COMPUTER-SIMULATION OF HIGH-MOUNTAIN TERRACETTES AS INTERACTION BETWEEN VEGETATION GROWTH AND SEDIMENT MOVEMENT

Terracettes are common features on steep slopes between 1700 and 2700 m a.s.l. in the central Pyrenees. These microforms can be understood a s the result of the interaction between the growth of bunch grass (Fes tuca eskia) and geomorphic processes; the first element provides a dis continuity of the physical properties of the slope, and the second ele ment modifies the growing pattern and affords the characteristic micro profile of the slope. In order to analyse the former relationship, ass uming that the sediment transport is only caused by a surface process, a computer simulation experiment has been performed. The first purpos e of this model is the construction and verification of a set of hypot heses, and the production of a guide for field investigations. This mo del may then be used to investigate several questions concerning the g eoecology of these features such as the role of geomorphic processes a nd vegetation behaviour, whether they represent a steady-state or an a ggradation or degradation succession, and their efficiency as soil sta bilisation features. The first results, obtained during sensitivity an alysis, show that it is easy to simulate the formation and evolution o f individual forms although the biological model parameters for the ge neration of a continuous trend of terracettes are rather narrow, due t o the risk of demographic instabilities or explosions. Once a paramete r set is achieved, simulated terracettes are self reproducing in a dyn amic equilibrium condition. Simulated sediment transfer is lowered whe n a good terracette pattern is obtained, as a consequence not only of the vegetation cover but also of the topographical organisation. The m ain questions posed are the role of reproduction from seeds and the fa ct that the model routes sediment but not water. The model is unable t herefore to produce features related to concentrated runoff, indirectl y simulating the role of montane processes which disturb the organisat ion of incipient channel forms.