Geomorphological and Geoecological Controls and Processes Following Gully Development in Alpine Mires, Lesotho

Abstract

Despite the international reputation of Lesotho’s severely eroded landscape, there have been no previous quantitative accounts of soil erosion processes and associated consequences from the alpine belt. This paper examines sedimentological, geomorphological, and geoecological controls and processes following gully development within alpine mires in eastern Lesotho. Contemporary gully extension is controlled by exposure to various sedimentary sequences, by gully sidewall crack development, and by topographic aspect. Significant vertical gully denudation rates of 8 cm (mineral sediment) to 13 cm (peat) were recorded over an 18-mo period. During a 5-d field experiment in July 1999, the continuously frozen south-facing gully walls recorded considerable horizontal movement of peat blocks (avg. = 19.8 mm/5 d), whereas needle ice–induced horizontal particle movement rates on north-facing walls averaged 10.2 mm/5 d. Soil moisture transect data show a pronounced reduction in surface soil moisture toward and between the gullied areas of a mire. The percentage of moisture loss toward midwinter was found to be greater between the gullies (47.4%) than in the adjoining zones, where moisture loss averaged 33.6%. Similarly, vegetation transects indicate a reduction in vegetation cover at the drier and more intensely burrowed (by Otomys, or “ice rats”) zones close to the gullies. Invasive dwarf Karroid shrubs (e.g., Chrysocoma ciliata) are now establishing themselves alongside gullies, burrowed sites, and fringe areas of mires. We found that dryland plant invasions around grazing posts and heavily grazed areas on the slopes subsequently spread along alpine hydrological systems, particularly where gully erosion has created a suitable habitat.