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Effects of fire and herbivory on the stability of savanna ecosystems

By F. van Langevelde, C.A.D.M. van de Vijver, L. Kumar, J. van de Koppel, N. de Ridder, J. van Andel, A.K. Skidmore, J. Hearne, L. Stroosnijder, H.H.T. Prins and M.G. Rietkerk


Savanna ecosystems are characterized by the co-occurrence of trees and grasses.\ud In this paper, we argue that the balance between trees and grasses is, to a large extent,\ud determined by the indirect interactive effects of herbivory and fire. These effects are based\ud on the positive feedback between fuel load (grass biomass) and fire intensity. An increase in\ud the level of grazing leads to reduced fuel load, which makes fire less intense and, thus, less\ud damaging to trees and, consequently, results in an increase in woody vegetation. The system\ud then switches from a state with trees and grasses to a state with solely trees. Similarly,\ud browsers may enhance the effect of fire on trees because they reduce woody biomass, thus\ud indirectly stimulating grass growth. This consequent increase in fuel load results in more\ud intense fire and increased decline of biomass. The system then switches from a state with\ud solely trees to a state with trees and grasses. We maintain that the interaction between fire\ud and herbivory provides a mechanistic explanation for observed discontinuous changes in\ud woody and grass biomass. This is an alternative for the soil degradation mechanism, in which\ud there is a positive feedback between the amount of grass biomass and the amount of water\ud that infiltrates into the soil. The soil degradation mechanism predicts no discontinuous changes,\ud such as bush encroachment, on sandy soils. Such changes, however, are frequently observed.\ud Therefore, the interactive effects of fire and herbivory provide a more plausible\ud explanation for the occurrence of discontinuous changes in savanna ecosystems

Topics: Milieukunde, alternate stable states, browsing, bush encroachment, fire, grazing, herbivory, positive feedback, savanna, stability, tree–grass balance
Year: 2003
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