Effects of herbivory on twig dynamics of a Sonoran desert shrub Simmondsia chinensis (Link) Schn.
Density of jojoba (Simmondsia chinensis) was similar but plant size and cover were greater inside than outside a 48-yr-old livestock (cattle) exclosure established in 1937 in central Arizona. Branch diagrams and destructive samples of ungrazed, moderately grazed and heavily grazed shrubs were taken over a 2-yr period (1985-87) to determine twig responses to herbivory. Twig growth was greatest in the spring while greatest herbivory occurred in late spring and summer after senescence of annual herbaceous plants. Grazed shrubs had greater twig growth than ungrazed shrubs resulting in similar net increases in twig length and biomass. The ability of jojoba to regrow from lower lateral or apical buds after removal of outer twig ends allows it to tolerate herbivory in the ecological context of the study site. Grazed shrubs had lower male and female flower densities than ungrazed shrubs. Periodic spring rest or control of stocking to limit grazing intensity is recommended to maintain shrub size and total production.