Simulated shrubland ecosystem dynamics in the semi-arid Karoo, South Africa

In arid regions, the effects of grazing or sparing management on natural communities of long-lived plants generally take decades to become evident. Event driven dynamic behaviour, unpredictable and low rainfall and complicated interactions between species make it difficult to gather sufficient understanding of plant community dynamics to be able to develop guidelines for sustainable management. To gain a better understanding of the main processes and mechanisms involved in vegetation change, we have developed a spatially explicit individual based model that simulates changes in plant communities over long time spans. The specific aim of the model is to identify key processes of vegetation change and to calculate probabilities and timespans for transitions between different vegetation states. The model shows that the dynamics of the shrub community are typified by episodic and discontinuous changes in species composition with intervening quasi-stable phases lasting some decades. The short-term community dynamics (years to decades) are very sensitive to the sequence of rainfall events. In all simulation experiments the final vegetation state varied by more than 37% after a 60 year simulation period. However, time-scale for changes of the dynamic state of the system are long compared with human lifespans. Simulating resting of an overgrazed part of the shrub community indicated that little improvement in rangeland condition was likely during a period of 60 years. Overgrazing of a rangeland in good initial condition, simulated by reducing seed production in preferred forage plant species, only became obvious 40 or 50 years after the initiation of heavy grazing, and after 70 years the mean vegetation state eventually reached that of an overgrazed rangeland.