Soil and water components of banded vegetation patterns

Abstract

Banded landscapes are comprised of alternating bands of vegetation and bare ground aligned along the contours in arid and semi-arid regions (50–750 mm rainfall), on very gentle and uniform slopes (0.2–2%). Vegetated bands can be perpendicular to the direction of the dominant wind, or more frequently of the slope. Under given climatic conditions, slope gradient is the controlling factor of the type of pattern (`spotted', `broadly' or `typically' banded). For a given slope gradient, mean annual rainfall determines the contrast between the vegetated and bare phase, as well as the band length and the interband width:band width ratio. A typical transect through such two-phase mosaic includes a dynamic succession of surface crusts which generates a run-off–run-on system. This favours the capture of limited water resources and thus a biomass production greater for banded patterns than spotted patterns or uniformly scattered vegetation. Moreover, vegetated bands act as natural bench structures that greatly limit soil erosion. As suggested by modelling, banded vegetation patterns can equally be derived from nearly bare areas or from dense vegetation patterns. Because banded vegetation patterns have often been considered as a form of degradation of previous continuous vegetation cover, many attempts have been made by foresters to restore this initial cover by reforestation of the bare interbands. However the numerous failures of reforestation of the bare interbands illustrate the key role of these mosaic components in the maintenance of the tiger bush ecosystem. Banded vegetation patterns are more resilient to climate change than to human disturbances. The loss of landscape patchiness due to the clearing of the bands either for fuelwood or for ephemeral cropping induces a rapid decline in soil fertility and water infiltration potential. Long-term monitoring shows that the interband width:band width ratio adapts to rainfall variations that are intrinsic of semi-arid zones. Moreover, climatic changes might also induce a change in the type of patterning, suggesting that banded vegetation patterns are highly self-sustainable. The ecological differences between the pioneer and the decaying edges of the vegetated bands, the temporal/spatial succession of soil crusts, as well as other indices such as the spatial distribution of soil organic matter and termite nests suggest the upslope migration of the bands. Nevertheless field evidence of such a shift remains scarce. Profound lessons can be learnt from the study of banded landscapes in terms of ecosystems functions. When designing water-harvesting systems, or restoring degraded arid and semi-arid land, banded vegetation patterns should be imitated to optimize biomass production and limit land degradation.