Disturbances In Savanna Ecosystems: ModellingThe Impact Of A Key Determinant

Abstract

Savannas are not only subject to intense, recurrent human-generated disturbances of major extent. Also natural disturbances are common: savannas typically occur in regions where the climate is characterized by spatial and temporal variability, with precipitation varying drastically in space and time. In addition to the resulting climatic extremes, recurrent grass fires and large herbivores often cause major disturbances. Some large scale disturbances, such as drought, grass fires and grazing are considered to be driving forces in savanna ecosystems. These factors are also hypothesized to allow for a long-term coexistence of trees and grasses which is one of the distinguishing features of savanna ecosystems. We use a grid-based spatially explicit simulation model to analyse the impact of various types of disturbances on questions of long-term tree-grass coexistence. The model is based on information of the southern Kalahari and simulates the dynamics of the dominating life forms in semi-arid savannas, i.e., trees, shrubs, perennial grasses and herbs, and annuals. We investigate the impact of various scenarios of rainfall, grass fires and grazing as well as the role of small-scale disturbances of different type and spatio-temporal correlation. The model results indicate that the impact on the savanna ecosystem significantly depends on the interaction of different processes as well as on the disturbance type, rate and correlation. We show that the interaction and relative intensity of the two large scale disturbances fire and grazing may lead to both woodland and long-term coexistence of trees and grasses. However, small-scale heterogeneities and disturbances such as colonies of seed caching rodents may increase this range of long-term coexistence. Especially in the case of highly correlated smallscale disturbances more than half of the explored rain scenarios led to a persisting coexistence of the two dominant savanna life forms.