Studies of Soil and Vegetation Development on Fresh Landslide Scars in the Mgeta Valley, Western Uluguru Mountains, Tanzania

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Abstract
In February 1970, a heavy rainstorm triggered off more than 1000 landslides in the Mgeta Valley in the western Uluguru mountains, Tanzania. The area, originally forested, is densely populated and under intensive land use, mainly annual cropping. The slides are described by Temple and Rapp (1972), who studied the geomorphological and economic effects of the catastrophe. This is a follow up study of that report. Continued erosion in the areas affected by the landslides, topsoil development and recolonisation of vegetation in the landslide scars and land reclamation by the local farmers have been investigated on fourteen landslides during the period 1970 to 1977. Sheet, rill and gully erosion on the bare surface of the landslide scars and retrogressive slumping of the slide edges have been of such a magnitude that the amounts of material eroded from the slide in some cases are as much as that removed in the initial landslide. Gullies up to 4 m deep and areas of slumps up to 150 m2 have been measured. The surface soil layer of the landslides, which has been analysed for organic carbon content, bulk density, pH, texture, and plant available and reserve phosphorus and potassium contents, shows weak development during the period. Organic carbon content has increased from 0.16% to 0.44%, compared to 1.8% in the topsoil of the surrounding areas. The clay content has decreased from 13% to 8%, compared to 15% in the corresponding topsoil, due to the high erosion rate. Available potassium content has increased. The other parameters show very weak or no change during the period. Recolonisation of vegetation has been investigated in 2×2 m sample plots on the slide scars. The vegetation shows a clear succession from annual herbs to perennial grasses, though the total average cover is still small—appr. 25%—after seven years. Recolonisation has mainly occurred through seed germination. Vegetative growth from “floes” and slide edges seems to have played an unimportant role. Only species found in the neighbourhood, i.e. weeds and species from grazing land, have been found in the slides. Water availability has most likely been the major factor restricting vegetation establishment, since the coarse textured soils of the landslides readily dry out. Setting aside the affected areas for grazing, planting trees, and terracing the recultivated slides are conservation measures used by the farmers. But these measures have been haphazard and generally not effective.

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