Simulated long-term vegetation response to alternative stocking strategiesin savanna rangelands

Increasing cover by woody vegetation, prevalent in semiarid savanna rangela nds throughout the world, is a degrading process attributed to the grazing impact as a major causal factor. We studied grazing effects on savanna vege tation dynamics under alternative stocking strategies with a spatially expl icit grid-based simulation model grounded in Kalahari (southern Africa) eco logy. Plant life histories were modeled for the three major life forms: per ennial grasses, shrubs, annuals. We conducted simulation experiments over a range of livestock utilization intensities for three alternative scenarios of small scale grazing heterogeneity, and two alternative strategies: fixe d stocking versus adaptive stocking tracking herbage production. Additional ly, the impact of the duration of the management planning horizon was studi ed, by comparing community response and mean stocking rates after 20 and 50 years. Results confirmed a threshold behavior of shrub cover increase: at low, subcritical utilization intensity little change occurred; when utiliza tion intensity exceeded a threshold, shrub cover increased drastically. For both stocking strategies, thresholds were highly sensitive to grazing hete rogeneity. At a given critical utilization intensity, the long term effect of grazing depended on the level of grazing heterogeneity: whereas under lo w heterogeneity, shrub cover remained unchanged, a large increase occurred under highly heterogeneous grazing. Hence, information on spatial grazing h eterogeneity is crucial for correct assessment of the impact of livestock g razing on vegetation dynamics, and thus for the assessment of management st rategies. Except for the least heterogeneous grazing scenario, adaptive sto cking allowed a more intensive utilization of the range without inflating t he risk of shrub cover increase. A destabilizing feedback between rainfall and herbage utilization was identified as the major cause for the worse per formance of fixed compared to adaptive stocking, which lacks this feedback. Given the usually high grazing heterogeneity in semiarid rangelands, adapt ive stocking provides a management option for increasing herbage utilizatio n and thus returns of livestock produce without increasing degradation risk s.