently not different from each other. Female shrubs at this site were presumably affected more negatively by cattle-browsing than were male shrubs. I proposed 3 explanations for this apparent pattern : (1) Female shrubs had been browsed more heavily than had been male shrubs ; (2) Cattle browsing was promoting sex phenotype shifts towards maleness ; and/or (3) Female shrubs were more sensitive to browsing than were male shrubs. I conducted a series of experiments to test these hypotheses and found that moderate cattle-browsing in winter or late summer resulted in higher levels of utilization of female shrubs relative to both males and juveniles. In early spring, when cattle fed mostly on grasses, gender specific browsing differences in adult shrubs disappeared, but utilization of juveniles increased dramatically. These trends persisted under a three-fold increase in cattle density. I also found that release from cattle-browsing triggered sexual-phenotype shifts toward femaleness that occurred mostly in monecious shrubs. The magnitude of these shifts, however, did not translate into measurable changes in overall shrub sex ratios. Cattle-browsing, irrespective of season and animal density, was associated with an increase of nonflowering shrubs but was not related to a decrease in percent utricle fill. Finally, the results of a greenhouse experiment I conducted suggested that female Atriplex canescens clones were more negatively affected by artificial defoliation than were males. Sex biased-herbivory and (to a lesser degree) shrub gender specific responses to defoliation may have contributed to gender related mortality and, ultimately, to the alteration of shrub sex ratios at this site. Browsing-induced sex shifts are probably not an important factor controlling shrub sex ratios at my research site.

Mots clés : Ecology, Atriplex canescens, Zoology, Shortgrass steppe, Herbivory, Cattle-shrub interactions Biological sciences, Range management

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Page publiée le 16 février 2015, mise à jour le 17 novembre 2018