Spatial aspects of grazing in savanna rangelands - a modelling study of vegetation dynamics

摘要

Increasing shrub cover density poses a widespread problem in semiarid savanna rangelands. Recently, vegetation dynamics has been studied with the aid of spatially explicit models, aiming to highlight the response of shrub cover to livestock grazing. Assuming that on a local scale, grazing and trampling act as a disturbance utilmately facilitating local extinction, small scale local variability of grazing intensity might influence density and vegetation cover of forage plants and thus affect shrub cover dynamics. We used a spatially explicit grid based simulation model based on southern Kalahari ecology. Plant life histories were modelled at the level of the three major life forms: perennial grasses and herbs, shrubs, annuals. To study the role of small scale local variability of grazing intensity under various levels of livestock grazing pressure we conducted factorial simulation experiments for two large scale grazing patterns: firstly, assuming homogenous grazing, secondly, assuming a grazing gradient as observed around artificial watering points. Our results confirm the previously reported threshold behavior of shrub encorachment, with threshold levels close to the recommended stocking rates of the study area. stocking rate threshold levels and rates of shrub encroachment were sensitive to small scale local variability of grazing pressure. For gazing regimes with lower local grazing variability, thrshold levels of shrub encroachment shifted towards higher grazing pressures, and at given critical stocking rates, shrub encroachment was slower than for grazing regimes with larger local variability. Equally, for grazing regimes with lower local grazing variability, piosphere formation arund boreholes required higher grazing pressures, and was slower at given critical stocking rates than for grazing regimes with larger local variability. We conclude that information on small scale spatial variability of grazing intensity is crucial for correct assessment of the impact of livestock grazing on vegetation dynamics in savanna rangelands.