Simulation of the cyclical process of heathlands: induction of mosaic structures, evolution to irreversible states

A simulation (random process with a discrete control of time) of cyclical regeneration of the monospecific heathlands established on the volcanic Chaîne des Puys (Massif Central, France) after the general and sudden surrender of grazing in the middle of this century, is proposed. On the high sloping zones on the south and south-west, spontaneous regressive dynamics of the heathlands appears, causing intense erosion. In other cases, some mosaic structure (Calluna vulgaris-Brachypodium pinnatum) cover the volcanoes. This highly simplified model takes into account the following processes: (i) the senescence process and vegetative reproduction of Calluna, (ii) the reproduction and intraspecific competition of Pinus sylvestris, (iii) erosion. The main results demonstrate that such a system exhibits an oscillatory behaviour. This phenomenon is strongly dependent on the initial parameters of the Calluna populations and may or may not reach an interior equilibrium: a stationary phase. An activity state function, taking into account the average transformation rapidity and modifications with constant surfaces, allows following the whole activity of the system. The origin of the mosaics could lie in this behaviour and could be more clearly topologic than strictly ecological. The oscillatory behaviour induces a multiple-steps settlement by pines. The suspension of grazing in several times would favour the health colonization by multimodal age-population of Calluna and then lessen the oscillations as well as erosion and regularizes pine colonization. The optimal density of the initial pool of pines seems to be equivalent to 1 tree per 700 m².