Tiger bush

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Tiger bush is a patterned vegetation community consisting of alternating bands of trees or shrubs, separated by bare ground or low herb cover, that run roughly parallel to contours. The patterns occur on low slopes in arid and semi-arid regions, such as in Australia, Sahelian West Africa, and the USA.

Due to the natural water harvesting capacity, many species in tiger bush usually occur only under a higher rainfall regime.

How tiger bush forms

The alternating patterns arise from the interplay of hydrological, ecological, and erosional phenomena. In the regions where tiger bush is present, plant growth is water-limited - the shortage of rainfall prevents vegetation from covering the entire landscape. Instead, trees and shrubs are able to establish by either tapping soil moisture reserves laterally or by sending roots to deeper, wetter soil depths. By a combination of plant litter, root macropores, and increased surface roughness, infiltration into the soil around the base of these plants is enhanced. Surface runoff arriving at these plants will thus likely to become run-on, and infiltrate into the soil.

By contrast, the areas between these larger plants contains a greater portion of bare ground and herbaceaous plants. Both bare soil, with its smoother surface and soil crusts, and herbaceaus plants, with fewer macropores, inhibit infiltration. This causes much of the rainfall that falls in the inter-canopy areas to flow downslope, and infiltrate beneath the larger plants. The larger plants are in effect harvesting rainfall from the ground immediately up-slope.

More water will infiltrate at the up-slope edge of the canopies than down-slope. This favours the establishment and growth of plants at the up-slope edge, and mortality of those down-slope. Differences in growth and mortality across the vegetation band result in the band moving gradually upslope.

Tiger bush never develops on moderate to steep slopes, because in these cases surface runoff concentrates into narrow threads or rills instead of flowing over the surface as sheet flow. Sheet flow distributes water more evenly across hillslope, allowing a contiguous vegetation band to form.

The exact roles and importance of the different phenomena is still the subject of research.

Exploitation and conservation

The woody plants comprising tiger bush are used for fire wood and as a source of foliage for grazers. The extensive loss of tiger bush around Niamey, Niger, now threatens local giraffe populations. In neighbouring Burkina Faso, the tiger bush vegetation is also declining.

See also

• Ecohydrology