Abstract
Vegetation patterns in arid regions are influenced by chance causes, including the nonuniformity of soil or topography, but these patterns themselves are not a matter of chance. We present here the imperatives underlying vegetative patterning with the aid of mathematical modeling, as well as the implications of these desimperatives for the phenomenon of desertification, which is threatening about a quarter of the Earth's landmass.
The mechanism responsible for creating these patterns is the positive feedback between vegetation and water. On one hand, when the soil contains more water, vegetation grows more rapidly. On the other hand, when the plants increase in size they are able to take up more water. This is due to longer root systems or to increased water infiltration at the plants' locations. When precipitation is less than ideal, uniform vegetative regions will start to dry out, producing patterned growth. Vegetation in areas containing larger plants and a greater total biomass than a neighboring area will extract water at the expense of the nearby smaller plants. As a result, the gap in biomass of the two areas will increase, further increasing the gap in water availability of the areas. After a time, the neighboring vegetation will completely dry out.
A mathematical model developed by researchers at the Blaustein Institutes for Desert Research enables an investigation of vegetative pattern formation along the rainfall gradient. The model predicts the orderly change of patterns when precipitation decreases over a flat plane of soil: uniform vegetation to vegetation with holes to growth in bands to spotted growth to bare soil. The model also predicts the possible coexistence of different stable vegetation patterns under the same level of precipitation, with the specific pattern seen in the field depending on the history of changes in water supply and vegetation coverage.
The coexistence of two stable patterns under identical external conditions provides the key for understanding the process of desertification. This phenomenon, which is exemplified by the transition from one vegetation pattern to another, is characterized by a reduction of biological productivity (such as the change from a spotted pattern to bare soil); it may result from climatic change or human activity, or both. When bare soil appears, the restoration of former patterns is not trivial. Because of the relatively stable nature of bare soil, removal of local human endeavors or a cyclic return of precipitation to its previously higher levels (which supported a spotted pattern) will not automatically restore the vegetation cover.
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