Shrub control by browsing: Targeting adult plants

Reconciling the well known benefits of shrubs for forage with environmental goals, whilst preventing their dominance, is a major challenge in rangeland management. Browsing may be an economical solution for shrubby rangelands as herbivore browsing has been shown to control juvenile shrub growth. Less convincing results have been obtained for adult plants, and long-term experiments are required to investigate the cumulative effects on adult plants. We therefore assessed the impact of different levels of browsing intensity on key demographic parameters for a major dominant shrub species (broom, Cytisus scoparius), focusing on adult plants. We assigned individual broom plants to one of three age classes: 3–5 years (young adults); 5–7 years (adults); and 7–9 years (mature adults). These plants were then left untouched or had 50% or 90% of their total edible stem biomass removed in simulated low-intensity and high-intensity browsing treatments, respectively. Morphological, survival and fecundity data were collected over a period of four years. Browsing affected the morphology of individual plants, promoting changes in subsequent regrowth, and decreasing seed production. The heavily browsed plants were 17% shorter, 32% narrower, and their twigs were 28% shorter. Light browsing seemed to control the growth of young adult plants more effectively than that of older plants. Reproductive output was considerably lower than for control plants after light browsing, and almost 100% lower after heavy browsing. High-intensity browsing had a major effect on survival causing high levels of plant mortality. We conclude that suitable browsing practices could be used to modify adult shrub demography in the management of shrub dominance and forage value.