INTERACTIONS OF PLANT-GROWTH AND SEDIMENT MOVEMENT ON SLOPES IN A SEMIARID ENVIRONMENT

The morphology, and architecture of the tussocks of alpha grass (Stipa tenacissima) as well as its associated micro topography have been des cribed in several sites on the south versant of the Filabres Range, in southern Spain. The annual rainfall of this area is between 225 and 2 75 mm, and plant cover ranges from 20 to 45%. The results show the for mation of micro topographic mounds upslope of the tussocks which are i nterpreted as deposition bodies originating from the interception of d ownslope sediment movement by the vegetated patches. The tussock archi tecture shows an outer growing zone, a middle mature zone and a centra l senescent zone. The outward expansion of the tussock is performed by tillers in the growing zone which bend at the nodes and root when the y come into contact with the soil surface. The mat of dead leaves insi de the tussock may prevent the stems from bending and this factor play s a significant role in controlling tussock growth. A similar effect o n slopes is produced by the mound of sediments which accumulate upslop e of the tussock and press upon the growing stems. A simple simulation model has been built from the above field observations. The model is able to reproduce the tussock shapes found in nature and predicts that at low plant growth rates, the tussocks become unstable and collapse, while at higher levels of plant vigour and sediment movement rates, s edimentation occurs upslope of the tussock and erosion downslope. Thes e foci of sedimentation and erosion are crescent shaped and their rela tive importance depends on the splash factor.