A geomorphological based banded (`tigerʹ) vegetation pattern related to former dune fields in Sokoto (Northern Nigeria)

A banded vegetation pattern has been observed on aerial photographs of Sokoto province in Northern Nigeria. It was successfully used as a land mapping characteristic in the soil and vegetation (land unit) reconnaissance survey see FAO (1969) [FAO, 1969. Soils Survey and Land Classification, Soils and Water Resources Survey of the Sokoto Valley (Nigeria). Final report Vol. V, FAO/SF:67/Nr3.], Zonneveld et al. (1971) [Zonneveld, I.S., de Leeuw, P.N., Sombroek, W.G., 1971. An ecological interpretation of aerial photographs in a savanna region in northern Nigeria. Publication of the International Institute for Aerial Survey and Earth Sciences (ITC) Enschede, The Netherlands, 41 pp.]. The banded pattern is visible through vegetation density related to differences in surface hydrology. The latter are caused by variation in soil sealing that is connected with a by sheet erosion (pediplanisation) levelled former early Holocene to late Pleistocene dune landscape (Sangiwa coversand landscape). The difference in sealing is related to a (very small) difference in silt content between the levelled former dunes and the filled in valleys in combination with extreme low organic matter content. The lower silt content in the valley filling is connected with the re-sedimentation process, a feature well known in coversand formation. The sealing is a present day process. Fresh loosened soil material (by ploughing and soil pit digging) is after one rain shower already covered with a sealed crust of several millimetres through which no water penetrates. The orientation of the bands (about NNW–SSE) is perpendicular to the prevailing wind direction (ENE–WSW) during the period of formation of the former dunes. The genesis of this pattern of regional scale appears to be quite different from local scale banded patterns known to us in dry zones in Africa and Asia and described, in the same period of our study as `Brousse tigréeʹ, being moving vegetation arcs related to sheet runoff and by consequence oriented perpendicular to that water flow.