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TheImprovement ofTropical and Subtropical Rangelands

TheImprovement ofTropical and Subtropical Rangelands

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28 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS<br />

The effects of rangel<strong>and</strong> degradation often extend well beyond<br />

the rangel<strong>and</strong>s themselves. Dust originating in degraded rangel<strong>and</strong>s<br />

is transported by dry-season winds to distant areas, causing annoyance,<br />

health hazards, <strong>and</strong> costly interruptions in air <strong>and</strong> ground<br />

traffic. The rapid release of runoff in degraded rangel<strong>and</strong>s following<br />

rains contributes greatly to destructive flooding in downstream<br />

lowl<strong>and</strong>s, <strong>and</strong> sediment entering drainage systems in degraded rangel<strong>and</strong>s<br />

shortens the useful life of reservoirs <strong>and</strong> irrigation systems.<br />

Less obvious effects would include the impact ofrangel<strong>and</strong> devegetation<br />

on climatic regimes. For example, it is now widely believed<br />

that precipitation is strongly influenced by biogeophysical feedback<br />

mechanisms (Charney, 1976). According to this hypothesis, drought<br />

is reinforced through changes in vegetative cover. Large-scale losses<br />

of vegetation would increase surface albedo, which, in turn, would<br />

affect the atmospheric energy budget in such a way that the subsidence,<br />

which promotes aridity, would be intensified.<br />

Further, it is now believed that levels ofprecipitation are strongly<br />

influenced by soil moisture locally rele~d into the atmosphere<br />

through evapotranspiration. Hence, reduced vegetative cover <strong>and</strong> decreased<br />

soil moisture would result in reduced local precipitation. Finally,<br />

losses of vegetation affect surface roughness in the atmospheric<br />

boundary layer. Surface roughness contributes to the destabilization<br />

of moisture-laden air masses, thus encouraging precipitation. Devegetation<br />

also reduces carbon dioxide uptake in the planetary biomass.<br />

The greater concentration of carbon dioxide in the atmosphere contributes<br />

to global warming, causing changes in atmospheric circulation<br />

<strong>and</strong> rising sea levels through the melting ofcontinental ice sheets<br />

(Study of Man's Impact on Climate, 1971; Woodwell, 1984).<br />

The science of range management originated in North America,<br />

<strong>and</strong> North American approaches to range management are<br />

described in several well-known volumes, such as A. W. Sampson<br />

(1952), Stoddart <strong>and</strong> Smith (1955), R. R. Humphrey (1962), <strong>and</strong><br />

National Research Council (1962, 1984). Historically, attempts to<br />

transfer experience gained in the management of North American or<br />

European rangel<strong>and</strong>s to the management of tropical <strong>and</strong> subtropical<br />

rangel<strong>and</strong>s have been unsuccessful (Heady <strong>and</strong> Heady, 1982).<br />

In improving tropical <strong>and</strong> subtropical rangel<strong>and</strong>s, it is important to<br />

carefully characterize the physical system being managed in order to<br />

better underst<strong>and</strong> the biological potential of the system <strong>and</strong> assure<br />

that critical ecological processes are restored <strong>and</strong> mainta.ined. It is<br />

equally important to relate efforts in range improvement to the needs,

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