Vegetation arcs and litter dams: Similarities and differences

Vegetation arcs are near parallel bands of denser vegetation aligned perpendicular to slope and separated by barer inter-arc zones. These patterns occur in arid and semi-arid areas of gentle to very gentle gradient. Similar, but much smaller scale patterns, here referred to as 'litter dams and microterraces', occur in a variety of climatic regions, from tropical to temperate to arid, on steeper slopes, as well as on very gentle slopes. We describe similarities between arcs and dams in attributes such as shape, orientation, cross-sectional profile, spatial organisation, and surface features (arrangement of plants, surface crusts). Differences are in spatial and temporal scale. Range of wavelengths, or spacing between bands, is 25-300 m for arcs and 0.1-2 m for dams. With regard to time, arcs may be stable for tens to a hundred or more years while dams may be ephemeral or remain stable for more than a decade. Overland flow involving the transport of organic material (leaves, charcoal, faecal pellets) as floating load over comparatively bare surfaces is an important factor in the maintenance of vegetation arcs and formation of and maintenance of litter dams. Recognising the importance of gradient to transport by overland flow a preliminary investigation of the relationship between wavelength and gradient was conducted. A combined analysis of a limited set of arc and dam data describes parallel lines which may indicate that similar processes are operating at the two scales. This would allow the convenience of process studies on dams as an aid in understanding the origin of arcs. However, the difference in the intercept of the two lines with the In(wavelength) axis may indicate changes in process that preclude extrapolations from one spatio-temporal scale to the other.