Geography Reference
In-Depth Information
the vegetation is damaged, the soil becomes much more susceptible to erosion. Herein
lies one of the greatest limiting factors to the use of mountain soils for agricultural pur-
poses. In North America, mountain areas are rarely used for agriculture, while in other
parts of the world (e.g., the Alps, Andes, and Himalaya), pressures of dense populations
and other factors, such as the lack of fertile, low-relief agricultural lands and land own-
ership systems, have led to centuries of intensive agriculture, grazing, and other an-
thropogenic stresses on steep slopes, as discussed in more detail in Chapter 11.
Cultivation of mountain lands is limited not only by steep surfaces and thin soils,
but also by climate. If the climate is adequate and the need is great, even the steepest
slopes may be cultivated. If steep slopes are to be brought under cultivation, terracing
is often employed to decrease soil erosion and water loss, but it is an expensive and
high-maintenance endeavor. In most cases, terracing has been accomplished by organ-
ized cultures, which have continued to maintain the fields and terraces. Many of the
world's mountainsides, notably those in the Philippines and Southeast Asia, have been
extensively terraced for agriculture. Before the mid-1980s, many “Western experts” and
institutions assumed that conversion of hillsides from forest to terraced agriculture was
causing an environmental catastrophe in the Himalaya, an assumption that has been
largely disproved (Ives and Messerli 1989). Terracing for rain-fed agriculture has been
shown to decrease soil erosion rates in the Middle Hills of Nepal (Gardner and Ger-
rard 2003) and in Jamaica, El Salvador, and Taiwan (Sheng 1981). Where terraces are
actively maintained and used for agriculture, the soil can become enriched in plant nu-
trients and biological activity. In the Colca Valley of Peru, after 1,500 years of cultiva-
tion, terraces, both those still cultivated and abandoned, have higher concentrations of
phosphorus, organic carbon, and nitrogen, and higher levels of three soil enzymes than
nearby uncultivated slopes (Sandor and Eash 1995). Terrace abandonment, however, is
often associated with higher rates of soil erosion (Inbar and Llenera 2000) and a higher
incidence of land-sliding (Gerrard and Gardner 2002).
Without terraces, most mountain agriculture is less sustainable and more vulnerable
to soil erosion. An intermediate situation is found in the Alps, which have a long history
of settlement and agriculture. These landscapes have been preserved remarkably well,
considering the length of time they have been used; nevertheless, the replacement of
natural vegetation by crops has depleted the soil (Bouma 1974). In the northern Andes,
a recent increase of greenhouse-based agriculture created a market for highly organic
mountain soils (Fig. 6.9). The resulting excavation and removal of soil from sites in the
high-elevation grasslands creates an anthropogenically accelerated process of soil loss
in the uplands.
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