• Geologic grazing refuges and grassland diversity: A shortgrass steppe study

      Milchunas, Daniel G.; Noy-Meir, Imanuel (Society for Range Management, 2004-03-01)
      Geologically isolated, natural grazing refuges that have never been grazed by domestic livestock can be foci of diversity for rare species. This study compared refuges protected from grazing by ravines to adjacent grazed sites in an uncommon grassland type in eastern Colorado. We also tested whether differences between refuge and grazed sites were due to protection from grazing versus abiotic conditions, based on temporary caging of little bluestem plants (Schizachyrium scoparium [Michaux] Nash). Regionally rare species were not exclusive to refuges, but occurred also on adjacent grazed sites. However, refuges showed greater species richness, as well as greater abundance of several tall stature species that are uncommon in the surrounding shortgrass steppe. Temporary protection of little bluestem plants in grazed sites resulted in significant changes in growth and reproductive output, in the direction of refuge plants. Though this geological refuge did not manifest an exclusive set of rare species as reported from other systems, it does preserve strong source populations of uncommon species that are sensitive to grazing and thereby contributes to regional grassland diversity.
    • Successional Transitions and Management of a Phosphorus-Limited Shrubland Ecosystem

      Henkin, Zalmen; Seligman, No'am G.; Noy-Meir, Imanuel (Society for Range Management, 2007-09-01)
      The decline of traditional pastoral systems has highlighted the problem of managing shrub encroachment on successional shrublands in the Mediterranean region, especially in marginal habitats. A long-term study of the response of ecosystem dynamics to phosphate amelioration and shrub control was initiated in 1988 on an area of phosphorus deficient terra rossa, dominated by dwarf shrubs that had been burnt in the summer of that year. The treatments were imposed in a replicated factorial design once at the beginning of the study. The area was previously grazed yearlong by goats, but during the experiment beef cattle grazed the area during the summer of each year. Without herbicide control, shrub cover reached its preburn level within 5 years, but with shrub control after 17 years, it had not yet reached the preburn level. The average shrub cover over the whole experimental period was 41.9%-49.1% without herbicide and 13.5%-24.4% with (P<0.0001, SE of the difference = 3.99). The effect of phosphate application on shrub cover was not significant, but cover of herbaceous vegetation increased (P < 0.0016, SE of difference = 5.03). A ‘‘state and transition’’ scheme was constructed that defines the interventions necessary to buffer any one of the states against the pressures of successional processes. Vegetation states were defined by the dominance of either herbaceous vegetation or one of two spiny shrub species, Prickly burnet (Sarcopoterium spinosum, Rosaceae) and Calicotome villosa (Fabaceae). The timing and scale of the interventions depend largely on landscape management objectives and on available economic and logistic resources. We conclude that appropriate management of grazing, periodic control of the shrub component, and occasional soil nutrient amelioration can lead to the development of attractive open woodland with a productive herbaceous understory that provides a wider range of ecological services than a landscape dominated by the undisturbed successional shrub thickets.