Measurements at one-year interval of rock-fragment fluxes by sheep trampling on degraded rocky slopes in the Mediterranean

Field experiments were conducted to assess the impact of trampling by sheep on the downslope movement of rock fragments on steep, degraded rangeland slopes in the western part of Lesvos (Greece). Marked rock fragments were used as tracers which were monitored over a one-year period. It was shown that the slope gradient and the development of a fast encroaching shrub Sarcopoterium spinosum appeared to be major controlling factors of the rock fragment displacement distances. In this paper, downslope transport fluxes of rock fragments were calculated for different combinations of surface rock fragment mass and Sarcopoterium cover (%). The rock fragment displacement was also described as a diffusion type process. The diffusion constants (0.1 to 121 kg m(-1)) were strongly influenced by the Sarcopoterium cover. The highest transport rate, using regression equations for the displacement distance, was estimated to be 33.8 kg(-1) y(-1) or 3.4 ton ha(-1) for high rock fragment covers (34 kg m(-2)) and low Sarcopoterium cover (0 %, after burning) on a 100 in long and 20 % steep slope. Locally, measured values indicated a maximum flux of 61.2 kg(-1) y(-1) on a 50 % slope with a Sarcopoterium cover of 1 %. The results show that the rock fragment fluxes are significant when compared to other processes of hillslope erosion, in extreme cases comparable to (but still lower than) published rates for tillage erosion. An exponential decrease in rock fragment loss with increasing Sarcopterium cover was found, similar to such relationships between soil loss and vegetation cover for water erosion. The disturbance of the rock fragment pavement by trampling also results in exposure of fine earth to erosion by water, as indicated by erosion pavements and gullies on neighbouring slopes, especially after burning of the Sarcopoterium cover. Rock fragment movement by trampling therefore significantly contributes to soil degradation in these Mediterranean environments.status: publishe