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

Titre : Dynamics of tropical and subtropical vegetation : from an equilibrium to a non-equilibrium modelling approach

Auteur : ACCATINO, FRANCESCO

Université de soutenance : Politecnico di Milano

Grade : Tesi di dottorato 2014

Résumé
Understanding and modelling tropical and subtropical vegetation systems is of high relevance. Firstly, these ecosystems can respond to climate changes with irreversible transformation from one biome to another ; secondly, they sustain rangelands which provide livelihood for millions of people. There are two main paradigms for modelling and interpreting the dynamics of these ecosystems : the equilibrium and the non-equilibrium paradigm. The equilibrium paradigm implies that the vegetation systems stay permanently in a stable steady state. If there are alternative stable steady states, the ecosystem can undergo abrupt (catastrophic) transitions from one state to another in response to variations in the environmental conditions. The non-equilibrium paradigm implies that the system always fluctuates in response to disturbances (rainfall, fire, herbivory) and never reaches permanently a steady state. In this thesis I focus on a comparison between these two different approaches. For the equilibrium paradigm I consider a non-linear deterministic differential equation model, with the potential fire frequency considered as a constant parameter. For the non-equilibrium model I consider a time-discrete stochastic matrix model, with fire occurrence modeled as a stochastic phenomenon whose probability of occurrence depends on the amount of grass in the ecosystem. I show how the non-equilibrium model provides a more complete view in specific debate fields of tropical and subtropical vegetation systems. In the debate around the Savanna Question the non-equilibrial model gives an interpretation of the tree-grass co-existence in line with other theories in literature and catches the difference between \textitarid and \textitmoist savannas. The non-equilibrial model interprets the bimodality showed by woody cover for high rainfall values (1000 – 2500 mm/yr), and does not involve the bistability and the consequent catastrophic transitions arising from non-linear deterministic differential equation models. Last but not least, the non-equilibrium approach provides a framework to plan sustainable rangeland management strategies accounting for the environmental variability of rangelands.

Mots Clés  : vegetation dynamics ; tropical and subtropical ecosystems ; modelling ; equilibrium dynamics ; non-equilibrium dynamics ; savanna question ; woody cover bimodality ; rangelands

Présentation