Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Accueil du site → Doctorat → Pays-Bas → < 2000 → Catastrophic vegetation dynamics and soil degradation in semi-arid grazing systems

Wageningen Universiteit Pays Bas (1998)

Catastrophic vegetation dynamics and soil degradation in semi-arid grazing systems

Rietkerk, M. 

Titre : Catastrophic vegetation dynamics and soil degradation in semi-arid grazing systems

Auteur : Rietkerk, M. 

Université de soutenance : Wageningen Universiteit

Grade : PhD thesis 1998

Résumé partiel
When vegetation is drastically reduced as a result of drought or an increase in herbivore numbers, it does not simply recover if periods with normal rainfall follow or if herbivores are removed. These are commonly recognized catastrophic phenomena of semi-arid grazing systems in general and of the African Sahel in particular. The main aims of this thesis are to provide an effective explanation of the catastrophic properties of vegetation dynamics in these systems and to predict under which conditions they might be expected.

We start with a description of Sahelian rangeland vegetation dynamics, to reveal its catastrophic properties. This exercise appeared a very useful first step in the growth of our ideas about catastrophic vegetation dynamics because : 1) it translated rather vague concepts into a verifiable format by deducing hypotheses about the conditions under which catastrophic vegetation dynamics might be expected, and 2) it generated the notion that soil degradation could somehow be an important factor attributing to catastrophic vegetation dynamics in semi-arid grazing systems. This is in contrast with models that emphasize herbivore feeding characteristics or plant competition as possible mechanisms underlying catastrophic vegetation dynamics. We tested the hypothesis that soil degradation, i.e. soil erosion by run-off and wind and the consequent loss of water and nutrients, is sufficient to explain catastrophic vegetation dynamics by mathematical modelling.

Our model studies indeed show that soil degradation can effectively explain the catastrophic properties of semi-arid grazing systems. Soil degradation can cause a positive feedback between reduced resource (soil water and nutrients) availability and reduced vegetation biomass which may lead to collapse of the system. This positive feedback loop can be triggered by grazing. We argue on the basis of a large body of literature that this is an important mechanism causing catastrophic vegetation dynamics in semi-arid grazing systems. Furthermore, our model studies predict for which site-specific properties catastrophic vegetation dynamics may be expected, that is on loamy or clayey soils in case of water-limited vegetation biomass production, and on sandy soils in case of nutrient-limited biomass production. This is because sandy soils have higher water infiltration rates but are more vulnerable to nutrient loss through erosion than loamy or clayey soils.

Mots clés : grazing systems / arid zones / soil degradation / desertification / vegetation / soil water / sahel / tanzania / burkina faso

Présentation

Version intégrale (5 Mb)

Page publiée le 10 juin 2008, mise à jour le 2 juin 2022